WATER GLOBAL PRACTICE WASTEWATER TREATMENT AND REUSE A Guide to Help Small Towns Select Appropriate Options Jean-Martin Brault, Konrad Buchauer, and Martin Gambrill About the Water Global Practice Launched in 2014, the World Bank Group’s Water Global Practice brings together financing, knowledge, and implementation in one platform. By combining the Bank’s global knowledge with country investments, this model generates more firepower for transformational solutions to help countries grow sustainably. Please visit us at www.worldbank.org/water or follow us on Twitter: @WorldBankWater. About GWSP This publication received the support of the Global Water Security & Sanitation Partnership (GWSP). 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WASTEWATER TREATMENT AND REUSE A Guide to Help Small Towns Select Appropriate Options Je n-M rtin Br ult, Konr d Buch uer, nd M rtin G mbrill © 2022 International Bank for Reconstruction and Development / The World Bank 1818 H Street NW, Washington, DC 20433 Telephone: 202-473-1000; Internet: www.worldbank.org This work is a product of the staff of The World Bank with external contributions. The findings, interpretations, and conclusions expressed in this work do not necessarily reflect the views of The World Bank, its Board of Executive Directors, or the governments they represent. The World Bank does not guarantee the accuracy of the data included in this work. The boundaries, colors, denominations, and other information shown on any map in this work do not imply any judgment on the part of The World Bank concerning the legal status of any territory or the endorsement or acceptance of such boundaries. Rights and Permissions The material in this work is subject to copyright. Because The World Bank encourages dissemination of its knowledge, this work may be reproduced, in whole or in part, for noncommercial purposes as long as full attribution to this work is given. Please cite the work as follows: Brault, Jean-Martin, Konrad Buchauer, and Martin Gambrill. 2022. “Wastewater Treatment and Reuse: A Guide to Help Small Towns Select Appropriate Options.” World Bank, Washington, DC. Any queries on rights and licenses, including subsidiary rights, should be addressed to World Bank Publications, The World Bank Group, 1818 H Street NW, Washington, DC 20433, USA; fax: 202-522-2625; e-mail: pubrights@ worldbank.org. Cover design: Sue McGillivray, [e]merge Creative, and Bill Pragluski, Critical Stages, LLC. Report design: Circle Graphics, Inc. Contents Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi 1 About This Guide 1 Introduction and Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Objective and Target Audience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Scope and Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 How to Use This Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Overview of the Guide’s Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 The Citywide Inclusive Sanitation Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Considerations for Small-Town Wastewater Treatment 5 Definition of a Small Town . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Common Characteristics of Small Towns Relevant for Wastewater Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Wastewater Resource Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3 Appropriate Wastewater Treatment Technology for Small Towns 9 Background to the “Appropriateness” of Technologies . . . . . . . . . . . . . . . . . . 9 Types of Wastewater Treatment Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Levels of Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Pretreatment Options and Process Considerations for Small Towns . . . . . . . 10 Preselection of Wastewater Treatment Technologies Appropriate for Small Towns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Wastewater Treatment and Reuse iii Technology Sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 The Optimum Combination of Technologies for Primary and Secondary Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 The Optimum Combination of Treatment Technologies for Wastewater Reuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Note . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4 Factors to Address for WWTPs in Small Towns 71 Project Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Feasibility of Sewers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Total Connections to the WWTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Fecal Sludge/Septage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Regulations for Wastewater Treatment, Effluent, and Sludge Discharge and Reuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Available Land for the WWTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Power Supply to the WWTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Technology Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Treatment Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Ease of Upgrading to Enhanced Nutrient Removal . . . . . . . . . . . . . . . . . . 81 Land Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Labor Qualification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Availability of Replacement Parts and O&M Inputs . . . . . . . . . . . . . . . . . . 86 Wastewater Sludge Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Energy Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 O&M Costs (OPEX) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Investment/Capital Costs (CAPEX) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Reuse Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Climate Change Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 5 Applying This Guide in Practice: A Step-by-Step Approach 104 Methodology: Overview of Suggested Five-Step Approach . . . . . . . . . . . . 104 Step 1: Familiarize Yourself with the Guide’s Methodology . . . . . . . . . . . . . 105 Step 2: Convene Key Stakeholders to Discuss the Project Criteria . . . . . . . 105 Schematic Work Plan for Step 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Step 3: Convene Key Stakeholders to Discuss the Project Criteria . . . . . . . . 107 Step 4: Identify and Apply Nonnegotiable or Exclusion Criteria . . . . . . . . . 108 iv Contents Step 5: Assign Weighting to Technology Criteria and Calculate Total Score for Remaining Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Schematic Work Plan for Steps 3 to 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 How to Weight Criteria and Calculate Total Scores . . . . . . . . . . . . . . . . . 109 6 Case Studies 113 Case 1: Small Town in Morocco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Case 2: Small Town in Vietnam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Case 3: Small Town in El Salvador . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Appendix A: Extended Aeration versus Conventional Activated Sludge 137 References 142 Wastewater Treatment and Reuse v Figures Figure 1.1 When to Apply this Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Figure 3.1 Examples of Combinations of Treatment Options for Different Wastewater Reuse Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Figure 4.1 Defining Project Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73 Figure 4.2 Relative Increase in BOD Load in a WWTP as a Function of the Combined Discharge of Municipal Wastewater and Different Fecal Sludge Volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Figure 4.3 Summary of BOD5 Effluent Quality Ranges of Different Wastewater Treatment Technologies for Medium-Strength Wastewater . . . . . . . . . . . . . . 80 Figure 4.4 Summary of Land Requirement Ranges of Different Wastewater Treatment Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Figure 4.5 Economy of Scale Effect on Land Requirements of WWTPs for Different Wastewater Treatment Technologies . . . . . . . . . . . . . . . . . . . . . . . . . 85 Figure 4.6 Summary of Sludge Production Ranges of Different Wastewater Treatment Technologies (Assuming a Sludge Dry Solids Content of 20 Percent SS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Figure 4.7 Summary of Electric Power Consumption Ranges of Different Wastewater Treatment Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Figure 4.8 Summary of OPEX Ranges of Different Wastewater Treatment Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Figure 4.9 Economy of Scale Effect on OPEX of WWTPs with Different Wastewater Treatment Technologies and Treatment Standards (2019 Price Level) . . . . . . 94 Figure 4.10 Summary of CAPEX Ranges of Different Wastewater Treatment Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Figure 4.11 Economy of Scale Effect on CAPEX of WWTPs with Different Wastewater Treatment Trains (2019 Price Level) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Figure B4.2.1 NPV Results for Different Wastewater Treatment Technologies . . . . . . . . . . . 98 Figure 5.1 Overview of the Key Steps in the Application of this Guide . . . . . . . . . . . . . 105 Figure 5.2 Project Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Figure 5.3 Schematic Work Plan for Step 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Figure 5.4 Technology Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 Figure 5.5 Schematic Work Plan for Steps 3 to 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Figure 6.1 Summary of Weighted Scoring for Remaining Technologies after Step 4 for the Morocco Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Figure 6.2 Summary of Weighted Scoring for Remaining Technologies after Step 4 for the Vietnam Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Figure 6.3 Summary of Weighted Scoring for Remaining Technologies after Step 4 for the El Salvador Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Figure A1.1 Schematic Diagram of an Oxidation Ditch EA . . . . . . . . . . . . . . . . . . . . . . . . 139 Figure A1.2 Schematic Diagram of a Carousel Type EA . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Figure A1.3 Schematic Diagram of a Plug-Flow Type EA . . . . . . . . . . . . . . . . . . . . . . . . . . 140 vi Contents Tables Table 2.1 Population for Small Towns, by Country and Region . . . . . . . . . . . . . . . . . . . . . 5 Table 3.1 Levels of Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Table 3.2 Typical Pretreatment Options and Process Considerations for Small-Town WWTPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Table 3.3 Long List of Treatment Technologies and Preselection of Appropriate Technologies for Small-Town WWTPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Table 3.4 List of Wastewater Treatment Technologies that Met the Preselection Criteria of Being Appropriate for Small Towns . . . . . . . . . . . . . . . . . . . . . . . . . 17 Table 3.5 Typical Wastewater Treatment Trains for Preselected Treatment Technologies for Small-Town WWTPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Table 3.6 Typical Sludge Treatment Trains for Preselected Treatment Technologies for Small-Town WWTPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Table 3.7 Correspondence between Log Units and Removal Efficiency Percentages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Table 4.1 Summary of Treatment Efficiency Scores for Different Effluent Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Table 4.2 Examples of Different Scenarios of Required Treatment Performance . . . . . 82 Table 4.3 Summary of Scoring for Ease of Upgrading to BNR and Examples of Scores for Different Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Table 4.4 Summary of Scoring for Relative Land Requirements and Corresponding Examples of Scores for Different Scenarios of Land Requirements . . . . . . . . 84 Table 4.5 Summary of Scoring for O&M Labor Needs and Corresponding Examples of Scores for Different Scenarios of O&M Labor Needs . . . . . . . . . 86 Table 4.6 Summary of Scoring for O&M Inputs and Replacement Parts and Corresponding Examples of Scores for Different Scenarios . . . . . . . . . . . . . . 87 Table 4.7 Summary of Scoring for Needed Frequency of Sludge Removal . . . . . . . . . . 89 Table 4.8 Summary of Scoring for Energy Demand and Examples of Scores for Different Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Table 4.9 Energy Consumption and Treatment Efficiency . . . . . . . . . . . . . . . . . . . . . . . . 92 Table 4.10 Summary of Scoring for O&M Costs (OPEX) and Corresponding Ranges . . . 94 Table 4.11 Summary of Scoring for Investment Costs and Examples of Scores for Different Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Table 4.12 Analysis of the Reuse Potential of Products Resulting from a Treatment Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Table 5.1 Summary of Suggested Scores for Each Technology (Standard Defaults) . . 110 Table 5.2 Summary of Weighted Scoring for Each Technology, Based on Suggested Standard Defaults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Table 6.1 Project Criteria for the Morocco Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Table 6.2 Technology Criteria and Exclusion Criteria for the Morocco Case . . . . . . . . 115 Table 6.3 Summary of Excluded Technologies for the Morocco Case . . . . . . . . . . . . . 118 Table 6.4 Summary of Scoring for Remaining Technologies after Step 4 for the Morocco Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Wastewater Treatment and Reuse vii Table 6.5 Summary of Weighted Scoring for Remaining Technologies after Step 4 for the Morocco Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Table 6.6 Project Criteria for the Vietnam Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Table 6.7 Technology Criteria and Exclusion Criteria for the Vietnam Case . . . . . . . . .123 Table 6.8 Summary of Excluded Technologies for the Vietnam Case . . . . . . . . . . . . . . 125 Table 6.9 Summary of Scoring for Remaining Technologies after Step 4 for the Vietnam Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Table 6.10 Summary of Weighted Scoring for Remaining Technologies after Step 4 for the Vietnam Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .126 Table 6.11 Project Criteria for the El Salvador Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Table 6.12 Technology Criteria and Exclusion Criteria for the El Salvador Case . . . . . . 131 Table 6.13 Summary of Excluded Technologies for the El Salvador Case . . . . . . . . . . . 133 Table 6.14 Summary of Scoring for Remaining Technologies after Step 4 for the El Salvador Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Table 6.15 Summary of Weighted Scoring for Remaining Technologies after Step 4 for the El Salvador Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Boxes Box 3.1 Examples of Selection of Technology for Agricultural Wastewater Reuse . . . 70 Box 4.1 Disinfection Considerations: Formation of Chlorination By-Products . . . . . . 79 Box 4.2 Life-Cycle Cost Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 viii Contents Acknowledgments Jean-Martin Brault led the preparation of this guide with the support and guidance of Konrad Buchauer, Nishtha Mehta, and Martin Gambrill. The guide benefited from the contributions of relevant background literature, the sharing of experiences, and the feedback provided by many people at different stages of its development. We are grateful for such contributions and insights from Clémentine Stip, Rebecca Gilsdorf, Klaus Neder, Dimitri Xanthoulis, Daniel Nolasco, Gustavo Heredia, Sean Nelson, Nathan Engle, and Richard Abdulnour. The authors are also grateful to the numerous colleagues and peer reviewers from inside and outside of the World Bank for their valuable comments and support during the preparation of the guide, including Ravikumar Joseph, Habab Taifour, Irma Setiono, Ernesto Sanchez- Triana, Andreas Rohde, Gang Qin, Gustavo Saltiel, and Colette Génevaux. Finally, we want to thank Seema Thomas and Erin Barrett for helping us finalize and publish the guide. Wastewater Treatment and Reuse ix Executive Summary Small towns in low- and middle-income countries (LMICs) are growing rapidly and struggling to meet the increased demands of wastewater collection and treatment. To avert public health crises and continued environmental degradation, small towns are actively seeking safely managed sanitation solutions, appropriate for their scale, institutional capacity, financial resources, and overarching needs. This document is designed to provide a guide of small-town wastewater treatment processes in order to assist engineers, managers and other stakeholders responsible for wastewater service provision in identifying and selecting appropriate wastewater treatment processes for small towns. This guide is part of a World Bank suite of tools and other material to support World Bank teams and their government counterparts in the planning, design, and implementation of sanitation projects in urbanizing areas. Addressing the specific context of small towns, the format of this guide begins with an introduction of key concepts for a decision maker to understand and then applies a suggested five-step approach to exploring appropriate wastewater treatment technologies, culminating with case studies from three regions applying this approach. The guide’s introduction delves into the unique considerations for small-town wastewater treatment and the exploration of corresponding technologies. Before demonstrating the application of the approach, the guide also navigates (a) factors external to the technologies that define the characteristics and environment of a given small town and that will affect technology choice, coupled with (b) technology-specific information that will ultimately influence decision making. Before embarking on the formal planning and design process, the user is highly encouraged to become familiar with the guide methodology in its entirety while drawing on the principles of the Citywide Inclusive Sanitation (CWIS) approach. 1 Sewers and wastewater treatment should be pursued only in small towns where such a service-delivery approach is deemed the most appropriate, following the comparison of its advantages and disadvantages with onsite sanitation and fecal sludge management alternatives, as espoused by the CWIS approach. Note 1. For more information about the Citywide Inclusive Sanitation approach, see the World Bank’s CWIS website at www.worldbank.org/cwis. x Executive Summary Abbreviations ABR anaerobic baffled reactor MBR membrane bioreactor AL aerated lagoon MLD million liters per day (1 MLD = 1,000 m3/d) ANDA Administración Nacional de Acueductos MLSS mixed liquor suspended solids y Alcantarillados MPN most probable number ANF anaerobic filter N 2O nitrous oxide AS activated sludge NPV net present value AT aeration tank; O&M operation and maintenance BAF biological aerated filters ONEE Morocco’s National Electricity and Water BD biogas digester Office (Office National de l’Électricité BNR biological nutrient removal et de l’Eau Potable) BOD biochemical oxygen demand OPEX operating expenditures cap capita PE population equivalent CAPEX capital expenditures PE60 population equivalent of 60 g BOD5 CAS conventional activated sludge per capita per day CEPT chemically enhanced primary treatment P&ID piping and instrumentation diagram CH4 methane PP polishing pond Cl chlorine or chlorination PST primary sedimentation tank CO2 carbon dioxide RBC rotating biological contactor CO2e carbon dioxide equivalents RDF rotary disc filter COD chemical oxygen demand RF rock filter CW constructed wetland SBR sequencing batch reactor CW(1-st) single-stage constructed wetland SDG Sustainable Development Goal CW(hybrid) hybrid constructed wetland SF sand filter CWIS Citywide Inclusive Sanitation SpTP septage treatment plant DBP disinfection by-products SS suspended solids EA extended aeration (= low-load activated ST septic tank sludge) TF trickling filter FAB fluidized aerated bed TF/SC trickling filter/solids contact process FC fecal coliforms THM trihalomethane F/M food to microorganism ratio TSS total suspended solids FSM fecal sludge management UASB upflow anaerobic sludge blanket reactor FST final sedimentation tank UASB-TF UASB followed by a TF GHG greenhouse gas UASB-WSP UASB followed by a WSP GWP global warming potential UV ultraviolet [disinfection] IFAS integrated fixed film activated sludge WSP waste stabilization pond (here consisting IMH Imhoff tank of the classical configuration of anaerobic, ISF intermittent sand filter facultative, and maturation ponds) LMIC low- and middle-income countries WWTP wastewater treatment plant MBBR moving bed biological reactor Wastewater Treatment and Reuse xi About This Guide 1 Introduction and Background Low- and middle-income countries (LMICs) generally lack adequate wastewater infrastructure, and although 39 percent of the global population used a safely managed sanitation service in 2015, only 27 percent of the global population used facilities connected to sewers that led to wastewater treatment plants (WHO and UNICEF 2017). This gap between collection and treatment varies across regions—for example, 69 percent of the wastewater collected in Arab States is safely treated (LAS, ESCWA, and ACWUA 2016), compared with 30 to 40 percent in Latin America (Rodriguez and others 2020) and roughly 10 to 20 percent in Asia and the Pacific region (UNESCO World Water Assessment Programme 2017). This gap is important because it poses a critical obstacle to reaping the benefits of improved human health, environmental protection, and water security, particularly as wastewater is increasingly seen as a valuable resource that should be managed effectively. Investment needs associated with closing this gap are substantial, contributing to the need for a paradigm shift with respect to wastewater planning, management, and financing. There is a need for adaptive solutions that can be incrementally implemented, building off what is already in place. This paradigm shift is particularly relevant for countries dealing with rapid urbanization. In these countries, small towns create a unique challenge as they exist at the nexus of urban and rural dynamics and can thus play a strategic role in bridging the gap between wastewater collection and treatment. For this to happen, appropriate wastewater treatment solutions should be selected to allow small towns to cope with the challenges of providing services without the potential for economies of scale offered in larger urban centers, and with the limited human and financial resources that are often found in small towns but which need to be considered when assessing the operation and maintenance (O&M) requirements of treatment plants. Identifying appropriate wastewater treatment solutions for small towns in LMICs requires thinking beyond the conventional technologies applied in developed contexts and requires an understanding of how local constraints on human and financial resources, road connectivity and/or available inputs, such as chemicals and replacement parts, could influence technology choice. Although ultimately technology recommendations and designs will be the responsibility of a technical specialist or consultant, those responsible for wastewater service provision—engineers, managers and decision makers more broadly— should oversee this selection process and have the necessary information to discriminate between different treatment trains. Wastewater Treatment and Reuse 1 This guide was inspired by a report, “Definition of priorities, including minimizing investment costs a Tool for Evaluating Unconventional Wastewater and ensuring operational sustainability. Treatment Technologies,” commissioned under the World Bank-financed Oum Er Rbia Sanitation Project to provide Morocco’s Office of Electricity Scope and Limitations and Drinking Water (Office National de l’Électricité Although the guide details a methodology to et de l’Eau Potable [ONEE]) with a decision-making determine appropriate wastewater treatment tool to diversify its menu of technological options processes for small towns, it does not aim to provide for small towns as part of the rollout of the country’s a definitive answer as to which wastewater treatment National Sanitation Master Plan. This guide has technology would be “optimal” for a given small complemented the evaluation criteria proposed town. The reader should be mindful about the therein to highlight the priorities of wastewater variability of contexts and the interpretation of the treatment for small towns and aims to bring a more different aspects explored. global perspective to the associated challenges (Golla 2014). In adapting the criteria used in the As a result, it is likely that, after applying the Morocco report, the guide relies on available data methodology proposed in the guide, more than and publications from developed countries that the one appropriate solution will be identified and authors consider relevant to LMIC contexts. The peer a more detailed analysis (particularly regarding review process also allowed for practitioners working costs) may be necessary to further narrow down the in LMICs to provide inputs on the applicability and selection. A more experienced user of the guide relevance of the guide’s recommendations and its may still wish to include other technologies for methodology in these contexts. additional comparison. The user should evaluate these additional technologies with the same criteria that are applied in the guide so as to be Objective and Target Audience able to compare them with the technologies The objective of this guide is to assist engineers preselected here. Furthermore, the guide does not and managers responsible for wastewater service provide specific guidance on, or standards for, the provision in understanding which solutions are engineering design of each technology, as a large technically feasible and in line with the priorities number of such resources already exist.1 of their small town. It provides a methodology for The guide emphasizes opportunities for cotreatment these decision makers to identify the characteristics of wastewater with fecal sludge, where appropriate, of their service area that will be most important although it does not provide guidance on fecal sludge in choosing appropriate wastewater treatment management (FSM) or treatment.2 solutions and to understand the trade-offs between different solutions that meet their needs. The information presented in this guide therefore aims How to Use This Guide to support decision makers in reviewing the work of This guide supports decision making in the an engineering consultant but not to supplant the prefeasibility and feasibility phases of a project work of such a consultant. cycle, as illustrated in Figure 1.1. Thus, the guide The guide highlights key factors in the decision- is meant to help optimize, at a very early project making process, such as treatment facility design, stage, when such optimization is easiest and most possibilities for reuse, and receiving water quality. effective, the direction and the content of subsequent The comparison of technologies considers several more detailed analysis. 2 About This Guide FIGURE 1.1 When to Apply This Guide Project initiation • Detailed review of available technical alternatives Prefeasibility • Definition and examination of commercail, technical, financial, economic, Apply environmental, social, and regulatory prerequisites and feasibility • Life-cycle costing analysis guide studies • Risk analysis here • Preliminary and final project selection, respectively Preliminary • Establishment if design bases and criteria • Development of mass and energy balances, process flowsheets engineering • Development of preliminary P&ID, plans, and layouts design • Definition and sizing of major equipment • Definition of all construction details, by discipline (civil, mechanical, chemical, Detailed process, electrical, instrumentation and control, etc.) engineering • Finalization of P&IDs, plans and layouts, equipment drawings, sizing and costing design • Development of commissioning and start-up procedures • Development of O&M manuals and procedures and “as-built” drawings Procurement, construction, commissioning, and start-up Operation and maintenance Note: O&M = operation and maintenance; P&ID = piping and instrumentation diagram. Overview of the Guide’s treatment. This section introduces the concept of a small town and presents the unique challenges Structure of wastewater service provisions in such settings. Chapter  1 describes the guide’s purpose, target Chapter 2 also presents wastewater resource audience, contents and organization, and provides recovery considerations for small towns. guidance to the reader on how to use it. Chapter 3 introduces basic concepts of wastewater Chapter  2 presents specific considerations to treatment technology for small towns. This section understand the context of small-town wastewater addresses different wastewater treatment levels Wastewater Treatment and Reuse 3 incorporated in a treatment train and presents sanitation. The CWIS approach promotes a range individual technology sheets for the different of technical solutions—both onsite and sewers, technologies considered appropriate for small centralized or decentralized—which are tailored towns, as well as presenting appropriate treatment to the dynamics of the world’s burgeoning trains. cities and their large pockets of informality by integrating financial, institutional, regulatory and Chapter 4 builds upon the foundational knowledge social dimensions, and by harmonizing the sanitation in the prior chapters and delves into the factors solutions with related urban services, including water influencing the choice of small-town wastewater supply, drainage and solid waste management. treatment solutions. It identifies specific criteria that should be employed when using the guide. Criteria As part of the implementation of these CWIS are split into those that are specific to a given town principles, the World Bank is developing a suite of or context (project criteria) and those that relate to tools and other material3 to support Bank teams technology (technology criteria). and their government counterparts when engaging on CWIS initiatives. This suite of tools and other Chapter  5 applies the suggested five-step material are intended for use by World Bank task methodology to identify the appropriate wastewater teams and their government counterparts for the treatment solution in a given small town, drawing planning, design, and implementation of urban on the theory and background provided in the prior sanitation projects, and they may also be of use to sections. It details the aim of each step and the others working on sanitation challenges in urban corresponding process to employ in carrying out areas around the world. the step. Chapter  6 provides examples of the guide’s application through the use of three case studies Notes from Morocco, Vietnam and El Salvador. 1. See, for example: (a) G. Chen, G. A. Ekama, M. C. M. van Loosdrecht, and D. Brdjanovic, Biological Wastewater Treatment: Principles, Modelling and Design (London: IWA The Citywide Inclusive Publishing, 2020); (b) S. R. Qasim and G. Zhu, Wastewater Treatment and Reuse: Theory and Design Examples Sanitation Approach (Boca Raton: CRC Press, 2018); and (c) Metcalf and Eddy, Wastewater Engineering: Treatment and Reuse, 4th ed. The World Bank Water Global Practice, in (New York: McGraw-Hill, 2003). partnership with sector partners, have together 2. For more information on FSM, see, for example, the Fecal Sludge Management Alliance at https://fsm-alliance.org/ advanced an approach to and L. Strande, M. Ronteltap, and D. Brdjanovic, Faecal tackling urban sanitation Sludge Management-Systems Approach for Implementation challenges termed Citywide and Operation (London: IWA Publishing, 2014). For fecal Inclusive Sanitation (CWIS). sludge treatment plant design, see K. Tayler, Faecal Sludge This comprehensive and Septage Treatment: A Guide for Low and Middle Income Countries (Rugby: Practical Action Publishing, approach aims to shift the paradigm regarding 2018), https://practicalactionpublishing.com/book/693/ urban sanitation interventions by promoting a faecal-sludge-and-septage-treatment. range of technical solutions that help ensure 3. For more information about the CWIS approach, see the that everyone has access to safely managed World Bank website at www.worldbank.org/cwis. 4 About This Guide Considerations for Small-Town Wastewater Treatment 2 Definition of a Small Town Although there is no universally agreed upon definition of a small town, in most countries there is an understanding (formal or otherwise) of what areas to classify as small towns, which are typically based on population size and density. The lower bound for the population of a small town is typically between 2,000 and 5,000 people, though in some areas (especially in Asia), the lower bound can be as high as 10,000 residents. The upper size limit varies even more, from 20,000 to 50,000 to as high as 100,000 people (again, the latter limit is found mostly in Asian TABLE 2.1 countries). The population densities in Population for Small Towns, by Country and Region small towns also vary widely: In Niger, for example, the average small town REGION COUNTRY POPULATION population density is14 people per square Africa Benin 2,000–20,000 kilometer, whereas in Bangladesh it is Ethiopia 2,000–60,000 1,033 people/km2 (Economic Consulting Mozambique 2,000–100,000 Associates 2015). Table 2.1 shows examples Uganda 5,000–25,000 of the population ranges for small towns in Asia Bangladesh 25,000–200,000 different regions. These values were drawn India 10,000–50,000 from legal definitions and from data from Indonesia 10,000–100,000 World Bank staff. Philippines 10,000–100,000a Some definitions of small towns include Europe Eastern Europe 2,000–10,000 additional criteria. For example, small towns Latin America and Bolivia 2,000–20,000 may be defined as having certain key the Caribbean Ecuador 12,000–50,000 pieces of infrastructure (for example, types Haiti 3,000–10,000 of public buildings or roads) or an average Honduras 5,000–30,000 household income above or below given Nicaragua 2,000–50,000 values. Geographical location can also Peru 2,000–30,000 differentiate small towns from other urban North Africa Morocco 10,000–50,000 centers, as small towns are geographically Tunisia 2,000–50,000 more remote and are more separated In the Philippines, the definition further specifies that small towns are places where people are from major markets than are primary a mostly not farming, where it is not a predominant activity, and where the population density is or secondary cities. Nevertheless, small greater than or equal to 500 people/km2. Wastewater Treatment and Reuse 5 towns are often well connected to major roads Common Characteristics of and/or waterways, giving them better access to markets and other urban centers than rural areas. Small Towns Relevant for Although small towns typically have access to Wastewater Management markets (for both buying and selling goods), it may In most parts of the world, urban areas, including take longer to get to these markets, and the cost small towns, are growing faster than rural areas. of goods may consequently be higher than in larger In small towns with high growth rates, planning urban centers. Their comparative remoteness also on traditional time scales can be challenging, means that small towns typically have fewer highly and towns often struggle to keep pace with their trained technical professionals. Additionally, small growing populations. These fast growth rates call towns cannot generally take advantage of existing for more flexible and adaptive urban planning to service provision from large cities, such as the main allow for continued expansion of the population electricity grid or their water supply and sanitation and of the town more broadly (for example, any services. industrial expansion). Small towns targeted by this guide also tend to This adaptive approach to planning can be be closer to rural areas and thus to agricultural particularly challenging in small towns, where fields. This aspect of small towns is important for institutions are often less developed. This is several reasons. First, such small towns often serve especially true in agglomerations that have only as a central location for collecting food before recently grown large enough to be considered a sending it to larger markets, making agriculture key town. In these areas, water supply and sanitation to their economies. Second, these small towns are may have historically been managed by community close to an ideal market for end-use products of boards, but these models may no longer be wastewater treatment systems (treated wastewater appropriate. Where utilities do exist, they are often for irrigation, biosolids for fertilizer, and so on). In newer and less established. However, a wide range addition, combined with collected animal waste, of institutional models exists, from community-run these wastewater end-use products offer increased systems to centralized management handled by a options for biogas production. More broadly, small nearby larger town. To have sufficient institutional towns are often closer to natural resource extraction capacity (especially in terms of technical skills) and activities, such as mining—and, like agriculture, to more generally use economies of scale, it may the mining sector provides another possible market make sense to link multiple nearby small towns for end-use materials (for example, reusing treated together. Legal institutions and frameworks may also wastewater). Overall, as with most urban areas, be less evolved, which can affect the development the economies of small towns can be diverse, of guidelines for both wastewater treatment and though they are often dominated by one of the reuse—if reuse is to be permitted at all. aforementioned sectors. In small towns already experiencing industrial Finally, please note that the definition of small towns growth, the institutional framework selected for presented in this guide excludes rural villages with treatment plant operation will undoubtedly be populations below the ranges stated above for small affected by the choice of treatment technology, towns, and excludes periurban areas surrounding and vice versa, because the roles of regulation and major urban centers and large cities. monitoring will increase if industrial wastewater is 6 Considerations for Small-Town Wastewater Treatment also collected and treated. Additionally, the type help map and assess demand for reuse products, of industry in a small town may affect not only the which are differentiated under the following broad treatment technology but also possible markets categories: for product reuse. For example, both the mining and agricultural sectors may use treated wastewater, 1. Water, consisting of wastewater effluent but the water quality standards for each will differ, treated to a level appropriate to the end as will the capacity and skillset required to produce, use, such as for groundwater recharge, for monitor and enforce them. irrigation of parks and lawns or of agricultural crops, for industrial processes, and so on At present, small towns use a wide range of technologies for managing sanitation. This variation 2. Sludge,1 such as reuse as a soil amendment is mostly explained by the different status of or as a fuel sanitation services in small towns in LMICs. Some 3. Nutrients, through the treated wastewater small towns still have high rates of open defecation, effluent or the treated biosolids2 whereas others may fully rely on onsite solutions (for example, latrines and/or septic tanks) or count 4. Energy, through the conversion of biogas on a single centralized sewer system. This guide into electric power and/or thermal heat, focuses specifically on wastewater treatment, thus and through the combustion of processed excluding any small town using only onsite sanitation. solids (when these are converted into fuel Nevertheless, although the guide focuses on briquettes) instead of fossil fuels contexts in which sewers are the dominant technical solution for conveyance of waste (that is, sewered For example, nearby agricultural activity could collection of blackwater and graywater), the represent a reuse market for treated wastewater treatment technologies presented here may still effluent and biosolids, or it could also present be appropriate for small towns handling sewage the opportunity to carry out the codigestion of combined with a certain amount of fecal sludge/ agricultural waste with the sludge from the WWTP. septage (see “Fecal Sludge/Septage” in Chapter 4). Alternatively, a mining company may be interested in the treated effluent from a small town’s WWTP to Wastewater Resource Recovery use directly in its processes. The ability to recover resources generated in In addition, when considering the recovery of wastewater treatment plants (WWTPs) has become wastewater treatment by-products, it is important to increasingly important in recent years, as several assess the expected production or supply of reuse treatment by-products can have significant products in a realistic manner. Treatment plants economic value for the utility or for the small town tend to be oversized, and it can take many years, in the vicinity of a treatment plant, and as awareness even decades, to achieve the design flow. This, in grows regarding the importance of circular economy turn, can result in the much smaller production of approaches in development. The evaluation of treated wastewater effluent, biogas and biosolids wastewater treatment alternatives for a given context than the amount originally planned for. Smaller than should consequently always assess potential demand expected by-product outputs result in oversized for and supply of these resources. The proximity reuse structures, generate less revenue, and can of certain economic activities to a small town can cause a project to fail. These negative outcomes Wastewater Treatment and Reuse 7 are especially likely in biogas recovery projects, by-products also requires specific organizational in which designers often tend to blur the thresholds arrangements to ensure process operationalization; between the potential for biogas generation and the utility responsible for the WWTP may not, the amount of biogas that can indeed be captured however, be interested in, or able to be directly for reuse, resulting in a financial burden for the involved in, the resource recovery process. A project sustainability of the related infrastructure. A realistic should therefore identify both the demand for estimate of treatment by-products is also important by-products and the players who will be responsible in estimating the potential income generation from for system management before wastewater treatment the sale of these by-products. by-product recovery is considered. This is particularly true for small towns, which may be well positioned The evaluation of wastewater treatment alternatives should also take the existing legal and institutional to connect with potential users of reuse by-products framework for reuse into account—considering but may require support from regional or national environmental, public health and economic agencies to help operationalize a reuse scheme. regulations, and identifying key players involved in its operationalization. In some contexts, reuse has no Notes legal status or existing environmental/public health 1. Sewage sludge refers to the solids separated during the standards may make reuse unattractive—for example, treatment of wastewater. the cost to treat to the necessary standard would be 2. Biosolids refers to sewage sludge treated to a degree greater than any possible revenue from the sale of that meets pollutant and/or pathogen requirements for the end product. The use of wastewater treatment beneficial reuse. 