Caribbean Rainwater Harvesting Handbook   Caribbean Rainwater Harvesting HandbookUnited Nations Environment ProgrammeP.O. ...
Caribbean Rainwater Harvesting Handbook                            Caribbean Rainwater Harvesting HandbookHandbook for Rai...
Caribbean Rainwater Harvesting Handbook                                           Caribbean Rainwater Harvesting HandbookA...
Caribbean Rainwater Harvesting Handbook                                                  Caribbean Rainwater Harvesting Ha...
Caribbean Rainwater Harvesting Handbook                                Caribbean Rainwater Harvesting Handbook            ...
Caribbean Rainwater Harvesting Handbook                 Caribbean Rainwater Harvesting Handbookaquifers, while the possibi...
Caribbean Rainwater Harvesting Handbook                                                        Caribbean Rainwater Harvest...
Caribbean Rainwater Harvesting Handbook                                                            Caribbean Rainwater Har...
Caribbean Rainwater Harvesting Handbook                                                                             Caribb...
Caribbean Rainwater Harvesting Handbook                                            Caribbean Rainwater Harvesting Handbook...
Caribbean Rainwater Harvesting Handbook                                                         Caribbean Rainwater Harves...
Caribbean Rainwater Harvesting Handbook                                    Caribbean Rainwater Harvesting HandbookAcronyms...
Caribbean Rainwater Harvesting Handbook                                                                        Caribbean R...
Caribbean Rainwater Harvesting Handbook                                  Caribbean Rainwater Harvesting Handbook 1. Backgr...
Caribbean Rainwater Harvesting Handbook                                                    Caribbean Rainwater Harvesting ...
Caribbean Rainwater Harvesting Handbook                                                Caribbean Rainwater Harvesting Hand...
Caribbean Rainwater Harvesting Handbook                                         Caribbean Rainwater Harvesting Handbook•  ...
Caribbean Rainwater Harvesting Handbook                                             Caribbean Rainwater Harvesting Handboo...
Caribbean Rainwater Harvesting Handbook                                             Caribbean Rainwater Harvesting Handboo...
Caribbean Rainwater Harvesting Handbook                                            Caribbean Rainwater Harvesting Handbook...
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
Caribbean Rainwater Harvesting Manual
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Caribbean Rainwater Harvesting Manual

  1. 1. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookUnited Nations Environment ProgrammeP.O. Box 30552, Nairobi, KenyaTel: +254 2 621334Fax: +254 2 624489/90Web: www.unep.orgCaribbean Environmental Health Institute (CEHI)P.O. Box 1111The Morne, Castries, St. LuciaTel: (758) 452-2501Fax: (758) 453-2721E-mail: cehi@candw.lcWeb: www.cehi.org.lc 64
  2. 2. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookHandbook for Rainwater Annex 3: Useful websitesHarvesting for the Caribbean Chennai Metro Water – Rainwater Harvesting http:// www.chennaimetrowater.com/rainwatermain.htmA practical guideline featuring best practices for eToolkit on Rainwater Harvesting http://www.rainwater-toolkit.net/ index.php?id=75rainwater harvesting in small island Caribbeanenvironments International Rainwater Catchment Systems Association (IRCSA) http://www.eng.warwick.ac.uk/ircsa/ Rainwater Harvesting http://www.rainharvesting.com.au/Prepared for UNEP by the Caribbean Environmental rain_water_harvesting.aspHealth Institute Rainwaterharvesting.org http://www.rainwaterharvesting.org/ SearNet – Southern and East Africa Rainwater Network http:// www.searnet.org/searnetfinal/home.asp Technology Informatics Design Endeavour (TIDE) http://www.tide- india.org/products/06polyhouse.html (example of RWH for agricultural greenhouse) The Web of Rain - Online resources on rainwater harvesting http:// www.gdrc.org/uem/water/rainwater/rain-web.html YouTube videos on rainwater harvesting http://www.youtube.com/ results?search_type=&search_query=rainwater+harvestingOctober 2009 2 63
  3. 3. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookAnnex 2: Planning guide for water use (cont’d) First published by United Nations Environment Programme and the Caribbean Environmental Health Institute Type of Water used Unit Copyright© 2009, United Nations Environment Programme/CEHI Establishment (litres/day)Livestock Per animal Cattle (drinking) 45 Dairy (drinking and servicing) 132 This publication may be produced in whole or in part and in any form for Goat (drinking) 8 educational or non-profit purposes, without special permission from the copyright Hog (drinking) 45 holder(s), provided acknowledgement of the source in made. The United Nations Environment Programme and the Caribbean Environmental Health Institute would Horse (drinking) 45 appreciate receiving a copy of any publication that uses this report as a source. Mule (drinking) 45 Sheep (drinking) 8 No use of this publication may be made for resale or for any other commercialMotel Bath, toilet and kitchen facilities (per 189 purpose whatsoever without