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Chapter 6: Safe Water Technology

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Chapter 6: Safe Water Technology Chapter 6 Final Draft – January 31, 2001 Chapter 6: Safe Water Technology Richard Johnston, Han Heijnen and Peter Wurzel Previous chapters have documented the serious health effects that are posed by ingestion of arsenic through drinking water. While some palliative treatment of arsenicosis patients is possible, it is clear that the first step in treating patients, and preventing others from falling sick, is to identify safe sources of water for drinking and cooking in arsenic-affected areas. This chapter will present a brief overview of safe drinking water supply technologies that can provide arsenic-free1 drinking water, either through identifying an arsenic-free source or by removing arsenic from contaminated water. The main focus of this chapter is on rural and peri-urban settings, though many of the technologies discussed are applied in central water supply systems as well, and a few examples of centralized arsenic removal are given. Conventional water supplies are briefly discussed, but as these systems are extensively described elsewhere, the reader is referred to authoritative documents for more detailed information. Arsenic removal technologies are less well documented in standard water supply texts, though a large and growing literature exists in technical journals. In addition, many other valuable papers are found in seminar and conference proceedings, particularly in Latin America and Asia. Since many of these resources are less accessible for some readers, this chapter presents a detailed review of arsenic removal technologies. Many of these technologies are under development, and a sample protocol is presented for evaluation of new technologies for arsenic removal, as well as a more general framework for selection of an appropriate approach in a given socioeconomic and environmental context. A series of case studies are presented of technological responses to arsenic contamination at municipal, community, and household levels. The final section presents some general conclusions and identifies areas where more knowledge or research is required. 1 No water is ever completely free of arsenic – trace levels will always be present, even if they cannot be measured. In this chapter the term ‘arsenic-free’ is taken to mean free from unsafe levels of arsenic. 1 Chapter 6 Final Draft – January 31, 2001 Executive Summary ................................................................................................................6 6.1 Safe drinking water supply............................................................................................10 6.2 Sources of arsenic-free drinking water ..........................................................................12 6.2.1 Groundwater ........................................................................................................12 6.2.2 Rainwater .............................................................................................................15 6.2.3 Surface Water .......................................................................................................18 6.2.4 Disinfection..........................................................................................................23 6.3 Technologies for arsenic removal..................................................................................25 6.3.1 Oxidation .............................................................................................................27 6.3.2 Coagulation and filtration .....................................................................................28 6.3.3 Ion-Exchange Resins ............................................................................................36 6.3.4 Activated Alumina................................................................................................38 6.3.5 Membrane methods ..............................................................................................41 6.3.6 Emerging technologies .........................................................................................43 6.3.7 Waste disposal......................................................................................................50 6.3.8 Summary..............................................................................................................53 6.4 Evaluation and selection of arsenic-free water supply technologies................................54 6.4.1 Technical criteria ..................................................................................................55 6.4.2 Socioeconomic criteria .........................................................................................57 6.4.3 Feasibility.............................................................................................................61 6.4.4 Environmental Technology Verification ...............................................................62 6.5 Case study: application of evaluation protocol in Bangladesh ........................................63 6.5.1 Water supply and treatment options for consideration ...........................................63 6.5.2 Technical Criteria .................................................................................................67 6.5.3 Socioeconomic criteria .........................................................................................72 6.5.4 Conclusions..........................................................................................................78 6.6 Application of safe drinking water supply technologies in arsenic-affected areas...........79 6.6.1 Municipal level.....................................................................................................79 6.6.2 Community level ..................................................................................................81 6.6.3 Household level arsenic removal ..........................................................................84 6.7 Conclusions and continuing needs.................................................................................86 6.8 References ....................................................................................................................88 2 Chapter 6 Final Draft – January 31, 2001 Figures: Figure 6.1: Arsenic and well depth in Bangladesh .....................................................................14 Figure 6.2: Rooftop catchment with tank ..................................................................................16 Figure 6.3: Multiple barriers to microbiological contamination in surface water treatment .........19 Figure 6.4: Multi-stage filtration ...............................................................................................22 Figure 6.5: Bank infiltration ......................................................................................................23 Figure 6.6: Temperature and pathogen removal in SODIS treatment .........................................24 Figure 6.7: Arsenate removal by coagulation ............................................................................30 Figure 6.8: Arsenate adsorption onto preformed HFO ...............................................................31 Figure 6.9: Arsenite adsorption onto HFO ................................................................................33 Figure 6.10: Bed volumes treated with ion exchange resin .........................................................37 Figure 6.11: Effect of pH on arsenate adsorption onto activated alumina ...................................38 Figure 6.12: Bed volumes treated with activated alumina...........................................................39 Figure 6.13: Pore size of various membranes, and size of materials subject to filtration .............42 Figure 6.14: 18-DTP arsenic removal plant design.....................................................................45 Figure 6.15: Pore size of various membranes.............................................................................65 Figure 6.16: Annual precipitation in Bangladesh .......................................................................69 Tables: Table 6.1: Drinking water and sanitation access in developing countries ....................................10 Table 6.2: Typical removal efficiencies in slow sand filtration...................................................21 Table 6.3 : Molecular weights of compounds used in arsenic removal........................................29 Table 6.4 : Effects of anions on removal of arsenite and arsenate in coagulation .......................34 Table 6.5: Arsenic removal in three 18-DTP plants....................................................................45 Table 6.6: Summary of technologies for arsenic removal ...........................................................54 Table 6.7 : Summary of protocol scores.....................................................................................78 Table 6.8 : Field testing of Alufloc and Ferrifloc in Salta, Argentina..........................................85 3 Chapter 6 Final Draft – January 31, 2001 Boxes: Box 6.1: Deep aquifer in Bangladesh.........................................................................................13 Box 6.2: Bank infiltration in Hungary........................................................................................23 Box 6.3: Sorbent capacity..........................................................................................................29 Box 6.4: Residual aluminum in treated waters ...........................................................................40
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