8 Considerations for Small-Town Wastewater Treatment Appropriate Wastewater Treatment Technology for Small Towns 3 Background to the “Appropriateness” of Technologies Wastewater treatment is undertaken in a series of steps that can have increasing effectiveness and complexity, depending on the financial means and the human resources available to operate the systems. To guide the user through the identification of a wastewater project’s physical, technical and financial boundaries, this section provides a list of criteria and a methodology that can help identify a subset of “appropriate” wastewater treatment technologies or process configurations for a specific project’s particular context. Depending on the context, it may also be important to adopt an adaptive and incremental approach to wastewater treatment to better respond to the realities found in the small towns of low- and middle-income countries (LMICs) and to ensure that desired effluent quality levels and/or treatment objectives can be realistically met. This section will therefore focus on introducing wastewater treatment technologies that are deemed appropriate for small towns. To support this guide, a series of two-page technology sheets has been developed. These provide an overview of the technology itself, the level of treatment that can be expected from each technology, selection criteria, and design considerations. The technology sheets, which are presented in Chapter 3, were developed with the considerations and criteria presented in this section in mind, and with the understanding of actual operating conditions of wastewater treatment plants (WWTPs) in small towns. Experience indeed shows that poor performance of treatment plants in LMICs, particularly for small towns, is often a result of a lack of operational expertise and of financial resources for adequate operation and maintenance (O&M), as well as whether the plant design included plans for O&M based on the available resources in the first place. That being said, it should be noted that the present document is not meant to serve as a design or an O&M manual, nor should the list of technologies presented hereafter be considered exhaustive. The aim of this section is to assist the user in intuitively making appropriate and informed decisions about technology selection by providing basic information that can be relevant for the design, financing, implementation, monitoring and O&M of cost-effective wastewater treatment systems in small towns. In addition, as wastewater treatment systems are composed of combinations of technologies in the primary, secondary and tertiary treatment steps, this section will also present appropriate wastewater treatment and sludge “treatment trains” for small towns. Wastewater Treatment and Reuse 9 Types of Wastewater along with other basic process considerations for small-town WWTPs. Treatment Systems Wastewater treatment systems can be extensive (or natural) and intensive (or primarily mechanically Levels of Wastewater driven) systems. In extensive systems, such as anaerobic and facultative lagoons, treatment rates Treatment are typically relatively slow, requiring large retention Wastewater treatment plants are typically grouped times and land requirements to achieve acceptable into different levels of treatment, commonly referred treatment levels. Intensive systems, such as aerated to as pretreatment, primary, secondary and tertiary lagoons, are based on higher reaction rates, resulting treatment. Additional treatment steps include in more compact reactor volumes and a smaller advanced treatment and sludge treatment. These treatment plant footprint, but at the cost of treatment levels group a variety of unit operations engineering complexity, and thus typically requiring and processes of wastewater treatment, as presented continual operational support, regular routine in Table 3.1. maintenance, and a continuous, reliable external The technology sheets presented in this guide source of energy. focus on primary, secondary and tertiary levels of Extensive, or natural, treatment systems should be wastewater treatment. No specific technology sheets prioritized as much as possible for small towns as are provided for pretreatment or for the sludge they are typically robust and are associated with low treatment stages; however, general guidance is energy consumption. Where space is limited, however, provided on these treatment stages in this section. alternative WWTP solutions are available along The sequence that treatment facilities typically the broad extensive-intensive spectrum combining use, consisting of primary, secondary and tertiary more technologically complex configurations that treatment stages, are known as wastewater treatment aim to increase treatment rates using a smaller layout trains. Similarly, sludge treatment trains describe footprint. Examples of these technologies include the multiple stages that are needed to treat the upflow anaerobic sludge blanket (UASB) reactors, sludge generated from the wastewater treatment trickling filters (TFs) or anaerobic baffled reactors train. Appropriate wastewater treatment and sludge (ABRs). These so-called seminatural systems are treatment trains are presented later in this chapter relatively robust and simple to operate but require (see “The Optimum Combination of Technologies more operational attention than natural systems, for Primary and Secondary Treatment” below). and they typically require additional steps to achieve a secondary level of treatment. The technology sheets clarify the level of treatment Pretreatment Options and that can be expected from each technology, except Process Considerations for the pretreatment technologies for which no specific sheets were created. Nevertheless, given for Small Towns their importance in enhancing the performance As mentioned in Table 3.1, pretreatment (also of downstream treatment processes, typical referred to as preliminary treatment) is critical to pretreatment unit operations are presented in the protect downstream treatment process units and next section (“Levels of Wastewater Treatment”), equipment from materials or substances that 10 Appropriate Wastewater Treatment Technology for Small Towns TABLE 3.1 Levels of Wastewater Treatment LEVEL OF TREATMENT DESCRIPTION Pretreatment The importance of pretreatment for small-town wastewater treatment solutions cannot be stressed enough. (also referred to Pretreatment of wastewater protects the units and equipment further downstream in the treatment process as preliminary from materials or substances that could hamper their performance or that could excessively increase the treatment) frequency or intensity of their maintenance needs. Pretreatment can help provide sustainable and cost- effective wastewater treatment solutions to small towns and, depending on the quality of the wastewater to be treated, several pretreatment processes could be required. Primary Primary treatment consists of the partial removal of suspended solids, organic matter and nutrients from wastewater. It produces a liquid effluent suitable for downstream secondary biological treatment and separates out solids as a sludge that should be treated before its ultimate disposal or reuse. Primary wastewater treatment is typically achieved by means of physical processes, such as sedimentation, but other types of treatment units can also be considered to provide a primary level of treatment, either on a stand-alone basis (septic/Imhoff tanks or digesters) or as the first step of a longer treatment chain (anaerobic ponds). Primary treatment can also help reduce fecal coliforms,a but secondary, and potentially tertiary, treatment will generally be required to make it fit for agricultural reuse. Secondary Secondary treatment aims at removing soluble and colloidal organic matter and suspended solids from wastewater, and it converts biodegradable organic matter into biomass, or sludge, through microbiological processes. Effective treatment can be achieved through aerobic processes, which require oxygen typically supplied by intensive mechanical aeration, facultative processes in which oxygen is supplied to bacteria through atmospheric reaeration and algal respiration in the water layer near the surface of lagoons, or anaerobic processes that harness anaerobic bacteria to convert organic matter into biogas. Secondary treatment can help further reduce fecal coliforms, but most options will still require tertiary treatment to produce effluent fit for agricultural reuse. Tertiary Tertiary treatment further improves the treatment level, beyond secondary treatment, of specific wastewater effluent parameters, such as nitrogen, phosphorus and suspended solids, as well as its hygienic quality (i.e., the removal of bacteria, viruses and other pathogens). The most common tertiary treatment process is a final disinfection stage, using ultraviolet radiation or chlorination. Other processes, such as polishing ponds, rock filters and other filter technologies, may also be used to meet specific effluent quality requirements. Such tertiary treatment effluent levels may, for instance, be required for agricultural reuse, groundwater recharge or discharge to recreational or protected waters. A small set of reuse options (for example, for potable reuse) would involve the use of additional steps, typically referred to as advanced treatment, including technologies such as reverse osmosis, ultrafiltration and microfiltration. Sludge All types of wastewater treatment plants produce sludge/biosolids as a by-product. In most small towns, sludge will require volume reduction before its disposal or reuse. Simple drying beds are typically a common solution, as they dewater the sludge and provide pathogen reduction. Additional treatment could be required to ensure further pathogen reduction before agricultural reuse. a Fecal coliforms are bacterial organisms that are used to indicate the presence of fecal contamination. could hamper their performance and/or their Protecting wastewater treatment systems in this maintenance needs. Examples of wastewater way is particularly relevant to small-town contexts in contents or properties that could excessively increase terms of promoting sustainable and cost-effective maintenance needs include coarse materials, wastewater treatment solutions in them. Table 3.2 grit, oil and grease, as well as acute variations in presents typical pretreatment options and process wastewater concentrations and flow volumes. considerations for small-town WWTPs. Wastewater Treatment and Reuse 11 TABLE 3.2 Typical Pretreatment Options and Process Considerations for Small-Town WWTPs PRETREATMENT COMMON OPTION OR PRETREATMENT PROCESS ISSUE CONSIDERATION DETAILED DESCRIPTION Coarse material (rocks, Screening/sieving Bar screens can be used to remove coarse material (rocks, sticks, leaves, sticks, leaves, garbage devices garbage and other debris) from wastewater that would otherwise damage and other debris) can pumps and other equipment or interfere with plant operability. Depending damage pumps and on the downstream needs, there are various types of screening devices other equipment from coarse (100 to 25 mm) to medium (20 to 10 mm) to fine (10 to 3 mm), and/or interfere with as defined by the gap separating the parallel screen bars, and there are plant operability manually and mechanically cleaned screens. Sieves feature further improved retention of solid matter because of small square or circular openings (mostly 1 to 5 mm in opening size), with their shape avoiding the passage of slim and longitudinal materials that can otherwise pass even fine screens. Sieves have, for instance, become common standard equipment upstream of UASBs to minimize scum formation in the latter. Rotating microscreens are special types of screens or sieves, in which the wastewater enters a slowly rotating drum, with the effluent passing through its cylindrical screen/sieve surface while solids are retained inside the drum. To avoid clogging, the retained matter is removed automatically by special cleaning and removal systems. In most cases, rotating microscreens have only small openings, ranging from about 0.1 to 3 mm, and the smaller the openings, the more the treatment efficiency of rotating microscreens resembles that of conventional primary settling tanks. Grit with the potential Grit removal systems Grit is the inert matter present in wastewater, which is heavier than the to create clogging, biodegradable organic solids to be degraded in the downstream treatment damage equipment processes. If not removed, grit can clog downstream systems, reduce and reduce efficiency treatment efficiency by occupying valuable reactor volume, and cause abrasion damage and wear in equipment. Grit removal equipment should be located after screening devices and before primary treatment units.  orizontal flow grit chamber. In small installations, grit can be removed 7 H by maintaining a low flow velocity in specific pretreatment channels or reactors, allowing grit to settle and lighter organic solids to be maintained in suspension and thus transported out of the channel. The settled grit can be manually or mechanically collected, though the former is typically favored in small plants. Vortex-type grit chamber. These units make beneficial use of 7  hydraulically induced vortex flow conditions. The grit spirals down along the perimeter of the cone- or cylinder-shaped reactor and is collected and removed at the bottom, and the degritted effluent is usually collected at the top. 7 Aerated rectangular grit chamber. Aerated rectangular grit chambers or aerated channels are typically used in larger works. In these installations, aerators diffuse coarse bubbles and produce a rolling motion, perpendicular to the wastewater flow. The heavier grit, washed free from organic matter by the turbulent flow, is collected at the bottom of the tank while lighter organic particles are suspended and eventually carried out. These systems also effectively allow for a preaeration of the wastewater and can be used to eliminate oil and grease within the same unit. (continues on next page) 12 Appropriate Wastewater Treatment Technology for Small Towns TABLE 3.2 Typical Pretreatment Options and Process Considerations for Small-Town WWTPs (Continued) PRETREATMENT COMMON OPTION OR PRETREATMENT PROCESS ISSUE CONSIDERATION DETAILED DESCRIPTION Varying levels of Oil and grease Oil and grease removal from wastewater involves separating substances viscosity and density removal or compounds that have a lighter density than water from the wastewater stream, and is commonly achieved through gravity separation, assisted flotation or chemical treatment. Whereas oil describes liquid products, such as vegetable oils, mineral oils and light hydrocarbons, the term grease refers to solid products or substances that originate from animal or vegetable sources and that may end up aggregating with suspended solids. Unit operations for oil and grease removal can also help collect other floating products, such as debris, soaps, foams, scum, detergents, plastics and so on. Variable conditions, Equalization Wastewater treatment processes, particularly biological ones, work best such as uneven with uniform conditions, and shocks in the form of sudden changes in the concentrations or flow concentration of organic matter or of nutrients in the wastewater can lead to process upsets. Equalization can be done either to eliminate or dampen wastewater flow variations that may arise during the day (flow equalization) or to dampen concentration variations in wastewater (concentration equalization) that may be associated with heavy storms or industrial contributions, for example. In certain cases, it might be recommendable to include an equalization step in the treatment train in order to: 7 Provide constant wastewater flows for the subsequent treatment steps and avoid feeding sudden concentration peaks to the biological steps of treatment processes. This can also help reduce the use of chemicals, as increased stability will require minimal dosage readjustments, thus minimizing wastage; 7 Avoid by-passing the treatment plant during heavy storms; and 7 Discharge an effluent of more constant quality into the receiving environment, thus reducing the risk of noncompliance with effluent standards. Need to continually Flow measuring All wastewater treatment plants require efficient flow measurement devices, monitor/control flow devices and flow at a minimum for both influent and effluent flows. Devices include Parshall in the system for distribution flumes, venturi flow meters, electromagnetic or ultrasonic flow meters, and improved stability of a variety of weirs in open channels. Weirs and flumes tend to be the most wastewater treatment common devices as they offer a simple way to measure flow. Flow distribution devices, such as distribution boxes, flow splitters or tipping buckets, are a key element of treatment plants in that they allow the influent flow to be shared between two or more parallel treatment trains. Wet weather flow Stormwater detention For combined stormwater and wastewater sewer networks, it is often exceeds wastewater basins necessary to add an additional treatment step to handle combined sewer treatment plant overflow events. Stormwater detention basins refer to the combination of capacity units that can help mitigate events when wet weather flows exceed the wastewater treatment plant capacity by diverting excess flows away from the treatment plant and providing a limited level of treatment (settling) before discharge into the environment—and/or after heavy rain events, the water stored in the stormwater basins can be progressively pumped back toward the wastewater treatment plant. This type of basin can also be part of a system for flow equalization. Note: UASB = upflow anaerobic sludge blanket reactor; WWTP = wastewater treatment plant. Wastewater Treatment and Reuse 13 Preselection of Wastewater Nevertheless, the long list does not include technologies for which only few large-scale Treatment Technologies references exist or for which design rules are Appropriate for Small Towns still under discussion within the engineering and academic communities. For this reason, Although a wide array of wastewater technologies technologies such as evaporative systems exists, not all of them are well suited for the requirements of small-town WWTPs. Therefore, or epuvalisation were not included in the to narrow down the technology options included long list. in this guide as likely to be most appropriate (b) Default to the most recent variation to small towns, twenty-one have been shortlisted of an established technology. Some using preselection criteria and these preselected technologies have been improved and technologies are the focus of this guide. upgraded into variations, which are now This shortlist was developed in two stages: widely considered to be safer and more cost- effective and show improved performance ◾ First, a long list of technologies was established. and general process robustness. In such ◾ Then, the long list was subjected to preselection cases, the improved version of the criteria, which led to the exclusion of certain technology was included and not the older technologies from further consideration if precursor technologies. For instance, the deemed unsuitable to small-town wastewater infiltration-percolation technology (also treatment. Only the remaining technologies called intermittent sand filters [ISFs]) is now are further developed as part of this guide. being abandoned for new construction projects globally in favor of constructed For the establishment of the long list of wastewater wetlands (CWs), which are de facto sand treatment technologies, the following inclusion filters complemented by vegetation. These criteria were used: have proved to be even more efficient in (a) Only well-established WWTP technologies terms of organic, solids, nutrients and are included. These are technologies that pathogen removal, with improved stability have been applied frequently in large- against hydraulic peak loads and with scale projects and for which generally considerably less risk of clogging compared acknowledged design rules exist. with ISFs. Applied three Applied four preselection criteria to Numerous criteria to determine Long list of Shortlist of wastewater determine well-established thirty-two twenty-one treatment suitability for wastewater technologies technologies technologies small-town treatment wastewater technology treatment 14 Appropriate Wastewater Treatment Technology for Small Towns (c) New technologies that are proving Small towns are defined in Chapter  2 as to be efficient and that are gaining having a population of mostly less than prominence. The guide includes various 50,000 people, sometimes less than new developments that have recently 100,000 people and, very rarely, even more become increasingly popular, such as hybrid than 100,000 people. In addition, the CWs , and combinations of secondary expected per capita wastewater pollution treatment components, such as UASB-waste level in LMICs and transition countries, where stabilization ponds (WSPs) and UASB-trickling this guide is intended to be applied, is most filters (TFs). Such combinations can prove likely to be less than 60 g BOD5/cap/d (for quite advantageous when compared to example, in the range of 30 to 50 g BOD5/ non-combined single process units because cap/d). An appropriate design capacity of such combinations usually reduce costs such WWTPs should typically be < 50,000 and simultaneously increase treatment PE60 (roughly equivalent to < 5 MLD), with efficiencies. a maximum of < 100,000 PE60 (< 10 MLD) in (d) Not all variations or modifications of a rare cases. Therefore, WWTP design sizes technology are appropriate or relevant. It appropriate for small towns are < 50,000 PE60 is common practice in wastewater treatment and < 5 MLD (with rare maxima of up to to use simple terms to describe complex 100,000 PE60 and < 10 MLD).1 technologies, such as activated sludge, trickling filter, anaerobic treatment, and (b) Technologies should be simple to operate so on. However, such simplifications and present low operational risks. can mask a wide range of quite different Finding sufficient personnel to operate technological variations, not all of which and maintain WWTPs in small towns can may be appropriate for specific projects—in present a challenge, so the technologies this case, for small towns. This is particularly chosen should be simple to operate with true for activated sludge and its extended low operational risks. aeration (EA) or low-load modifications. (c) Capital expenditures (CAPEX) and This applies to both batch-wise variations, operating expenditures (OPEX) associated such as sequencing batch reactors (SBRs), and flow-through type facilities, such as with technologies should be affordable. oxidation ditches (ODs). A comparative Financial aspects play a crucial role in the description and analysis of conventional sustainable running of small-town WWTPs, activated sludge (CAS) and EA is presented so CAPEX and OPEX should be kept within in Appendix A. the service provider’s financial capacity. This is particularly important to consider The resulting long list of well-established technologies for electromechanical installations, as these is presented in Table 3.3. This long list was further tend to get over-proportionally expensive narrowed down using the three following specific to purchase, operate and maintain as criteria in order to identify a short list of appropriate they get smaller. Technologies such as WWTP options for small towns: chemically enhanced primary treatment (a) The technology design capacity should (CEPT), flotation or thermal sludge dryers be appropriate for small town sizes. were therefore not considered as part of the Wastewater Treatment and Reuse 15 TABLE 3.3 Long List of Treatment Technologies and Preselection of Appropriate Technologies for Small-Town WWTPs APPLIED FOR SIMPLE TO FINANCIALLY WWTP DESIGN OPERATE WITH COMPETITIVE SIZES < 50,000 LOW OPERATIONAL FOR SMALL/ # WASTEWATER TECHNOLOGY a ABBREV. PE60 AND < 5 MLD RISKS MEDIUM WWTPS b Primary treatment (only) 1 Septic tank ST (only for clusters of Yes Yes houses) 2 Biogas digester BD Yes Yes 3 Imhoff tank IMH Yes Yes Yes Primary + secondary treatment 4 Anaerobic baffled reactor ABR Yes Yes Yes 5 Anaerobic filter ANF Yes Yes Yes 6 Waste stabilization pond WSP Yes Yes Yes 7 Aerated lagoon AL Yes Yes Yes 8 Single-stage constructed wetland CW(1-st) Yes Yes Yes 9 Hybrid constructed wetland CW(hybrid) Yes Yes Yes 10 Upflow anaerobic sludge blanket reactor UASB Yes Yes Yes 11 Conventional activated sludge process CAS (> 20,000 PE60) No No 12 Sequencing batch reactor (conventional) SBR (conv.) (> 20,000 PE60) No No 13 Extended aeration (AS type) EA Yes Yes Yes 14 Extended aeration (SBR type) SBR EA Yes Yes Yes 15 Trickling filter TF Yes Yes Yes 16 Rotating biological contactor RBC Yes Yes Yes Activated sludge variations 17 Nereda® c NEREDA Yes No No 18 Membrane bioreactor MBR Yes No No 19 Two-stage AS with high-loaded first stage AB No No No Attached biomass growth system variations 20 Biological aerated filter d BAF Yes No No Combinations of AS and attached growth 21 Integrated fixed film activated sludgee IFAS Yes No Yes 22 Moving bed biological reactor  f MBBR Yes No Yes 23 Trickling filter/solids contact process TF/SC No Yes Yes 24 UASB-WSP UASB-WSP Yes Yes Yes 25 UASB-TF UASB-TF Yes Yes Yes 26 UASB-AS UASB-AS Yes No Yes Tertiary treatment (additional) 27 Disinfection with UV system UV Yes Yes Yes 28 Disinfection with chlorine Cl Yes Yes Yes 29 Polishing pond PP Yes Yes Yes 30 Rock filter RF Yes Yes Yes 31 Sand filter SF Yes No No 32 Rotary disc filter RDF Yes Yes Yes Note: AS = activated sludge; CAPEX = capital expenditures; MLD = million liters per day; OPEX = operating expenditures; PE = population equivalent; UV = ultraviolet; WWTP = wastewater treatment plant. a Appropriate WWTP technologies are presented in green text. b Technologies that have considerably higher CAPEX and/or OPEX figures than other technologies are not considered to be financially competitive. c Nereda is a proprietary variation of AS based on aerobic granulation. d These systems can come under different proprietary variations and trademarks such as BIOFOR® and BIOSTYR®. e These systems can come under different proprietary variations and trademarks such as STM-Aerotor™. f These systems can come under different proprietary variations and trademarks such as Kaldnes™, Linpor™ and Captor™. long list of this guide. Economies of scale TABLE 3.4 effects also exist for civil works-intensive List of Wastewater Treatment Technologies technologies, but this effect is usually less That Met the Preselection Criteria of pronounced than for electromechanical Being Appropriate for Small Towns installations. TECHNOLOGY SHEET: APPROPRIATE Technologies were excluded for being inappropriate # TECHNOLOGY FOR SMALL TOWNS for small towns if they did not meet all three of the Primary treatment only aforementioned criteria. For example, the CAS 1 Septic tank (ST) process (high-load) or membrane bioreactors 2 Biogas digester (BD) (MBRs) were not considered part of the guide 3 Imhoff tank (IMH) because they are widely known to require a Primary and secondary treatment combination of higher levels of CAPEX and OPEX, 4 Anaerobic baffled reactor (ABR) highly skilled staff, a constant electricity supply, 5 Anaerobic filter (ANF) high levels of chemical consumption and a highly 6 Waste stabilization pond (WSP) developed management system that ensures that 7 Aerated lagoon (AL) the facility is correctly operated and maintained. 8 Single-stage constructed wetland (CW(1-st)) In addition, given the economies of scale and the 9 Hybrid constructed wetland (CW(hybrid)) reduced fluctuation of influent characteristics in 10 Upflow anaerobic sludge blanket reactor (UASB) larger towns and cities, these options are deemed 11 Extended aeration – activated sludge type (EA) more appropriate for the treatment of large flows Extended aeration – sequencing batch reactor type in such settings. 12 (SBR(EA)) In Table 3.3, wastewater treatment technologies 13 Trickling filter (TF) that meet all of the preselection criteria, and are 14 Rotating biological contactor (RBC) therefore deemed appropriate for small towns, 15 UASB followed by WSP (UASB-WSP) are presented in green text, and those excluded 16 UASB followed by TF (UASB-TF) technologies, based on the fact that they do not Tertiary treatment meet the preselection criteria for small towns, are 17 Disinfection with ultraviolet system (UV) presented in red text. 18 Disinfection with chlorine (Cl) 19 Polishing pond (PP) Twenty-one options met the preselection criteria 20 Rock filter (RF) of being appropriate for small towns, as presented 21 Rotary disc filter (RDF) in Table 3.4. More experienced users of this guide may still wish to include other technologies for additional comparison. However, we suggest that any additions be assessed against the same criteria Technology Sheets that are applied in this guide for comparability To help better navigate the reader around the with the preselected technologies here. technology sheets, we present here an outline For those technologies that met the preselection template that provides an overview of how each criteria, technology sheets were developed. These technology sheet is structured and an explanation are presented in the next section (“Technology of how to interpret the different figures that are Sheets”). used to characterize each technology. Wastewater Treatment and Reuse 17 TECH SHEET #1 Septic Tank (ST) DESCRIPTION REUSE Primary anaerobic treatment POTENTIAL The septic tank is the most common, small-scale and decentralized treatment tech- ▶ Effluent not fit for nology worldwide. The septic tank is a watertight chamber that performs preliminary reuse. treatment through sedimentation and anaerobic digestion. Physical treatment happens ▶ Not enough biogas through the retention of solids: the gravity separation of solid particles between produced for reuse. flotation (formation of a grease cap) and sedimentation (formation of a sludge bed) produces a totally liquid effluent. Biological treatment occurs through anaerobic digestion which liquefies solids retained in the pit and produces some biogas. The effluent is infiltrated onsite and spread through a leach field, IA ER where further filtration occurs. Treatment I T efficiencies vary greatly depending on C R i t a l co /cap ) s ts Treatm ent t operation and maintenance and L e n tm (CA PE X ef f ic i e climatic conditions. I A Inve s nc y 3 C N en A E a nce N ha )X se d n PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r ili t y b RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL 18 Appropriate Wastewater Treatment Technology for Small Towns access covers vent inlet outlet scum sedimentation zone sludge Source: Tilley et al. 2014. PROJECT DESIGN OPERATION ▶ Connected population: ▶ Simple and robust technology ▶ Regular desludging must be (Household) or with long service life; can be ensured. (Cluster of houses) sized and constructed by non- ▶ A septic tank is appropriate ▶ Population growth can be expert. where there is a way of accounted for as the sizing is ▶ Small land area required (can be dispersing or transporting the relatively flexible to a maximum built underground). effluent. of 200 population equivalent. ▶ Treated wastewater can be ▶ Consider existing capacity for dispersed into the soil for onsite sludge treatment in neighboring infiltration. areas. Mixed wastewater flow is ▶ If septic tanks are used in not allowed. densely populated areas, onsite infiltration should not be used, otherwise, the ground will become oversaturated and contaminated, and wastewater may rise up to the surface, posing a serious health risk. ▶ Even though septic tanks are watertight, it is not recommended to construct them in areas with high groundwater tables or where there is frequent flooding. Wastewater Treatment and Reuse 19 TECH SHEET #2 Biogas Digester (BD) DESCRIPTION REUSE Primary treatment, anaerobic process, sludge treatment POTENTIAL The biogas digester consists in a chamber where blackwater, sludge, and/or bio- ▶ Effluent not fit for degradable waste is introduced with no aeration to create the ideal conditions for reuse. anaerobic bacteria to break down (digest) the organic matter from the inputs into ▶ Market for reuse simpler chemicals components. Anaerobic digestion is a process which take place in exist for biogas and low oxygen or anoxic environments. In these conditions, anaerobic bacteria thrive and sludge (digestate) break down organic carbon into biogas (methane and carbon dioxide) and produces valorization. a digested slurry (digestate) rich in organics and nutrients, almost odorless and where pathogens are partly inactivated. Because this digester is used for strong sub- ▶ Gas production is strate only, biogas production is high; however, significant gas production cannot be directly related to achieved if blackwater is the only input. This process can be very useful to treat aris- the organic fraction ing organic waste such as sewage sludge, organic farm waste, municipal solid waste, of the substrate. green waste and industrial organic waste. ▶ Digestate is rich in stabilized organic matter and nutrients and can be reused IA as a fertilizer. T ER C RI /cap i t a l co s ts Treatm e nt e L m ent PEX) f f ic IA t (CA ien es cy Inv 3 C N en A E a nce N ha )X se d n PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa e g y us ERIA En e r bili t y RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL 20 Appropriate Wastewater Treatment Technology for Small Towns inlet gas outlet access cover overflow outlet biogas slurry Source: Tilley et al. 2014. PROJECT DESIGN OPERATION ▶ Connected population: ▶ Often, biogas reactors are ▶ The main parameter is the (Household) or directly connected to private or hydraulic retention time, which (Cluster of houses) public toilets with an additional should not be less than 15 and ▶ Power and water supply: access point for organic 25 days in hot and moderately Does not require electricity or materials. warm climate, respectively. constant water supply. Will not ▶ For economic reasons, it is Below 15 °C biogas digesters accommodate only wastewater not suitable for weak liquid are less appropriate for colder and therefore benefits from wastewater, as the total volume climates as the rate of organic nearby agricultural or industrial of wastewater must be agitated matter conversion into biogas is activity to supplement inputs and kept for full retention time very low. with animal manure, green waste inside the digester. This leads to or organic waste, or a food large digester volumes and thus, waste collection system. to high construction costs. ▶ Mixed wastewater and organic ▶ Can be built underground if waste is allowed. However, the soil conditions and initial space influent should remain strong allows. and the system will not deal well ▶ Construction requires masonry with dilution. knowledge. ▶ To minimize distribution losses, the reactors should be installed close to where the gas can be used. Wastewater Treatment and Reuse 21 TECH SHEET #3 IMHOFF Tank (IMH) DESCRIPTION REUSE Primary anaerobic technology POTENTIAL The Imhoff tank is a communal settling tank that treats raw wastewater by sep- ▶ Effluent not fit for arating solids and liquids. The settled solids are then digested and partially reuse. stabilized in the lower chamber through anaerobic digestion. The V shape ▶ Treated wastewater allows solids to trickle into the digestion compartment while preventing gas can be discharged from rising back up and disturbing the settling process. Gas vents direct the in ocean or large gas to the sides, transporting sludge particles and creating a scum layer. river only. Imhoff tanks work for domestic or mixed wastewater flows, though the effluent requires additional treatment. The combination of solid- ▶ Not enough biogas liquid separation and sludge stabilization in one produced for reuse. unit is advantageous. IA T ER C RI a pi t a l c os ts Treatm e nt/c EX) nt e L e tm (CA P f f ic IA ien es cy Inv 3 C N en A E a nce N ha )X se d n PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r ili t y b RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL 22 Appropriate Wastewater Treatment Technology for Small Towns gas vents scum flow tank / cleanout settling compartment sludge outlet gas bubbles sludge digestion compartment Source: Tilley et al. 2014. PROJECT DESIGN OPERATION ▶ Connected population: ▶ Due to depth of tank, the height ▶ Process operation in general is (Cluster of houses) or of the groundwater table should not required, and maintenance (Town) be considered carefully. is limited to the removal of ▶ Consider existing capacity for ▶ Moderate area requirement (can accumulated sludge and scum sludge treatment in neighboring be built underground). every 1 to 3 years. areas. ▶ Low odors due to containment ▶ Mixed wastewater flow is of gas. allowed. Resistant against ▶ Performance depends on organic shock loads, but not temperature. In colder climates, suitable for hydraulic overloads. a larger tank may be needed for longer retention. ▶ Usually, the biogas produced in an Imhoff tank through anaerobic digestion is not collected because of its insufficient amount. Wastewater Treatment and Reuse 23 TECH SHEET #4 Anaerobic Baffled Reactor (ABR) DESCRIPTION REUSE Primary anaerobic treatment POTENTIAL An anaerobic baffled reactor (ABR) is an improved Septic Tank with a series of baffles ▶ Effluent not fit for under which the wastewater is forced to flow through several compartments. The reuse. ABR also treats of non-settleable and dissolved solids by bringing them in close con- ▶ Treated wastewater tact with active bacterial mass that accumulates on the reactor walls. The increased can be discharged contact time with the active biomass in ocean or large results and the upflow chambers river only. provide enhanced removal IA ER and digestion of organic ▶ Not enough biogas T matter. RI pi t a l C c os t s Treatm /ca ) ent produced for reuse. L ent X ef f tm (CAPE ic i e IA e s nc Inv y 3 C N en A E a nce N ha )X se d n PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r ili t y b RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL 24 Appropriate Wastewater Treatment Technology for Small Towns settler anaerobic baffled reactor (ABR) access covers vent inlet outlet scum sedimentation zone sludge baffles Source: Tilley et al. 2014. PROJECT DESIGN OPERATION ▶ Connected population: ▶ Moderate area requirement ▶ Process operation in general is (Cluster of houses) or (can be built underground). not required, and maintenance (Town) is limited to the removal of ▶ Consider existing capacity for accumulated sludge and scum sludge treatment in neighboring every 1 to 3 years. areas. ▶ Low sludge production; the ▶ Mixed wastewater flow is sludge is stabilized. allowed. Resistant to organic and hydraulic shock loads. Wastewater Treatment and Reuse 25 TECH SHEET #5 Anaerobic Filter (ANF) DESCRIPTION REUSE POTENTIAL Primary anaerobic treatment The anaerobic filter, also known as fixed bed or fixed film reactor, consists in an anaer- ▶ Effluent not fit for obic baffle reactor structure equipped with additional material that forms a filter on reuse. which bacteria can grow. This increases the surface area where wastewater is in con- ▶ Treated wastewater tact with active biomass and improves treatment. The treatment of non-settleable and can be discharged dissolved solids occurs through contact with this surplus of active bacterial mass. The in ocean or large bacteria affix themselves to solid particles and river only. on the reactor walls. Filter material, such as gravel, rocks, cinder or ▶ Not enough biogas produced for reuse. plastic pieces designed as IA T ER such media provide addi- I CR s ts i t a l co Treatm tional surface area for t /cap ) ent L m n e PE X ef f ic i e IA s t (CA nc bacteria to settle. Inve y 3 C N en A E a nce N ha )X se d n PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa use gy ERIA En e r bili t y RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL 26 Appropriate Wastewater Treatment Technology for Small Towns access covers vent inlet outlet scum filter sedimentation zone sludge filter support anaerobic filter units settler Source: Tilley et al. 2014. baffles PROJECT DESIGN OPERATION ▶ Connected population: ▶ Hydraulic retention time is ▶ Risk of clogging, depending on (Cluster of houses) or the most important design pre-treatment. (Town) parameter influencing filter ▶ Consider existing capacity for performance. The hydraulic sludge treatment in neighboring retention time should be in the areas. range between 1.5 and 2 days. ▶ Mixed wastewater flow is ▶ For domestic wastewater, allowed. constructed gross digester volume (voids plus filter mass) may be estimated at 0.5 m3/ capita. ▶ Moderate area requirement (can be built underground). Wastewater Treatment and Reuse 27 TECH SHEET #6 Waste Stabilization Ponds (WSP) DESCRIPTION Primary/secondary/tertiary anaerobic treatment Secondary/tertiary aerobic treatment Waste Stabilization Ponds are man-made ponds and can be used at all stages of wastewater treatment, in series or as one step in a broader treatment chain.  