permission in writing from the United Nations bed space) Environment Programme and the Caribbean Environmental Health Institute. Bed and toilet per (per bed space) 151Park Overnight, flush toilets (per camper) 95 Application for such permission, with statement of purpose and extent of production should be addressed to the Director, DCPI, P.O. Box 30552, Nairobi, Trailer individual bath units no sewer 95 00100 Kenya. connection (per trailer) Trailer, individual baths, connected to 189 The designation employed and the presentation of material in this publication do sewer (per person) not imply the expression of any opinion whatsoever on the part of the UnitedPicnic Bathhouses, showers and flush toilets 76 Nations Environment Programme concerning the legal status of any country, (per picnicker) territory or city or its authorise, or concerning the delimitation of its frontiers of Toilet facilities only (per picnicker) 38 boundaries.Poultry Chickens (per 100) 19-38 Turkeys (per 100) 38-68 Mention of a commercial company or product in this publication does not imply endorsement by the United Nations Environment Programme. The use ofRestaurant Toilet facilities (per patron) 9-11 information from this publication concerning proprietary products for publicity or Bar and cocktail lounge (additional 8 advertising is not permitted. quantity per patron)School Boarding (per pupil) 284-379 Layout: Christopher Cox, Avril Isaac/CEHI Day, cafeteria, gymnasiums and showers 95 (per pupil) Day cafeteria, no gymnasiums or 76 showers (per pupil) Day no cafeteria, gymnasiums or 57 showers (per pupil)Service station per vehicle 38Store per toilet room 1514Swimming pool per swimmer 38Theatre Drive in (per car space) 19 Movies (per auditorium seat) 19Worker Construction (per person per shift) 189 Day (school or offices per person per 57 shift) 62 3
  4. 4. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookAcknowledgements Annex 2: Planning guide for water use Water used Type of Establishment UnitThis Handbook was produced under a United Nations Environment Programme (litres/day)(UNEP)-funded initiative and executed by the Caribbean Environmental Health Per passenger 11-19Institute (CEHI). The project was two-staged with a first stage involving the devel- Apartment (multiple family) Per resident 227opment of a national programme for the promotion of Rainwater Harvesting Bathhouse Per bather 38(RWH) in Grenada that subsequently informed the development of a regional-levelRWH programme. It produced a suite of public educational materials and mapping Camp Construction semi permanent 189of water availability. The outputs from this first stage are available on the CEHI (per worker)website at www.cehi.org.lc Day no meals served (per 57 camper)The finalization of this Handbook was achieved under a second stage of the UNEP Luxury (per camper) 379-568initiative where best-practice RWH demonstration models were developed in An- Resort day and night, limited 189tigua and Barbuda. The contents of the Handbook were derived from lessons plumbing per camperlearnt from the Antigua and Barbuda demonstration project and the vast resources Tourist central bath and toilet 132on the subject have been published on-line and in hard-copy publications. CEHI facilities (per person)acknowledges these contributions as providing valuable input in the production of Cottage, seasonal occupancy 189this publication. (per resident) Court, tourist, individual bath 95CEHI and UNEP wish to express its sincere gratitude to Alphonsus Daniel, Con- units (per person)sulting Engineer of Grenada who served as technical advisor and co-contributor, Club Country (per resident member) 379Dr. Henry Smith, of the University of the Virgin Islands for technical review and Country (per non resident 95guidance and Dr. Everson Peters, of University of the West Indies St. Augustine, member)Andrew P. Hutchinson, of Stantec Caribbean, and Sven Homscheid, Caribbean Dwelling Boarding house (per boarder) 189Renewable Energy Development Programme (CREDP-GTZ) for their additional Additional kitchen requirements 38technical contributions. for non-resident boarders Luxury (per person) 379-568CEHI and UNEP wish to acknowledge the contributions of staff of the Grenada Multi-family apartment (per 151National Water and Sewerage Authority (NAWASA) who provided insights into resident)the operation and management of the community rainwater harvesting systems on Rooming guesthouse (per 227the islands of Carriacou and Petit Martinique. Thanks are also extended to the resident)staff of the Antigua Public Utilities Authority (APUA), in particular Ivan Rodrigues, Single family (per resident) 189-284Hastin Barnes, and McClure Simon, for their contributions to the development of Estate Per resident 379-568the demonstration projects on Antigua and Barbuda which are featured in this Factory Gallons per person per shift 57-132Handbook. Acknowledgements are also given to Cornelius Isaac, DemonstrationProject Manager, the Global Environment Facility-funded Integration Watershed Highway rest area Per person 19and Coastal Areas Management (GEF-IWCAM), for contributions from the RWH Hotel Private bath (2 persons per 227installations in the Project demonstration site in St. Lucia. room) No private baths 189The Project Team also thanks Elizabeth Khaka, Programme Officer in the Division Institution other than hospi- Per person 284-473of Environmental Policy Implementation at UNEP, Nairobi, Kenya and Christopher talCorbin, Assessment and Management of Environmental Pollution (AMEP) Pro- Hospital Per bed 946-1514gramme Officer of UNEPs Caribbean Regional Coordinating Unit for their support Laundry Self serviced (gallons per wash- 189and guidance. ing per customer) 4 61