As primary anaerobic treatment, ation and algal respi­ ration, while anaerobic lagoons or ponds operate the sludge settles at the bottom much like open septic tanks and are and provides an anaerobic environ- used as the first step to treat strong ment where decomposition occurs wastewater and reduce organic load. and the sludge has to be regularly The depth of anaerobic ponds pro- extracted. Usually, a facultative pond motes sedimentation: settleable receives settled water from an anaer- solids fall to the bottom of the pond obic pond and therefore operates to form a sludge layer, where they undergo anaerobic digestion. The anaerobic bacteria (acidogenic, ace- togenic, and methanogenic) operate IA T ER RI at temperatures above 15°C and ts s i t a l co Treatm transform the organic carbon in C /cap ent PEX) e nt e L m f f ic IA t (CA ien the solids into biogas (metha- es cy Inv neand carbon dioxide), leav- 3 C N en ing a nutrient-rich sludge. A E a nce N ha ) The scum layer that often X se d n PE FI of utr (O forms on the surface up ien 2 ts gra t rem cos does not need to be d in M removed. Anaerobic O& g to val ponds are particu- o larly well adapted for 1 warm countries. Land availa  As secondary treat- g y use ment, facultative ERIA ponds rely on both En e r ili t y b aerobic and anaer- obic processes. Fac- RIT ultative ponds stratify L C influent by working on n La tio TA two levels: the top layer bo uc r qu od contains dissolved oxygen EN a li pr e f ic dg ati due to atmospheric reaer- on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL 28 Appropriate Wastewater Treatment Technology for Small Towns under lighter organic loading than REUSE POTENTIAL anaerobic ponds. Wastewater flows into the pond in a continuous man- ner. Facultative ponds are used to ▶ If the anaerobic treat raw municipal wastewater in pond is covered, small communities and for primary the biogas can be or secondary effluent treatment for recovered for reuse. small or large cities. ▶ Maturation lagoon  As tertiary treatment, aerobic effluent is fit for non- or maturation ponds rely on nat- restrictive irrigation. ural aeration, sedimentation and UV disinfection to treat wastewa- ▶ Due to high algae ter. This process mirrors the nat- production, use ural treatment occurring in a river through drip body. Natural oxygenation occurs irrigation requires through atmospheric reaeration filtration to remove and algal respiration, promoting the suspended organic degradation and nutrient solids. removal. Wastewater flows in con- tinuously, and the shallow depth of the pond allows sunlight to reach the whole pond depth, combining with the oxygen to promote patho- gen removal. All these processes contribute to good fecal bacte- rial removal. Photosynthetic algae release oxygen in the water while consuming the carbon dioxide pro- duced by bacterial activity. They are also used as the polishing step after an anaerobic system, such as USABs (see sheet 10). Maturation ponds contribute significantly to pathogen removal and effectively remove the majority of nitrogen and phosphorus if used in combi- nation with algae harvesting. The algal population in maturation ponds is much more diverse than in the facultative ponds. Wastewater Treatment and Reuse 29 anaerobic facultative maturation Source: Tilley et al. 2014. PROJECT DESIGN OPERATION ▶ Connected population: ▶ Works best in series. ▶ Odor release (mainly hydrogen (Cluster of houses) or ▶ Requires relatively large areas of sulfide) is a major disadvantage (Town) land, and therefore is still best of anaerobic ponds. ▶ Can accommodate high organic suited for peri-urban areas or ▶ Anaerobic treatment requires loading. large, rural settlements. a longer start-up time, alkaline ▶ Mixed wastewater flow is ▶ Anaerobic lagoons are usually addition and anaerobic microbes allowed. Population growth can 2 to 5 m deep and the height of are sensitive to toxic substances. be accounted for as the sizing is the groundwater table should be ▶ Anaerobic lagoons must be relatively flexible. considered carefully. de-sludged approximately ▶ Facultative ponds usually are once every 2 to 5 years, when 1.5 m deep, although depths the accumulated solids reach between 1 m and 2.0 m are one third of the pond volume. used. Depths less than 0.9 m are Sludge accumulation is slower not recommended, as rooted for other lagoons. plants may grow in the pond ▶ The classic ponds configuration and provide a shaded habitat (anaerobic pond + facultative suitable for mosquito breeding. pond + maturation ponds) ▶ Maturation ponds are usually usually reaches complete 1–1.5 m deep. removal of protozoan cysts and helminths eggs. ▶ Anaerobic lagoons receive raw wastewater with high organic ▶ Mosquitoes and similar insect loading (>100g BOD5/m­ 3 per day). vectors can be a problem if emergent vegetation is not ▶ Aerobic lagoons can be built controlled. in series for most effective treatment and to provide a high ▶ Facultative ponds have good level of pathogen removal. resistance to temporary organic overloads. ▶ Although fecal bacteria are partially removed in the ▶ Maturation ponds achieve a high facultative ponds, the size and reduction of solids, BOD and number of the maturation ponds pathogens and high nutrient determine the quantity of fecal removal if combined with algae bacteria in the final effluent. harvesting (Tilley et al. 2014) and should reach high coliform removal efficiency (3–4 Ulog). 30 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #7 Aerated Lagoons (AL) DESCRIPTION REUSE Secondary aerobic treatment POTENTIAL The aerated lagoon (also known as aerated pond) consists of a man-made pond ▶ Treated water can be receiving mechanical aeration. This process mirrors the natural treatment occurring used for restrictive in a river body but is aided through mechanical or diffused aeration. The oxygen irrigation (fruit trees, promotes organic degradation and nutrient removal. industrial crops). The wastewater flows in continuously and the wastewater. The treatment of waste­ ▶ Effluent requires solids are maintained in suspension by water by lagoon processes is charac- disinfection the aeration. Dissolved oxygen and sus- terized by its high buffering capacity treatment for non- pended solids are maintained uniform with respect to variations in organic restrictive reuse. throughout the basin. If aeration is main- or hydraulic loads, due to its hydraulic tained in the upper layer only, the pond retention time being much higher than is called a facultative aerated lagoon. that of other processes. In that case, a portion of the suspended solids settle to the bottom of the basin, where they undergo anaerobic decomposition. In the settling stage, IA the suspended solids agglomerate in T ER the form of sludge, which has to C RI /cap i t a l co s ts Treatm e nt e L m ent PEX) f f ic IA be regularly extracted. t (CA ien es cy Inv 3 C Aerated lagoons can be built N en A in series for most effective E a nce N ha )X se d n PE FI treatment, with modulated of utr (O up ien aeration along the series. 2 ts gra t rem cos The wastewater first goes d in M O& g to val through the facultative lagoon and the effluent o is then polished in an 1 aerated or high perfor- Land availa mance aerated lagoon. g y use Facultative lagoons are ERIA larger, shallower and less En e r b aerated than high perfor- ili t y mance aerated lagoon. RIT The aerated lagoon is L C particularly adapted to n La tio TA bo communities where artisanal uc r qu od EN a li pr or industrial activities have dg e on ati f ic M Slu a significant influence on the N O pa r uts IR Avail ts and O&M inp ent nature of the organic pollutant in NV ability of replacem /E TECHNI CAL Wastewater Treatment and Reuse 31 influent S effluent S (aerator) sludge PROJECT DESIGN OPERATION ▶ Connected population: ▶ Work best in series. ▶ The power level needed to (Cluster of houses) or ▶ Requires relatively large areas of maintain uniform dissolved (Town) land, and therefore is still best oxygen in the basin—or top ▶ Water and power supply: suited for peri-urban areas or layer of the basin—depends on No need for continuous water large, rural settlements. the aeration equipment (if any) supply. Continuous energy and the influent quality. ▶ As the process is resistant to supply is required to operate the organic and hydraulic shock ▶ Energy use will be higher for aeration mechanism. loads, no equalization step is aerated than for facultative ▶ Mixed wastewater flow with needed. aerated lagoons. organic industrial wastewater ▶ The sludge must be removed is accepted, as are variations in from the aerated pond, or from organic or hydraulic loads. the subsequent sedimentation pond, for continued performance. 32 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #8 Single-Stage Constructed Wetlands (CW 1-stage) DESCRIPTION Secondary aerobic treatment Constructed wetlands are man-made areas mirroring the structure of natural wetlands to take advantage of natural treatment processes for wastewater. They consist in a porous layer of rock, gravel or sand and a planted bed. The porous layer performs filtration functions and traps some of the suspended solids. The planted bed absorbs some of the pollutants and promotes the development of invertebrates and micro- organisms that further treat the water as it flows through by degrading the organic pollutants. Nutrients are also taken up by microorganisms and plants. The bottom is usually lined with an impermeable liner to control wastewater flow and protect the surrounding area. Constructed wetlands can be distinguished according to criteria such as hydrology (water surface flow and subsurface flow), macrophyte growth form (emergent, submerged, free-floating and floating leaved plants) and direction of flow (horizontal and vertical).  Vertical flow constructed wetlands (VFCW) require less area than horizontal flow wetlands given the downward flow of RIA the wastewater, which is loaded I TE CR s ts from the top as uniformly i t a l co Treatm t /cap ) e nt e L e n tm (CAPE X f f ic IA as possible to allow for ien es cy Inv oxygenation. Intermittent 3 C N loading, using a pump or en A E a nce N siphon, further increases ha )X se d n PE FI of utr the oxygenation and (O up ien 2 ts gra t rem aerobic phase. An cos d in M anaerobic phase O& g to val then follows once o the wastewater 1 infiltrates further into the medium and Land availa until it is collected g y use and discharged at ERIA En e r the bottom of the ili t y b system. RIT  In horizontal flow constructed wetlands L C (HFCW), wastewater n La tio TA bo uc flows horizontally r qu od EN a li pr f ic through the basin and dg e on ati M Slu undergoes filtration as it N O pa r uts IR Avail ts and O&M inp ent makes its way to the other side NV ability of replacem of the wetland, pushed by new /E TECHNI CAL Wastewater Treatment and Reuse 33 influent. The vegetation transfers REUSE POTENTIAL a small amount of oxygen to the root zone so that aerobic bacteria can colonize the area. The soil ▶ Effluent not fit remains saturated, providing limited for unrestricted nitrifying capacity compared to reuse but fit for vertical flow constructed wetlands. restricted irrigation However, mechanisms to feed the (trees, crops eaten wastewater into HFCW are simpler, cooked). making them the preferred choice ▶ Treated wastewater unless nitrification is required to can be discharged meet discharge standards. in stream.  In free water surface constructed wetlands (FWSCW), water flows above ground and plants are rooted in the sediment layer at the base of the basin or floating in the water. Compared to subsurface wetlands (horizontal flow or vertical flow), FWSCW can be vegetated with emergent, submerged and floating plants. In these systems, the water surface of the wetland is exposed to the atmosphere which can theoretically provide oxygen to the water and UV disinfection. 34 Appropriate Wastewater Treatment Technology for Small Towns Vertical flow constructed wetlands (VFCW) distribution pipe influent impermeable liner sand/gravel layers effluent drainage pipe Horizontal flow constructed wetlands (HFCW) water level influent effluent impermeable liner coarse sand/fine gravel layer water level control Free water surface constructed wetlands (FWSCW) inlet outlet sludge impermeable liner sediment layer PROJECT plants, such as the common reed, OPERATION cattail and bulrush.  Connected population:  In VFCW, 4 to 10 times a day  The main function of the plants (Cluster of houses) or feeding of wastewater, whereas is to counteract clogging of the (Town) HFCW is continuous. filter.  Power supply: When using a  For HFCW, the water level in  VFCW will require a pump or pump for wastewater loading, the wetland is maintained at sufficient gradient for a siphon will require electricity supply on 5 to 15 cm below the surface pulse-loading system. a set schedule. Works with both to ensure subsurface flow and continuous and intermittent  Oxygen transfer rates can be avoid bad smells. wastewater inflow. improved by using sand and/or  The quantity of sludge is affected gravel beds and ensuring  Mixed wastewater flow is not by the liquid temperature but intermittent loading, so that the recommended. remains below that of other beds are not water saturated. secondary treatment processes.  In HFCW, the outlet should be DESIGN The vegetation transfers a small  variable so that the water surface amount of oxygen to the root can be adjusted to optimize  Typical depths range from 0.5 to zone so that aerobic bacteria treatment performance. 1.0 m for HFCW, from 0.8 to 1.4 m can colonize the filter media. for VFCW and 0.15 to 0.60 m for  FWSCW typically require a  The risk of mosquito breeding is FWSCW. larger area than subsurface reduced in HFCW compared to systems (HFCW and VFCW),  Wetland species of all growth VFCW and FWSCW since there as the porous subsurface filter forms have been used in is no standing water. medium in subsurface systems constructed wetlands. However, provides a greater contact area  In HFCW, the filter material at the the most commonly used species for treatment activities. inlet zone will require replacement are robust species of emergent every 10 or more years. Wastewater Treatment and Reuse 35 TECH SHEET #9 Hybrid Constructed Wetlands (CW-Hybrid) DESCRIPTION REUSE Secondary aerobic treatment POTENTIAL Hybrid Constructed Wetlands (Hybrid CWs) make primarily use of the components ▶ Effluent not fit described in the Technology Sheet on 1-stage CWs, i.e. horizontal flow CW (HFCW) for unrestricted and vertical flow CW (VFCW). At least 2 of such components are employed in series, reuse but fit for but also 3, 4 or even more stages are sometimes used. This brings about distinctive restricted irrigation advantages, as compared to 1-stage CWs, such as: (trees, crops eaten cooked). ▶ Total land requirement is reduced ▶ Hybrid CWs can be designed ▶ Treated wastewater to roughly 50% of 1-stage CW. A for enhanced biological nutrient can be discharged common plant footprint of Hybrid removal (BNR). The VFCW in in stream. CWs equals 2.0–2.5 m2/capita in such designs usually serve for moderate climates, possibly less in nitrification, the HFCW usually hot climates. serves for denitrification. ▶ Treatment efficiency becomes more stable, and even improves (in spite of smaller footprint). E RIA IT CR i ta l c os t s Treatm /cap e nt e L m ent PEX) f f ic IA t (CA ien es cy Inv 3 C N en A E a nce N ha )X se d n PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r ili t y b RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL 36 Appropriate Wastewater Treatment Technology for Small Towns sequential batch first stage preliminary feeding horizontal flow treatment (HFCW) second stage system vertical flow (VFCW) influent Source: Dotro et al. 2017. effluent PROJECT DESIGN OPERATION ▶ Connected population: ▶ For construction details typical ▶ The same principles apply as (Cluster of houses) or characteristics described in for 1-stage CWs. That means, (Town) the Technology Sheet for O&M is very easy, and does not ▶ Feasibility of sewerage: 1-stage CWs apply with small require particular qualifications. sufficient water supply needed. modifications, e.g. use of slightly coarser gravel in the first stage. ▶ Fecal sludge: not suited for direct treatment of fecal ▶ For design maximum permitted sludge; but can be applied after load criteria for hydraulic and effective primary treatment such organic loading need to be as BAR. considered. Treatment efficiency can be derived via kinetic ▶ Regulations for treated parameters. discharge & reuse: high-quality secondary treatment level can be achieved; better disinfection than in most other secondary treatment technologies. ▶ Available land for WWTP: relatively high footprint (even though less than 1-stage CW). ▶ Power supply to WWTP: usually only needed for wastewater pumping; but mere gravity flow is possible in case of advantageous topography. Wastewater Treatment and Reuse 37 TECH SHEET #10 Upstream Anaerobic Sludge Blanket reactor (UASB) DESCRIPTION REUSE Primary anaerobic treatment POTENTIAL The Upstream Anaerobic Sludge Blanket (UASB) reactor consists in a tank at the bottom ▶ Treated wastewater of which a ‘sludge blanket’ forms and anaerobic digestion takes place. Wastewater can be discharged is introduced as uniformly as possible over the reactor bottom, passes through the in ocean or large sludge bed, and enters the settling zone where solids will further settle. The active river only. sludge is suspended in the lower part of the digester and serves directly as a filter ▶ Biogas produced medium. This blanket is made of granular sludge where anaerobic bacteria thrive and can be used. process the wastewater as it flows through it. The most characteristic device of the UASB reactor is the phase separator. This device, placed at the top of the reactor, divides it into a lower part, the digestion zone, and an upper part, the settling zone. Wastewater will enter the settling zone via the aperture of the phase separators as it flows upwards. Upstream velocity and settling speed of the sludge are in equilibrium and forms a locally IA stable but suspended sludge blanket. T ER After some weeks of maturation, gran- R I s ts i t a l co Treatm C t /cap ) e nt e ular sludge forms which improves L e n tm (CAPE X f f ic IA ien es cy the physical stability and the filter Inv 3 C capacity of the sludge blanket. N en A E a nce N ha )X se d n PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r ili t y b RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL 38 Appropriate Wastewater Treatment Technology for Small Towns gas collector gas outlet sludge bed effluent sludge outlet Source: Helmer et al. 1997. PROJECT DESIGN OPERATION ▶ Connected population: ▶ The technology is relatively ▶ To maintain a stable sludge (Cluster of houses) or simple to design and build blanket, the flow rate must be (Town) but requires several months to controlled and properly geared ▶ Existing capacity for sludge mature and to develop sufficient in accordance with fluctuation of treatment in a neighboring granular sludge for treatment. the organic load. In smaller units, urban center can help to use ▶ There is no need for primary it is not possible to stabilize this technology. A UASB is not settling. the process by increasing the appropriate for small or rural hydraulic retention time without ▶ If biogas capture is not a priority, communities without a constant lowering the upstream velocity. can be built underground water supply or electricity. to optimize the space and ▶ The fully controlled UASB ▶ Mixed wastewater flow is structure. is used for relatively strong allowed. Appropriate for heavy industrial wastewaters. load urban wastewater and ▶ The UASB reactor has the industrial wastewater. potential to produce higher quality effluent than Septic Tanks and can do so in a smaller reactor volume. ▶ Sludge production is very low. Wastewater Treatment and Reuse 39 TECH SHEET #11 Extended Aeration: Activated Sludge Type (EA) DESCRIPTION Secondary aerobic treatment Extended Aeration (EA) is a well-established variation of the activated sludge pro- cess. Contrary to other, more complicated representatives of that process, EA is built around the principle of simplicity. There are no Primary Sedimentation Tanks, and the waste activated sludge is subjected to such long retention times in the aeration tanks, that no sludge digesters are needed for sludge digestion / stabilization. That is, waste sludge removed from the tanks can be directly thickened and dewatered. EA comes in several variations, of which the most common ones are characterized as follows: ▶ Oxidation ditch EA: In this ▶ Carrousel type EA: Similvar configuration the Aeration Tank to Oxidation Ditches, however is constructed as a closed loop employing larger tanks with more channel, leading to what is U-turns; there are typically 4 lanes. called “completely mixed” flow Water depth conditions. Typically, water depth is in the order of 2 m only, and thus enables the use of horizontal shaft mechanical aerator brushes, or E RIA IT CR similar installations. Sometimes i ta l c os t s Treatm /cap e nt e also vertical shaft aerators L m ent PEX) f f ic IA t A ien es (C cy are used, and located Inv 3 C at the U-turning point N en A towards the end of E a nce N ha )X se d n PE FI those tank loops. The of utr (O up ien aerators provide the 2 ts gra t rem cos necessary oxygen for d in M O& g to val microorganisms, and o they also provide horizontal thrust 1 to facilitate good Land availa mixing conditions. g y use In a subsequent ERIA Secondary En e r b Sedimentation Tank ili t y the sludge flocs are RIT allowed to settle by L C gravity at the tank n La bottom, from where the tio TA bo uc r qu sludge is pumped back d EN ro a li ep ati f ic to the Aeration Tank. g lu d on M N S O pa r uts R Avail ts and O&M inp ent VI ability of replacem N A L/E TECHNIC 40 Appropriate Wastewater Treatment Technology for Small Towns is sometimes increased to about REUSE POTENTIAL 5 m, which facilitates better energy efficiency of aeration. ▶ Plug-flow EA: The Aeration Tanks ▶ Effluent not fit for are shaped such that flow enters on irrigation. one end, and leaves at the other ▶ Treated wastewater end (“longitudinal” flow, also called can be discharged “plug-flow” conditions). Mostly in stream. this is done to improve efficiency: that is, pressurized aeration is used, water depth is increased to mostly 5–6 m, aerated zones and non-aerated zones are installed intermittently, and smart automation systems for the control of air supply are introduced, complete with effluent quality control sensors and frequency-controlled blowers. ▶ SBR type EA: For this variation a separate Technology Sheet has been prepared, due to the very different flow conditions of that process. The advantages of EA are a very robust process with large reactor volumes that can also cope with brief organic and hydraulic shock loads. It can be employed in any climate conditions, and it can be designed for any secondary treatment level. Due to economy of scale effects on cost, this technology should not be considered for very small facilities. Disadvantages are a requirement for sound process understanding by oper- ators, regular maintenance needs, high energy consumption for aeration, high OPEX and CAPEX, and a risk of the for- mation of filamentous micro-organisms, which negatively hamper sedimentation and may thus seriously affect treatment efficiency. Wastewater Treatment and Reuse 41 compressed air clarifier inlet outlet sludge recirculation extracted sludge Source: Tilley et al. 2014. PROJECT DESIGN OPERATION ▶ Connected population: ▶ The aeration tank volume ▶ O&M requires process (Town) sizing is done such that the understanding by well-trained ▶ Feasibility of sewerage: sludge stays sufficiently long staff. This involves finding the sufficient water supply needed. in the Aeration Tank so that it right balance between incoming can be considered stabilized pollution loads and adequate ▶ Fecal sludge: only very limited (represented by what is called biomass, and at the same time volumes of fecal sludge can be “high aerobic sludge age,” or permitting stabilization of the co-treated. “high aerobic sludge retention sludge. But also appropriate ▶ Regulations for treated time (SRT),” or “low F/M (food/ control of the sludge depth discharge & reuse: high-quality microorganisms) ratios”). in the sedimentation stage is secondary treatment level can needed, and appropriate and ▶ Typical design values are SRT in be achieved. fast trouble-shooting to sludge a range of 15–25 days, with cold ▶ Available land for WWTP: small climates in the upper range, and sedimentation problems may be footprint. warm climates towards the lower necessary, too. ▶ Power supply to WWTP: high range. ▶ Regular maintenance of pumps (particularly for aeration, but also ▶ The Secondary Sedimentation and aeration system (diffusers + for pumping purposes); constant Tanks are designed as classical blowers, or mechanical aerators) and reliable power supply sedimentation tanks, based on is needed. needed. parameters such as retention time and hydraulic surface charge. 42 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #12 Extended Aeration: Sequencing Batch Reactor Type (SBR-EA) DESCRIPTION REUSE Secondary aerobic treatment POTENTIAL The sequencing batch reactor (SBR) constitutes a particular variant of activated sludge ▶ Treated water can be and in the case of small towns is most relevantly used for extended aeration (EA). EA used for restrictive applies best to smaller waste loads and requires longer mixing times given that all irrigation (fruit trees, processes (agitation of sludge and decantation) occur in the same clarifier, leading industrial crops). to high sludge age. The SBR follows the same basic principles as activated sludge: ▶ Effluent requires biological treatment, such as the formation of suspended biomass, the concentration disinfection of biomass in the reactor and the separation of biomass from the treated effluent. The treatment for non- special feature of this variant is that the settling of the biomass is carried out directly restrictive reuse, in the aeration tanks rather than in a separate clarifier. and filtration and The process operates in batch mode in a sequence typically comprising the following disinfection for reuse phases: filling, reaction (aeration and mixing), decantation, and withdrawal of the by drip irrigation. supernatant or effluent. The performance of this system is theoretically equivalent ▶ Sludge needs to to the conventional “activated sludge” process be digested for associated with a clarifier. applications. EA can also be carried out in oxida- IA T ER RI tion ditches, which tend to be s ts i t a l co Treatm considered an older technol- C /cap ent PEX) e nt e L m f f ic IA t (CA ien ogy and require more space es cy Inv 3 C than SBR. N en A E a nce N ha )X se d n PE FI of utr (O 2 up ien ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r ili t y b RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL Wastewater Treatment and Reuse 43 STAGE 1 STAGE 2 STAGE 3 STAGE 4 influent filling aeration + mixing settling effluent withdrawal treated effluent sludge wasting PROJECT DESIGN OPERATION ▶ Connected population: ▶ To optimize the performance ▶ The choice of SBR-EA is not (Cluster of houses) or of the system, two or more recommended for applications (Town) batch reactors are used in a where the wastewater is diluted ▶ Power supply: Requires predetermined sequence of or where there is a high flow of continuous electricity supply. operations. parasitic water. Functionality with low loads ▶ SBR are typically used at ▶ The choice of SBR is not must be evaluated where there flowrates of 20.000 m3/d or less recommended for irregular is a combined sewer system. but the most SBR installations applications with periods of low ▶ Mixing incoming wastewater are used for smaller wastewater loads or absence of loads which with industrial wastewater could systems of less than 8.000 m3/d. could lead to deterioration of the impact treatment performance. ▶ Flexible sizing means potential biomass, though sometimes it is Resistant to shocks in organic population growth can be complemented with additional and hydraulic loading. considered at design. feed in low load periods. ▶ Potential capital cost savings by ▶ Potential plugging of aeration eliminating clarifiers and other devices during selected equipment. operating cycles. ▶ Due to the high level of sophistication and complexity of the process, not all parts and materials may be locally available. 44 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #13 Trickling Filter (TF) DESCRIPTION REUSE Secondary aerobic treatment POTENTIAL Trickling filters consist in a structure containing a substrate (rocks, gravel, shredded ▶ Treated water can be PVC pipes, pozzolana) that acts as support for the development of microorganisms. used for restrictive These form the biological film, which is composed of aerobic bacteria on the surface irrigation (fruit trees, and anaerobic bacteria deeper in the medium. As previously decanted wastewater is industrial crops). sprinkled and infiltrates through the medium, the biofilm grows around the support and ▶ Effluent requires detaches when the water percolates. At the outlet of the trickling filter, the biofilm is disinfection trapped by settling in a secondary clarifier and forms treatment for non- sludge. The water separated in the settling restrictive reuse. tank is often recirculated to improve per- formance and maintain the filter wet. RIA I TE CR i ta l c os t s Treatm In most cases, the wastewater /cap e nt e L m ent PEX) f f ic IA is distributed at the top of t (CA ien es cy Inv the bed by a rotary distrib- 3 C N utor (sprinkler), though en A E a nce N it can also be supplied ha )X se d n PE FI of utr by gravity. (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r ili t y b RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL Wastewater Treatment and Reuse 45 rotary influent distributor sprinkler filter media inlet air filter support outlet collection Source: Tilley et al. 2014. PROJECT DESIGN OPERATION ▶ Connected population: ▶ Best suited for peri-urban or ▶ Influent distribution must be (Cluster of houses) or large rural settlements. uniform to allow for treatment (Town) ▶ Requires primary clarification to and avoid preferential paths. ▶ Continuous flow of influent is avoid clogging. ▶ Periods of non-supply to important to avoid drying of ▶ The sprinkler is the most suitable the trickling filter lead to its the biofilm, and continuous and widely used distribution desiccation and are to be energy supply is required if system with a sufficient flow avoided. used to transport and/or supply rate to generate a rotational ▶ Replacement parts are the wastewater. In this sense, movement. needed for the pumps and the continuity of water supply may distribution system (sprinkler). ▶ Must be coupled with secondary also affect performance, or settler to remove suspended ▶ If influent distribution is done require a storage/equalization solids. with gravity, then there is no tank. energy input need. ▶ Limited ability to accept mixed wastewater flow. Good resistance to transient organic overloads (50% organic load increase is accepted). 46 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #14 Rotating Biological Contactor (RBC) DESCRIPTION REUSE Secondary aerobic treatment POTENTIAL A rotating biological contactor (RBC) is a biological aerobic process. Discs serve as ▶ Treated water can be the supports for microflora growth. They are partially immersed in the wastewater and used for restrictive driven by a rotational movement along a horizontal axis, which ensures both mixing irrigation (fruit trees, and aeration. The microorganisms develop and form an active biological film on the industrial crops). disc surface. The rotation alternates the immersion state of the biomass, allowing both ▶ Effluent requires its oxygenation and absorption of organic matter. The rotational speed, which controls disinfection the contact intensity between the biomass and the wastewater and the rate of aeration, treatment for non- can be adjusted according to the organic load in the restrictive reuse. wastewater. The influent is previously decanted to IA avoid clogging of the support mate- T ER I CR s ts rial. When the biomass layer is i t a l co Treatm t /cap ) e nt e sufficiently thick (about 5 mm) L e n tm (CAPE X f f ic IA ien es cy some biomass detaches Inv 3 C N and is deposited at the en A bottom of the unit. The E a nce N ha )X se d n PE FI sludge is separated of utr (O up ien 2 ts from the treated water gra t rem cos d in by secondary clari- M O& g to val fication. The treat- o ment performance 1 is of the same order of magnitude as Land availa activated sludge or g y use SBR. Also very effec- ERIA En e r tive in the removal of bili t pathogenic bacteria. y RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL Wastewater Treatment and Reuse 47 rotating disk primary clarifier final clarifier influent water level effluent basin sludge optional aeration pipe sludge treatment underflow solids PROJECT DESIGN OPERATION ▶ Connected population: ▶ Must be coupled with secondary ▶ Requires operating personnel (Cluster of houses) or settler to remove suspended with electromechanical skills. (Town) solids. ▶ Additional oxygen supply may ▶ Power supply: Requires a ▶ Typical arrangement for secondary be particularly helpful when the continuous electricity supply. treatment comprises 3 or 4 stages. loads of the influent are high. ▶ The process is highly stable, In small installations these stages ▶ The sludge from the secondary resistant to shock hydraulic or can be on the same shaft, the clarifier must be extracted daily organic loading. sections of support medium to prevent sludge buildup and separated by baffles to produce effluent losses. a series of hydraulically ▶ Must be protected against independent compartments. sunlight, wind and rain (especially ▶ The addition of an air injection against freezing in cold climates). system to the wastewater in the disc tank is optional. ▶ If the organic load of the influent is variable, an aerated equalization basin is needed upstream. 48 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #15 UASB Followed by WSP (UASB-WSP) DESCRIPTION Secondary aerobic treatment The UASB reactor as first biological stage removes the bulk of organic pollution, and the sludge from this stage is well-digested. Combined with the ponds for disinfection and polishing this treatment technology is thus ideal for a focus on removal of organic pollution, combined with disinfection.  Several advantages exist: Anaerobic sible use of the biogas from UASB sludge yield is generally low, which— for energy generation may be an combined with the efficient stabi- attractive side-effect. Taking the lization and thickening inside the low total energy consumption into UASB reactors—permits for direct account, such systems can hence cost-efficient sludge dewatering. even become energy independent Fecal sludge can be efficiently co- from the public grit. digested in UASB. The high organic load reduction in UASB permits the polishing ponds to be designed with an optimized focus on disinfection RIA (e.g. optimum water depth). I TE CR s ts The disinfection of ponds i t a l co Treatm t /cap ) ent L m n e PEX ef f ic i e is efficient, and typically I A Inve s t (CA nc y no tertiary disinfection 3 C N stage is needed for en A E a nce N direct effluent reuse ha )X se d n PE FI of utr in non-restrictive (O up ien 2 ts gra t rem irrigation. Sludge cos d in M removal from the O& g to val ponds can be o limited to pro- 1 longed intervals > 10 years fre- Land availa quently. Even- g y use tually, the pos- ERIA En e r bili t y RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL Wastewater Treatment and Reuse 49  Disadvantages are that UASB reac- REUSE POTENTIAL tors generate biogas, rich in meth- ane, which must be properly man- aged to minimize risks of explosion. ▶ Effluent is fit for non- That also implicates a need for well- restrictive irrigation. trained personnel. Further, prelimi- ▶ Due to high algae nary treatment must be efficient and production, use well operated, too. If this is not done through drip the formation of scum on the sur- irrigation requires face of UASB can be considerable; filtration to remove removing such scum from the inside the suspended solids. of reactors that are designed to be possibly gas-tight, is a challenge. ▶ Treated wastewater This challenge is further complicated, can be discharged in since the scum tends to solidify, and stream. then proves hard to remove. Even if the UASB is properly managed, some biogas always remains dissolved in its liquid effluent, and then escapes to the open air, contributing to GHG emissions. Upgrading to BNR can be done, but requires extra stages for nitrification. Finally, it remains to mention that the ponds require con- siderable land footprint, and are not feasible in case of very limited land availability. 50 Appropriate Wastewater Treatment Technology for Small Towns gas outlet gas collector preliminary treatment anaerobic pond facultative pond maturation pond O2 O2 O2 O2 O2 O2 O2 O2 sludge bed digested receiving body sludge (for dewatering) Source: de Lemos Chernicharo 2007. PROJECT DESIGN OPERATION ▶ Connected population: ▶ For UASB design see separate ▶ O&M of UASB requires (Cluster of houses) or Technology Sheet. While careful attention to keeping (Town) there are a series of different preliminary treatment efficient ▶ Feasibility of sewerage: parameters that need to be and functional, as well as regular sufficient water supply needed. taken into account, for very rough scum removal, and proper sizing an average retention time biogas management. This is not ▶ Fecal sludge: to some of 6–12 hours may be assumed particularly time-consuming, but reasonable extent fecal sludge (6 h for wastewater temperature requires well-trained operators. can be co-treated in UASB. > 26°C, 12 h for 18°C). Typical ▶ The maturation ponds only need ▶ Regulations for treated UASB water depth is 4–6 m. regular trimming of grass on its discharge & reuse: secondary ▶ The maturation ponds are embankments and intermittent treatment level can be achieved; designed for hydraulic surface cleaning. however, organic parameters charge and retention time may even increase in pond (minimum 3–4 days to permit effluent due to formation of proliferation of algae). Water algae (algal BOD5 is not the depth is about 1 m. same as raw wastewater BOD5, but nonetheless is shows up in analysis); nutrient removal is limited. ▶ Available land for WWTP: high footprint, particularly for ponds. ▶ Power supply to WWTP: low; if power is needved, it is primarily for wastewater pumping and for operation of preliminary treatment. Biogas from UASB could be used for energy generation. Wastewater Treatment and Reuse 51 TECH SHEET #16 UASB Followed by TF (UASB-TF) DESCRIPTION Secondary aerobic treatment The UASB reactor as first biological stage removes the bulk of organic pollution, and the sludge from this stage is well-digested. Combined with a Trickling Filter (TF) the effluent quality can be further improved, even to BNR standards. For disinfection a tertiary stage is needed.  Several advantages exist: Anaerobic may be an attractive side-effect. Tak- sludge yield is generally low, which— ing the low overall energy consump- combined with the efficient stabiliza- tion into account, such systems can tion and thickening inside the UASB hence even become energy indepen- reactors—permits for direct cost- dent from the public grit, or at least efficient sludge dewatering. The reach a high percentage of power waste sludge from the TF stage can coverage from the biogas. also be co-digested in the UASB, as well as fecal sludge. The high organic load reduction in UASB permits the TF volume to be designed signifi- RIA cantly smaller than in classical TF I TE CR plants. The combined effects l c os t s Treatm a pi t a ent nt/c EX) of 2 high-rate reactors L e tm (CAP ef f ic i e I A I nves nc y (UASB + TF) leads to a 3 C N WWTPs with low foot- en A E a nce print, comparable to N ha )X se d n PE FI of utr Activated Sludge (O 2 up ien ts systems. In addi- gra t rem cos tion, such a sys- d in M O& g to val tem with two o separate stages 1 can cope well with hydrau- Land availa lic and organic g y use shock-loads in ERIA En e r raw wastewater. ili t y b The possible RIT use of the bio- gas from UASB for L C energy generation n La tio TA bo uc r d qu EN ro p f ic a li ge d on ati M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem A L/E TECHNIC 52 Appropriate Wastewater Treatment Technology for Small Towns  Disadvantages are that UASB reac- REUSE POTENTIAL tors generate biogas, rich in methane, which must be properly managed to minimize risks of explosion. That also ▶ Effluent is fit for implicates a need for well-trained restricted irrigation personnel. Further, preliminary treat- or can be discharged ment must be efficient and well in stream. operated, too. If this is not done the formation of scum on the surface of UASB can be considerable; remov- ing suchscum from the inside of reac- tors that are designed to be possibly gas-tight, is a challenge. This chal- lenge is further complicated, since the scum tends to solidify, and then proves hard to remove. Even if the UASB is properly managed, some biogas always remains dissolved in its liquid effluent, and then escapes to the open air, contributing to GHG emissions. Upgrading to BNR can be done, but requires extra TF volume for nitrification. Wastewater Treatment and Reuse 53 gas outlet gas collector trickling filter rotary influent secondary to preliminary distributor settler receiving treatment sprin- body kler sludge bed air sludge collection filter filter effluent support media recycle digested sludge (for dewatering) return excess sludge Source: de Lemos Chernicharo 2007. PROJECT DESIGN OPERATION ▶ Connected population: ▶ For UASB design see separate ▶ O&M of UASB requires (Town) Technology Sheet. While careful attention to keeping ▶ Feasibility of sewerage: there are a series of different preliminary treatment efficient sufficient water supply needed. design parameters that need and functional, as well as regular to be taken into account, for scum removal, and proper ▶ Fecal sludge: to some very rough sizing an average biogas management. This is not reasonable extent fecal sludge retention time of 6–12 hours may particularly time-consuming, but can be co-treated in UASB. be assumed (6 h for wastewater requires well-trained operators. ▶ Regulations for treated temperature > 26°C, 12 h for ▶ The operation of the TF stage discharge & reuse: secondary 18°C). Typical UASB water depth does not require particular treatment level can be achieved; is 4–6 m. process know-how; however, nutrient removal can be ▶ The TFs are designed for keeping the electro-mechanical incorporated. volumetric organic loading installations well maintained, is ▶ Available land for WWTP: low and hydraulic surface charge, not up to unskilled labor. footprint. dependent on specific ▶ Power supply to WWTP: conditions. Filter depth is usually low; power serves primarily 3–5 m in modern filters. for wastewater pumping and ▶ Secondary Sedimentation for operation of preliminary Tanks are designed similar treatment. Biogas from UASB to such installations after could be used for energy Activated Sludge tanks (see generation. e.g. Technology Sheet on EA). Albeit after TFs they can even be designed somewhat smaller due to good settling characteristics of the TF sludge. 54 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #17 Disinfection with Ultraviolet System (UV) DESCRIPTION REUSE Tertiary treatment, water disinfection POTENTIAL Ultraviolet (UV) disinfection uses mercury arc lamps to expose wastewater to con- ▶ Effluent fit for centrated UV light, which kills pathogenic microorganisms. Wastewater flows per- nonrestrictive pendicular or parallel to the lamps, which are encased in a protective quartz sleeves irrigation. (instead of glass) to protect them from the cooling effects of the wastewater. The concentrated light inactivates microbial cells and prevents them from reproduc- ing. The process takes place in an opaque tube in order to protect operators from exposure. The effectiveness of a UV disinfection system depends on the charac- teristics of the wastewater, the intensity of UV radiation, the time the microorganisms are exposed to the radiation, and the reac- IA tor configuration. Some simplified T ER I CR UV tubes have been developed l c os t s Treatm a pi t a ent nt/c EX) for household-level use. As L e tm (CAP ef f ic i e this disinfection process is I A I nves nc y 3 C purely physical, it provides N en A an interesting alterna- E a nce N ha )X se d n PE FI tive where by-products of utr (O up ien 2 ts from chlorination are a gra t rem cos d in concern. M O& g to val o 1 Land availa g y use ERIA En e r bili t y RIT L C n La tio TA bo uc r d qu EN ro p f ic a li ge d on ati M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem A L/E TECHNIC Wastewater Treatment and Reuse 55 inlet outlet UV light module PROJECT DESIGN OPERATION ▶ Connected population: ▶ UV disinfection equipment ▶ UV disinfection is a physical (Cluster of houses) or requires less space than other process rather than a chemical (Town) methods. disinfectant; thus eliminating ▶ Power supply: Requires a the need to generate, handle, constant electricity supply. transport, or store toxic/hazardous or corrosive chemicals. ▶ Low dosages may not effectively inactivate some biological organisms. ▶ Organisms can sometimes repair and reverse the destructive effects of UV. ▶ Turbidity and total suspended solids (TSS) in the wastewater can render UV disinfection ineffective. ▶ Inadequate cleaning is one of the most common causes of a UV system’s ineffectiveness. ▶ Lamps need to be replaced every 6–12 months. 56 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #18 Disinfection with Chlorine (Cl) DESCRIPTION REUSE Tertiary treatment, water disinfection POTENTIAL Chlorine kills most bacteria, viruses, and other microorganisms that cause disease. ▶ Effluent fit for Wastewater and chlorine are first mixed completely and then enter a baffled con- nonrestrictive reuse tact chamber to allow time for disinfection to occur. The radicals formed when the for irrigation. chlorine dissolves in the water ‘attack’ microorganisms and pathogens by breaking molecular bonds and cells. The effluent is then discharged to the receiving water body or reused, as applicable. The effluent contains residual chlorine, which ensures it is not re-contaminated for a certain amount of time. Disinfection is usually accomplished with liquid chlorine (sodium hypochlorite), elemental chlo- rine gas, calcium hypochlorite (solid), or IA ER chlorine dioxide (gas). The chemi- cal should be selected after due I T CR s ts i t a l co Treatm t /cap ) e nt e consideration of wastewa- L e n tm (CAPE X f f ic IA ien es cy ter flow rates, application Inv 3 C and demand rates, pH of N en A wastewater and chemi- E a nce N ha )X se d n cal availability. PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r bili t y RIT L C n La tio TA bo uc r qu od EN a li pr e f ic dg ati on M Slu N O pa r uts IR Avail ts and O&M inp ent NV ability of replacem /E TECHNI CAL Wastewater Treatment and Reuse 57 chlorine addition contact basin inlet outlet mixing unit PROJECT DESIGN OPERATION ▶ Connected population: ▶ Chlorine is a well-established ▶ The chlorine residual that (Household), technology, easy to use, remains in the discharged (Cluster of houses) or solubilize in water and rinse wastewater can prolong (Town) with water. disinfection even after initial ▶ Form of chlorine to be used will ▶ Presently, chlorine is more cost- treatment and also provides a depend on local availability and effective than other disinfection measure of the effectiveness. connectivity to suppliers. methods. ▶ Chlorine by reacting with certain natural organic compounds creates toxic or ecotoxic by-products. However, the WHO considers that the health risks of these by-products are still low compared to those caused by inadequate disinfection of water. ▶ All forms of chlorine are highly corrosive and toxic. Thus, storage, shipping, and handling pose safety risks. ▶ Corrosion or embrittlement of certain plastics and corrosion of many metals (including stainless steel) if the pH of the medium is lower than 8. 58 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #19 Polishing Pond (PP) DESCRIPTION Tertiary treatment for removal of effluent suspended solids (SS) Polishing Ponds (often also called Sedimentation Ponds) are e.g. employed in the final effluent of Aerated Lagoons, to minimize effluent suspended solids. This is usually done to improve effluent quality as such, since reduced SS also implies reduced BOD5, COD, TN, TP. Or it may be indirectly necessary to permit UV radiation for disinfection (UV radiation only works efficiently if SS is low.) Polishing Ponds permit to achieve effluent SS in the range of 20 to 60 mg/L. IA T ER C RI /cap i ta l c os t s Treatm e nt e L m ent PEX) f f ic IA t A ien es (C cy Inv 3 C N en A E a nce N ha )X se d n PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r bili t y RIT L C n La tio TA bo uc r d qu EN ro p f ic a li ge d on ati M Slu N O pa r uts R Avail ts and O&M inp ent VI ability of replacem N A L/E TECHNIC Wastewater Treatment and Reuse 59 influent S effluent S effluent aerated lagoons (AL) polishing pond PROJECT DESIGN OPERATION ▶ Connected population: ▶ Hydraulic retention time in ▶ Embankments need to (Cluster of houses) or Polishing Ponds should be be checked regularly and (Town) chosen between 1 to 2 days. maintained free from large ▶ Feasibility of sewerage: To meet this requirement at plants; grass needs to be sufficient water supply needed. all times, it is recommended trimmed from time to time. designing for 1 day at design In certain intervals sludge Regulations for treated ▶ ▶ horizon. In order to minimize removal is required. To that ends discharge & reuse: dependent algae formation, unnecessarily it is either necessary to empty on design conditions of prior large Polishing Ponds should be the pond first, and then enter treatment. avoided. with machinery to remove the ▶ Available land: low-medium ▶ Construction of Polishing Ponds sludge. Or floating rafts may be footprint. follows the principles described employed which have sludge ▶ Power supply: usually not in the Technology Sheet on pumps mounted. needed. WSPs. Common water depth ▶ Sludge removal becomes is 1.5 m. necessary as soon as the sludge is covered by less than 1.0 m water, to minimize odor emissions. Hence with a typical total liquid depth of about 1.5 m in Sedimentation Ponds, the maximum sludge depth is limited to about 33% = 0,5 m. 60 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #20 Rock Filter (RF) DESCRIPTION REUSE Tertiary treatment for algae removal POTENTIAL Rock filters provide low-cost, low-maintenance polishing of pond effluents. Their ▶ Effluent fit for prime effect is removal of algal suspended solids. The system consists of a submerged nonrestrictive bed of rocks. Rock filters can be located either in the lagoon / pond effluent zone, or irrigation. they can be installed as separate units downstream of the lagoon / pond. The algal solids settle and/or attach to the rock, where they are then decomposed by bacteria. Typical SS removal rates are in the order IA of 40 to 60%. Consequently, properly T ER designed rock filters can achieve C RI /cap i ta l c os t s Treatm e nt e effluent SS of ≤ 30 mg/L. L m ent PEX) f f ic IA t A ien es (C cy Inv 3 C N en A E a nce N ha )X se d n PE FI of utr (O up ien 2 ts gra t rem cos d in M O& g to val o 1 Land availa g y use ERIA En e r bili t y RIT L C n La tio TA bo uc r d qu EN ro p f ic a li ge d on ati M Slu N O pa r uts R Avail ts and O&M inp ent VI ability of replacem N A L/E TECHNIC Wastewater Treatment and Reuse 61 anaerobic pond facultative pond maturation pond O2 O2 O2 O2 O2 O2 O2 O2 inlet outlet rock filter PROJECT DESIGN OPERATION ▶ Connected population: ▶ The design of rock filters usually ▶ Optimum cleaning procedures (Cluster of houses) or is done via hydraulic loading are not clearly established, but (Town) rate (HLR). Typical loadings are periodic removal of accumulated ▶ Water supply: sufficient water in the order of 1,0 m3 effluent/d humus may be recommendable. supply needed. being applied to 1,0 gross m3 of rock filter. ▶ Regulations for treated discharge & reuse: dependent ▶ The system consists of a on design conditions of prior submerged bed of rocks, mostly WSP. 75 to 100 (50 to 200) mm in size, with a bed depth of about ▶ Available land: low-medium 1,5–2,0 m, through which footprint. the lagoon effluent flows ▶ Power supply: usually not horizontally. The rocks should needed. extend at least 100 mm above the water level, to minimize mosquito breeding and to avoid odor emissions from cyanobacteria that like to develop on wet surfaces exposed to sunlight. 62 Appropriate Wastewater Treatment Technology for Small Towns TECH SHEET #21 Rotary Disc Filter (RDF) DESCRIPTION REUSE Tertiary treatment POTENTIAL Rotary Disc filters (RDF) are a physical treatment process relying on the filtration of ▶ Effluent fit for wastewater through disc-shaped filters affixed in a rotating drum to remove residual restrictive irrigation. suspended solids from secondary effluents. The rotating drum is divided into segments, ▶ Fit for unrestrictive themselves covered with filter media. The wastewater is introduced at the center of the reuse after drum through a feed tube and pressed through the fil- disinfection. ter media by the pressure differential between the filter channel and the collection area RIA outside. Treated water is collected at the bottom of the drum and con- I TE CR l c os t s Treatm veyed. Sludge accumulates in a pi t a ent nt/c EX) L m e P ef f ic i e the filter media and, once it I A Inv e s t (C A nc y reaches a certain thickness, 3 C N activates the backwashing en A E a nce N ha ) process which consists X se d n PE FI of utr (O in spraying effluent up ien 2 ts gra t rem cos (clean) water on the d in M filter while the drum O& g to val is rotating, collecting o the washwater into 1 a specific pipe for discharge. Filtration Land availa g y use can be either con- stant with continu- ERIA En e r ous backwashing or intermittent. bili t y RIT L C n La tio TA bo uc r d qu EN ro p f ic a li ge d on ati M Slu N O pa r uts R Avail ts and O&M inp ent VI ability of replacem N A L/E TECHNIC Wastewater Treatment and Reuse 63 washing water for backwashing water pressed through filter media effluent effluent sludge influent Source: Enerhall and Stenmark 2012. PROJECT DESIGN OPERATION ▶ Connected population: ▶ When using as a tertiary filter, ▶ Backwash automatized; (Cluster of houses) a very fine pore size is required backwash filter cleaning every six ▶ No flexibility in changing leading to low hydraulic months. influent quality. Activities capacity. ▶ Acid cleaning can be used for increasing Suspended solids in ▶ With prefiltration, less energy is mineral fouling if needed. effluent feeding the ISF are not required. accepted. 64 Appropriate Wastewater Treatment Technology for Small Towns The Optimum Combination both of which serve to reduce the water content in sludge, thereby decreasing the sludge volume. of Technologies for Primary Thickening is the first step of water reduction and and Secondary Treatment is done mostly by gravity. It can also be achieved mechanically on moving belts or in rotating drums. There are many components and treatment stages Flotation, which is also a means of thickening, was available, and selecting the optimum combination excluded in the preselection stage because it is of technologies—that is, treatment trains—can be a not considered financially competitive for small- challenge. For this reason, this guide also presents town WWTPs. Dewatering, the second step in water several predefined and well-established treatment reduction, is usually done extensively in drying beds trains and their main components. or intensively in different types of centrifuges or Tables 3.5 and 3.6 present the commonly employed presses, such as belt filter presses or screw presses. wastewater treatment trains for small-town WWTPs. If sludge is not properly stabilized in the wastewater These draw from the preselected technologies listed treatment train—that is, if it continues degrading and in Table 3.4, with Table 3.5 focusing on wastewater emitting bad odors after removal from the treatment and Table 3.6 focusing on sludge treatment. The train—there is a need for sludge stabilization . following points should be taken into account: Anaerobic digesters and aerobic stabilization are the most commonly used options. UASB reactors ◾ Pretreatment is an indispensable requisite for can also be used to digest both the primary sludge, almost any treatment train, apart from a few stand- which accumulates inside those reactors, and the alone primary technologies. secondary sludge from the subsequent stages. ◾ Primary treatment options can be used as Finally, if the dewatering is still insufficient for the stand-alone technologies, albeit with reduced disposal or reuse of the sludge, sludge drying may treatment efficiency. also be employed. Possible technologies range from ◾ The most common WWTP technologies are simple drying beds to solar drying greenhouses. those that employ a combination of primary and Thermal driers are excluded here because they secondary treatment elements. are considered too costly and too operationally demanding for the purpose of small-town WWTPs. ◾ Tertiary treatment is considered as further improvement after primary and secondary treatment. It is usually not applied after primary The Optimum Combination treatment. of Treatment Technologies In addition, Tables 3.5 and 3.6 present components for Wastewater Reuse of treatment trains that are typically found in small The challenges of achieving the Sustainable towns (indicated with black cells), as well as those Development Goals (SDGs), combined with water that are considered to be optional in that they security, have driven countries to identify ways can complement “typical” treatment chains or of deriving value from wastewater streams. The replace some of their components (indicated with potential for wastewater reuse for agricultural, brown cells). environmental, industrial, residential or municipal In sludge treatment trains, the most common uses has consequently become a key factor in treatment stages are thickening and dewatering, WWTP designs. As mentioned in previous chapters, Wastewater Treatment and Reuse 65 TABLE 3.5 Typical Wastewater Treatment Trains for Preselected Treatment Technologies for Small-Town WWTPs WASTEWATER TREATMENT TRAIN PRIMARY TERTIARY PRETREATMENT TREATMENT SECONDARY TREATMENT TREATMENT MATURATION POND ROTARY DISC FILTER FACULTATIVE POND GRIT/FAT REMOVAL ANAEROBIC FILTER AERATED LAGOON ANAEROBIC POND PLASTIC MEDIA TF DISINFECTION–UV BIOGAS DIGESTER PLANTED GRAVEL POLISHING POND STONE MEDIA TF DISINFECTION– EQUALIZATION IMHOFF TANK LIQUID/SOLID SEPTIC TANK ROCK FILTER SEPARATION CHLORINE SCREEN FILTER SIEVE UASB ABR RBC SBR PST FST AT # TECHNOLOGY ABBREV. Primary treatment (only) 1 Septic tank ST 2 Biogas digester BD 3 Imhoff tank IMH Primary + secondary treatment 4 Anaerobic baffled reactor ABR 5 Anaerobic filter ANF 6 Waste stabilization pond WSP (as needed) 7 Aerated lagoon AL 8 Single-stage constructed wetland CW (1-st) 9 Hybrid constructed wetland CW (hybrid) 10 Upflow anaerobic sludge blanket reactor UASB 11 Extended aeration (AS type) EA 12 Extended aeration (SBR type) SBR (EA) 13 Trickling filter TF 14 Rotating biological contactor RBC 15 UASB-WSP UASB-WSP (as needed) 16 UASB-TF UASB-TF   Typical component   Optional component (either additional or replacing another component) Note: The term waste stabilization pond (WSP) refers to the classical configuration consisting of anaerobic, facultative and maturation ponds. The term polishing pond is used for an optional component to complement technologies and treatment trains, whereas the term maturation pond is strictly used as part of WSP systems in this guide. AT = aeration tank; FST = final sedimentation tank; PST = primary sedimentation tank; UV = ultraviolet; WWTP = wastewater treatment plant. TABLE 3.6 Typical Sludge Treatment Trains for Preselected Treatment Technologies for Small-Town WWTPs SLUDGE TREATMENT TRAIN SEDIMENTATION SLUDGE DRYING SOLAR DRYING STABILIZATION DIRECT REUSE COMPOSTING MECHANICAL MECHANICAL DEWATERING TREATMENT ANAEROBIC THICKENER THICKENER THICKENER WETLAND DIGESTER AEROBIC SEPTAGE GRAVITY POST- TANK UASB BED # TECHNOLOGY ABBREV. Primary treatment (only) 1 Septic tank ST 2 Biogas digester BD 3 Imhoff tank IMH Primary + secondary treatment 4 Anaerobic baffled reactor ABR 5 Anaerobic filter ANF 6 Waste stabilization pond WSP Wastewater Treatment and Reuse 7 Aerated lagoon AL 8 Single-stage constructed wetland CW (1-st) 9 Hybrid constructed wetland CW (hybrid) 10 Upflow anaerobic sludge blanket reactor UASB 11 Extended aeration (AS type) EA 12 Extended aeration (SBR type) SBR (EA) 13 Trickling filter TF 14 Rotating biological contactor RBC 15 UASB-WSP UASB-WSP 16 UASB-TF UASB-TF Typical component   Optional component (either additional or replacing another component) 67 Note: WWTP = wastewater treatment plant. small towns present unique opportunities for reuse wastewater are fecal coliforms (FC) and helminth in that there is a likely advantage for the treated eggs (particularly intestinal nematode ova), the wastewater to be generated closer to potential removal efficiency of which is typically expressed reuse sites. This is particularly true for agriculture. using a logarithmic scale (log units). For example, a reduction in FC concentration from 107 FC/100 mL Whereas the primary and secondary technologies to 104 FC/100 mL would correspond to a reduction presented herein are effective, to varying degrees, of 3 log units, or 99.9 percent, as shown in Table 3.7. at removing suspended solids and organic matter Furthermore, it should be borne in mind that although from wastewater, they are generally not sufficient 90 percent removal efficiencies may seem high, this for the removal of pathogenic microorganisms to represents only a 1 log unit reduction. Much higher an acceptable level (WHO 2006). Given the health pathogen removal rates will generally be required hazards associated with direct and indirect treated to achieve low effluent concentrations given the wastewater use, pathogen elimination and monitoring high incoming pathogen concentrations in raw of control measures should be considered an integral sewage, which is particularly the case in LMICs part of the wastewater treatment train. Similar to which are often characterized by higher pathogen wastewater treatment in general, the optimal prevalence in the population and lower overall combination of technologies to reach a certain level water usage, with both leading to higher pathogen of pathogen removal in a given situation will depend concentrations in the wastewater. For example, even on a variety of factors. Different combinations with a 3-log unit reduction, there would still be involving extensive and intensive treatment options 10,000 FC/100 mL left in the effluent, falling short can be used to achieve the desired effluent quality of the required microbial quality to irrigate root levels required for reuse, as shown in Figure 3.1. crops (unrestricted reuse), according to the 2006 The most commonly used indicator parameters World Health Organization (WHO) guidelines for to monitor the presence of pathogens in treated the safe use of wastewater, excreta and graywater, FIGURE 3.1 Examples of Combinations of Treatment Options for Different Wastewater Reuse Scenarios Extensive treatment Use Intensive treatment • Wastewater stabilization • Irrigation of fruit trees, • Extended aeration pond forest, meadows • Trickling filter • Constructed wetland • Industrial crops • Rotating biological • Crops eaten cooked contactor • Industrial Additional tertiary • Nonrestrictive irrigation of Additional tertiary treatment and/or crops eaten uncooked treatment and/or pathogen removal by: • Urban use for irrigation of pathogen removal by: • Maturation pond parks, golf courses, etc. • Disinfection • Soil infiltration • Groundwater recharge • Removal of nitrogen • Industrial and phosphorus Source: Authors’ own work. 68 Appropriate Wastewater Treatment Technology for Small Towns TABLE 3.7 Correspondence between Log Units and Removal Efficiency Percentages PATHOGEN INDICATOR PATHOGEN INDICATOR CONCENTRATION IN REMOVAL CONCENTRATION IN RAW WASTEWATER EFFICIENCIES EFFLUENT (FC/100 mL) (Log units) (%) (FC/100 mL) 10 7 1 90 106 107 2 99 105 107 3 99.9 104 107 4 99.99 103 107 5 99.999 102 Note: FC = fecal coliforms. and representing a potential public health risk if effluent requirements with disinfection may not be the treated wastewater were to be reused without able to sufficiently reduce effluent concentrations of further treatment. viruses, helminth eggs or protozoa, such as Giardia or Cryptosporidium, thus potentially contributing Average pathogen removal efficiencies for several to public health risks if its effluent is discharged technologies and combinations of technologies can to surface waters that are used downstream as be found in the literature, together with information drinking water sources, or if the treated wastewater on the removal levels achievable by various control is used for the irrigation of crops. It is therefore measures aimed at protecting the health of workers critical to also carefully consider the importance and consumers from wastewater pathogens, of pathogens that may be a local or regional public particularly in the case of treated wastewater reuse health concern, such as protozoa and helminths for irrigation (Oakley and Mihelcic 2019; WHO 2006). Such protection can be achieved through the (instead of just focusing on FC, for example) when establishment of several barriers to contamination, selecting treatment technologies for reuse. namely: (a) barriers upstream of the reuse perimeter, Box 3.1 provides two examples of agricultural through the wastewater treatment process itself; wastewater reuse, where a combination of (b) barriers at the place of reuse; and (c) barriers at technologies would need to be selected to achieve the consumer and household level. For example, certain effluent quality objectives. In both cases, the although WSPs can typically achieve a reduction of selection is dictated by the end use of the treated 3 to 5 log units, adopting localized (drip) irrigation wastewater or the type of crop to be irrigated. could provide an additional pathogen reduction of 2 to 4 log units, depending on whether the harvested parts of the crops are in contact with the Note soil; the cooking of produce can provide additional 1. Based on a production rate of 100 L wastewater/cap/d. In pathogen reduction of 5 to 6 log units. addition, BOD5 refers to the five-day biochemical oxygen demand; PE60 refers to the per capita BOD5 loading In addition, it is important to note that a well- produced during 24 hours, or population equivalent (PE), of operated treatment plant meeting its bacterial 60 g BOD5/cap/d; and MLD refers to million liters per day. Wastewater Treatment and Reuse 69 BOX 3.1 Examples of Technology Selection for Agricultural Wastewater Reuse EXAMPLE 1: Intensive treatment option to irrigate lettuce crops. In this case, costs associated with land acquisition are prohibitively high and an intensive treatment combination could be implemented so that investment costs associated with the civil works and the earth works are minimized. As per the 2006 WHO guidelines (and bearing in mind the need to protect the health of workers in wastewater-irrigated fields against excessive risks of viral, bacterial, protozoan and helminth infections), we see that only a 3 to 4 log unit pathogen reduction will be achieved by the wastewater treatment, whereas a conservative total reduction of 7 log units is needed to ensure the safe consumption of wastewater-effluent-irrigated lettuce. Similarly, additional technologies may be required for the effluent to be considered safe in terms of helminth egg concentrations, which should be reduced below or equal to 1 helminth egg/L, as per these same guidelines. The treatment process could thus include: TREATMENT PATHOGEN REMOVAL HELMINTH EGG REMOVAL LEVEL TECHNOLOGY (LOG UNITS) (LOG UNITS) Pretreatment Screening, oil/grease removal 0 0 Primary Primary sedimentation <1 <1 Secondary Trickling filters and sedimentation tank 1–2 1–2 Tertiary Chlorination 2–6 < 1a Tertiary Disc filters with a mesh size of ≤ 10 µmb <1 > 3c,d a As part of a recent research project, chlorination was found to provide removal efficiencies of up to 20% (< 0.7 log units). Cornel, P., Kneidl, S., Bishop, F., Schmaußer, S., Merkl, A., and Dehnert, M. 2016. “Elimination of Helminth Eggs.” Closing event for the EXPOVAL Federal Ministry of Education and Research (BMBF) Joint Project, Essen, Germany, October 5–6. b Disc filters are increasingly being used not only for solids but also for helminth eggs removal. c Cornel, P., Kneidl, S., Bishop, F., Schmaußer, S., Merkl, A., and Dehnert, M. 2016. “Elimination of Helminth Eggs.” Closing event for the EXPOVAL Federal Ministry of Education and Research (BMBF) Joint Project, Essen, Germany, October 5–6. d Quinzaños, S., Dahl, C., Strube, R., and Mujeriego, R. 2008. “Helminth Eggs Removal by Microscreening for Water Reclamation and Reuse.” Water Science and Technology 57 (5): 715–20. In this example, chlorination is used to reach this high level of pathogen removal, but such tertiary treatment could also be substituted by posttreatment control measures, such as drip irrigation, exposure to the sun, or rinsing and washing of the lettuce at home. In terms of helminth eggs, the efficiency of their removal will depend on the ova content in the influent wastewater, which can vary significantly, particularly in LMICs (Jiménez and Galván 2007). Assuming a high content of helminth eggs, such as 2,000 eggs/L, the proposed treatment process would be able to reach the recommended limit of £ 1 helminth egg/L, but only with the addition of the disc filters. EXAMPLE 2: Extensive treatment option to irrigate olive tree plantations. In this case, the costs associated with land acquisition are not prohibitive, and land is available near the small town. An extensive treatment solution could thus be implemented, and the operation and maintenance costs could be minimized. An additional 2 to 4 log units of pathogen removal can be achieved through the inclusion of a control measure at the place of reuse, and because olive trees are a high-growing crop, drip irrigation should allow the reuse system to reach a removal of an additional 4 log units. The treatment process could thus include:process could thus include: TREATMENT LEVEL TECHNOLOGY PATHOGEN REMOVAL (LOG UNITS) Pretreatment Screening, oil/grease removal 0 Primary Primary sedimentation <1 Secondary Constructed wetland 3–4 Posttreatment control measure Drip irrigation 2–4 70 Appropriate Wastewater Treatment Technology for Small Towns Factors to Address for WWTPs in Small Towns 4 Users of this guide will be directed through the selection of technologies with the help of two categories of criteria: (a) project criteria, which are external to the technologies and define the characteristics and environment of a given small town and which will affect the technology choice; and (b) technology criteria, which include the technology-specific information (for example, technical performance and characteristics) which will ultimately influence decision making. This section describes each criterion, provides examples, as appropriate, and offers guidance on refining them for a specific context. Project Criteria Project criteria aim to identify small-town characteristics that will affect technology choice. The guide suggests six core project criteria that outline important characteristics of the small town, which should be considered when selecting a wastewater treatment system. These highlight the importance of several different aspects that decision makers need to take into account relating to population, growth, local activities and existing services and practices. Feasibility of Sewers The presence and quality of other urban services in the target small town will affect the selection of wastewater treatment options. The institution responsible for wastewater management will likely need to engage with other urban service providers to ensure alignment of activities and parameters. The most important urban services which have an influence on the feasibility and efficiency of sewer systems are typically water supply, drainage and solid waste management. The density of housing, and the distance between neighboring houses, also has an important impact on the viability of sewered sanitation as compared to on-site sanitation approaches, such as those provided by septic tanks and pit latrines. The denser the housing in the small town in question, the shorter the sewer extensions, and the more viable are sewers from a financial perspective. Some service providers, such as eThekwini Water and Sanitation in South Africa, have used upfront analyses of the capital cost of laying sewers in comparison to the cost of installing properly designed and constructed on-site sanitation alternatives, in order to identify which approach makes the most financial sense to the utility in a given neighborhood. Wastewater Treatment and Reuse 71 Water supply: costs of the WWTP. Combined sewer systems also Water supply is a key factor when assessing the increase the likelihood of overflow events leading feasibility of sewers. If there is only intermittent to untreated wastewater being directly discharged water supply, or if households do not have their own to the environment, which may be of particular water connections, a sewered sanitation solution concern in areas where the receiving body is may not be appropriate, or it may be appropriate environmentally fragile or where humans may come only in certain parts of the town. The same also into direct contact with the receiving body. applies if the water supply consumption per capita Nevertheless, planning for a separate sewer system is very low and/or if the population is using most (in which wastewater and stormwater are conveyed of the generated wastewater or graywater for separately) is no guarantee of well-functioning sewers, irrigation purposes—for example, in private as there are numerous examples of defunct or gardens or vegetable allotments—leaving almost poorly maintained stormwater drainage systems no wastewater for discharge into sewers. that have serious negative impacts on the sewer Where water consumption is sufficient and regular, system. In situations in which the drainage system not only can it help estimate the volume of is not working properly, residents may try to divert wastewater generated by each household with stormwater flows to the sewer system, even if this is simple assumptions about the wastewater return not allowed, and the sewers may consequently be coefficient, but the consumption volumes are also hydraulically overloaded. This can lead to combined closely related to wastewater strength, as measured wastewater-stormwater flows being inadvertently by its five-day biochemical oxygen demand (BOD5) discharged at certain points of the sewer network or chemical oxygen demand (COD). Where water and possibly overwhelming the hydraulic capacity consumption is high, wastewater tends to be weaker/ of the WWTP. In addition, drainage systems may be more diluted, whereas in many LMICs where water deliberately intercepted and discharged to sewers, consumption can be relatively low, wastewater and in such cases, the dilute nature of the flows is correspondingly stronger. Knowing whether would also need to be duly taken into account when households also use their water supply for irrigation conceptualizing and designing the WWTP. purposes will help define the return factor, or the portion of water use that is discharged to the sewer Solid waste management: as wastewater. Usually, a value of 0.8 is used, but If solid waste is not properly managed in the town, if a larger part of the water is used for irrigation, a excess solid waste may end up in the sewers and at the factor of 0.6 could be taken. In addition, if roofs are treatment site. Common implications associated with connected to the sewers (even if that is against local this include clogged sewer pipes and wastewater regulations), peak wastewater flow values during pumping stations, emitting bad odors and leading rainfall events will be correspondingly larger than to wastewater spillage, as well as the transmission usual, thereby also affecting wastewater treatment of the solid waste to the WWTP. Solid waste that plant (WWTP) process selection and sizing. arrives at the WWTP can be managed but must be planned for and may require additional steps Drainage/stormwater management: of pretreatment and operation and maintenance. If the town uses a combined sewer system (in which Ideally, the solid waste should be collected at wastewater and stormwater are both collected), the source and not allowed to enter the sewers, WWTPs will need to be sized accordingly—and this where it typically requires subsequent elaborate may affect the associated capital and operational removal efforts. 72 Factors to Address for WWTPs in Small Towns Total Connections to the WWTP FIGURE 4.1 Defining Project Boundaries Total connections to a WWTP are usually expressed in terms of capita (equivalents), reflecting the permanent Low-density areas population and the nonpermanent population, the sewer connection rates, industrial discharges and any fecal sludge that may be disposed of at the WWTP. Isolated areas In addition, the WWTP capacity requirements need to take future growth into account to avoid overloading, and WWTP design horizons are nowadays typically Project areas defined on the basis of forecast developments of about 15 to 20 years. Connected population: The connected population defines the minimum treatment technologies, particularly as it relates treatment capacity that needs to be installed for to graywater.1 The characteristics of graywater a given wastewater collection system. It should depends on several factors, including lifestyle, include not only permanent residents but also living standards, social and cultural habits, types people passing through or commuting to work in and quantities of household chemicals used, food the small town. Such nonpermanent residents are residues, and so on. The biochemical characteristics usually multiplied by a factor of 0.3 to 0.5 and then of graywater can vary greatly, which can influence added to the number of permanent residents. The the selection of wastewater treatment options. For resulting total number is often termed as population example, in areas where manual laundry washing is equivalents or capita equivalents, with each capita common, an increased amount of fiber could make equivalent representing the typical pollution its way to the WWTP, requiring fine screening to generated by one permanent resident. improve the pretreatment’s efficiency. Graywater can also represent an important part of the total In some cases, only parts of a town will be covered water consumption of a household (and thus of the by the sewer system, whereas others will remain wastewater flow generated), and an understanding with other forms of sanitation services. Political, of whether it is discharged into the street, to drains topographical, urban development and density or to sewers will help further guide the selection factors should be considered when defining the of wastewater treatment processes for a given small sewer project boundaries. Even when a project is town. Variations in diet can also influence the amount meant to cover the whole town, the boundaries of organic waste produced per person per day (as between urban and rural areas may not be clearly measured by BOD5 or COD), and graywater from defined, and decision makers will need to justify kitchen sinks can contain elevated amounts of oil whether to include low-density or isolated areas and grease, which would require grease traps at (see Figure 4.1) while ensuring that the project is the treatment facility. Again, as described earlier, economically sustainable. in situations in which not all of the daily wastewater Having a good understanding of the social norms generated by a subgroup of the population is and behavioral characteristics of a relevant sample discharged to the sewers (such as that of visitors/ of the targeted population for the new sewer commuters), the population equivalent of that network can also be beneficial when selecting subgroup is reduced by a factor reflecting the Wastewater Treatment and Reuse 73 percentage of pollution that they do, in fact, discharge should also be taken into account where such to the sewers. facilities exist. The outcome of this exercise then needs to be converted into capita equivalents, Another key aspect related to defining the wastewater either through flow- or pollution-specific per-capita flow and treatment capacity of a given system is assumptions (for instance, based on 100 liters/cap/d whether households in the target area end up being or 50 g BOD 5/cap/d). These theoretical capita actually connected to the sewage network. In many equivalents should then be added to the connected cases around the world, we often see situations in population equivalents, as described earlier. which secondary sewer networks are installed and pass in front of houses but not all households connect to them. This can occur for several reasons Fecal sludge/septage: including, for example, a lack of financial resources Similarly to the case of industrial pollution, fecal to pay for the connection fee or for the necessary sludge/septage discharged to a WWTP also needs intradomiciliary works, unwillingness to forgo their to be taken into account when estimating the total existing sanitation solution, and/or an inability to capacity requirements for a small-town WWTP. The bear the cost of sealing a septic tank. Maximizing the fecal sludge volumes are most likely to be of minor connection rate to the sewer network will help service relevance compared with the volumes originating providers and the broader community realize the from the sewer system, but fecal sludge is usually financial, public health and environmental benefits highly concentrated and the pollution load per cubic associated with the investments in sanitation. For meter that is sent to treatment facilities could still be more information on how to design and implement rather high. This fecal sludge pollution load should sewer connection programs, see the “Connecting therefore be considered when estimating the total the Unconnected” guidance document (Kennedy- connections to a WWTP and be converted into Walker and others 2020). population equivalents. The volume of fecal sludge/ septage produced will depend on several factors, Connected industries: including containment type, groundwater infiltration and emptying frequency. The volume of sludge taken Another source of pollution originates from industrial to a WWTP will be influenced by septage tanker wastewater flows connected to the municipal sewer sizes, tanker numbers and the tanker working hours. system. Estimating the characteristics of these flows Bearing these factors in mind, the following rule- can prove difficult, given that industries are often of-thumb estimate can be used to calculate the not forthcoming with relevant information and that equivalent load associated with septage discharge: their water supply schemes may be drawing from 100 people serviced by septage collection and private boreholes instead of the public water supply discharge to a WWTP is equivalent to the load of network. These factors notwithstanding, an estimate one person serviced by a sewer system.2 For more of the relevant parameters is needed and, ideally, the details on the issue of fecal sludge and wastewater effluents of major industries should be monitored cotreatment, see the “Fecal Sludge/Septage” criterion and analyzed for a period of time in advance of below. designing the WWTP. If this is not possible, guides on industrial pollution can offer rule-of-thumb values for pollution generated per ton of input processed, Urban and industrial growth: per ton of output produced, or per ton of live weight When designing WWTPs, it is important to assess killed for slaughterhouses, and so on. Pollution current and future changes in the characteristics of reduction by pretreatment of industrial effluents a given small town that may affect the treatment 74 Factors to Address for WWTPs in Small Towns system. For example, the nature of the local economy, following comments refer only to situations in which especially the growth of local industry and/or the cotreatment may occur. For the separate treatment likelihood that increased or more diverse industrial of fecal sludge/septage in those situations in which activity could move into a certain area, may affect cotreatment is not undertaken, see the bibliography the nature of the wastewater influent and therefore listed in this section. the type of treatment needed. Not unlike any The main issue associated with cotreatment of fecal feasibility study of treatment alternatives, investigating sludge is that WWTPs are typically not designed the dynamics of a small town in terms of population for such cotreatment. Consequently, overloading is and industrial growth is thus a critical part of the frequent because even small volumes of fecal sludge selection process. can represent high pollution and solids loads for The connected population should include not only a small-town WWTP. This can manifest itself at the current (permanent and nonpermanent) residents the pretreatment stages, where septage is usually but also an appropriate estimate of the population discharged from tankers. Screens are not designed growth over the life span of the WWTP (i.e., the project to treat waste with such a high solids content, and horizon). Both vegetative growth and migration the raking installations to remove screenings can from nearby rural areas should be considered. If the be overwhelmed. Likewise, grit removal units often population growth rate is already particularly high cannot cope with the additional solids, and grit or estimated to increase in a significant way over cannot be separated properly from the fecal sludge. the coming years, it may make sense to consider This results in a potential domino effect, whereby primary settling tanks and sludge removal units treatment plant options that are modular or that are overloaded with both grit and sludge, in turn allow for incremental capacity to be added over time overloading the secondary treatment stages and as population grows (rather than overdesigning at ultimately negatively affecting the final effluent the onset and then operating with a substantial idle quality. In extraordinary cases, the fecal sludge may capacity for several years). even contain toxic substances, and because inlet In addition, designers and decision makers should quality control is often weak or nonexistent in small- always bear in mind that planning for future town WWTPs, then the whole treatment train can generations should not come at the detriment of be brought to a standstill, requiring emptying of first ensuring that all of the existing population has treatment units and a complete restart of the WWTP access to sanitation services. processes. Given the nature of fecal sludge, the problems mentioned herein also often come hand- in-hand with the emission of bad odors, leading Fecal Sludge/Septage to even stronger rejection of cotreatment practices, both by WWTP operators and by neighboring Fecal sludge/septage can be treated separately residents. It is therefore not surprising that success in fecal sludge treatment plants or cotreated at stories of cotreatment, particularly at small WWTPs WWTPs. There is a growing body of knowledge, in LMICs, are rare. This is not to say that cotreatment experience and literature available concerning is unfeasible. It should, however, be incorporated typical characteristics of fecal sludge, and its collection, properly into WWTP design and be managed and transport and treatment.3 However, global practical monitored carefully. experience of cotreatment of fecal sludge/septage at WWTPs is mixed, and failures are frequent. Since Consequently, this guide advises the limiting of this guide focuses on wastewater treatment, the cotreatment of fecal sludge at small-town WWTPs Wastewater Treatment and Reuse 75 and the allowance of such practices to take place volumes on the organic load to be treated only if all of the following four conditions are met: at a WWTP is presented in Figure 4.2. For example, whereas a fecal sludge volume (a) The disposal of fecal sludge is documented representing 2 percent of the total influent reliably at the WWTP, including the truck discharged to a WWTP can have limited driver’s name and the origin of the delivered affect on the BOD load at a fecal sludge fecal sludge. concentration of 1,000 mg/L (representing a 10 percent increase in the organic loading (b) The accepted daily volume of fecal sludge of the WWTP), a more concentrated fecal should not lead to overloading of the sludge of 5,000 mg/L, discharged at this same WWTP and should be carefully checked. 