  5. 5. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting Handbook Preface This rainwater harvesting handbook is a practical guideline to assist homeowners, contractors, architects, farmers, and business owners in offering tips on how they can introduce rainwater harvesting to aug- ment their water supply. Access to a safe and reliable source of water still remains a number one development priority in the Caribbean. Over the past decades many coun- tries have made tremendous strides toward meeting this priority through the establishment of centralized municipal water supply systems that service the majority of households and businesses. Only in the smallest islands, most notably the Grenadines and some of the Bahamas, rainwater remains the sole source of water. In several of the more arid islands such as Antigua & Barbuda, the Bahamas, and the Virgin Islands, rainwater harvesting is still practised in spite of access to the municipal potable supply, where residents on these islands still favour rainwater in meeting their drinking and cooking needs. In recent years however, some of the water-scarce countries have made significant investments in desalini- zation technologies, where sea water is converted to fresh water. In this case, the availability of a virtually unlimited supply of water is now meeting the development needs for water on these islands, but it comes at a significant cost due to the energy inputs required among other things. On these islands, as well as on many others, with expanded access to potable water whether ground, surface or sea water, there has been a trend to move away from RWH, due to the general perception that the practice is outdated; to be relegated to a past era before the advent of the conventional water supply and distribution system. The applications of RWH however are gaining prominence in national water resources management planning. This is in the context of the pressures that water resources (both ground and surface sources) have been facing from land degradation and pollution due to expanded urbanization, industrialization, agricultural and other commercial development. If pollution of the regions waters is not addressed, climate change will exacerbate diminishing water availability at a more rapid pace. Rising sea levels will intrude vital coastal 60 5
  6. 6. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting Handbookaquifers, while the possibility of declining rainfall will negatively affect aquiferrecharge rates. The anticipated more frequent occurrence of destructivehurricanes, floods and droughts will compound the situation. The post-Hurricane Ivan experience of Grenada (2005) served to highlight the issue ofwater scarcity, and how RWH can go a long way towards meeting post-disaster water and sanitation needs. Grenada’s sister island, Carriacou, whichis served only by harvested rainwater, was mainland Grenada’s principlesource of water in the aftermath of the hurricane.Policies and attitudes toward the practice of RWH must change if we are tocontinue to make our communities more water-secure into the future. TheCaribbean Environmental Health Institute has been working with partnerssuch as the United Nations Environment Programme (UNEP) and the GlobalWater Partnership (GWP) in advancing water resources management in theCaribbean region with particular focus on Integrated Water Resources Man-agement (IWRM) where rainwater harvesting is an element. This Handbookis accompanied by a range of general and technical information products thathave been developed with the assistance of UNEP and includes policy guide-lines for RWH, educational posters, brochures, technical fact sheets, radiopublic service announcements, a television production, stormwater RWHmanagement recommendations for aquifer recharge and computer-assistedmapping to estimate rainwater harvesting potentials.The Handbook primarily details the application of domestic RWH systemswith emphasis on aspects such as rooftop rainwater capture potential to stor-age capacity, determining storage capacity based on household size, andreducing contamination of stored water. The guidelines set out in the Hand-book were derived from the wealth of resources on thesubject from a variety of different websites. The primaryreference in the preparation of this document was the guide-line Harvesting the Heavens - Guidelines for Rain-water Harvesting in Pacific Island Countries (2004)that was compiled by the South Pacific Applied GeoscienceCommission (SOPAC) for the United Nations EnvironmentProgramme (UNEP) in conjunction with the Tonga Commu-nity Development Trust (TCDT) and funded by the SwedishInternational Development Agency (SIDA). The SOPAC publication is avail-able on-line at http://www.sopac.org/data/virlib/JC/JC0178.pdfIt is anticipated that the Handbook will be utilized by contractors, home-owners, commercial operators, and farmers as an easy guide in the installationand operation of RWH systems. Although the material presented in this 6 59