2  percent influent volume, could quickly Although cotreatment may be realistic at lead to the overloading of the WWTP (as it large WWTPs with well-trained and qualified would represent a 50 percent increase in the personnel, and where the necessary devices organic loading of the plant). for fecal sludge input control are available (c) The fecal sludge has been factored into the and properly maintained, small WWTPs WWTP design. usually do not count on these features. Only very small amounts of fecal sludge should, (d) The fecal sludge reception station is therefore, be accepted. An example of the equipped with a coarse screen and an effects of cotreatment of different fecal sludge equalization basin or tank that has a FIGURE 4.2 Relative Increase in BOD Load in a WWTP as a Function of the Combined Discharge of Municipal Wastewater and Different Fecal Sludge Volumes 5 FS BOD of 1,000 mg/L Fecal sludge volumes as percent of influent (%) 4 FS BOD of 2,000 mg/L 3 2 FS BOD of 5,000 mg/L 1 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Relative increase in BOD load (%) Source: Authors’ calculations. Note: Expressed as a percentage of the total influent discharged to the plant (considering a constant wastewater BOD concentration of 200 mg/L). BOD = biochemical oxygen demand; FS = fecal sludge; WWTP = wastewater treatment plant. 76 Factors to Address for WWTPs in Small Towns minimum volume equivalent to the volumes ◾ Select an area that is not too central and/or of two conventional vacuum trucks used for surrounded by residential areas, in order to avoid the collection and transport of fecal sludge.4 complaints about odor issues, traffic, noise, and From there, the fecal sludge should then so on, but that is also not too distant from the be progressively dosed into the wastewater small town to avoid high capital expenditures treatment train. (CAPEX) associated with pipe procurement and laying and high operating expenditures (OPEX) needed for any pumping required; Regulations for Wastewater ◾ Avoid elevated grounds that would require higher Treatment, Effluent, and Sludge OPEX for pumping; Discharge and Reuse ◾ Avoid flood-prone areas in order to minimize During the design process, stakeholders need to ask CAPEX needed for flood protection and to themselves several questions regarding the legal and guarantee the WWTP’s operational safety. regulatory framework in which a particular project Selecting the location should be based on the is to be set: “Are there regulations on wastewater best climate change information available and treatment plant design, effluent discharge, sludge not, for example, only on historical flood data; management, emissions, and so on?” “Is reuse an issue?” “If so, what are the existing regulations, and ◾ Ensure that the area possesses adequate which effluent quality standards are required to be geotechnical characteristics to sustain the met?” “How are the existing regulations enforced, construction of heavy structures and thus if at all?” Alternatively, “are there water quality or minimizes CAPEX for foundation works; and environmental standards that would influence reuse, ◾ Ensure that it offers some reserve areas for even if these are not specifically geared toward its potential expansions of the treatment capacity/ regulation?” footprint. In certain cases, there may not be any regulations at all, and stakeholders will need to establish their Even if some of the aforementioned criteria cannot be fully adhered to, it is likely that there will be expectations and derive certain minimum quality several alternative locations for the WWTP and standards that the design of the WWTP should meet. the maximum available land footprint at those In general, the key parameters that are relevant for locations will be broadly known. This information WWTP design, and that need to be cross-checked in will be critical for the comparison of the different the available regulations, are BOD5, COD, suspended technologies available for a given WWTP because, solids, nitrogen, phosphorus and fecal contamination as mentioned earlier, the treatment technologies indicators, such as fecal coliforms (FC). selected have a direct correlation with their land area requirements. Available Land for the WWTP When initiating the prefeasibility and feasibility Power Supply to the WWTP phases of the project cycle, it is likely that stakeholders Before the selection of appropriate technologies, have already identified suitable locations for the the availability of a reliable power supply to the planned WWTP. When selecting the location, decision planned WWTP location will need to be verified, makers should also, to the extent possible: and where it doesn’t exist, it should be confirmed Wastewater Treatment and Reuse 77 whether one can be installed. In addition, certain Treatment Efficiency key characteristics of the available power supply will When comparing the treatment performance of need to be well understood, such as the maximum different technological options, it should be kept in possible capacity of that power connection and the mind that this assessment can be performed through duration of power blackouts in the town’s power grid. various lenses: If the power supply were interrupted, for example, flow conveyance could be discontinued, resulting ◾ Removal of organic loads, as measured by BOD5 in upstream flooding of pumping stations and and COD an interruption to the normal operation of the downstream wastewater conveyance and treatment ◾ Removal of pathogens, including viruses, bacteria, facilities. This limitation is typically addressed by protozoa and helminths, as conventionally providing an emergency power supply, which will measured with biological indicator parameters, add to the CAPEX requirements. such as FC and helminth eggs (particularly intestinal nematodes) Many wastewater treatment technologies require a continuous external supply of electricity. If electricity ◾ Removal of nutrients, namely nitrogen and is not reliably available in the town, these solutions phosphorus will likely not be appropriate. Alternatively, other technologies require only medium to low power Wastewater treatment should result in water quality requirements, or they may not require any power at which is compatible with the sensitivity of the area where the treated effluent will be discharged (i.e., all. In some cases, the necessary power may even be the receiving environment) and which is suitable for generated onsite from renewable resources, such any particular reuse application that is envisaged, as from biogas and/or from photovoltaic modules as well as for the regulatory requirements for both which, when fully and appropriately assessed, could discharge and reuse. If people will come into direct increase the case of the WWTP not requiring a contact with the body of water to which the effluent dedicated energy supply line. In many cases, an stream is discharged, pathogen concentrations are unreliable public electricity grid connection may typically of greatest concern, whereas in areas where serve only as a backup to a dedicated power line human contact is unlikely, the adverse effect on or to an onsite power generation system, when the receiving water quality of high organic and unexpected system failures occur or as a response nutrient concentrations may be the issue deserving to peaks in power demand. the most attention. On some occasions all of these parameters may be of relevance. Ultimately, the technology chosen will need to comply with the Technology Criteria discharge standards in effect locally. Technology criteria are considered to be treatment When selecting a treatment option, the user should technology-specific, and this guide uses eleven bear in mind that trade-offs between these treatment core technology criteria to consider, together objectives may need to be made, including between with suggested scoring. Chapter  5 (see “How to the types of pathogens to be removed. Although weight criteria and calculate total scores”) provides “natural” systems, such as lagoons or constructed a summary and an example of the calculation of wetlands, are effective in removing helminth eggs, total scores, based on a set of suggested standard bacteria, protozoa and viruses, disinfection methods, scores and weights. such as chlorination and ultraviolet (UV) radiation, 78 Factors to Address for WWTPs in Small Towns which are typically coupled with more energy- medium-strength raw wastewater (i.e., concentrations intensive treatment processes, do not remove of about 300 mg of BOD5/L). This is considered the helminth eggs as these are very resistant and key parameter for identifying the content of organic behave differently from bacteria and viruses during pollution present in raw and treated wastewater, treatment (Jimenez and others 2010). Box  4.1 hence it is ideally suited to represent the treatment presents additional considerations when selecting efficiency in terms of removal of organic pollution. an adequate disinfection method for a small-town If raw wastewater quality were to deviate strongly WWTP. from this medium-strength figure, the indicated effluent BOD5 levels could then go up or down It is also important to note that the location of accordingly, but this figure serves as a basis for the WWTP could affect the required treatment comparison. To allow an assessment of different performance, as plants located closer to urban categories of achievable effluent qualities, the areas or next to small or sensitive water bodies may effluent BOD5 concentrations are further compared require higher efficiency levels, demanding more with three arbitrarily defined standards that represent complex treatment systems and higher investment the common range of typical standards found around costs than WWTPs located further from urban areas. the world: 20 (strict), 60 (relaxed) and 120 (very relaxed) mg of BOD5/L. For the purpose of this guide, BOD5 will be used as the proxy to illustrate and compare the treatment Figure 4.3 presents minimum, mean and maximum efficiency of different technologies using typical effluent BOD5 concentrations5 for a wide range of BOX 4.1 Disinfection Considerations: Formation of Chlorination By-Products Selecting an adequate disinfection method is an important part of the appropriate disposal and possible reuse of treated effluents, not only in terms of removing potentially pathogenic agents but also in terms of controlling potentially harmful disinfection by-products (DBPs). Disinfection processes can indeed result in the formation of both organic and inorganic DBPs, such as trihalomethane (THM) compounds and haloacetic acids when chlorine is used, and the presence of these compounds is an emerging public health concern to both human health and the aquatic environment, with some compounds having carcinogenic, mutagenic and genotoxic properties (“Science for Environment Policy” 2018). Because chlorination continues to be an important method of disinfecting municipal wastewater—particularly with sodium hypochlorite, which is considered to be a simple and cost-effective process not requiring extensive technical expertise—a prudent course of practice should be pursued to balance the need for removing pathogenic agents and reducing or eliminating the formation of DBPs. In addition, it has been found that the formation of halogenated organic by-products, such as THMs, is higher in the absence of ammonia and that in WWTPs that do not nitrify, THM formation may not be a problem (Black & Veatch Corporation 2010; Rebhun, Heller-Grossman, and Manka 1997). Since the design and operating conditions associated with small-town WWTPs are unlikely to be favorable to nitrification, THM formation is likely to be minimized in such settings. Chlorination can thus remain an acceptable disinfection option for small-town WWTPs without nitrification. Nevertheless, operating conditions observed in underloaded WWTPs may still lead to nitrification and, notwithstanding the aforementioned consideration, the THM issue may arise in such circumstances. It is therefore important to reliably forecast sewer connection rates (see “Feasibility of Sewers” in Chapter 4) when selecting the optimum disinfection technology for a small town. Wastewater Treatment and Reuse 79 FIGURE 4.3 Summary of BOD5 Effluent Quality Ranges of Different Wastewater Treatment Technologies for Medium-Strength Wastewater 300 Mean effluent concentration 250 Effuent BOD5 (mg/L) 200 150 120 mg/L Very relaxed standard 100 60 mg/L Relaxed standard 50 20 mg/L Strict standard 0 ST BD IMH ABR ANF WSP AL CW(1-st) CW(hybrid) UASB EA SBR(EA) TF RBC UASB-WSP UASB-TF Source: Data collected for this guide. Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; BOD5 = five-day biological oxygen demand; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; RBC = rotating biological contactor; SBR(EA) = sequencing batch reactor (extended aeration variant); ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond. technologies to help the user make some preliminary being complemented with tertiary treatment. comparisons between the available options. The Examples of complementary tertiary treatment following key conclusions and recommendations units include: can be drawn in terms of treatment efficiency: ▪ Rock filters, which are typically used as tertiary treatment after ponds (WSP, UASB-WSP ◾ It is clear that primary treatment options alone and AL), including to remove algae from the (septic tank [ST], biogas digester [BD] and Imhoff effluent, and can help bring total suspended tank [IMH]) cannot comply with any of the typical solids (TSS) and BOD5 levels down to about BOD5 discharge standards. These technologies 30 mg/L, if properly designed and operated; are thus usually not applicable as stand-alone and treatment regimes in situations where discharge ▪ Polishing or sedimentation ponds, which are standards apply. typically used as tertiary treatment after AL ◾ Secondary treatment options, either of anaerobic and can help bring TSS and BOD5 levels down type or those involving ponds (anaerobic baffled to about 20  mg/L, if properly designed and reactor [ABR], anaerobic filter [ANF], waste operated. Polishing or sedimentation ponds stabilization pond [WSP], aerated lagoon [AL], are characterized by shorter retention times upflow anaerobic sludge blanket reactor [UASB], than maturation ponds—usually less than one UASB-WSP), can only rarely meet typical strict or day—and operate under conditions that allow relaxed BOD5 discharge standards. Depending for some algae to settle and for algal biomass on the specifics of a particular project, such production to be minimized or eliminated, technologies could thus be eliminated or require leading to improved effluent parameters. 80 Factors to Address for WWTPs in Small Towns ◾ In addition, ANF and UASB are rarely used may consider the scenarios presented in Table 4.2, with tertiary treatment, as these technologies in which treatment performance is linked to the final are most often followed by another secondary destination of the effluent to be discharged. treatment stage, leading to treatment trains, In terms of pathogen removal, the majority of the such as the ones included in Figure 4.2, namely technologies presented in Figure 4.3 cannot meet UASB-WSP or UASB-trickling filter [TF]. typical standards for indicators of pathogens in ◾ Several types of secondary treatment, such as wastewater effluent, which are typically defined as single-stage constructed wetland (CW(1-st)), FC < 1,000 to 10,000 MPN/100 mL, where MPN is the hybrid constructed wetland (CW(hybrid)), “most probable number,” and as ≤ 1 helminth egg/L.6 extended aeration (EA), sequencing batch Only WSPs, if properly designed and operated, may reactor (extended aeration variant) (SBR(EA)), meet such requirements. However, with appropriate TF, rotating biological contactor (RBC) and tertiary treatment, such as UV or chlorination and UASB-TF, can meet strict BOD 5 discharge filtration, all technologies would be able to meet standards directly, without tertiary treatment. these pathogen standards. It remains to be said that fecal sludge treatment With this in mind, scores for treatment efficiency are plants (typically using WSPs or CWs) can also meet presented in Table 4.1. standards similar to what has been described above. In most cases, effluent discharge standards are Nevertheless, and as mentioned in Chapter  1, often already prescribed by the local legislation, this guide focuses on wastewater treatment. For particularly for BOD5, TSS and pathogens (although further information on fecal sludge treatment plant they may be less so for nutrients), and designers technologies, see the sources indicated in “Fecal and decision makers will use these effluent quality Sludge/Septage.” standards as a starting point to plan for wastewater treatment investments. However, the story can be Ease of Upgrading to Enhanced quite different when it comes to, for example, reuse for irrigation, in which case designers may have to Nutrient Removal decide the extent to which the targeted effluent Both primary and secondary treatment technologies quality must go beyond the discharge regulation. remove nutrients from wastewater, in particular In that sense, and in addition to respecting local nitrogen and phosphorus. The typical removal discharge regulations, designers and decision makers mechanisms involved are sedimentation, adsorption, TABLE 4.1 Summary of Treatment Efficiency Scores for Different Effluent Concentrations RELATIVE TREATMENT EFFLUENT EFFICIENCY SCORE CONCENTRATION TECHNOLOGIES Very relaxed 1 120 mg BOD5/L and higher Primary treatment only options Relaxed 2 Between 60 and 120 mg BOD5/L ABRs, ANFs, WSPs, ALs and UASBs Strict 3 Less than 60 mg BOD5/L CWs, EA, SBR(EA), TFs, RBCs, and UASB-TF and UASB-WSP Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BOD5 = five-day biochemical oxygen demand; CW = constructed wetland; EA = extended aeration; RBC = rotating biological contactor; SBR(EA) = sequencing batch reactor (extended aeration variant); TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond. Wastewater Treatment and Reuse 81 TABLE 4.2 Examples of Different Scenarios of Required Treatment Performance TREATMENT SITUATION OBJECTIVE(S) EXPLANATION Effluent to be discharged Removal of organic The focus of treatment can be limited to the removal of coarse solids into a river with a large loads and settleable organic matter. Primary treatment could thus be sufficient. dilution effect (that is, a dilution factor of 1 in 100, for example) Effluent to be reused for Removal of pathogens The focus of treatment can be on the removal of pathogens (to protect irrigation of a tree crop and organic loads workers’ health) and organic loads. Natural systems, such as lagoons-WSPs, (for example, for olive or other secondary treatment options with disinfection, for example, trees) would be appropriate. In this particular case, nutrient removal could even be considered counterproductive as the nutrients will naturally help crop growth without the need for artificial fertilizers; additional TSS removal could be needed if drip irrigation is to be used (so as not to clog the drippers). In cases in which there exists a risk of eutrophication of surface or coastal waters, or of phosphorus-induced deficiency of micronutrients in soil, for example, technologies that can achieve high nutrient removal rates might be better suited for the situation, provided that the effluent discharge regulations require nutrient removal. Effluent to be discharged Removal of Removal of pathogens would be required as the effluent could come into in a lake requiring water pathogens, organic direct contact with people, whereas the removal of organic loads and quality for recreational loads and nutrients nutrients would be required to preserve water quality and contribute to uses curbing the potential for eutrophication. Secondary or tertiary treatment options would be required, depending on their potential for nutrient and pathogen removal and based on the effluent guidelines in place. Note: TSS = total suspended solids. and the use of those nutrients as building blocks to nutrients. In such cases, consideration should be for microbial growth, although the efficiency of given to the ease with which a particular technology each of these mechanisms, even when combined, can be upgraded to include BNR standards. Bearing is relatively limited, ranging from 10 to 30 percent this in mind, scores for the ease of upgrading to nutrient removal (see, e.g., Metcalf & Aecom 2014). BNR are presented in Table 4.3. This is why, when employing technologies that It is important to highlight that upgrading for are able to provide nutrient removal rates that go enhanced nitrogen removal is generally particularly beyond this conventional range, the terms enhanced costly. The CAPEX requirements for such an nutrient removal or biological nutrient removal (BNR) improvement typically amount to an additional are used. With such technologies, nitrogen and 20 to 30 percent of the original WWTP investment phosphorus removal efficiencies can climb to 60 to figures. The OPEX of the WWTP will also increase 90 percent or even beyond (Metcalf & Aecom 2014). accordingly, as per the higher power requirements associated with increased aeration, return pumping When designing a WWTP, effluent standards and cycles and/or additional mixers. The case for the discharge legislation prevailing at that time may phosphorus is somewhat less costly, but the most not require BNR. However, standards evolve and common technology used for enhanced phosphorus may eventually become more stringent with regard removal—that is, chemical precipitation—requires the 82 Factors to Address for WWTPs in Small Towns TABLE 4.3 from cost constraints, may be a challenge for other Summary of Scoring for Ease of Upgrading reasons. Space requirements can be a limiting to BNR and Examples of Scores for factor where population density already constrains Different Scenarios new land development, where the space for the treatment plant is already allotted and cannot be EASE OF expanded, and/or where topography constrains the UPGRADING TO BNR SCORE TECHNOLOGIES availability and/or the suitability of sites for certain Difficult – Upgrading 1 7 ST technologies. Space constraints and proximity to to BNR standards is 7 BD populations may also trigger the need to eliminate difficult or not possible 7 IMH 7 ABR certain technologies that can be associated with 7 ANF undesirable odors, for example, and may also 7 WSP 7 AL require the adoption of treatment systems that are 7 UASB enclosed or that are complemented with adequate 7 UASB-WSP odor minimization methodologies or treatment Medium – Upgrading 2 7 CW(1-st) process units. The proximity of the WWTP to urban/ to BNR standards is 7 TF residential areas will affect the cost of land (which possible, involving 7 RBC medium-level difficulties 7 UASB-TF may also be higher the closer the plant is to the and medium-level urban center) and may trigger the NIMBY effect.7 financial resources Easy – Upgrading 3 7 CW(hybrid) For the purpose of this guide, relative space to BNR standards is 7 EA requirements are provided for each technology, as technically easy and 7 SBR(EA) specific requirements will be largely dependent can be done with relatively limited on the number of capita (population equivalents) financial resources the plant serves and on local conditions (particularly Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic for natural treatment systems). filter; BD = biogas digester; BNR = biological nutrient removal; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = Table  4.4 presents scores for the relative land extended aeration; IMH = Imhoff tank; RBC = rotating biological contactor; SBR = sequencing batch reactor; ST = septic tank; TF = trickling filter; UASB = requirements of different treatment technologies upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; and examples of the scores allocated for different UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond. scenarios of land requirements. Figure  4.4 presents typical land requirements constant dosage of chemicals, implying an elevated per capita (population equivalents) for different OPEX and a reliable supply of those chemicals. technologies to help the user make some preliminary It is also to be noted that tertiary treatment comparisons between the available options. Septage technologies, which are already considered upgrades treatment plants (SpTPs)8 using WSPs and CWs and thus are serving a specific purpose, are not are also included here to allow for comparison with considered to be suited for upgrading to BNR— the different wastewater treatment technologies. for example, tertiary disinfection does not assist in In addition, the following key conclusions and biological nutrient removal. recommendations can be drawn in terms of land requirements: Land Availability ◾ The different treatment technologies presented As noted earlier, land requirements affect the overall here show a wide range of land requirements. cost of the investment, but land availability, separate As a rule of thumb, one may conclude that the Wastewater Treatment and Reuse 83 TABLE 4.4 Summary of Scoring for Relative Land Requirements and Corresponding Examples of Scores for Different Scenarios of Land Requirements RELATIVE LAND REQUIREMENTS SCORE TECHNOLOGIES High 1 7 All types of ponds/lagoons and CWs, including combinations, such as UASB-WSP. Medium 2 7 Although generally considered to be rather compact processes, UASBs present medium land requirements, particularly because of the need for them to be followed by posttreatment steps, such as TFs or lagoons. 7 PPs and RFs are also associated with medium land requirements. Low 3 7 Technologies more suitable for clusters of households rather than entire small towns, such as BDs, ANFs and STs, present low land requirements. In addition, these systems and ABRs can typically be built underground. 7 Activated sludge-based technologies and TFs are typically considered to be among the most compact technologies. 7 IMHs, RBCs, RDFs and disinfection by chlorination and UV are also associated with low land requirements. Note: ABR = anaerobic baffled reactor; ANF = anaerobic filter; BD = biogas digester; CW = constructed wetland; IMH = Imhoff tank; PP = polishing pond; RF = rock filter; RBC = rotating biological contactor; RDF = rotary disc filter; ST = septic tank; TF = trickling filter; UASB = upflow anae robic sludge blanket reactor; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. FIGURE 4.4 Summary of Land Requirement Ranges of Different Wastewater Treatment Technologies 9.00 8.00 7.00 6.00 (m2/cap) 5.00 4.00 3.00 2.00 1.00 0.00 ST BD IMH ABR ANF WSP AL CW(1-st) CW(hybrid) UASB EA SBR(EA) TF RBC UASB-WSP UASB-TF CI PP RF RDF SpTP-(WSP) SpTP-(CW) UV Source: Data collected for this guide. Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; Cl = chlorination; CW = constructed wetland; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); SpTP = septage treatment plant; ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. 84 Factors to Address for WWTPs in Small Towns easier to a technology is to operate, the more Labor Qualification land it requires, and vice versa. The level of complexity of the O&M tasks associated ◾ Three aspects mainly influence the footprint of with a given treatment system has implications on an individual technology, namely: (a) wastewater the required labor force’s qualifications to perform temperature; (b) required effluent quality; and these tasks. The existence or absence of this kind of (c) economies of scale. In general, if temperature support is an important factor in selecting a treatment is low, effluent quality standards are strict and/or process. The qualifications and technical knowledge the WWTP capacity is projected to be small, land level of the local workforce may need to be assessed, requirements are likely to be as indicated by the weighing the demands of each treatment alternative upper end of the whisker plots shown in Figure 4.4 against the effective capacity of the entity responsible for each technology. Conversely, if the temperature for meeting them. The institutional arrangements is high, effluent quality standards are relaxed for running the WWTP will also influence the ease and/or the WWTP capacity is projected to be high, of access of staff with the necessary qualifications. land requirements are likely to be as indicated A small town that is disconnected from an urban by the lower end of the whiskers. As is the case for hub, for example, might not be able to ensure the the other technology criteria listed in this section, presence of trained or skilled personnel onsite at the importance of the size of a particular facility is all times to operate a UASB system. Alternatively, high, as shown in Figure 4.5, which presents the a regional utility could decide to assign one operator land requirements for different technology trains to supervise the O&M of several isolated treatment designed to treat different volumes of wastewater plants using simpler technology, such as anaerobic in India and in Europe (ARAconsult 2018). and facultative lagoons, which typically require a lower skill set and presence. FIGURE 4.5 Economy of Scale Effect on Land This labor qualification criterion, therefore, Requirements of WWTPs for Different incorporates two dimensions: Wastewater Treatment Technologies ◾ Required qualification level for O&M — that is, 0.40 skilled labor (trained or specialized technician UASB with minimum background in wastewater treatment or an equivalent field) or unskilled Land requirement (ha/MLD) 0.30 labor (someone who does not require any prior training or certification to perform the required 0.20 task) AS WWTPs in Chennai (AS) ◾ Frequency of the O&M tasks — in particular, 0.10 whether a permanent presence is required BIOFOR WWTPs in Europe (AS, SBR, TF) onsite because of the complexity of the tasks or MBBR/FAB 0.00 because of the need to perform frequent analyses, 0 50 100 150 200 250 which can inform treatment plant operation, for Capacity (MLD) example Note: AS = activated sludge; BIOFOR® = biological aerated filter; MBBR/FAB = moving bed biological reactor/fluidized aerated bed; MLD = million liters per day; SBR = sequencing batch reactor; TF = trickling filter; Since all technologies require a certain number of UASB = upflow anaerobic sludge blanket reactor; WWTP = wastewater treatment plant. unskilled laborers to be on site at least temporarily, Wastewater Treatment and Reuse 85 this criterion focuses on the type and frequency of support structure to provide a regular supply of required skilled labor inputs. Certain technologies consumables and spare parts. Similarly, the system/ may also require only unskilled and periodic support, engineering design should ensure that the expected such as in the case of ABRs, which require very limited O&M costs of the treatment plant being proposed attention to operation and for which maintenance remain within budget and/or within the income- is generally limited to periodic inspections and the generating potential of the intervention—such a removal of accumulated sludge and scum. costing analysis should be undertaken in coordination with a financial specialist. With this in mind, Table 4.5 presents the scoring of O&M labor requirements and examples of scores for different scenarios of O&M labor needs. Availability of Replacement Parts In addition to considering technical capacity, it may and O&M Inputs be appropriate and necessary to evaluate human Service providers in small towns with limited resource capacity for administrative and financial connectivity to urban or industrial centers, or that management tasks. A treatment plant demands host a limited range of economic activities, may lack technical expertise and a minimal institutional and resources to purchase or procure replacement parts administrative capacity. Keeping a treatment plant for the wastewater treatment system equipment in adequate condition requires not only a qualified and other necessary inputs for O&M, such as team of professionals but also an administrative chemicals, inputs for testing, monitoring, and so on. TABLE 4.5 Summary of Scoring for O&M Labor Needs and Corresponding Examples of Scores for Different Scenarios of O&M Labor Needs LABOR NEEDS SCORE TECHNOLOGIES Several skilled 1 7 EA, SBRs, ALs and UASBs typically require several permanent skilled laborers to operate the laborers required system, monitor and adjust operation, as needed, and maintain and repair equipment. Smaller on site UASB systems may only require one skilled laborer onsite, but because UASBs tend to be followed by posttreatment (WSPs or TFs, for example), there may be the need for additional personnel, even in those cases. 7 UV, chlorination and RDFs are also associated with higher levels of training and skill. One skilled 2 7 TFs typically require one skilled laborer to monitor the filter, regularly clean and maintain the laborer required rotary distribution system and repair pumps, as needed. on site 7 Small RBCs typically require one onsite skilled laborer, with the support of various unskilled or semiskilled personnel for the various maintenance elements, such as replacing seals and motors, servicing bearings and spray-washing discs to clean the attached-growth media. Periodic support 3 7 WSPs mostly require unskilled laborers to remove aquatic plants in the ponds and scum, which from skilled may have built up on pond surfaces, and to keep vegetation in check around the banks of the laborer required ponds. Periodic support and visual inspection from skilled operators can help adjust operation, maintain treatment efficiency and plan sludge dredging campaigns for the anaerobic ponds, but it is not required daily. 7 All primary treatment options and CWs, PPs and RFs also require only periodic support from skilled laborers. Note: AL = aerated lagoon; CW = constructed wetland; EA = extended aeration; O&M = operation and maintenance; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR = sequencing batch reactor; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UV = ultraviolent; WSP = waste stabilization pond. 86 Factors to Address for WWTPs in Small Towns Alternatively, proximity of the small town to certain logistical and technical challenges. The same could suppliers and the reach of the suppliers in a be said regarding the procurement of technical particular country, though advantageous for certain studies, engineering designs, and construction and well-established technologies, could complicate supervision services. Consequently, a market study access to products needed for undertaking the could help identify potential contractors, equipment O&M of other treatment technologies, which are suppliers and consultants in order to understand their not typically used or are not part of the menu of size and limitations and thereby inform and improve options currently offered by the local market. When procurement planning with regard to civil works and selecting a treatment technology, an assessment related services, particularly if a decision has been of the supply market for these O&M elements made to expand the menu of available wastewater would therefore be useful to help characterize treatment technological options in small towns. the likelihood of providing acceptable treatment performance and compliance at all times for a given With these considerations in mind, scores for the option, particularly in environments in which market availability of replacement parts and O&M inputs competition for technological equipment is likely are presented in Table 4.6, together with examples to be limited, as is the case in remote areas with for different scenarios. TABLE 4.6 Summary of Scoring for O&M Inputs and Replacement Parts and Corresponding Examples of Scores for Different Scenarios O&M INPUTS AND REPLACEMENT PARTS SCORE TECHNOLOGIES O&M inputs and 1 7 On top of their regular O&M inputs, technologies that include aeration equipment, replacement parts are such as EA, SBRs(EA) and ALs (although ALs typically have simpler aeration equipment both needed on a regular than SBRs), will also require readily available replacement parts to prevent extended basis downtimes that would otherwise result in the creation of anaerobic conditions in the associated reactors. 7 Tertiary treatment options, such as UV and RDFs, require O&M inputs and replacement parts on a regular basis. For example, the proper O&M of a UV disinfection system includes cleaning of all surfaces between the UV radiation source and the target organisms, as well as the periodic replacement of lamps, quartz sleeves and ballasts. 7 Systems that require the constant use of chemicals to enhance sedimentation or help with the conditioning of sludge, for example, would also receive a score of 1. Regular O&M inputs but 2 7 RBCs, TFs and UASB-TF combinations require few regular O&M inputs, but a readily few replacement parts available supply of seals, motor parts and bearings would be needed. are needed 7 Chlorination requires regular O&M inputs to clean the various components of the system, as well as needing replacement parts for the chemical dosing pumps and chlorine residual analyzers, for example. Few regular O&M inputs 3 7 Primary and secondary treatment options, such as IMH, BDs, STs, ABRs, ANFs, WSPs, and replacement parts CWs and UASBs (and UASB-WSP), require few O&M inputs and few replacement parts. are needed 7 Tertiary treatment options, such as PPs and RFs, are also assigned a score of 3. Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; CW = constructed wetland; EA = extended aeration; IMH = Imhoff tank; O&M = operation and maintenance; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR = sequencing batch reactor; SBR(EA) = sequencing batch reactor (extended aeration variant); ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. Wastewater Treatment and Reuse 87 Wastewater Sludge Production the only approach is to discharge the WWTP sludge to a dumping site, for example, then this criterion Wastewater sludge production and treatment, and will suggest prioritizing a wastewater treatment the frequency of sludge removal that is required technology that minimizes sludge production. to maintain optimal treatment performance, can together have a significant effect on the O&M costs With these considerations in mind, scores for sludge of a WWTP, such as those associated with sludge production are presented in Table 4.7. dredging, stabilization, conditioning, thickening, Where a separate wastewater sludge treatment dewatering and/or landfilling, as well as on its plant/step exists, the complexity of the O&M tasks capital costs by affecting the size of the WWTP’s at the plant can also be considered as a criterion. footprint when including sludge drying beds, In other words, even if the sludge removal frequency is for example. Sludge handling can represent a low (every two to five years), complex sludge removal particularly important cost for small WWTPs a and treatment might provide an added burden to transport to a municipal landfill site, after dewatering, the plant’s overall O&M. In such cases, the ease of is often the default solution for small towns unless access for removing and transporting the sludge there is an economically viable land application should be considered. For example, difficult access reuse opportunity for the sludge. Alternatively, to sludge accumulated in anaerobic or facultative depending on the connectivity of the small town lagoons could either render the dredging process with larger agglomerations, sludge from small incomplete or costlier, so ease of such maintenance WWTPs may be transported to larger plants where should be incorporated into the design. further sludge treatment could take place, offsetting some of the transport costs with opportunities It is difficult to pinpoint a precise frequency of such as generating biogas at scale at the larger desludging that can be associated with a particular plant. However, it is important to note that in many technology because it will largely depend on the LMICs, the distances separating small towns from selected pretreatment and primary treatment steps, larger urban centers may make such considerations as well as on the design, operation and maintenance unaffordable. of the system. For example: an SBR operated in The amount of sludge production will also be extended aeration mode will require daily sludge influenced by the existence of a fecal sludge/ removal; a septic tank may require monthly, yearly septage management and treatment system in or even less frequent desludging, depending on or near the small town under consideration. If a the size of the tank; and facultative ponds will need to separate septage treatment facility exists, the be dredged once every two to five years, or when WWTP should not be burdened with such additional the accumulated solids reach approximately one- discharges, but if cotreatment at the WWTP is third of the pond’s volume. Nevertheless, the scores pursued, it is important to account for the volume presented here are intended to help further guide the and characteristics of the fecal sludge/septage user through the selection process by categorizing as compared with the wastewater (given the appropriate technologies according to typical sludge comparatively high strength and high solids content removal needs. of the former, as discussed early in this guide). Figure  4.6 presents sludge production ranges It is important to note that on rare occasions increased for a selection of technologies to help the user sludge production is beneficial—for example, if there compare the available options in a preliminary is a market for the reuse of the treated sludge. In way. The literature refers to sludge production of other situations, where no such market exists and different technologies with a wide array of units, 88 Factors to Address for WWTPs in Small Towns TABLE 4.7 Summary of Scoring for Needed Frequency of Sludge Removal NEEDED FREQUENCY OF SLUDGE REMOVAL SCORE TECHNOLOGIES Daily 1 7 EA, SBR(EA) and CW(1-st) require daily sludge removal, and TFs (and UASB-TF) and RBCs also require daily handling of the sloughed sludge. 