  7. 7. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting Handbook Handbook is mainly based on single household applications, the con- cepts can be applied or scaled up to other applications in agricultural,Quick Reference: commercial, industrial and municipal services sectors. Advice from pro- fessional engineers, contractors or water services specialists should be solic-Calculating the amount of water you can ited when investing in RWH systems.capture off your roof Patricia AquingMapping your roof areas - determining the Executive Director Projecting the sloped roof area to horizontalcontribution catchments catchment area Caribbean Environmental Health InstituteThe following are formulas to estimatethe areas of roof section shapes Projecting the sloped roof area to horizontal catchment area - Sine of the roof angle; multiply by roof dimensions 58 7
  8. 8. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookTable of Contents Annexes Annex 1: Quick reference calculationsAcknowledgements ................................................................................................4Preface .......................................................................................................................5Table of Contents...................................................................................................7Technical boxes, tables and figures....................................................................8Annexes.....................................................................................................................9Acronyms and units ...............................................................................................10Glossary ....................................................................................................................111. Background ..........................................................................................................12 1.1 Water is life! ...............................................................................................12 1.2 Rainwater harvesting in the Caribbean................................................12 1.3 Rainwater harvesting – the pros and cons..........................................142. Rainwater harvesting system design..............................................................16 2.1 Main RWH components ..........................................................................16 2.2 The catchment area ..................................................................................18 2.2.1How to calculate the volume of water that can be captured by a roof catchment ..............................................................................18 2.3 The Conveyance system ..........................................................................21 2.3.1 Gutters..................................................................................................21 2.3.2 First-flush diverter..............................................................................22 2.3.3 Screens..................................................................................................24 2.3.4 Baffle tanks ...........................................................................................25 2.4 Storage .........................................................................................................26 2.4.1 Above-ground versus underground tanks ...................................26 2.4.2 Tank materials.....................................................................................27 2.4.3 Tank inlet and outlet configurations..............................................30 2.4.4 Tank overflow configurations .........................................................31 2.4.5 Sizing of the storage facility .............................................................31 2.4.6 Pressurized water supply systems .................................................33 8 57
  9. 9. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookTWDB (2005) The Texas Manual on Rainwater Harvesting. Third Edi- 2.5 Post-storage filtration ................................................................................ 35 tion. Texas Water Development Board. Available on-line at: http:// www.twdb.state.tx.us/publications/reports/ RainwaterHarvesting- 3. Maintenance and vector control ................................................................... 36 Manual_3rdedition.pdf 3.1 Contamination sources ............................................................................. 36 3.1.1 Contamination from the atmosphere ............................................ 36TWDB (2009) Rainwater Harvesting System Sizing Calculator. Texas Wat er Develop men t Board. Av ailab le on -lin e at: h t tp :// 3.1.2 Contamination during rainwater capture...................................... 36 w w w .t w db . s t at e .t x .u s / a ss i s t a n ce / c o n s e r v a t io n / A l t e rn a - 3.1.3 Contamination during storage ......................................................... 37 tive_Technologies/Rainwater_Harvesting/Rain.asp 3.1.4 Contamination during maintenance................................................ 37UNEP (2002) Rainwater Harvesting and Utilisation - An Environmentally 3.2 Summary: Protecting collected rainwater from contamination...... 38 Sound Approach for Sustainable Urban Water Management: An Introductory Guide for Decision-Makers United Nations Environ- 4. Managing water quality..................................................................................... 40 ment Programme. Available on-line at: http://www.unep.or.jp/Ietc/ 4.1 Disinfection by boiling ............................................................................... 40 Publications/Urban/UrbanEnv-2/index.asp 4.2 Disinfection by chlorinating...................................................................... 40University of Warwick (2007) Introduction to domestic roofwater harvest- 4.2.1 Chlorine disinfection procedure ..................................................... 41 ing. Development Technology Unit, School of Engineering. Available 5. Cost of installation of RWH systems........................................................... 42 on-line at: http://www2.warwick.ac.uk/fac/sci/eng/research/civil/dtu/ rwh/index.html 6. Non-domestic RWH applications................................................................. 6.1 Municipal applications............................................................................. 44University of Warwick (2006) Sizing the DRWH system. Development Technology Unit, School of Engineering. Available on-line at: http:// 6.2 Agricultural applications.......................................................................... 44 www2.warwick.ac.uk/fac/sci/eng/research/dtu/rwh/sizing/ 6.3 Commercial and industrial applications .............................................. 45 Bibliography ............................................................................................................. 46 Annexes.................................................................................................................... 48 56 9
  10. 10. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookTechnical boxes, tables and figures Bibliography CEHI (2006) A Programme for Promoting Rainwater Harvesting in theTechnical boxes Caribbean Region. Collaborator: United Nations EnvironmentTechnical Box 1: How to use the Rational Method to calculate the volume of rain- Programme (UNEP). Available on-line at: http://cehi.org.lc/ water which could be captured rwhindex_files/Programme%20to%20Promote%20RWH%20in%Technical Box 2: How to calculate the volume of water you need to divert using a 20the%20Cabbean%20Region.pdf first flush systemTechnical Box 3: How to estimate the cistern/tank size you need Hutchinson, A (2009) Rain Water Harvesting - The Barbados Experience Caribbean Water and Wastewater Association (CWWA) St. Lucia Water Week, May 2009. Stantec CaribbeanTablesTable 1 Runoff coefficients for various catchment types OAS (1997) Source Book of Alternative Technologies for FreshwaterTable 2 Guide to sizing gutters and down-pipes for RWH systems in tropical regions Augmentation in Latin America and the Caribbean - RainwaterTable 3 Advantages and disadvantages of aboveground and underground storage harvesting from rooftop catchments Organization of American systems States Washington, D.C.Table 4 Chlorine disinfection mixing ratios for stored water Available on-line at: http://www.oas.org/osde/publications/Unit/Table 5 Estimated unit costs for standard supplies used in RWH systems in the oea59e/ch10.htm CaribbeanTable 6 Typical cost of rainwater system installations Peters, E. (2003) Sizing of Rainwater Cisterns for Domestic Water Sup- ply in the Grenadines. Available on-line at: http:// www.uwichill.edu.bb/bnccde/svg/conference/papers/peters.htmlFiguresFigure 1: Typical household rainwater harvesting in the Caribbean Smith, H. (2009) Rainwater Harvesting in the US Virgin Islands. CaribbeanFigure 2: Map of the Caribbean Water and Wastewater Association (CWWA) Conference. Octo-Figure 3: Old public water supply cistern and a hillside catchment surface on ber 2009. University of the Virgin Islands St .Thomas, U.S. Virgin IslandsFigure 4: Domestic RWH system with cistern in St. Thomas, U.S. Virgin Islands SOPAC (2004) Harvesting the Heavens - Guidelines for Rainwater Har-Figure 5: Cistern serving apartment building in St. Thomas, U.S. Virgin Islands vesting in Pacific Island Countries. South Pacific Applied Geo-Figure 6: Surface catchment for communal RWH system at Ashton on Union Island, science Commission for the United Nations Environment Pro- St. Vincent and the Grenadines gramme (UNEP). Collaborators: the Tonga Community Develop-Figure 7: Typical domestic RWH systems with (a) below-ground and (b) above- ment Trust (TCDT) and the Swedish International Development ground storage Agency (SIDA). Available on-line at http://www.sopac.org/data/virlib/Figure 8: Dividing your roof into sections for area estimation JC/JC0178.pdfFigure 9: Typical PVC guttering and downpipeFigure 10: Simple first-flush diverter SOPAC (2004) Harvesting the Heavens – A Manual for ParticipatoryFigure 11: First-flush diverter Training in Rainwater Harvesting South Pacific Applied Geo-Figure 12: Examples of first-flush systems using float-ball mechanism science Commission for the United Nations Environment Pro-Figure 13: Upflow filter design. Used in UNEP/ CEHI RWH Project in Antigua and gramme (UNEP). Collaborators the Tonga Community Develop- Barbuda ment Trust (TCDT) and the Swedish International DevelopmentFigure 14: Examples of up-flow filter first-flush diverters (single and dual tank Agency (SIDA). Available on-line at: http://www.sopac.org/data/ configurations). virlib/MR/MR0544.pdf 10 55