7 Sludge is also removed daily from RDFs, typically with a scraper placed at the top of the filter. Monthly 2 7 ANFs, CW(hybrid) and UASBs (and UASB-WSP) are associated with a monthly sludge removal frequency. Every year or more 3 7 All primary treatment options and WSPs, ALs, PPs and RFs require low sludge removal frequencies. 7 For example, anaerobic ponds in WSPs may need desludging every year, whereas facultative and maturation ponds typically require lower frequencies of two to five years and 10 to 20 years, respectively. Note: AL = aerated lagoon; ANF = anaerobic filter; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR = sequencing batch reactor; SBR(EA) = sequencing batch reactor (extended aeration variant); TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond. FIGURE 4.6 Summary of Sludge Production Ranges of Different Wastewater Treatment Technologies (Assuming a Sludge Dry Solids Content of 20 Percent SS) 70 60 50 40 (L/cap/y) 30 20 10 0 ST BD IMH ABR ANF WSP AL CW(1-st) CW(hybrid) UASB EA SBR (EA) TF RBC UASB-WSP UASB-TF CI PP RF RDF SpTP (WSP) SpTP (CW) UV Source: Data collected for this guide. Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; Cl = chlorination; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); SpTP = septage treatment plant; SS = suspended solids; ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. Wastewater Treatment and Reuse 89 including: liters per capita per year (L/cap/y); grams of from these systems is generally transported to suspended solids (SS) per capita per day (gSS/cap/d); the treatment site. Part of the fecal load is, in fact, grams of SS per gram of COD removed; and grams infiltrated from septic tanks into soak pits or similar of SS per gram of BOD5 at the inlet; among others. devices and that from pit latrines infiltrates directly Understanding these units and using them to into the surrounding soil, which explains the lower compare technologies can quickly become a difficult sludge production figures shown in Figure 4.6. task for non-specialists. So, in order to facilitate the comparison of the various technologies, sludge Energy Use production data are presented here in L/cap/y and Energy use associated with wastewater treatment is based on a sludge dry solids content of 20 percent depends on a variety of factors, including the location of SS, a common dewatering result. of the WWTP, the treatment process, effluent quality A few key conclusions and recommendations can requirements, the experience of its operators, and be drawn in terms of sludge production: the age of the plant and its size (in terms of population equivalent or organic or hydraulic loads).9 For small ◾ Primary treatment technologies usually produce towns, the size of a plant is a particularly important a typical sludge volume of about 20 to 30 L/cap/y. factor affecting energy consumption, as smaller ◾ Similar sludge volumes are also produced by plants tend to use more energy on a per-unit basis several secondary treatment options, even though and can present a limited ability to use energy in a these produce better effluent quality than primary more efficient way, as opposed to larger plants. treatment options. Particularly outstanding for Electricity costs in water and wastewater utilities their appealing combination of low sludge typically vary from 5 to 30 percent of a utility’s running production and excellent effluent quality are CWs costs (ESMAP  2012) and have been reported and UASB-TF (see Figure 4.3 for effluent quality). to comprise between 15 and 50  percent of the total operating costs of WWTPs, with higher costs ◾ Secondary treatment options based on aerobic most likely for very small WWTPs because of the treatment only, such as EA, SBR(EA), TF and implications of economies of scale, lower efficiency RBC, have the highest sludge production rates, of installations, less sophisticated automation and typically averaging 50 L/cap/y. This is roughly lower staff skills (Vazquez Alvarez and Buchauer 2014). double the sludge production of any other option. As energy use can be a large part of the O&M costs ◾ Tertiary treatment options are associated with of WWTPs, selecting treatment technologies that either no additional sludge production (such fit the responsible entity’s capacity to cover these as for UV and chlorine disinfection) or low costs, and the availability and reliability of the additional volumes of sludge, in the order of electricity supply, will thus be critical to ensure the about 5 L/cap/y. It should be noted that that sustainability of sanitation services in small towns these are additional volumes of sludge that employing WWTPs. Depending on the reliability of should be added to those of primary and/or the electricity supply available in the small town in secondary sludge volumes as tertiary treatment question, this technology criterion could be given is never employed as a stand-alone step. more weight, with intermittent or expensive energy ◾ Fecal sludge/SpTPs typically present the lowest supply skewing technology selection toward solutions sludge production rates among treatment that do not require continuous supply or that present solutions, although it is important to bear in mind low-energy consumption. In addition, the distance that the full amount of the fecal/pollution material of the WWTP from the town center may imply higher 90 Factors to Address for WWTPs in Small Towns energy use linked to conveyance of the wastewater Figure  4.7 presents electric power requirement (that is, the greater this distance, the greater the ranges for an array of technologies to help the user piping and the pumping costs). There may also be make some preliminary comparisons between the energy requirements if the flow at the inlet to the available options, from which several key conclusions WWTP needs to be elevated to provide gravity and recommendations can be drawn: flow throughout the WWTP or if pumping to an equalization tank or basin is needed, such as in ◾ All primary and tertiary treatment options and the case of flow-sensitive treatment technologies. SpTPs are associated with very low energy Nevertheless, it should be noted that the energy consumption. required for the pumping of the wastewater from the ◾ Within the range of secondary treatment options, small town to the WWTP is generally not considered there are technologies that have low, medium and here because this may or may not be required high energy consumption. Energy consumption depending on local conditions; furthermore, this is mostly driven by treatment efficiency—that energy requirement will be identical for any of the is, a higher energy consumption goes hand in possible WWTP technologies/treatment trains in hand with a higher treatment efficiency, and vice such a small town setting. versa. However, this is not always necessarily the With these considerations in mind, scores for energy case, as shown in Table 4.9: CWs and RBCs, for use are presented in Table 4.8, including different instance, have excellent treatment efficiency but scenarios of energy demand. very low energy consumption. TABLE 4.8 Summary of Scoring for Energy Demand and Examples of Scores for Different Scenarios ENERGY DEMAND SCORE TECHNOLOGIES Energy required 1 7 EA variations are associated with high energy consumption as a constant and reliable continuously and/or source of electricity is required to maintain an aerobic environment. They sometimes require on a set schedule aeration to be provided according to a planned schedule; thus, reliability is also critical. Low to medium 2 7 ALs require a reliable source of electricity to maintain an aerobic environment, either in a energy demand, constant manner or according to a planned schedule. with energy required 7 Although energy demand may be lower than for aerated systems, attached growth non-continuously or systems, such as TFs (and UASB-TF) and RBCs, require a continuous power supply to on a non-scheduled function properly. For example, TFs require pumping to dose wastewater to the top of the supply filter, and for recirculation, sludge pumping, digester mixing and centrifuges when these are included in the treatment chain. 7 Certain types of CWs if pumping is needed for flow distribution. No energy required 3 7 WSPs and certain types of CWs do not require energy if gravity is used for the flow between process units. 7 UASBs consume considerably less energy than aerobic systems but require a constant wastewater flow as these reactors tend to be less robust in the face of organic and hydraulic variability at the inlet. Nevertheless, as upstream pumping energy requirements are not considered here, UASBs are ranked as also consuming negligible amounts of energy. 7 Primary treatment options, such as ABRs, ANFs, IMH and STs, do not require electrical energy inputs. Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; CW = constructed wetland; EA = extended aeration; IMH = Imhoff tank; RBC = rotating biological contactor; ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; WSP = waste stabilization pond. Wastewater Treatment and Reuse 91 FIGURE 4.7 Summary of Electric Power Consumption Ranges of Different Wastewater Treatment Technologies 60 50 40 (kWh/cap/y) 30 20 10 0 ST BD IMH ABR ANF WSP AL CW(1-st) CW(hybrid) UASB EA SBR (EA) TF RBC UASB-WSP UASB-TF CI PP RF RDF SpTP (WSP) SpTP (CW) UV Source: Data collected for this guide. Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; Cl = chlorination; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); SpTP = septage treatment plant; ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. TABLE 4.9 Energy Consumption and Treatment Efficiency TREATMENT EFFICIENCY ENERGY CONSUMPTION LOW MEDIUM HIGH Low energy consumption ABR, ANF, WSP, — CW(1-st), CW(hybrid), UASB, UASB-WSP RBC Medium energy consumption — AL TF, UASB-TF High energy consumption — — EA, SBR(EA) Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; RBC = rotating biological contactor; SBR(EA) = sequencing batch reactor (extended aeration variant); TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond. 92 Factors to Address for WWTPs in Small Towns ◾ Power requirements also depend on the effects of savings. For more on the effects of climate change on economies of scale, with larger plants consuming technology selection, see “Climate Change Impact” less energy per capita of wastewater treated in this chapter. than smaller ones. However, this effect is not as pronounced for energy consumption as it is O&M Costs (OPEX) for land requirements or for OPEX and CAPEX implications. Figure  4.8 presents OPEX cost ranges for a wide range of technologies to help the user compare the In addition to energy use, greenhouse gas (GHG) available options in a preliminary manner, keeping emissions are among the aspects that have in mind that O&M costs can vary based on local become increasingly critical in assessing the overall markets and other factors. performance of WWTPs and a deciding factor in With these considerations in mind, scores for O&M technology selection. Wastewater treatment facilities costs (OPEX) are presented in Table 4.10. are potential sources of GHG emissions, such as carbon dioxide (CO2), methane (CH4) and nitrous A few key conclusions and recommendations can oxide (N2O), contributing to climate change and air be drawn in terms of OPEX costs, namely: pollution. CO2 is also emitted during the production ◾ Primary treatment usually involves OPEX costs of the energy required for the plant operation, and of less than 1 US$/cap/y. OPEX costs associated it can be directly reduced by enhancing energy with SpTPs are also generally at about this level; efficiency at WWTPs, thus creating opportunities to simultaneously reduce environmental effects ◾ Secondary treatment, depending on the chosen and treatment costs by seeking to maximize energy technology and project specific conditions, FIGURE 4.8 Summary of OPEX Ranges of Different Wastewater Treatment Technologies 60 50 40 (USD/cap/y) 30 20 10 0 ST BD IMH ABR ANF WSP AL CW(1-st) CW(hybrid) UASB EA SBR (EA) TF RBC UASB-WSP UASB-TF CI PP RF RDF SpTP (WSP) SpTP (CW) UV Source: Data collected for this guide. Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; Cl = chlorination; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; OPEX = operating expenditures; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); SpTP = septage treatment plant; ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. Wastewater Treatment and Reuse 93 TABLE 4.10 Summary of Scoring for O&M Costs (OPEX) and Corresponding Ranges RELATIVE OPEX RATING SCORE COST RANGE TECHNOLOGIES High average 1 More than 20 US$/cap/y EA, SBR(EA) Medium average 2 3–20 US$/cap/y ALs, TFs, UASBs (as well as UASB-TF and UASB-WSP), including a nonnegligible part for scum removal, and RBCs Low average 3 Less than 3 US$/cap/y Primary treatment alone, tertiary treatment options and ABRs, ANFs, WSPs and CWs Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; CW = constructed wetland; EA = extended aeration; OPEX = operating expenditures; RBC = rotating biological contactor; SBR(EA) = sequencing batch reactor (extended aeration variant); TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond. implies OPEX in the range of 0.5 to 50 US$/cap/y. Investment/Capital Costs (CAPEX) This cost range holds true globally for small- Investment costs include all construction and town WWTPs with design sizes ranging from equipment costs for the treatment processes, about 5,000 to 100,000 capita. These values may including electric and mechanical equipment supply even be higher for very small WWTPs serving and installation, materials, civil engineering, auxiliary less than 5,000 capita; buildings and contractor overheads. ◾ A key factor for the estimation of appropriate OPEX of WWTPs is the impact of economies of FIGURE 4.9 scale. The smaller a facility, the higher its OPEX Economy of Scale Effect on OPEX of per capita are likely to be, and vice versa, which WWTPs with Different Wastewater may indicate that the wide ranges of OPEX shown Treatment Technologies and Treatment in Figure 4.8 for various treatment solutions are Standards (2019 Price Level) only partially influenced by the locally prevailing 60 unit cost levels. In addition, the design size of a given facility is of major relevance, as illustrated 50 in Figure  4.9, which demonstrates this effect Central Europe: SBR (BNR) for the OPEX of different technologies. As OPEX (US$/cap/y) 40 can be seen from the data collected in different 30 regions/countries, there is generally a unit cost Brazil: UASB+TF (C) increase by a factor of about two between a 20 WWTP designed for 100,000 capita and a WWTP India: SBR (C) designed for 5,000 capita. Although this OPEX- 10 China: EA (BNR) related economy of scale effect is not as strong 0 as it is for CAPEX, it is significant enough to be 0 20,000 40,000 60,000 80,000 100,000 taken into account; and Design population equivalent Source: Data collected for this guide. ◾ Tertiary treatment contributes to a WWTP’s Note: EA (BNR) = extended aeration for biological nutrient removal; OPEX = operating expenditures; SBR (BNR) = sequencing batch reactor total OPEX costs but is usually of relative minor designed for biological nutrient removal; SBR (C) = sequencing batch importance when compared with the OPEX costs reactor designed for carbon removal only; UASB+TF (C) = upflow anaerobic sludge blanket reactor, followed by a trickling filter, designed of the secondary treatment options. for carbon removal only; WWTP = wastewater treatment plant. 94 Factors to Address for WWTPs in Small Towns Investment costs are typically expressed in local Although the total investment costs should also currency units per capita and, when possible, average include the cost of the land needed for the WWTP’s figures should be drawn from existing in-country footprint, it is treated as a separate criterion in this experience in installing each process unit, with guide (see “Land Availability”). This will have an these figures being reviewed with the local service effect on the classification of certain technologies provider. If no such information is available, costs with regard to investment costs, particularly waste could then be adapted from experiences in other, stabilization ponds/lagoons. All types of WSPs/ comparable, countries using a ratio comparing lagoons can present high investment costs because investment costs in the target country with those in of their large land requirements depending, of course, the comparator country for which data are available. on the local price of land. However, because, for the The values chosen for this criterion should be purpose of this guide, the cost dimension of land is defined with the service provider and based on the incorporated into the land requirements criterion, local market conditions. WSPs/lagoons are scored as technologies with For the purpose of this guide, each technology is medium investment costs. It should be noted that provided with a relative investment cost rating based a WWTP located further from the urban center or on typical experience. Table 4.11 presents the scoring residential areas may incur higher investment costs of investment costs and examples of scores for related to the wastewater conveyance infrastructure different scenarios of investment costs. (piping and pumping) but may result in lower land TABLE 4.11 Summary of Scoring for Investment Costs and Examples of Scores for Different Scenarios RELATIVE INVESTMENT COSTS SCORE COST RANGE TECHNOLOGIES High 1 More than US$ 150 per capita 7 EA and SBR(EA) is generally associated with high investment costs because of the importance of the civil works and the complex equipment needs. 7 The same applies to CWs, RBCs, TFs, UASB-TF and UASB-WSP. Medium 2 US$ 50–150 per capita 7 Primary treatment options, such as IMH, ANF and all types of lagoons and UASBs, are generally associated with medium investment costs. Low 3 Less than US$ 50 per capita 7 Technologies more suitable for clusters of households rather than entire small towns, such as BDs and STs, present low investment costs. 7 ABRs are generally associated with low investment costs. 7 Tertiary treatment options, such as PPs, RFs and RDFs, are associated with low investment costs. Disinfection technologies, such as chlorination and UV radiation, when taken on their own, have low investment costs, although they are typically incorporated into a larger treatment chain with higher investment cost implications. Note: ABR = anaerobic baffled reactor; ANF = anaerobic filter; BD = biogas digester; CW = constructed wetland; EA = extended aeration; IMH = Imhoff tank; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. Wastewater Treatment and Reuse 95 costs given the increased distance from the small- important for trickling filters, lagoons and UASBs, for town center. example, which are typically used for larger small- town population clusters. In addition, the costs It should also be kept in mind that certain technologies presented for individual technologies would need to included in this guide are meant to be used together be added, depending on the treatment train chosen. as part of a treatment train. In practice, this means that each step of the treatment train would need to A few key conclusions and recommendations can be costed to understand the full cost of treatment be drawn in terms of CAPEX costs, namely: for different treatment systems (each of which may ◾ Primary treatment usually corresponds to CAPEX include several treatment technologies). of less than US$ 50 per capita; Figure 4.10 presents typical construction costs per ◾ Depending on the technology employed and capita for different technologies to help the user on project specific conditions, the CAPEX for make some preliminary comparisons between the secondary treatment range from US$ 50 to available options. Nevertheless, it is important to 600 per capita. This cost range holds true globally acknowledge that although some technologies for small-town WWTPs with design sizes ranging may have similar per capita construction costs, from about 5,000 to 100,000 capita, and these certain technologies (for example, septic tanks values could even be higher for very small WWTPs or sand filters) are more appropriate for individual of less than 5,000 capita; households or clusters of households and therefore offer limited opportunities for economies of scale. ◾ A key factor for the estimation of appropriate The issue of economies of scale for capital costs is CAPEX of WWTPs is the effect of economies of FIGURE 4.10 Summary of CAPEX Ranges of Different Wastewater Treatment Technologies 700 600 500 (USD/cap) 400 300 200 100 0 ST BD IMH ABR ANF WSP AL CW(1-st) CW(hybrid) UASB EA SBR (EA) TF RBC UASB-WSP UASB-TF CI PP RF RDF SpTP (WSP) SpTP (CW) UV Source: Data collected for this guide. Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; Cl = chlorination; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); SpTP = septage treatment plant; ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. 96 Factors to Address for WWTPs in Small Towns scale. The smaller a facility, the higher its CAPEX systems are further reduced by the fact that no per capita are likely to be, and vice versa, which extensive sewer system is required. may indicate that the wide ranges of CAPEX The CAPEX and OPEX figures are presented shown in Figure  4.10 for various treatment separately here, which could lead to questions technologies are only partially influenced by the concerning the overall least cost solution when locally prevailing unit cost levels. In addition, combining the CAPEX and the OPEX. To guide design size of a given facility is of major relevance, the user with regard to this issue, Box 4.2 presents as illustrated in Figure 4.11, which demonstrates an example of a life-cycle cost analysis and the this effect for CAPEX of different technologies. calculated net present value (NPV) for different As can be seen from the data shown from different wastewater treatment technologies. The outcome regions/countries, there is a general unit cost offers some useful insights However, given that this increase by a factor of about 2.5 to 3 when analysis is based on the assumptions presented in comparing a WWTP designed for 100,000 capita the box, and since specific project conditions may and one designed for 5,000 capita; deviate considerably from these assumptions, the ◾ Tertiary treatment contributes to a WWTP’s total results should be interpreted accordingly. CAPEX but is usually of relative minor importance when compared with CAPEX costs of secondary Reuse Potential treatment options; and This section discusses which products are generated ◾ SpTPs are advantageous in terms of CAPEX, by a given treatment process and which of these lend and capital investment figures of such sanitation themselves to reuse. As mentioned in the “Wastewater Resource Recovery” section of Chapter  2, when FIGURE 4.11 selecting an appropriate technology for a small town, Economy of Scale Effect on CAPEX the quality of these end products and their potential of WWTPs with Different Wastewater uses should be matched with existing or potential Treatment Trains (2019 Price Level) local demand for the products. This criterion is thus most relevant where there is interest in the reuse 700 of such products. However, keeping this criterion 600 in mind can also be helpful in cases where informal Central Europe: SBR (BNR) reuse is ongoing and could be formalized, where 500 legislation is in place for such reuse but it is not CAPEX (US$/cap) 400 yet practiced, and/or where decision makers are Philippines: SBR/RBC (C) 300 considering or drafting legislation to enable the reuse Brazil: UASB+TF (C) of wastewater reuse products. 200 India: SBR (C) In addition, interrelations between wastewater 100 China: EA (BNR) treatment and the treatment, handling and disposal 0 0 20,000 40,000 60,000 80,000 100,000 of the generated wastewater sludge, need to be carefully studied when selecting and designing a Design population equivalent treatment option, particularly as sludge disposal or Source: Data collected for this guide. Note: EA (BNR) = extended aeration for biological nutrient removal; reuse may require a certain sludge quality, which in OPEX = operating expenditures; SBR (BNR) = sequencing batch reactor designed for biological nutrient removal; SBR (C) = sequencing batch turn calls for appropriate treatment of the sludges reactor designed for carbon removal only; UASB+TF (C) = upflow produced along the wastewater treatment chain anaerobic sludge blanket reactor, followed by a trickling filter, designed for carbon removal only; WWTP = wastewater treatment plant. (Andreoli, Von Sperling, and Fernandes 2007). For Wastewater Treatment and Reuse 97 BOX 4.2 Life-Cycle Cost Analysis The life-cycle cost analysis presented here is based on the following assumptions: ◾ Capital expenditure (CAPEX) figures are the average values presented in Figure 4.10. ◾ CAPEX is split into a civil works (CIV) component and a mechanical-electrical (ME) component according to typical percentages. ◾ Operating expenditure (OPEX) figures are the average values presented in Figure 4.8 and are assumed to be constant over the total calculation period. ◾ Life span of CIV = 30 years. ◾ Life span of ME installations = 15 years. ◾ Discount rate = 4 percent. ◾ The net present value (NPV) calculation was undertaken for a period of 15 years, with the ME component completely written off by then and with a 50 percent residual value for the CIV component at the end that 15-year period. FIGURE B4.2.1 NPV Results for Different Wastewater Treatment Technologies 700 600 500 (USD/cap) 400 300 200 100 0 ST BD IMH ABR ANF WSP AL CW(1-st) CW(hybrid) UASB EA SBR (EA) TF RBC UASB-WSP UASB-TF CI PP RF RDF SpTP (WSP) SpTP (CW) UV Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; Cl = chlorination; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; NPV = net present value; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); SpTP = septage treatment plant; ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. (continues on next page) 98 Factors to Address for WWTPs in Small Towns BOX 4.2 Life-Cycle Cost Analysis (Continued) The results show an extremely wide range of NPV values, and the following conclusions can be drawn from this exercise regarding life-cycle costs: ◾ Intensive secondary treatment technologies, such as extended aeration (EA), trickling filters (TFs) and rotating biological contactors (RBCs), are by far more expensive than extensive technologies, such as waste stabilization ponds (WSPs) and aerated lagoons (ALs), and are more expensive than primary treatment technologies and septage treatment plants (SpTPs). Additionally, tertiary treatment stages, such as disinfection and polishing, do not result in important additional life-cycle costs; ◾ Within the group of intensive technologies, the two investigated extended aeration variations of the activated sludge process, namely EA and sequencing batch reactor (SBR)(EA), are approximately 50 percent more expensive than other intensive alternatives, such as TF, RBC and upflow anaerobic sludge blanket reactor with ponds (UASB)-WSP and UASB with trickling filters (UASB)-TF; ◾ Among secondary treatment extensive technologies, WSPs, ALs and hybrid constructed wetlands (CWs) are the most cost-efficient options; and ◾ When additionally comparing the ability to meet effluent quality standards, in parallel to life-cycle cost considerations, hybrid CWs stand out as a cost-effective solution because this technology delivers an effluent quality comparable to activated sludge systems (see “Treatment Efficiency”) but for an NPV of just about 25 percent of that of the EA systems. example, the following additional sludge treatment (d) Pathogen removal when agricultural reuse steps may be required: is considered through, for example, lime treatment, composting and/or solar/thermal (a) Sludge stabilization is important for most drying. kinds of reuse because it minimizes bad odors emitting from the sludge. Hence, to The production and use of biogas from the sludge render sludge attractive for users, this is a may require additional treatment steps and process common minimum requirement. Stabilization units—for example, contaminants in the digester can be achieved by various means: anaerobic gas that should be reduced for co-generation digestion, extended aeration or the application include moisture, hydrogen sulfide and siloxanes of chemicals; (Kalogo and Monteith 2008; Vazquez Alvarez and Buchauer 2014). In all cases, the selection of a (b) Sludge conditioning, through the addition of technology for reuse purposes would need to meet chemicals (coagulants and polyelectrolytes) the corresponding reuse standards. to improve solids capture; Table  4.12 presents a template that can be used (c) Sludge dewatering, which has an important to evaluate the reuse potential of the different impact on transport and final destination wastewater treatment products. costs, as well as on ease of sludge handling (if a WWTP is close to agricultural land and It should be noted that no scores are assigned for sludge quantities are not significant, the this criterion, but the potential for reuse of either the dewatering step may be eliminated as the treated effluent, the solids and/or the nutrients, sludge could be applied directly to the land and/or the possibility of energy generation, should be in its liquid form); and factored in during the technology selection process. Wastewater Treatment and Reuse 99 TABLE 4.12 Analysis of the Reuse Potential of Products Resulting from a Treatment Process PRODUCT USE TREATMENT LEVEL REQUIRED Water Restricted irrigation (crops that are not eaten Secondary raw by humans) Unrestricted irrigation (crops such as fruit trees Secondary and olives, for example, which don’t come into direct contact with the ground/irrigation water) Unrestricted irrigation (root and leaf crops that Tertiary may be eaten uncooked) Urban landscape irrigation (parks, road margins, Tertiary sports facilities, and so on) Industrial uses Varies: in some situations, industries will purchase secondary effluent and handle tertiary/advanced treatment themselves Environmental/surface flow Secondary Seawater intrusion barrier through groundwater Secondary/tertiary recharge Aquifer recharge Tertiary/advanced Potable Advanced Soil amendment Sludge stabilization, conditioning, dewatering, drying and/ or composting Biosolids (solids Solid fuel Dewatering, drying and nutrients) Fuel briquettes Charring Construction materials Dewatering, drying Fertilizers (particularly phosphorus) Chemical extraction or crystallization Fuel Digestion + advanced conditioning Biogas Heat Digestion + boilers Electricity Digestion + treatment + combustion (through turbines, combustion engine/generator sets, direct drive engines or Stirling engines, for example) Note: See “Levels of Wastewater Treatment” in Chapter 3 for the definitions of primary, secondary and tertiary treatment. Climate Change Impact ◾ Greenhouse gas (GHG) emissions associated with WWTPs can be both direct and indirect. Climate change considerations are usually not used as an independent criterion when selecting ▪ Direct GHG emissions are associated with wastewater treatment technologies and have been gases that are released or produced during included in the discussions regarding the other wastewater and sludge treatment processes, criteria presented earlier in this guide. The following whether intentionally or as a by-product. text provides some insights with regard to the Methane (CH4) and nitrous oxide (N2O) are interface of wastewater treatment and climate the most important GHGs directly produced change and will help the user in better incorporating from excreta in sanitation systems. Over a these considerations at the prefeasibility and 20-year time horizon, the global warming feasibility phases of the project cycle: potential (GWP) of CH4 is 81.2 times larger 100 Factors to Address for WWTPs in Small Towns than for carbon dioxide (CO2), whereas CH4 recovery parameter. The amount of CH4 for N2O the GWP is 273. Over a horizon of transformed into CO2 through flaring or energy 100 years, the GWP of CH4 and N2O, is 27.9 generation should be included in the overall and 273 times larger than CO2, respectively.10 GHG emission calculation for the plant. Direct CO 2 emissions from wastewater With these considerations in mind, the following are not considered in the IPCC Guidelines can be said: because these are from a biogenic origin. ▪ Indirect GHG emissions are those caused ◾ When individual factors are viewed in isolation, by the use of energy and chemicals in the high GHG emissions are seen to be associated wastewater treatment process and in the with technologies that feature high electricity generation, production and transportation consumption, that target enhanced removal of of these chemicals to the WWTP. Electricity nitrogen, and/or that include anaerobic stages is particularly relevant for indirect GHG in which the generated biogas is not captured. emissions, especially in countries where it is ◾ When individual factors are viewed in isolation, largely generated using coal or other fossil low GHG emissions are more likely to be fuels. In such cases, the quantification of GHG observed for technologies with low electricity impacts should consider either the country- requirements that only target organic pollution specific mix employed in power generation (BOD5) removal, even when this is combined with or the site-specific energy mix if there will be disinfection, and that do not include anaerobic any investment in onsite energy generation treatment stages. (diesel, solar photovoltaic systems, biogas capture, and so on). In some cases, water and ◾ There are trade-offs between these factors. wastewater treatment plants are the largest For example, many anaerobic treatment energy consumers in certain municipalities technologies, such as deep ponds, have low and can account for 30 to 40 percent of the energy requirements but can still emit significant total energy consumed. Chemicals used in methane emissions if biogas is not captured.11 the treatment process also contribute to indirect GHG emissions because of the Electricity consumption and GHG emissions thus energy embedded in them, but chemicals show a similar trend: the higher a technology’s are typically not considered key components energy requirements, the higher its GHG emissions of WWTP operation. associated with energy usage and the lower its score. This dimension of the potential GHG impacts ◾ All direct and indirect GHG emissions at WWTPs of different WWTP technologies is partially captured are added together for each component of in the earlier section on the technology criterion the treatment train and are converted into “Energy Use.” Likewise, the GHG impact of ‘carbon dioxide equivalents’ (CO2e) based on treatment objectives is also indirectly captured in the corresponding GWP factor. this way since enhanced nitrogen removal typically ◾ Wastewater treatment facilities can include implies higher energy consumption. In addition, the anaerobic steps. CH4 generated at such facilities negative impact of anaerobic stages on the overall can be recovered and combusted in a flare or GHG balance can be reduced or eliminated by energy device, and the amount of CH4 handled collecting, capturing and flaring biogas or turning this way at the plant should be subtracted from it into energy. This applies to anaerobic ponds, total emissions, through the use of a separate UASBs and ABRs. Wastewater Treatment and Reuse 101 Consequently, for the purpose of this guide, no 2014); for fecal sludge treatment plant design, see (b) K. Tayler, Faecal Sludge and Septage Treatment: A Guide stand-alone GHG or climate change criterion is for Low and Middle Income Countries (Rugby: Practical included given the trade-offs between decisions Action Publishing, 2018), https://practicalactionpublishing. that can affect both direct CH4 and N2O emissions com/book/693/faecal-sludge-and-septage-treatment; as well as the indirect emissions from energy use, and for co-treatment of fecal sludge and wastewater, see as discussed here. It is recommended that, as part (c) D. Narayana, Co-treatment of Septage and Fecal Sludge in Sewage Treatment Facilities (London: IWA Publishing, of the prefeasibility and feasibility phases of the 2020). project, GHG analyses be undertaken along the   4. The typical volume of trucks used for the collection of above lines in order to compare treatment options fecal sludge in small towns ranges from 3 to 10  m3 (see and to assess whether capturing CH4 could bring K. Tayler, Faecal Sludge and Septic Treatment: A Guide for Low and Middle Income Countries (Rugby: Practical Action additional benefits. Publishing, 2018). In addition to GHG considerations, climate change   5. If the reader needs to compare with COD concentrations, COD effluent concentrations can be estimated (a) by adaptation is increasingly being recognized as multiplying effluent BOD5 values by a factor of about 2.5 important for defining the location and managing to 3.0 in case of effluent BOD5 concentrations higher than the performance of WWTPs. The potential effects 100 mg/L and (b) by multiplying BOD5 concentrations by of climate change on the design and operation a factor of 3.0 to 5.0 for cases of low effluent BOD5 (the of WWTPs should be factored in when selecting lower the BOD5 concentration is, the higher the factor).   6. See, for example, the 1989 and 2006 WHO guidelines: the location of the small town WWTP and when (a) WHO, “Health Guidelines for the Use of Wastewater defining an appropriate treatment train for it, in in Agriculture and Aquaculture. Report of a WHO Scientific order to improve its overall climate resilience (World Group” (Geneva, Switzerland, November 18–23, 1987, 1989); Bank 2020).12 For example, taking into account the and (b) WHO, “Excreta and Greywater Use in Agriculture,” vol. 4 in Guidelines for the Safe Use of Wastewater, Excreta hydrological risk associated with recurrent droughts and Greywater (Geneva, Switzerland: WHO, 2006). when designing a WWTP could help minimize   7. The not-in-my-backyard, or NIMBY, effect is the potential the impacts of reduced water consumption and rejection by neighboring communities to having a wastewater wastewater flows on its performance and on the treatment plant built and operating near their homes. associated CAPEX and OPEX. Similarly, it is worth   8. This guide refers to SpTPs as independent treatment plants that are specifically designed to treat septage considering the current and future climate change- delivered to these facilities in tankers. related flood risk when choosing a treatment site   9. Energy consumption in WWTPs is often reported as per and the location of onsite equipment. the volume of treated wastewater or unit of population equivalent (PE) on an annual basis—that is, kWh/m3/year or kWh/PE/year, respectively. Although international practice Notes typically points to the use of an average influent PE60   1. Graywater is defined as “water generated from washing where 1 PE60 = 60 g BOD5/d or PE120 where 1 PE120 = food, clothes and dishware, as well as from bathing, but 120 g COD/d, considered typical values of organic pollution not from toilets. It may contain traces of excreta (e.g., from discharged through wastewater by 1 capita in many washing diapers) and, therefore, also pathogens” (Tilley developed countries, for small towns in LMICs, the value and others 2014). of PE40 is considered more accurate. These figures   2. Based on the assumption of 0.25 L septage per capita can vary with, for example, 35 g BOD5/d (Morocco) and per day and a concentration of 2,000  mg BOD5/L (see, 50 g BOD5/d (Brazil) values commonly used as well. for example, sources cited in Footnote 3, comparing one 10. Preliminary figures from the Intergovernmental Panel on person serviced through a sewer system producing a Climate Change (IPCC) AR6 WGI, “Chapter 7: The Earth’s wastewater pollution of 50 g BOD5/cap/day). Energy Budget, Climate Feedbacks, and Climate Sensitivity –   3. For overall principles and issues concerning fecal sludge Supplementary Material,” 7SM-24, https://www.ipcc.ch/ management, see: (a) L. Strande, M. Ronteltap, and D. report/ar6/wg1/downloads/report/IPCC_AR6_WGI_ Brdjanovic, Faecal Sludge Management-Systems Approach Chapter_07_Supplementary_Material.pdf. Figures still for Implementation and Operation (London: IWA Publishing, subject to final editing as of October 20, 2021. 102 Factors to Address for WWTPs in Small Towns 11. It should also be noted that any emissions associated with 12. See, for example, A. Zouboulis and A. Tolko, “Effect latrine/septic tank use, with the fecal sludge/septage of Climate Change in Wastewater Treatment Plants: as well as wastewater collection systems, and with the Reviewing the Problems and Solutions,” in S. Shrestha, disposal of sludge at landfills or at other disposal sites, A. Anal, P. Salam, M. van der Valk (eds) Managing Water are all outside of the scope of those emissions that can Resources under Climate Uncertainty, pp 197–220 (Cham: be solely attributed to the treatment stages, and are thus Springer International Publishing, 2015). considered beyond the scope of this guide. Wastewater Treatment and Reuse 103 Applying This Guide in Practice: A Step-by-Step Approach 5 The selection of appropriate wastewater treatment technologies for small towns presents a challenge to national, regional and local policy makers and decision makers because (a) recent technological developments provide a large menu of options for the treatment of wastewater and (b) small towns often lack financial, technical and human resources to implement the treatment solutions commonly used for larger populations. The selection process will depend on where the solution is being implemented and which factors are deemed most important by the decision makers and other stakeholders. This section, drawing on concepts presented earlier, applies a suggested five-step approach for decision makers to identify appropriate wastewater treatment plants (WWTPs) for small towns. The approach will be detailed for each step, describing the aim, the suggested process to be followed, the expected result and any additional considerations. The guide methodology and overall selection process is demonstrated in Chapter  6 through the use of case studies. Methodology: Overview of Suggested Five-Step Approach The criteria detailed in the present subsection form the crux of the guide’s methodology, which aims to provide small towns with decision-making support in the identification of appropriate wastewater treatment solutions. To apply this guide to a real-life situation, decision makers should rely on a five-step approach (see Figure 5.1). 1. Familiarize themselves with the guide methodology, as described in prior sections. 2. Convene key stakeholders to discuss the project criteria and agree, through workshops and/or focus groups discussions, on the characteristics of the town(s) as per the different criteria presented herein. This guide suggests six core project criteria that outline important characteristics of the small town to consider for the choice of a wastewater treatment system as they relate to population, growth, local activities, and existing services and practices. 3. Convene key stakeholders to discuss the project criteria by holding discussions on the acceptable values for the technology criteria based on the local context. The technology criteria are based on each technology’s specifications, and their value 104 Applying This Guide in Practice: A Step-by-Step Approach FIGURE 5.1 remaining technologies, decision makers Overview of the Key Steps in the should arrive at a reduced list of applicable Application of This Guide technologies and/or treatment trains. Based on these options, a preselection and/or Step Familiarize with decision can be made regarding the 1 guide methodology appropriate technology train for the small town in question. Step Convene key stakeholders and 2 discuss project criteria Step 1: Familiarize Yourself with the Guide’s Methodology Step Convene key stakeholders and The aim of this step is to become familiar with 3 discuss technology criteria the foundational theory and application of the guide before following the subsequent steps of the suggested five-step approach. This entails Step Identify and apply nonnegotiable 4 or exclusion criteria understanding the context of the small town, and of wastewater treatment technologies for small towns, and then drawing up preliminary considerations of Assign weighting to technology criteria and the project and technology criteria. At the end of Step calculate total score for remaining 5 technologies Step 1, the user of the guide should have a strong understanding of the basic concepts of small-town wastewater treatment technologies and be prepared is therefore set in each technology sheet, to apply the guide in the subsequent planning/ independent of the local context. assessment process. 4. Identify the nonnegotiable or exclusion criteria to narrow down the list of potential Step 2: Convene Key technologies and treatment trains and agree upon the priorities (for example, minimal Stakeholders to Discuss energy use, minimizing space requirements, the Project Criteria potential for wastewater reuse for agriculture, The aim of Step 2 is to find agreement on the and so on). It is important to identify which project criteria through workshops and focus group technology criteria are nonnegotiable discussions. The discussion on project criteria allows because of local constraints or other priorities/ for relevant stakeholders to mutually agree on the factors. It is also important to determine conditions that will influence technology selection which provide more flexibility so they can for a given small-town WWTP. Although stakeholders be marked accordingly in the application are, at this stage, not yet likely to be able to define of the guide’s methodology and so help all project conditions very accurately, the order eliminate technologies that do not meet the of magnitude of certain criteria or their tentative identified requirements. importance need to be agreed upon before the 5. After assigning weighting to technology technology criteria can be applied. This relates, criteria and calculating total scores for the for instance, to issues such as the population to Wastewater Treatment and Reuse 105 be connected through sewers to the WWTP, how The guide suggests six core project criteria that much land is available for the WWTP, how reliable outline important characteristics of the small power supply is at the suggested WWTP site, town to consider for the choice of a wastewater whether the WWTP is expected to deliver a high- treatment system. The following brief discussion quality effluent or primarily only remove the bulk of and Figure  5.2 summarize the suggested project pollution, and so on. criteria (for more details, see Chapter  4 “Project FIGURE 5.2 Project Criteria For this criterion, an analysis of whether there is sufficient housing density and sufficient water Feasibility of supply—and thus wastewater discharge—available will be developed, which would justify the sewerage implementation of a sewer system and WWTP. A rough estimate of the total expected capita (equivalents) that shall be connected to the WWTP should be Total developed. This involves estimating, among other aspects, not only the actual population, connection connections percentages, and converting industrial discharges into capita equivalents but also forecasting future to the developments. The outcome of this estimation exercise will determine whether the total connections and WWTP thus expected WWTP capacity are indeed within the range for which this small town guide was developed. In addition, this criterion will help assess, for instance, absolute land and power requirements for the WWTP. This criterion will help define if the fecal sludge collected in the small town can also be transported Fecal to and treated at the WWTP or if a separate system for fecal sludge management and treatment sludge needs to be established. Whether the latter is required, there is no need for consideration of fecal loads in the WWTP design. Regulations The required level of treatment plays a key role in technology selection because not all technologies for treated can deliver any given quality requirement. It is thus important to agree on the required treatment discharge standards. and reuse Available As a rule of thumb, it can be stated that the smaller the available land area for a WWTP is, the more intensive the technology needs to be, and vice versa. Thus, large available land areas allow for the land for implementation of technologies that are cheaper to operate and that require less-qualified the WWTP personnel. Stakeholders also need to develop an understanding of the potential for power supply to the WWTP Power site. Some technologies depend fully on permanent and high levels of power supply, whereas others supply to may not require any power at all. High power needs usually require a robust grid connection and the WWTP reliable power supply, whereas medium to low power requirements might also be generated onsite from renewable resources, such as biogas or photovoltaic panels. Note: WWTP = wastewater treatment plant. 106 Applying This Guide in Practice:A Step-by-Step Approach Criteria”), explaining why each plays an important FIGURE 5.3 role in the selection of appropriate wastewater Schematic Work Plan for Step 2 treatment technologies. Schematic Work Plan for Step 2 Total connections > 5,000 and < No Guide not applicable As part of Step 2, key stakeholders need to determine 50,000* capita? the applicability of the guide methodology as illustrated in Figure  5.3 and in agreement with the Yes characteristics of the small town presented in chapter 4 “Project Criteria.” Feasibility of No Guide not At the end of Step 2, the stakeholders should sewerage? applicable have recorded their tentative agreement on the characteristics of the small town, including information on population, growth, local activities Yes and existing services and practices, which will inform Step 3. Acceptable No Develop separate amount of fecal solution for fecal sludge? sludge management Step 3: Convene Key Stakeholders to Discuss Yes the Project Criteria The aim of this step is to find agreement on the Decide on project Regulations for No acceptable values for the technology criteria discharge/reuse expectations for available? effluent quality through workshops and focus group discussions. Step 3 requires discussion about the technology criteria for which the guide suggests a total of Yes eleven criteria (see Chapter 4 “Technology Criteria” Decide on for details), as shown in Figure 5.4. maximum available land area Stakeholders may wish to add additional criteria for WWTP and/or eliminate some of the suggested criteria based on the local context. Similarly, if local cost Decide on data are available, stakeholders should modify maximum possible the operation and maintenance (O&M) cost/ power supply capacitites investment cost criteria with the use of specific values (according to acceptable cost levels for the * Can be as high as 100,000 people (mostly in Asian countries). Note: WWTP = wastewater treatment plant. town[s]) rather than use the relative low/medium/ high assessment given in Chapter  4 “Technology Criteria.” This process needs to be undertaken only for technologies that are being considered Wastewater Treatment and Reuse 107 FIGURE 5.4 Technology Criteria Technical/environmental criteria Financial criteria Other important considerations • Treatment efficiency • O&M costs (OPEX) • Reuse potential • Ease of upgrading to enhanced • Investment/capital costs • Climate change impact or biological nutrient removal (CAPEX) (BNR) • Land availability • Labor qualification • Availability of replacement parts and O&M inputs • Sludge production • Energy use Note: CAPEX = capital expenditures; O&M = operation and maintenance; OPEX = operating expenditures. for a given context (that is, some technologies may or exclusion criteria. Steps 3 and 4 will thus first define already have been ruled out). and apply the exclusion criteria, leading to a narrowed down list of technologies, which will be At the end of Step 3, the stakeholders should further analyzed in Step 5, as described in the next have agreement on potential changes to and/or section. specifications for the values that they have adopted for the technology criteria. For instance, technologies with a larger footprint requirement should be excluded if the land available for the WWTP is limited. Similarly, technologies Step 4: Identify and Apply that cannot achieve a specific required treatment efficiency should be eliminated, and those that Nonnegotiable or Exclusion present capital expenditure (CAPEX) or operating Criteria expenditure (OPEX) figures beyond the operating The objective of Step 4 is for the stakeholders to utility’s capacity should similarly not be included collectively determine which technology criteria are in the subsequent steps of the assessment. In nonnegotiable due to local constraints or priorities another example, the ability to meet the required and which provide more flexibility. These criteria discharge quality or space requirements may be nonnegotiable depending on the sensitivity of the should be assessed by following the guidance receiving body of water or the space available. provided herein and can thus help eliminate technologies that do not meet the identified By the end of this step, and after the application requirements. By making reference to the outcome of the nonnegotiable and/or the exclusion criteria, of the discussion on project criteria (Step 2), the stakeholders should have reduced the list of potential users will be able to approach technology selection technologies and treatment trains for consideration with an improved understanding of which criteria in the next step. In addition, stakeholders should will have a more significant impact on a specific agree upon local context priorities, such as minimal WWTP project and will be able to decide which energy use, minimizing space requirements, potential of these should be understood as nonnegotiable for wastewater reuse for agriculture, and so on. 108 Applying This Guide in Practice:A Step-by-Step Approach Step 5: Assign Weighting Schematic Work Plan for Steps 3 to 5 to Technology Criteria and The step-by-step methodology described earlier for Steps 3 to 5 is summarized in Figure 5.5. Calculate Total Score for Remaining Technologies How to Weight Criteria and The aim of Step 5 is to assign weights to the Calculate Total Scores technology criteria and to calculate the total scores for the remaining technologies. This can be seen This section provides an overview of how to perform as a subjective exercise as it will be dependent on the criteria weighting exercise. A summary table of nine technology criteria and their respective the perspectives of the decision makers involved scores is first presented, with the exception of two in the selection of the appropriate technologies of the criteria—reuse potential and climate change for the given small town. Nevertheless, because impact—for which qualitative guidance has been the objective of this guide is to support its users in provided earlier in the guide. Those scores are bringing together all the information considered thereafter weighted, and a total score is calculated. relevant for decision making, this weighting exercise is seen as a practical way to help narrow down Table 5.1 presents a matrix of the nine technology the number of technologies appropriate for a criteria and the preselected technology options. specific context. It presents the suggested standard scoring defaults in which a technology with a higher score would be For each technology that was considered considered more advantageous regarding a certain appropriate for small-town WWTPs, Chapter  4 criterion (3 being the highest score and 1 the lowest). “Technology Criteria” suggests a scoring table for all For example, in terms of land availability, an ABR technology criteria, except for the reuse potential and climate change impact aspects, for which qualitative guidance is instead provided. FIGURE 5.5 The user is free to apply the suggested scores or, Schematic Work Plan for Steps 3 to 5 alternatively, develop a set of customized scores for the technology criteria in question, which should Discuss technology criteria and decide which thereafter be weighted to arrive at the calculation ones are nonnegotiable of a total score. The highest score should then be considered as the best option based on the decision makers’ assumptions and conditions. Apply nonnegotiable criteria and eliminate technologies that do not meet these criteria Step 5 should culminate in the establishment of a reduced list of applicable technologies and/or treatment trains from which a preselection and/or Apply scoring and weighting to remaining decision can be made on the appropriate technology technologies, as described in Step 5 train for the small town in question. In addition, the case studies presented in Chapter 6 Discuss scoring and weighting results and provide working examples on how this approach make decision on recommended technologies can be applied. Wastewater Treatment and Reuse 109 TABLE 5.1 Summary of Suggested Scores for Each Technology (Standard Defaults) TECHNOLOGY TECHNOLOGY CRITERION 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7 4.2.81 4.2.9 EASE OF AVAILABLE TREATMENT UPGRADING LAND LABOR PARTS + O&M SLUDGE ENERGY # EFFICIENCY TO BNR AVAILABILITY QUALIFICATIONS INPUTS PRODUCTION USE OPEX CAPEX Primary treatment (only) 1 ST 1 1 3 3 3 3 3 3 3 2 BD 1 1 3 3 3 3 3 3 3 3 IMH 1 1 3 3 3 3 3 3 2 Primary + secondary treatment 4 ABR 2 1 3 3 3 3 3 3 3 5 ANF 2 1 3 3 3 2 3 3 2 6 WSP 2 1 1 3 3 3 3 3 2 7 AL 2 1 1 1 1 3 2 2 2 8 CW(1-st) 3 2 1 3 3 1 2 3 1 9 CW(hybrid) 3 3 1 3 3 2 3 3 1 10 UASB 2 1 2 1 3 2 3 2 2 11 EA 3 3 3 1 1 1 1 1 1 12 SBR (EA) 3 3 3 1 1 1 1 1 1 13 TF 3 2 3 2 2 1 2 2 1 14 RBC 3 2 3 2 2 1 2 2 1 15 UASB-WSP 3 1 1 1 3 2 3 2 1 16 UASB-Tf 3 2 3 1 2 1 2 2 1 Tertiary treatment (additional) 17 UV N/A N/A 3 1 1 N/A 3 3 3 18 CI N/A N/A 3 1 2 N/A 3 3 3 19 PP N/A N/A 2 3 3 3 3 3 3 20 RF N/A N/A 2 3 3 3 3 3 3 21 RDF N/A N/A 3 1 1 1 3 3 3 Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; BNR = biological nutrient removal; CAPEX = capital expenditures; Cl = chlorination; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; N/A = not applicable; O&M = operation and maintenance; OPEX = operating expenditures; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. that requires an average area of 0.30 square meters made to the presented approach, then experienced per capita is assigned a score of 3 and would be specialists should be included in the stakeholder considered more advantageous than a WSP that discussions to help make technically sound decisions. requires an average area of 4.75 square meters per In situations in which this may not be possible, or in capita and is thus assigned a score of 1. Detailed which the users of the guide are less experienced explanations of the scoring rationale can be found with technology selection, it is advised to use the in Chapter 4 “Technology Criteria”. standard recommendations provided here. In any case, whether the proposed standard approach To produce a total score for each technology using or a modified approach is used, definitions and the individual scores presented in Table  5.1, thus decisions should always properly reflect local permitting an overall comparison of technologies, conditions and the preferences of the relevant a weight should be assigned to each criterion, stakeholders. In addition, the users also need to taking into account that not all of the criteria may incorporate qualitative information provided by be of equal importance to a specific situation. The employing the reuse potential and climate change exercise described in this section will generate impact criteria when interpreting the results. scores for the seven technical criteria and the two financial criteria listed in the table. It is further It should also be kept in mind that the resulting total proposed to group the technical and the financial weighted score is not a fixed result but rather the criteria and to give equal weight to these two outcome of assumptions and subjective assessments groups—that is, the total of the scoring resulting and, as such, should be considered with the from criteria 1 to 7 receives an overall 50 percent flexibility inherent in the prefeasibility and feasibility weight, and the total of the scoring from criteria 8 phases of a project cycle. Furthermore, the user and 9 also receives a 50 percent weight. should continue to take into account the potential combinations of technology trains presented in The grouped scores are worked out as an average Table 3.5 (“Typical Wastewater Treatment Trains for of the total scores of each grouping. Table  5.2 Preselected Treatment Technologies for Small-Town presents an example of the outcome of a grouping WWTPs”) and Table 3.6 (“Typical Sludge Treatment and weighting exercise, with 3 continuing to be the Trains for Preselected Treatment Technologies for maximum achievable score per criterion. Small-Town WWTPs”) during the prefeasibility and The users of the guide are free to modify the feasibility phases of the project cycle, which will be standard approach described above, as deemed further illustrated in the case studies presented in appropriate for the small town in question. In doing Chapter 6. so, the proposed standard 1-2-3 scores for each The typical outcome of applying the guide’s criterion could be revised, as could the weighting. methodology should not point to a single optimum Furthermore, additional criteria together with their technology but rather to a group of technologies associated scores, could be added to the list of the that represent the best, or near-best, score, each eleven proposed criteria, and/or certain criteria of which thus potentially representing a sound and could be removed from consideration, depending appropriate wastewater treatment solution for a on the project circumstances. Nevertheless, it is specific small town and each of which then deserve strongly advised that if substantial revisions are further detailed analysis. Wastewater Treatment and Reuse 111 112 TABLE 5.2 Summary of Weighted Scoring for Each Technology, Based on Suggested Standard Defaults TECHNICAL/ENVIRONMENTAL Applying This Guide in Practice:A Step-by-Step Approach TECHNOLOGY CRITERIA FINANCIAL CRITERIA WEIGHTED SCORE AVERAGE SCORE WEIGHT OF AVERAGE SCORE WEIGHT OF # CRITERIA #1–7 CRITERIA #1–7 CRITERIA #1–7 CRITERIA #8–9 CRITERIA #1–7 CRITERIA #1–7 TOTAL Primary treatment (only) 1 ST 2.43 50% 3.00 50% 1.21 1.50 2.71 2 BD 2.43 50% 3.00 50% 1.21 1.50 2.71 3 IMH 2.43 50% 2.50 50% 1.21 1.25 2.46 Primary + secondary treatment 4 ABR 2.57 50% 3.00 50% 1.29 1.50 2.79 5 ANF 2.43 50% 2.50 50% 1.21 1.25 2.46 6 WSP 2.29 50% 2.50 50% 1.14 1.25 2.39 7 AL 1.57 50% 2.00 50% 0.79 1.00 1.79 8 CW(1-st) 2.14 50% 2.00 50% 1.07 1.00 2.07 9 CW(hybrid) 2.57 50% 2.00 50% 1.29 1.00 2.29 10 UASB 2.00 50% 2.00 50% 1.00 1.00 2.00 11 EA 1.86 50% 1.00 50% 0.93 0.50 1.43 12 SBR (EA) 1.86 50% 1.00 50% 0.93 0.50 1.43 13 TF 2.14 50% 1.50 50% 1.07 0.75 1.82 14 RBC 2.14 50% 1.50 50% 1.07 0.75 1.82 15 UASB-WSP 2.00 50% 1.50 50% 1.00 0.75 1.75 16 UASB-TF 2.00 50% 1.50 50% 1.00 0.75 1.75 Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; RBC = rotating biological contactor; SBR(EA) = sequencing batch reactor (extended aeration variant); ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond. Case Studies 6 The three case studies presented below—from Morocco, Vietnam and El Salvador, respectively—provide specific examples of applying the different criteria and working through the guide’s methodology, which can be helpful in conceptualizing the application of the guide’s approach to a specific context. Case 1: Small Town in Morocco The analysis of this case study follows the general methodology described in Chapter 5. Step 1: Familiarize with GUIDE METHODOLOGY Decision makers from ONEE, Morocco’s National Electricity and Water Office, convened and familiarized themselves with the approach. Step 2: PROJECT CRITERIA The project criteria described in Chapter 4 of the guide were discussed by the decision makers, and Table 6.1 was produced to summarize the outcome. TABLE 6.1 Project Criteria for the Morocco Case PROJECT CRITERION COMMENTS 1.  Feasibility of sewer Responsibility for water supply and Water, sanitation, and electricity are all handled by the same utility: ONEE. sanitation service delivery Water availability Most households in small towns are connected to the public water network (consumption is approximately 50 L/cap/day), though water availability may vary (water availability in Morocco has dropped over the past decades and has reached physical scarcity levels). Stormwater management Sewage and stormwater are managed separately (i.e., there is no combined system). It was also established that the drainage of stormwater is properly maintained and working well, enabling the construction of cost-efficient separate sewers. Solid waste management It was agreed that solid waste should have a minimal/negligible impact. It is collected and disposed of adequately. Conclusions Sewer system appears feasible (continues on next page) Wastewater Treatment and Reuse 113 TABLE 6.1 Project Criteria for the Morocco Case (Continued) PROJECT CRITERION COMMENTS 2.  Total connections to WWTP Project horizon 20 years Residential population 20,000 people Sewer connection rate A 90% connection rate was assumed. Industrial waste streams Industrial waste streams are present, particularly from olive oil mills/presses (margines), with a high concentration of phenols. All stakeholders agreed that the wastewater pollution from these sources may be high, though it is generated only seasonally. Without specific data on this waste stream, it was agreed to assume a maximum industrial pollution equal to about 5,000 PE. Fecal sludge and/or septage Dumping of fecal sludge at WWTPs is not common and was not considered a factor. Urban/industrial growth 1.5% annual growth, mostly attributable to vegetative growth. No industrial growth. Conclusions Total future population = ca. 27,000 PE Connected total future population = ca. 24.000 PE Industrial loads = 5,000 PE Total estimated capacity of WWTP = 29,000 PE 3.  Fecal sludge Conclusions (see item 2) Possible overloading of the WWTP by fecal sludge is not considered a factor. 4.  Regulations for treated discharge and reuse Discharge regulations Regulation for WWTP discharges to receiving waters exist in Morocco and focus exclusively on removal of organic pollution, with BOD5 ≤ 120 mg/L. There are no nutrient standards for nitrogen and phosphorus. Reuse regulations No major regulatory (environmental) constraints, although the legal and regulatory framework for wastewater reuse is incomplete, leading to common informal reuse of raw or treated wastewater, which poses important health risks. New regulations in this regard are currently under development. In general, irrigation standards in Morocco require a minimum hygienic quality, with fecal coliforms ≤ 1,000 MPN/100mL and an absence of nematode ova. Conclusions Standards for discharge quality exist and are defined primarily by requirements for removal of organic pollution—that is, BOD5 Ä 120 mg/L. Reuse for irrigation is not a project criterion, but if effluent is hygienically safe, this could constitute an added benefit. 5.  Available land for the WWTP Land assigned for WWTP Space is available and will not constrain any new construction for a wastewater and sludge treatment plant. The intention is to locate the WWTP relatively distant from the residential areas to avoid any odor or other issues. Elevation No major pumping head is required. The additional distance sought to minimize issues with odors may increase the pumping costs, but this is accepted as a nonavoidable cost. Flood protection There is no flooding risk at the potential WWTP sites. Geotechnical characteristics The soil is generally rather stony. No issues are expected with heavy structures. Reserves for later expansion There is sufficient land for future expansion. Conclusions No issues are foreseen at this point with finding suitable land for the WWTP. (continues on next page) 114 Case Studies TABLE 6.1 Project Criteria for the Morocco Case (Continued) PROJECT CRITERION COMMENTS 6.  Power supply to the WWTP Reliability of electricity Electricity is available and reliable. Maximum possible capacity No clear conclusion could be drawn concerning the maximum power capacity, and moderate to low power requirements should therefore be targeted. This will increase safety and reduce OPEX. Onsite generation of power Solar power generation could be an option because there is plenty of sunshine and strong solar radiation in the area. However, project stakeholders were uncertain as to whether such a source of electricity would be sufficient for the WWTP and/or whether it should serve to provide emergency power backup. Conclusions Electricity from the grid is safely and reliably available. Moderate to low power consumption is preferred. Solar panels may also be considered. Note: BOD5 = five-day biological oxygen demand; ONEE = Morocco’s National Electricity and Water Office; OPEX = operating expenditures; PE = population equivalent; WWTP = wastewater treatment plant. In summary, and using the decision tree presented in figure  5.3 as a guide, Step 2 concludes that (a) this guide is applicable; (b) a sewer system is indeed feasible; (c) fecal sludge disposal/cotreatment will not be a relevant factor or constraint; (d) there are clear definitions of the required treated wastewater quality; and (e) both land and power are sufficiently available. Step 3: TECHNOLOGY CRITERIA The technology criteria described in chapter  4 of the guide were discussed by the decision makers, and table 6.2 was produced to summarize the outcome. TABLE 6.2 Technology Criteria and Exclusion Criteria for the Morocco Case TECHNOLOGY CRITERION COMMENTS EXCLUSION OF TECHNOLOGIES? Treatment As described in table 6.1, effluent treatment targets Comparing the treatment targets with the information efficiency are defined by: provided in chapter 4 “Treatment Efficiency,” it becomes clear that primary treatment options only 7 BOD5 ≤120 mg/L; and (ST, BD, and IMH) cannot comply with the required 7 The desire (but not legally binding requirement) BOD5 limit and thus need to be excluded. for a hygienically safe effluent quality to minimize risks associated with (currently No technology should be excluded because of the unofficial) reuse in irrigation. hygienic requirements because all those technologies could be equipped with a separate tertiary disinfection stage. However, it is noted that WSPs could help avoid such an additional stage because they effectively remove pathogens as part of their treatment process, an advantage that should be considered at a later stage (weighting of technologies). (continues on next page) Wastewater Treatment and Reuse 115 TABLE 6.2 Technology Criteria and Exclusion Criteria for the Morocco Case (Continued) TECHNOLOGY CRITERION COMMENTS EXCLUSION OF TECHNOLOGIES? Ease of upgrading No such future requirements are expected by the This criterion is consequently considered to enhanced decision makers. irrelevant. nutrient removal Land availability Land availability is not considered an issue, as Notwithstanding, and although a smaller footprint concluded in table 6.1 is assumed to constitute an advantage, no technology is to be excluded because of this criterion. Labor qualification Technical capacity of the WWTP operator (ONEE) is Finding or hiring sufficient skilled laborers is of a high level, though low-tech treatment processes considered feasible for any technology, and are typically used. High-tech solutions are often used no technology exclusion is thus considered in cooperation with the private sector, and ONEE is necessary for this criterion. However, technologies in the process of building its capacity to implement with lower skill requirements should be scored more high-tech solutions for small towns. higher. Although the technical and financial capacities are both at a high level, staff numbers are limited. In addition, staff are often asked to operate or supervise numerous treatment plants often far apart from one another. Minimizing O&M labor requirements may thus be desirable. Availability of ONEE’s administrative capacity is sufficient at both No technology exclusion is required. replacement parts the central and regional levels to support operators and O&M inputs and provide a regular supply of consumables and spare parts. As this small town is relatively close to a larger city, the need for replacement parts or O&M inputs is not considered a risk factor. Sludge production Sludge production as such is not considered to No technology exclusion is required, though constitute a significant issue, as sludge can be weighting should give preference to technologies easily stored onsite at the WWTP or reused by with low desludging frequencies. local farmers. It is, however, recognized that a requirement for daily sludge removal may be problematic and/or will require higher personnel presence and thus OPEX. Energy use Energy supply is considered to be reliable. No technology exclusion is required, though weighting should give preference to technologies with low energy consumption. OPEX Decision makers agreed to select technologies All technologies with an OPEX score of 1 (see with low operating costs, even though tariff and table 5.1) are excluded, namely EA and SBR(EA). cost recovery levels allow for more expensive technologies to be implemented. (continues on next page) 116 Case Studies TABLE 6.2 Technology Criteria and Exclusion Criteria for the Morocco Case (Continued) TECHNOLOGY CRITERION COMMENTS EXCLUSION OF TECHNOLOGIES? CAPEX The National Urban Sanitation Master Plan (which All technologies with a CAPEX score of 1 (see includes small towns) defines financing for the table 5.1) are excluded, namely CW(1-st), sector as shared between ONEE (50%) and CW(hybrid), EA, SBR(EA), TF, RBC, and municipalities (50%—either provided themselves UASB-WSP and UASB-TF. or with support from the Ministry of the Interior). Financing from ONEE is generally available, but projects cannot move forward if the remaining portion has not been secured by municipalities, which may end up dictating the level of CAPEX that can be made available for these small towns. Reuse potential Reuse is not currently planned, though the informal No technology exclusion is required. reuse practice has already been phased into the assessment of technology criterion 1: treatment efficiency. No further considerations are deemed necessary. Climate change The information in chapter 4 “Climate Change Treatment technologies incorporating anaerobic impact Impact” states that higher GHG emissions are stages are here excluded, namely anaerobic typically associated with high energy consumption ponds and UASBs. In addition, ABR, ANF, and and with anaerobic stages. The former dimension all primary treatment stages are excluded, as is included in technology criterion 7: energy use they involve anaerobic processes.Nevertheless, and does thus not require further consideration. As if anaerobic technologies would constitute the only for the latter, and even though all decision makers remaining options after this exercise, this criterion could not fully agree whether GHG emissions should not be applied. should indeed be considered as a relevant criterion for their WWTP, it was decided that technologies incorporating anaerobic stages would be excluded from further consideration in this particular case. Note: ABR = anaerobic baffled reactor; ANF = anaerobic filter; BD = biogas digester; BOD5 = five-day biological oxygen demand; CAPEX = capital expenditures; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; GHG = greenhouse gas; IMH = Imhoff tank; O&M = operation and maintenance; ONEE = Morocco’s National Electricity and Water Office; OPEX = operating expenditures; RBC = rotating biological contactor; SBR(EA) = sequencing batch reactor (extended aeration variant); ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond; WWTP = wastewater treatment plant. Step 4: NONNEGOTIABLE or EXCLUSION CRITERIA As described in chapter 5 “Step 4: Identify and Apply Nonnegotiable or Exclusion Criteria,” the decision makers determined which technology criteria were nonnegotiable because of local constraints and priorities and which provide more flexibility. These criteria were marked accordingly in the application of this step and helped to eliminate technologies that did not meet the prior identified requirements. Wastewater Treatment and Reuse 117 TABLE 6.3 Summary of Excluded Technologies for the Morocco Case TECHNOLOGY TECHNOLOGY CRITERION LEADING TO EXCLUSION 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.3.9 4.3.10 4.3.11 AVAILABLE EASE OF PARTS + CLIMATE TREATMENT UPGRADING LAND LABOR O&M SLUDGE ENERGY REUSE CHANGE # EFFICIENCY TO BNR AVAILABILITY QUALIFICATION INPUTS PRODUCTION USE OPEX CAPEX POTENTIAL IMPACT Primary treatment (only) 1  ST excluded OK OK OK OK OK OK OK OK excluded excluded 2  BD excluded OK OK OK OK OK OK OK OK excluded excluded 3  IMH excluded OK OK OK OK OK OK OK OK excluded excluded Primary + secondary treatment 4  ABR OK OK OK OK OK OK OK OK OK OK excluded 5  ANF OK OK OK OK OK OK OK OK OK OK excluded 6  WSP OK OK OK OK OK OK OK OK OK OK OKa 7  AL OK OK OK OK OK OK OK OK OK OK OK 8  CW(1-st) OK OK OK OK OK OK OK OK excluded OK OK 9  CW(hybrid) OK OK OK OK OK OK OK OK excluded OK OK 10 UASB OK OK OK OK OK OK OK OK OK OK excluded 11 EA OK OK OK OK OK OK OK excluded excluded OK OK 12 SBR (EA) OK OK OK OK OK OK OK excluded excluded OK OK 13 TF OK OK OK OK OK OK OK OK excluded OK OK 14 RBC OK OK OK OK OK OK OK OK excluded OK OK 15 UASB-WSP OK OK OK OK OK OK OK OK excluded OK excluded 16 UASB-TF OK OK OK OK OK OK OK OK excluded OK excluded Tertiary treatment (additional) 17 UV OK OK OK OK OK OK OK OK OK OK OK 18 CI OK OK OK OK OK OK OK OK OK OK OK 19 PP OK OK OK OK OK OK OK OK OK OK OK 20 RF OK OK OK OK OK OK OK OK OK OK OK 21 RDF OK OK OK OK OK OK OK OK OK OK OK Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; BNR = biological nutrient removal; CAPEX = capital expenditures; Cl = chlorination; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; O&M = operation and maintenance; OPEX = operating expenditures; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. a WSP are acceptable; only anaerobic ponds should be excluded. The only remaining technologies meeting the decision makers’ criteria and preferences are waste stabilization ponds (WSPs), preferably without an anaerobic stage or pond, and aerated lagoons (ALs). In addition, because WSPs and ALs can be designed to meet the effluent quality requirements of this project, none of the tertiary treatment steps (UV [ultraviolet], Cl [chlorination], PP [polishing pond], RF [rock filter], and RDF [rotary disc filter]) would be necessary. Step 5: Assign WEIGHTING to technology criteria, calculate TOTAL SCORE for remaining technologies The scores proposed in table 5.1 are used here, and the suggested weighting approach of assigning equal weight to technical/environmental criteria and financial criteria. The criteria for ease of upgrading to BNR is excluded from consideration because it was considered irrelevant by the decision makers. Tables 6.4 and 6.5 present the outcome of that scoring and weighting exercise, with 3 continuing to be the maximum achievable score. TABLE 6.4 Summary of Scoring for Remaining Technologies after Step 4 for the Morocco Case TECHNOLOGY TECHNOLOGY CRITERION 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.3.9 4.3.10 4.3.11 AVAILABLE EASE OF PARTS + CLIMATE TREATMENT UPGRADING LAND LABOR O&M SLUDGE ENERGY REUSE CHANGE # EFFICIENCY TO BNRa AVAILABILITY QUALIFICATION INPUTS PRODUCTION USE OPEX CAPEX POTENTIALb IMPACTb Primary + secondary treatment 6 WSP 2 1 3 3 3 3 3 2 7 AL 2 1 1 1 3 2 2 2 Note: AL = aerated lagoon; BNR = biological nutrient removal; CAPEX = capital expenditures; O&M = operation and maintenance; OPEX = operating expenditures; WSP = waste stabilization pond. a Considered irrelevant and thus not considered here. b Not used for scoring. TABLE 6.5 Summary of Weighted Scoring for Remaining Technologies after Step 4 for the Morocco Case TECHNICAL/ ENVIRONMENTAL CRITERIA FINANCIAL CRITERIA WEIGHTED SCORE AVERAGE AVERAGE SCORE WEIGHT OF SCORE WEIGHT OF CRITERIA CRITERIA CRITERIA CRITERIA CRITERIA CRITERIA # #1–7 #1–7 #8–9 #8–9 #1–7 #8–9 TOTAL Primary + secondary treatment 6 WSP 2.50 50% 2.50 50% 1.25 1.25 2.50 7 AL 1.67 50% 2.00 50% 0.83 1.00 1.83 Note: AL = aerated lagoon; WSP = waste stabilization pond. Wastewater Treatment and Reuse 119 FIGURE 6.1 Summary of Weighted Scoring for CONCLUSION Remaining Technologies after Step 4 The analysis leads to two potentially for the Morocco Case suitable technologies for this small town, 3.0 namely WSP (without an anaerobic pond) Technical/environmental criteria and AL, with WSP showing a considerably 2.5 Financial criteria better score. These two technologies are 2.0 deemed appropriate for this particular small town and should be further analyzed as the Total score 1.5 project moves into the next phase. 1.0 0.5 0.0 WSP AL Note: AL = aerated lagoon; WSP = waste stabilization pond. Decision makers will also need to continue to take into account the potential combinations of technology trains for these two technologies presented in tables 3.5 and 3.6 during the prefeasibility and feasibility phases of the project cycle, for which the relevant rows of the tables are presented here. WASTEWATER TREATMENT TRAIN TERTIARY PRETREATMENT PRIMARY TREATMENT SECONDARY TREATMENT TREATMENT MATURATION POND FACULTATIVE POND GRIT/FAT REMOVAL ANAEROBIC FILTER AERATED LAGOON ANAEROBIC POND PLANTED GRAVEL- PLASTIC MEDIA TF DISINFECTION–UV BIOGAS DIGESTER POLISHING POND STONE MEDIA TF UASB REACTOR DISINFECTION– EQUALIZATION IMHOFF TANK LIQUID/SOLID SEPTIC TANK ROCK FILTER SEPARATION CHLORINE SCREEN FILTER SIEVE ABR RBC SBR PST FST AT OPTION TECHNOLOGY ABBREV. Primary + Secondary treatment 1 Waste stabilization pond WSP (as needed) 2 Aerated lagoon AL SLUDGE TREATMENT TRAIN POST-THICKENER SEDIMENTATION SLUDGE DRYING SOLAR DRYING STABILIZATION DIRECT REUSE COMPOSTING MECHANICAL MECHANICAL DEWATERING TREATMENT ANAEROBIC THICKENER THICKENER DIGESTION WETLAND AEROBIC SEPTAGE GRAVITY TANK UASB BED OPTION TECHNOLOGY ABBREV. Primary + Secondary treatment Typical component Optional component (either additional 1 Waste stabilization pond WSP or replacing another component) 2 Aerated lagoon AL Note: Note: ABR = anaerobic baffled reactor; AT = aeration tank; FST = final sedimentation tank; PST = primary sedimentation tank; RBC = rotating biological contactor; SBR = sequencing batch reactor; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UV = ultraviolet. 120 Case Studies Case 2: Small Town in Vietnam The analysis of this case study follows the general methodology described in chapter 5. Step 1: Familiarize with GUIDE METHODOLOGY Decision makers from the utility covering water and sanitation convened and familiarized themselves with the approach. Step 2: PROJECT CRITERIA The project criteria described in chapter 4 of the guide were discussed by the decision makers, and table 6.6 was produced to summarize the outcome. TABLE 6.6 Project Criteria for the Vietnam Case PROJECT CRITERION COMMENTS 1.  Feasibility of sewer Responsibility for water supply and Water and sanitation are handled by the same utility. sanitation service delivery Water availability Water availability is guaranteed. It is estimated that about 99% of the population has access to safe water. Stormwater management Sewage and stormwater are managed separately (that is, no combined systems), though there are imperfections in the system and stormwater infiltration can be high. Solid waste management Solid waste is collected separately. As is observed in the existing sewer system, solid waste has no relevant negative impacts on the sewer network. Conclusions Sewer system already exists and is working properly. 2.  Total connections to WWTP Project horizon 20 years Residential population 50,000 people Sewer connection rate More than 90% of the population is already connected. Industrial waste streams Not a major concern, but industries may contribute about 10%–20% of the overall wastewater pollution, which is currently being collected. Fecal sludge and/or septage Fecal sludge volumes are low because the majority of the population is already connected to a sewer system. Urban/industrial growth An annual growth rate of 2%–3% appears realistic. An average of 2.5% is assumed. Conclusions Total future population = ca. 80,000 PE Connected total future population = ca. 75,000 PE Industrial loads = 20% of population = 15,000 PE Total estimated capacity of WWTP = 90,000 PE 3.  Fecal sludge Conclusions (see item 2) Fecal sludge is not considered an important factor for the WWTP, even though a septage reception station should be installed. (continues on next page) Wastewater Treatment and Reuse 121 TABLE 6.6 Project Criteria for the Vietnam Case (Continued) PROJECT CRITERION COMMENTS 4.  Regulations for treated discharge and reuse Discharge regulations The quality of treated effluent is regulated in Vietnam through “QCVN 14/2008” (National Technical Regulation on Domestic Wastewater), which distinguishes between wastewater discharges into waters that are either used or not used for water supply. In this case study, the former applies, leading to the following criteria: BOD5 Ä 30 mg/L, ammonium-N Ä 5 mg/L, nitrate-N Ä 30 mg/L, and total coliforms Ä 3,000 MPN/100 mL, among others. Reuse regulations Direct reuse of treated wastewater is not envisaged in the foreseeable future. Conclusions Standards for discharge quality require both removal of organic pollution (BOD5) and oxidation of nitrogen (nitrification). Denitrification (removal of oxidized nitrogen) requirements are weak. In addition, disinfection is required. 5.  Available land for the WWTP Land assigned for WWTP Land is relatively expensive in the vicinity of the small town. Limiting the required WWTP footprint is thus considered to be important. Elevation Land is flat, requiring some pumping. Long conveyance distances to the WWTP should be minimized. Flood protection There is considerable flooding risk at the possible WWTP sites, requiring special attention in the design phase. Geotechnical characteristics Clay soil and alluvial sediments. Heavy structures will require geotechnical surveys and appropriate foundations. Reserves for later expansion Feasible, but because of high land prices, such expansions should be limited. Conclusions Suitable land for the WWTP exists but is expensive. Minimization of the WWTP footprint is thus important. 6.  Power supply to the WWTP Reliability of electricity Electricity is available and reliable. Energy cost is not high. Maximum possible capacity No particular known limits. Onsite generation of power If feasible, this is considered to be an interesting option. Conclusions Electricity from the grid is reliable and not too expensive. Note: BOD5 = five-day biological oxygen demand; PE = population equivalent; WWTP = wastewater treatment plant. In summary, and using the decision tree presented in figure  5.3 as a guide, Step 2 concludes that (a) this guide is applicable; (b) a sewer system is already in place and its use has been proven; (c) fecal sludge disposal/cotreatment will not be a relevant factor or constraint; (d) wastewater treatment requires very efficient removal of organics, nitrification, and disinfection; (e) land is available but costly; and (f) power supply is good. 122 Case Studies Step 3: TECHNOLOGY CRITERIA The technology criteria described in chapter 4 of the guide were discussed by the decision makers, and table 6.7 was produced to summarize the outcome. TABLE 6.7 Technology Criteria and Exclusion Criteria for the Vietnam Case TECHNOLOGY CRITERION COMMENTS EXCLUSION OF TECHNOLOGIES? Treatment As described in table 6.6, treatment targets are Comparing the treatment targets with the efficiency defined by: information provided in chapter 4 “Treatment Efficiency,” it becomes clear that only a limited 7 BOD5 ≤ 30 mg/L; range of technologies can comply with the BOD5 7 Ammonium-N ≤ 5 mg/L; limit. The only remaining technology options are 7 Nitrate-N ≤ 30 mg/L; and CW(1-st), CW(hybrid), EA, SBR(EA), TF, RBC, and 7 Total coliforms ≤ 3,000 MPN/100 mL. UASB-TF. No technology needs to be excluded because of the total coliforms requirement because all these technologies could be equipped with a separate tertiary disinfection stage. However, tertiary stages, such as PP, RF, and RDF, are not needed to achieve the treatment targets with the previously indicated remaining technologies and are hence excluded. Ease of upgrading Decision makers recognized that discharge Ease of upgrading is to be considered in the to enhanced requirements could become even more stringent in technology assessment. nutrient removal the future. Land availability Land is available but expensive. Technologies requiring large land areas should be excluded. In particular, and when considering the information presented in chapter 4 “Land Availability,” this means that WSP, CW(1-st), CW(hybrid), and UASB-WSP should be excluded from further consideration. Labor qualification It is assumed that the utility will be able to find No technology exclusion is required. and hire qualified personnel, as dictated by the technologies to be selected. Availability of Administrative capacity is sufficient to support No technology exclusion is required. replacement parts operators and provide a regular supply of and O&M inputs consumables and spare parts. The town is well connected to major cities, and availability of replacement parts is therefore not a challenge. Sludge production Sludge production is not considered to be a No technology exclusion is required. limiting factor. Energy use Reliable and relatively cheap energy supply can be No technology exclusion is required. provided. OPEX Is considered important to compare technologies. No technology exclusion is required. (continues on next page) Wastewater Treatment and Reuse 123 TABLE 6.7 Technology Criteria and Exclusion Criteria for the Vietnam Case (Continued) TECHNOLOGY CRITERION COMMENTS EXCLUSION OF TECHNOLOGIES? CAPEX Is considered important to compare technologies. No technology exclusion is required. Reuse potential Not of particular relevance, as long as the treated No technology exclusion is required. effluents meet the official requirements. Climate change Decision makers decided that this criterion may be No technology exclusion is required. impact applied, as deemed appropriate. Other Decision makers also considered the following: TF, UASB-TF, and RBC are excluded from further consideration. 7 In Vietnam, TFs are not allowed for WWTPs with a capacity beyond 50,000 PE. 7 It was mutually agreed that the RBC technology is usually employed only for plants smaller than the one in this case. Note: BOD5 = five-day biological oxygen demand; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; O&M = operation and maintenance; PE = population equivalent; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond; WWTP = wastewater treatment plant. Step 4: NONNEGOTIABLE or EXCLUSION CRITERIA As described in chapter 5 “Step 4: Identify and Apply Nonnegotiable or Exclusion Criteria,” the decision makers determined which technology criteria were nonnegotiable because of local constraints and priorities and which provide more flexibility. These criteria were marked accordingly in the application of this step and helped to eliminate technologies that did not meet the prior identified requirements. As most technologies are excluded, only two options will thus be subjected to scoring, namely extended aeration (EA) and sequencing batch reactor (extended aeration variant) (SBR(EA)). 124 Case Studies TABLE 6.8 Summary of Excluded Technologies for the Vietnam Case TECHNOLOGY TECHNOLOGY CRITERION LEADING TO EXCLUSION 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.3.9 4.3.10 4.3.11 4.3.12 EASE OF AVAILABLE CLIMATE TREATMENT UPGRADING LAND LABOR PARTS + SLUDGE ENERGY REUSE CHANGE OTHER # EFFICIENCY TO BNR AVAILABILITY QUALIFICATION O&M INPUTS PRODUCTION USE OPEX CAPEX POTENTIAL IMPACT CRITERIA Primary treatment (only) 1  ST excluded OK OK OK OK OK OK OK OK OK OK OK 2  BD excluded OK OK OK OK OK OK OK OK OK OK OK 3  IMH excluded OK OK OK OK OK OK OK OK OK OK OK Primary + secondary treatment 4  ABR excluded OK OK OK OK OK OK OK OK OK OK OK 5  ANF excluded OK OK OK OK OK OK OK OK OK OK OK 6  WSP excluded OK excluded OK OK OK OK OK OK OK OK OK 7  AL excluded OK OK OK OK OK OK OK OK OK OK OK 8  CW(1-st) OK OK excluded OK OK OK OK OK OK OK OK OK 9  CW(hybrid) OK OK excluded OK OK OK OK OK OK OK OK OK 10  UASB excluded OK OK OK OK OK OK OK OK OK OK OK 11  EA OK OK OK OK OK OK OK OK OK OK OK OK 12  SBR (EA) OK OK OK OK OK OK OK OK OK OK OK OK 13  TF OK OK OK OK OK OK OK OK OK OK OK OK 14  RBC OK OK OK OK OK OK OK OK OK OK OK OK 15  UASB-WSP OK OK OK OK OK OK OK OK OK OK OK excluded 14  RBC OK OK OK OK OK OK OK OK OK OK OK excluded 15  UASB-WSP excluded OK excluded OK OK OK OK OK OK OK OK OK 16  UASB-TF OK OK OK OK OK OK OK OK OK OK OK excluded Tertiary treatment (additional) 17  UV OK OK OK OK OK OK OK OK OK OK OK OK 18  CI OK OK OK OK OK OK OK OK OK OK OK OK 19  PP excluded OK OK OK OK OK OK OK OK OK OK OK 20  RF excluded OK OK OK OK OK OK OK OK OK OK OK 21  RDF excluded OK OK OK OK OK OK OK OK OK OK OK Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; BNR = biological nutrient removal; CAPEX = capital expenditures; Cl = chlorination; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; O&M = operation and maintenance; OPEX = operating expenditures; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. Step 5: Assign WEIGHTING to technology criteria, calculate TOTAL SCORE for remaining technologies The scoring employs the standard defaults suggested in this guide in table 5.1 (that is, the suggested scores for each technology and the standard default scores). Likewise, for the weighting applied here, the suggested approach of giving equal weight to the technical/ environmental criteria and the financial criteria is used. Tables 6.9 and 6.10 present the outcome of that scoring and weighting exercise, with 3 continuing to be the maximum achievable score. TABLE 6.9 Summary of Scoring for Remaining Technologies after Step 4 for the Vietnam Case TECHNOLOGY TECHNOLOGY CRITERION 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.3.9 4.3.10 4.3.11 AVAILABLE EASE OF PARTS + CLIMATE TREATMENT UPGRADING LAND LABOR O&M SLUDGE ENERGY REUSE CHANGE # EFFICIENCY TO BNR AVAILABILITY QUALIFICATION INPUTS PRODUCTION USE OPEX CAPEX POTENTIALa IMPACTa Primary + secondary treatment 13 EA 3 3 3 1 1 1 1 1 1 14  SBR (EA) 3 3 3 1 1 1 1 1 1 Tertiary treatment (additional) 27 UV N/A N/A 3 1 1 N/A 3 3 3 28 CI N/A N/A 3 1 2 N/A 3 3 3 Note: BNR = biological nutrient removal; CAPEX = capital expenditures; Cl = chlorination; EA = extended aeration; O&M = operation and maintenance; OPEX = operating expenditures; SBR(EA) = sequencing batch reactor (extended aeration variant); UV = ultraviolet. a Not used for scoring. TABLE 6.10 Summary of Weighted Scoring for Remaining Technologies after Step 4 for the Vietnam Case TECHNICAL/ ENVIRONMENTAL CRITERIA FINANCIAL CRITERIA WEIGHTED SCORE AVERAGE AVERAGE SCORE WEIGHT OF SCORE WEIGHT OF CRITERIA CRITERIA CRITERIA CRITERIA CRITERIA CRITERIA # #1–7 #1–7 #8–9 #8–9 #1–7 #8–9 TOTAL Primary + secondary treatment 13 EA 1.86 50% 1.00 50% 0.93 0.50 1.43 14  SBR (EA) 1.86 50% 1.00 50% 0.93 0.50 1.43 Note: EA = extended aeration; SBR(EA) = sequencing batch reactor (extended aeration variant). 126 Case Studies FIGURE 6.2 Summary of Weighted Scoring for CONCLUSION Remaining Technologies after Step 4 Because both of these remaining for the Vietnam Case technologies, EA and SBR(EA), received an 1.6 Technical/environmental criteria identical score, it can be anticipated that Financial criteria even a more detailed analysis may not lead to 1.4 major differences between these two. In such 1.2 situations, either a decision could be made 1.0 before bidding or both technologies could Total score 0.8 be permitted as part of the bidding process. 0.6 0.4 0.2 Decision makers will also need to continue to take into account the potential combinations of technology 0.0 trains for these two technologies presented in EA SBR (EA) tables  3.5 and 3.6 during the prefeasibility and feasibility phases of the project cycle, for which the Note: EA = extended aeration; SBR(EA) = sequencing batch reactor (extended aeration variant). relevant rows of the tables are presented here. WASTEWATER TREATMENT TRAIN PRETREATMENT PRIMARY TREATMENT SECONDARY TREATMENT TERTIARY TREATMENT MATURATION POND ROTARY DISC FILTER FACULTATIVE POND GRIT/FAT REMOVAL ANAEROBIC FILTER AERATED LAGOON ANAEROBIC POND PLASTIC MEDIA TF BIOGAS DIGESTER PLANTED GRAVEL DISINFECTION–UV POLISHING POND STONE MEDIA TF UASB REACTOR DISINFECTION– EQUALIZATION IMHOFF TANK LIQUID/SOLID SEPTIC TANK ROCK FILTER SEPARATION CHLORINE SCREEN FILTER SIEVE ABR RBC SBR PST FST AT OPTION TECHNOLOGY ABBREV. Primary + Secondary treatment 1 Extended Aeration (AS type) EA 2 Extended Aeration (SBR type) SBR (EA) SLUDGE TREATMENT TRAIN POST-THICKENER SEDIMENTATION SLUDGE DRYING SOLAR DRYING STABILIZATION DIRECT REUSE COMPOSTING MECHANICAL MECHANICAL DEWATERING TREATMENT ANAEROBIC THICKENER THICKENER DIGESTION WETLAND AEROBIC SEPTAGE GRAVITY TANK UASB BED OPTION TECHNOLOGY ABBREV. Typical component Primary + Secondary treatment Optional component (either additional or 1 Extended Aeration (AS type) EA replacing another component) 2 Extended Aeration (SBR type) SBR (EA) Note: ABR = anaerobic baffled reactor; AT = aeration tank; FST = final sedimentation tank; PST = primary sedimentation tank; RBC = rotating biological contactor; SBR = sequencing batch reactor; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UV = ultraviolet. To meet the disinfection requirements, it is also to be noted that both technology options will require either UV or chlorination as an additional (tertiary) disinfection stage. Wastewater Treatment and Reuse 127 Case 3: Small Town in El Salvador The analysis of this case study follows the general methodology described in chapter 5. Step 1: Familiarize with GUIDE METHODOLOGY Decision makers from the national water supply and sanitation utility convened and familiarized themselves with the approach. Step 2: PROJECT CRITERIA The project criteria described in chapter 4 of the guide were discussed by the decision makers, and table 6.11 was produced to summarize the outcome. TABLE 6.11 Project Criteria for the El Salvador Case PROJECT CRITERION COMMENTS 1.  Feasibility of sewer Responsibility for water supply and Responsibility for water supply and sanitation services lies with the national water sanitation service delivery supply and sanitation utility ANDA. Water availability Water services reach the majority of the population, mostly through the public water network (85%–90%) or via public standposts. Some houses also have private wells. Typical water consumption is about 100 L/capita/day. Stormwater management Stormwater is not properly managed. To the extent possible, it is directed toward the nearest quebrada, or “ravine.” Solid waste management Solid waste is poorly managed, with only 50% of solid waste collected throughout the municipality. Trash is often burned in gardens or open areas or left out in the street. Conclusions Sewer system appears feasible. However, it is to be noted that O&M of the sewer system will most likely experience several issues, such as clogging caused by solid waste or considerably increased flows during rainfall. The WWTP should be able to cope with such conditions. (continues on next page) 128 Case Studies TABLE 6.11 Project Criteria for the El Salvador Case (Continued) PROJECT CRITERION COMMENTS 2.  Total connections to WWTP Project horizon 20 years Residential population 17,000 people Sewer connection rate The existing sanitation system predominantly consists of onsite installations (mostly latrines, with a limited number of septic tanks) with no proper fecal sludge management. Some individual houses or clusters of houses may have a small local sewer network, which typically discharges into the nearest quebrada. These quebradas are formed by erosion caused by surface runoff and are a typical feature of many municipalities in El Salvador. The depth of such quebradas can range from a few meters to several dozen meters, and as the embankments are usually very steep, it is relatively easy to discharge into them without the risk of backflow. The project wants to do away with these sanitation systems and connect about two- thirds of the population in the town’s denser areas to a proper sewer system with centralized wastewater treatment. The remaining one-third of the population will continue using onsite sanitation facilities to be incorporated into a properly managed fecal sludge service chain in the future. Industrial waste streams Industrial waste streams are not considered a relevant factor. Only a few family businesses are making a living from agricultural and food processing, which should not contribute in a significant way to the waste streams. Fecal sludge and/or septage Fecal sludge and septage are currently not well managed, with a small-scale private sector offering emptying of septic tanks, but there is no clarity of where the waste is being transported and treated. After project implementation, any septage collected should be disposed of and treated at the new WWTP. Urban/industrial growth Population growth is relatively high but has been affected by migration to larger cities, particularly to the capital, San Salvador. A growth rate of 1%–2% may be realistic. A growth rate of 1.5% has been assumed. Conclusions Total future population = ca. 23,000 PE Connected total future population = ca. 15.000 PE Industrial loads: not relevant About 8,000 PE will continue with onsite sanitation. The majority of those will continue using latrines, which are backfilled once full. Only a limited number of residents will use septic tanks, and the septage volume hauled to the WWTP in future will not be large. Total estimated capacity of WWTP = 16,000 PE (including a provision of 1,000 PE for septage) 3.  Fecal sludge Conclusions (see item 2) Possible overloading of the WWTP by fecal sludge is not considered a factor as the expected volumes are not particularly high. (continues on next page) Wastewater Treatment and Reuse 129 TABLE 6.11 Project Criteria for the El Salvador Case (Continued) PROJECT CRITERION COMMENTS 4.  Regulations for treated discharge and reuse Discharge regulations The required treatment standards are set by ANDA’s “Normas Técnicas para Abastecimiento de Agua Potable y Alcantarillados de Aguas Negras.” This norm requires BOD5 Ä 60 mg/L and SS Ä 60 mg/L. Reuse regulations Some households have gardens in which graywater is reused for irrigation. Reuse of untreated wastewater for the irrigation of crops is also common. Conclusions Standards for discharge quality are defined by requirements for removal of organic pollution—that is, BOD5 and SS. Informal reuse for irrigation is common; thus, improved hygienic discharge quality would be an added benefit. 5.  Available land for the WWTP Land assigned for WWTP Space availability is generally low. The only location downstream of the small town that is suitable and available for the WWTP has an area of only about 5,000 m2. Elevation No pumping head is required. Flood protection Flooding is not considered an issue. Geotechnical characteristics Unknown. Nevertheless, the soil is typically prone to erosion, and heavy structures may thus require proper foundations. Reserves for later expansion Expansion is not possible. Conclusions The only suitable land for the WWTP has a very limited footprint. Because no expansion is possible at that site in the future, a small plant footprint is considered even more important. 6.  Power supply to the WWTP Reliability of electricity Electricity coverage is generally good, but power outages do happen. Hence, the lesser dependence on the public grid, the greater the possibility of safe operation. Maximum possible capacity Unclear maximum capacity. Onsite generation of power Solar generation of power could be an option. Conclusions Electricity supply is good, but outages do happen. Low power consumption is preferred. Solar panels may also be considered. Note: ANDA = Administración Nacional de Acueductos y Alcantarillados; BOD5 = five-day biological oxygen demand; O&M = operation and maintenance; PE = population equivalent; SS = suspended solids; WWTP = wastewater treatment plant. In summary, and using the decision tree presented in figure 5.3 as a guide, Step 2 concludes that (a) this guide is applicable; (b) a sewer system is feasible; (c) fecal sludge disposal/ cotreatment will not be a relevant factor or constraint, though some provision is included for septage in the total estimated capacity of the WWTP; (d) treatment focuses on the removal of organic pollution and (to the extent possible) on improving hygienic quality; (e) land availability is limited; and (f) power consumption should be minimized. 130 Case Studies Step 3: TECHNOLOGY CRITERIA The technology criteria described in chapter 4 of the guide were discussed by the decision makers, and table 6.12 was produced to summarize the outcome. TABLE 6.12 Technology Criteria and Exclusion Criteria for the El Salvador Case TECHNOLOGY CRITERION COMMENTS EXCLUSION OF TECHNOLOGIES? Treatment As described in the previous table, treatment Comparing the treatment targets with the information efficiency targets are defined by: provided in chapter 4 “Treatment Efficiency,” it becomes clear that only a limited range of • BOD5 ≤ 60 mg/L; and technologies can comply with the BOD5 limit. The • SS ≤ 60 mg/L. only remaining technology options are CW(1-st), CW(hybrid), EA, SBR(EA), TF, RBC, and UASB-TF. No technology needs to be excluded because of the hygienic requirements because all those technologies could be equipped with a separate tertiary disinfection stage. However, tertiary stages, such as PP, RF, and RDF, are not needed to achieve the treatment targets with the previously indicated remaining technologies and are thus excluded. Ease of upgrading No such future requirements are expected. No technology exclusion is required. to enhanced nutrient removal Land availability Land availability is considered an issue. The only Comparing land availability to the information in plot available has a footprint of about 5,000 m2, chapter 4 “Land Availability,” it becomes clear which relative to the envisaged WWTP capacity of that the CW(1-st) and CW(hybrid) technology 16,000 cap. equals 0.31 m2/cap. options both need to be excluded from further consideration. WSP and UASB-WSP should also be excluded because of their high land requirements. Labor qualification Technical capacity varies, depending on ANDA’s No technology exclusion is required. involvement in system management, though the number of highly trained staff is limited and concentrated in the larger urban areas. Nevertheless, it is expected that ANDA will be able to find and hire suitably qualified personnel for the selected technologies. Availability of Accessibility to larger urban centers has improved No technology exclusion is required. replacement parts over the years. Despite this, using technologies that and O&M inputs minimize the need for replacement parts or O&M inputs may be desirable. Sludge production Most of the sludge may be reused by local farmers No technology exclusion is required. in agriculture. Sludge volume is therefore not considered relevant for decision making. (continues on next page) Wastewater Treatment and Reuse 131 TABLE 6.12 Technology Criteria and Exclusion Criteria for the El Salvador Case (Continued) TECHNOLOGY CRITERION COMMENTS EXCLUSION OF TECHNOLOGIES? Energy use Although electricity supply is not considered a No technology exclusion is required. limiting factor for technology selection, low financial capacity may render a lower energy consumption (and therefore costs) desirable. OPEX High OPEX would definitely put a major strain on No technology exclusion is required. public finances; thus, low OPEX is preferable. This criterion is considered to be important to compare technologies. CAPEX Similar to OPEX. No technology exclusion is required. Reuse potential As mentioned in the project criteria, there is interest No technology exclusion is required. in water reuse options for agricultural uses. This requirement is already included in technology criterion 1: treatment efficiency. Climate change The information in chapter 4 “Climate Change No technology exclusion is required. impact Impact” states that high GHG emissions are typically associated with high energy consumption and with anaerobic stages. The former dimension is already included in technology criterion 7: energy use and thus does not require further consideration. The decision makers decided that no additional criteria should be applied in this regard. Note: ANDA = Administración Nacional de Acueductos y Alcantarillados; BOD5 = five-day biological oxygen demand; cap = capita; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; GHG = greenhouse gas; O&M = operation and maintenance; OPEX = operating expenditures; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); SS = suspended solids; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; WSP = waste stabilization pond; WWTP = wastewater treatment plant. Step 4: NONNEGOTIABLE or EXCLUSION CRITERIA As described in chapter 5 “Step 4: Identify and Apply Nonnegotiable or Exclusion Criteria,” the decision makers determined which technology criteria were nonnegotiable because of local constraints and priorities and which provide more flexibility. These criteria were marked accordingly in the application of this step and helped to eliminate technologies that did not meet the prior identified requirements. Five technologies and two disinfection options remain. 132 Case Studies TABLE 6.13 Summary of Excluded Technologies for the El Salvador Case TECHNOLOGY TECHNOLOGY CRITERION LEADING TO EXCLUSION 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.3.9 4.3.10 4.3.11 AVAILABLE EASE OF PARTS + CLIMATE TREATMENT UPGRADING LAND LABOR O&M SLUDGE ENERGY REUSE CHANGE # EFFICIENCY TO BNR AVAILABILITY QUALIFICATION INPUTS PRODUCTION USE OPEX CAPEX POTENTIAL IMPACT Primary treatment (only) 1  ST excluded OK OK OK OK OK OK OK OK OK OK 2  BD excluded OK OK OK OK OK OK OK OK OK OK 3  IMH excluded OK OK OK OK OK OK OK OK OK OK Primary + secondary treatment 4  ABR excluded OK OK OK OK OK OK OK OK OK OK 5  ANF excluded OK OK OK OK OK OK OK OK OK OK 6  WSP excluded OK excluded OK OK OK OK OK OK OK OK 7  AL excluded OK OK OK OK OK OK OK OK OK OK 8  CW(1-st) OK OK excluded OK OK OK OK OK OK OK OK 9  CW(hybrid) OK OK excluded OK OK OK OK OK OK OK OK 10 UASB excluded OK OK OK OK OK OK OK OK OK OK 11 EA OK OK OK OK OK OK OK OK OK OK OK 12 SBR (EA) OK OK OK OK OK OK OK OK OK OK OK 13 TF OK OK OK OK OK OK OK OK OK OK OK 14 RBC OK OK OK OK OK OK OK OK OK OK OK 15 UASB-WSP excluded OK excluded OK OK OK OK OK OK OK OK 16 UASB-TF OK OK OK OK OK OK OK OK OK OK OK Tertiary treatment (additional) 17 UV OK OK OK OK OK OK OK OK OK OK OK 18 Cl OK OK OK OK OK OK OK OK OK OK OK 19 PP excluded OK OK OK OK OK OK OK OK OK OK 20 RF excluded OK OK OK OK OK OK OK OK OK OK 21 RDF excluded OK OK OK OK OK OK OK OK OK OK Note: ABR = anaerobic baffled reactor; AL = aerated lagoon; ANF = anaerobic filter; BD = biogas digester; BNR = biological nutrient removal; CAPEX = capital expenditures; Cl = chlorination; CW(1-st) = one-stage constructed wetland; CW(hybrid) = hybrid constructed wetland; EA = extended aeration; IMH = Imhoff tank; O&M = operation and maintenance; OPEX = operating expenditures; PP = polishing pond; RBC = rotating biological contactor; RDF = rotary disc filter; RF = rock filter; SBR(EA) = sequencing batch reactor (extended aeration variant); ST = septic tank; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UASB-WSP = UASB followed by a WSP; UV = ultraviolet; WSP = waste stabilization pond. Step 5: Assign WEIGHTING to technology criteria, calculate TOTAL SCORE for remaining technologies The scoring employs the standard defaults suggested in this guide in table 5.1 (that is, the suggested scores for each technology and the standard default scores). Likewise, for the weighting applied here, the suggested approach of giving equal weight to the technical/ environmental criteria and the financial criteria is used. Tables 6.14 and 6.15 present the outcome of that scoring and weighting exercise, with 3 continuing to be the maximum achievable score. TABLE 6.14 Summary of Scoring for Remaining Technologies after Step 4 for the El Salvador Case TECHNOLOGY TECHNOLOGY CRITERION 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.3.9 4.3.10 4.3.11 AVAILABLE EASE OF PARTS + CLIMATE TREATMENT UPGRADING LAND LABOR O&M SLUDGE ENERGY REUSE CHANGE # EFFICIENCY TO BNR AVAILABILITY QUALIFICATION INPUTS PRODUCTION USE OPEX CAPEX POTENTIALa IMPACT  a Primary + secondary treatment 13 EA 3 3 3 1 1 1 1 1 1 14  SBR (EA) 3 3 3 1 1 1 1 1 1 15 TF 3 2 3 2 2 1 2 2 1 16 RBC 3 2 3 2 2 1 2 2 1 25 UASB-TF 3 2 3 1 2 1 2 2 1 Tertiary treatment (additional) 27 UV N/A N/A 3 1 1 N/A 3 3 3 28 CI N/A N/A 3 1 2 N/A 3 3 3 Note: BNR = biological nutrient removal; CAPEX = capital expenditures; Cl = chlorination; EA = extended aeration; N/A = not applicable; O&M = operation and maintenance; OPEX = operating expenditures; RBC = rotating biological contactor; SBR(EA) = sequencing batch reactor (extended aeration variant); TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF; UV = ultraviolet. a Not used for scoring. 134 Case Studies TABLE 6.15 Summary of Weighted Scoring for Remaining Technologies after Step 4 for the El Salvador Case TECHNICAL/ ENVIRONMENTAL CRITERIA FINANCIAL CRITERIA WEIGHTED SCORE AVERAGE AVERAGE SCORE WEIGHT OF SCORE WEIGHT OF CRITERIA CRITERIA CRITERIA CRITERIA CRITERIA CRITERIA # #1–7 #1–7 #8–9 #8–9 #1–7 #8–9 TOTAL Primary + secondary treatment 13 EA 1.86 50% 1.00 50% 0.93 0.50 1.43 14  SBR (EA) 1.86 50% 1.00 50% 0.93 0.50 1.43 15 TF 2.14 50% 1.50 50% 1.07 0.75 1.82 16 RBC 2.14 50% 1.50 50% 1.07 0.75 1.82 25 UASB-TF 2.00 50% 1.50 50% 1.00 0.75 1.75 Note: EA = extended aeration; RBC = rotating biological contactor; SBR(EA) = sequencing batch reactor (extended aeration variant); TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF. FIGURE 6.3 Summary of Weighted Scoring for CONCLUSION Remaining Technologies after Step 4 The outcome shows considerable for the El Salvador Case differences in the weighted scoring, with 2.0 Technical/environmental criteria EA and SBR(EA) clearly inferior to the 1.8 Financial criteria other three technology options, mainly for financial reasons. Consequently, it would be 1.6 recommended to consider only the three 1.4 best scorers—namely trickling filter (TF), rotating biological contactor (RBC), and 1.2 upflow anaerobic sludge blanket reactor Total score 1.0 (UASB)-TF—for further analysis. 0.8 0.6 0.4 0.2 0.0 EA SBR (EA) TF RBC UASB-TF Note: EA = extended aeration; RBC = rotating biological contactor; SBR(EA) = sequencing batch reactor (extended aeration variant); TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UASB-TF = UASB followed by a TF. Wastewater Treatment and Reuse 135 Decision makers will also need to continue to take into account the potential combinations of technology trains for these three technologies—as presented in tables 3.5 and 3.6— during the prefeasibility and feasibility phases of the project cycle, for which the relevant rows of the tables are presented here. WASTEWATER TREATMENT TRAIN PRETREATMENT PRIMARY TREATMENT SECONDARY TREATMENT TERTIARY TREATMENT MATURATION POND ROTARY DISC FILTER FACULTATIVE POND GRIT/FAT REMOVAL ANAEROBIC FILTER AERATED LAGOON ANAEROBIC POND PLASTIC MEDIA TF BIOGAS DIGESTER PLANTED GRAVEL DISINFECTION–UV POLISHING POND STONE MEDIA TF UASB REACTOR DISINFECTION– EQUALIZATION IMHOFF TANK LIQUID/SOLID SEPTIC TANK ROCK FILTER SEPARATION CHLORINE SCREEN FILTER SIEVE ABR RBC SBR PST FST AT OPTION TECHNOLOGY ABBREV. Primary + Secondary treatment 1 Trickling Filter TF 2 Rotating Biological Contractor RBC 3 UASB-TF UASB-TF SLUDGE TREATMENT TRAIN POST-THICKENER SEDIMENTATION SLUDGE DRYING SOLAR DRYING STABILIZATION DIRECT REUSE COMPOSTING MECHANICAL MECHANICAL DEWATERING TREATMENT ANAEROBIC THICKENER THICKENER DIGESTION WETLAND AEROBIC SEPTAGE GRAVITY TANK UASB BED OPTION TECHNOLOGY ABBREV. Primary + Secondary treatment 1 Trickling Filter TF Typical component Optional component (either additional or 2 Rotating Biological Contractor RBC replacing another component) 3 UASB-TF UASB-TF Note: ABR = anaerobic baffled reactor; AT = aeration tank; FST = final sedimentation tank; PST = primary sedimentation tank; RBC = rotating biological contactor; SBR = sequencing batch reactor; TF = trickling filter; UASB = upflow anaerobic sludge blanket reactor; UV = ultraviolet. To meet the reuse requirements, it may be noted that TF, RBC, and UASB-TF will require an additional (tertiary) disinfection stage, such as chlorination or UV. 136 Case Studies Appendix A: Extended Aeration versus Conventional Activated Sludge Issues with the CAS Process The conventional activated sludge (CAS) process is built around the idea that the total reactor volume should be minimized, and despite the fact that CAS is one of the most energy intensive of wastewater treatment technologies, energy consumption should nevertheless be reduced along the wastewater treatment train. This, however, comes at a price: capital expenditures (CAPEX) associated with electromechanical equipment increase, even if CAPEX associated with civil works decrease to minimize reactor volume. This brings about an increased dependence on control and automation systems, more challenging maintenance requirements, and the need to establish the capacity for swift repairs and to ensure efficient spare part management and procurement. These factors increase the complexity of plant operation and point to the basic need for efficient administration and skilled operators. CAS systems generate two types of sludge, namely fresh sludge from the primary sedimentation tanks and waste-activated sludge from the aeration tanks. Both require stabilization to minimize the emission of bad odors during disposal or reuse. To that end, CAS usually employs anaerobic sludge digesters, which are expensive to build and difficult to operate. Anaerobic digesters indeed require large volumes, and in low- and middle-income country (LMIC) contexts, about one-third of their total cost is associated with the electromechanical installations required both within and outside the digesters. In LMICs where the equipment often has a relatively higher price than the civil works, the electromechanical components can amount to more than 50 percent of the total digester cost. In addition, digesters are considered risky because methane is produced during sludge digestion, which has caused several explosion incidents at wastewater treatment plants (WWTPs) worldwide, including in high-income countries. Digester operation thus requires skilled operators and well-established procedures for preventive maintenance. Finally, it is important to underscore that the financial/economic assessment of CAS systems almost always seeks to take advantage of the potential for the conversion of the generated methane into electric energy—but such systems require high operation and maintenance (O&M) skill levels. Thus, they will make financial/economic sense only in situations in which energy unit costs are high and/or in which carbon credits for reducing greenhouse gas emissions can be leveraged. However, most of these conditions are typically not found in small-town settings of LMICs. Consequently, the CAS process—or at least key components of its treatment Wastewater Treatment and Reuse 137 train—frequently fails in such environments. In digesters are therefore not needed. Because of a fact, a WWTP using the CAS process but with larger total reactor volume (as compared with CAS), malfunctioning digesters is associated with odor- CAPEX figures associated with the EA wastewater related issues and complaints by neighbors and treatment train thus tend to increase, but the CAPEX operators, so it can pose a severe risk to the plant’s figures associated with the sludge treatment train security. In addition, if the digestion is not working are lower than those of CAS. In terms of total life- properly, a domino effect can set in: The sludge cycle costs, EA usually comes out equal to or more volume after digestion will be higher than designed, attractive than CAS for small and medium-sized often overwhelming the complete downstream WWTPs. In many LMICs, the breakpoint at which sludge treatment train. This in turn leads to even CAS becomes more financially attractive than EA poorer dewatering results, further increasing the is associated with WWTPs designed for 100,000 to dewatered sludge volume. Eventually, it may 500,000 population equivalents (PE). Only in high- become difficult for the landfill operator to accept income countries can this threshold be set lower sludge volumes that are larger than expected and than 100,000 PE. that are of inferior quality, and the plant operator will For small-town WWTPs, the aforementioned be forced to remove insufficient quantities of sludge factors—namely, financial costs, ease of operation, from the wastewater treatment train. Consequences reduced safety risks, less problematic sludge disposal, of such scenarios include an increase in the mixed and improved effluent quality compliance—all point liquor suspended solids (MLSS), increased energy toward favoring EA rather than CAS. Consequently, consumption, and a deterioration in effluent quality. CAS has been excluded from this guide at the In summary, the CAS process at medium-sized preselection stage (see table  3.3), whereas EA plants implies CAPEX figures that are comparable remains one of the technologies considered to be with those of many other advanced technologies, appropriate by the guide. such as extended aeration (EA), trickling filters EA systems can be grouped into two fundamentally (TF), and so on, but it also comes with serious risks different flow regimes: (a) flow-through and associated with unsuccessful O&M, increased (b) batch-wise treatment variations. The most operating expenditures (OPEX),1 and noncompliant common configurations are as follows: effluent quality. Such scenarios are in fact rather frequent. Flow-through EA-Activated Sludge Systems ◾ Oxidation ditch EA: In this configuration, the aeration tank is constructed as a closed-loop Contrary to CAS, the EA alternative is simpler in channel, leading to what are called completely that (a) it is not preceded by primary sedimentation mixed flow conditions. Water depth is only tanks, and (b) as the waste-activated sludge is typically in the order of 2 meters, thus enabling subjected to long retention times in the aeration the use of horizontal shaft mechanical aerator tanks, no digesters are required to stabilize the brushes or similar installations. Vertical shaft sludge. Sizing of the aeration tank volume ensures aerators can also sometimes be used and are that the sludge stays sufficiently long in the aerobic located at the turning points toward the end zones so that it can be considered stabilized of the loops. In general, aerators in oxidation (represented by a high aerobic sludge age or low ditches not only provide the necessary oxygen food to microorganism [F/M] ratios). Separate sludge for microorganisms but also provide horizontal 138 Appendix A: Extended Aeration versus Conventional Activated Sludge FIGURE A1.1 Schematic Diagram of an Oxidation Ditch EA carrousel type extended aeration clarifier brush aerator outlet sludge inlet brush aerator recirculation extracted sludge thrust to facilitate constant movement and mixing efficiency, given that pressurized aeration is of the wastewater-sludge mixture, thereby used; water depth is increased to 5 to 6 meters; avoiding settlement of the sludge MLSS. aerated zones and nonaerated zones are installed intermittently2 with high initial substrate ◾ Carrousel type EA: The tank configuration of concentrations, allowing for faster biological carrousel plants is a further development of reaction rates; and smart automation systems oxidation ditches, typically employed for larger for the control of air supply are introduced, WWTPs. Instead of a single closed-loop channel complete with effluent quality control sensors (with two 180-degree turning points, one at each and frequency-controlled blowers. end of the system), carrousel facilities typically use tanks with four turning points, before the loops are closed. Water depth is also often Batch-wise treatment increased to 5 meters or more. To increase the ◾ Sequencing batch reactor (SBR) type EA: This low-energy efficiency of mechanical surface configuration employs batch-wise treatment of aerators, the aeration system can be changed the wastewater. In its classical variation, there to a pressurized one at the bottom of the tank, are at least two parallel SBR tanks, where one but in such cases, horizontal flow thrust needs to tank receives fresh flow (filling and treatment), be introduced by the use of special mixers. and in the second tank sludge is settled and ◾ Plug-flow type EA: In this configuration, tanks are the supernatant is withdrawn and discharged shaped so that flow enters one end and leaves (sedimentation and discharge). After some time, at the other (providing longitudinal flow or plug- following a timed program, the two tanks switch flow conditions). This is mostly done to improve roles: The second tank receives fresh flow, and Wastewater Treatment and Reuse 139 FIGURE A1.2 Schematic Diagram of a Carrousel Type EA oxidation ditch extended aeration brush aerator clarifier outlet brush aerator sludge inlet recirculation extracted sludge FIGURE A1.3 Schematic Diagram of a Plug-Flow Type EA plug-flow type extended aeration inlet clarifier outlet sludge recirculation extracted sludge 140 Appendix A: Extended Aeration versus Conventional Activated Sludge the first one transitions to the sedimentation with no CAPEX advantages when compared and discharge mode. Although the flow pattern with EA in a small-town context and poses changes over the course of its operation, the serious O&M challenges, potentially leading biological principles in the SBR remain identical to process failures. Such failures usually start to the other EA configurations described earlier. with the malfunctioning of the digester, leading The key difference lies in the fact that the aeration to odor issues, increased sludge volumes, and the sedimentation take place in the same increased OPEX, problems with sludge disposal/ tank, allowing for the elimination of the piped reuse, and noncompliant effluent quality, not interconnections between the aeration tanks and to mention potential complaints from nearby the final sedimentation tanks, as well as for the populations and the plant operators. piping for the return sludge pumping. In addition, ◾ EA is a simpler form of activated sludge that the overall WWTP footprint can be reduced as may be suited for certain small-town WWTPs. rectangular tanks closely aligned to one another In this guide, two different variations of EA are can be used, and the classical traveling bridges presented: (a) EA representing the flow-through in the final sedimentation tanks are no longer configurations and (b) SBR(EA) representing the needed. Modern SBR systems also focus on batch-wise treatment configurations. efficiency, employing 5- to 6-meter-deep tanks and optimized aeration systems. In summary, Notes SBR type EA systems present slightly lower 1. OPEX may increase for various reasons: (a) high maintenance/ CAPEX figures when compared with the other repair costs associated with electromechanical installations; EA variations described earlier, whereas OPEX (b) inefficient digestion processes increasing the demand figures are comparable to those of optimized (and costs) in polymers for sludge dewatering; (c) inefficient digestion processes leading to higher sludge volumes and completely mixed or plug-flow EA types. thus higher sludge disposal costs; and (d) inefficient digestion processes producing little or no biogas—therefore, electricity has to be purchased fully from the grid to satisfy the WWTP’s Conclusions needs. 2. Operating in an intermittent aeration mode allows for an ◾ This guide does not consider CAS to be improved nutrient effluent quality while minimizing OPEX appropriate for small-town WWTPs: It comes associated with energy consumption. 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