  11. 11. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookRWH from farm building catch- Figure 15: Screens to exclude entry of insects and other potential contaminantsments and constructed surfaces can Figure 16: Design configuration for a self-cleaning screengreatly contribute to meeting water Figure 17: Baffle tank design to trap debrisdemands during the drier months Figure 18: Multiple PVC tanks for rainwater storagefor sustained production. Figure 19: Typical ferrocement tank Figure 20: Example of typical non-toxic concrete sealant for cisternsFigure 34 shows a typical example Figure 21: Typical block-type cisterns under constructionof using irrigation from harvested Figure 22: Typical plastic tanks for household RWH systemrainwater. Figure 23: Design configurations for (a) tank inflow and (b) outflow Figure 24: Design configurations for tank overflows Figure 34 Rainwater harvesting off a Figure 25: Graphical guide to tank size selection greenhouse for irrigation purpose Source: Figure 26: Pressure tank and pump Technology Informatics Design Endeavour (TIDE) Figure 27: Typical head-pressure RWH system http://www.tide-india.org/products/06polyhouse.html Figure 28: Solar energy water pump Figure 29: Off-the-shelf water filters 6.3 Commercial and industrial applications Figure 30: An example of a rat guard along a gutter to a storage tank in the GrenadinesRainwater harvested off roofs and surface catchments such as roads and park- Figure 31: An example of a poorly-maintained guttering lots can be stored and used as required to offset the need for use of the Figure 32: A typical ultra-violet lamp on a RWH installationpotable supply for non-drinking purposes. Figure 33(a): Rainwater capture from a municipal facility in Charlotte Amalie, St. Thomas, U.S. Virgin Islands Typical applications will include Figure 33(b): Rainwater harvesting for irrigation of Kensington Cricket Oval, St. washing and cleaning, cooling, fire Michael, Barbados fighting, bathing pool recharge and Figure 34: Rainwater harvesting off a greenhouse for irrigation purpose irrigation. Where water may be Figure 35: Rainwater harvesting diverted to surface storage at Mount Gay required for food preparation and Distilleries, St. Lucy, Barbados; irrigation and firefighting purposes other manufacturing processes, treatment will need to be applied. Annexes Figure 35 shows an example of RWH and storage by a commer- Annex 1: Quick reference calculations a) Calculating the amount of water you can cial enterprise in Barbados. capture off your roof b) Estimating storage requirements Figure 35 Rainwater harvesting diverted to surface storage at Mount Gay Distilleries, St. Annex 2: Planning guide for water use Lucy, Barbados; irrigation and firefighting pur- Annex 3: Useful websites poses (Photo courtesy: Andrew Hutchinson) 54 11
  12. 12. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookAcronyms and units 6. Non-domestic RWH applicationsCEHI Caribbean Environmental Health InstituteImp Imperial gallon 6.1 Municipal applicationshp horsepower Rainwater harvesting can be used in a variety of municipal applications. Directkm3 cubic kilometers roof capture off city buildings or capture of excess runoff from paved surfacesm2 square metres can be used to fill cisterns and other storage facilities that can be used for irrigation of green spaces and recreational facilities, washing and cleaning ofm3/d cubic meters per day streets and facilities, or for fire fighting.mg milligram per litreml millilitres Such bulk water storage can be used to augment emergency water suppliesmm millimeters following natural disasters when the potable supply may be out of operation. In this case filtration and treatment will need to be applied before distribution.PAHO Pan American Health Organization Figures 33a and 33b show a few examples.PVC Poly-Vinyl ChlorideRWH Rain Water HarvestingSIDS Small Island Developing States (A) (B)SOPAC South Pacific Applied Geoscience CommissionUNEP United Nations Environment ProgrammeUS$ United States dollarsUSVI United States Virgin IslandsUV ultraviolet Figure 33(A) Rainwater capture from a municipal facility in Charlotte Amalie, St. Thomas, USVI; 33(B) Rainwater harvesting for irrigation of Kensington Cricket Oval, St. Michael, Barbados (runoff from field and roof catchments diverted to perimeter sub-surface drain - see inset) (Photos courtesy: Henry Smith and Andrew Hutchinson) 6.2 Agricultural applications Crop irrigation and livestock watering have heavy water demands. In the Caribbean agricultural production is predominantly rain-fed with the excep- tion of larger commercial farm holdings where water is abstracted from irriga- tion drains, natural watercourses and wells. Under rain-fed production, crop yields will drop significantly during the dry season unless supplemental irriga- tion is applied. Livestock production is similarly impacted where water is in short supply. 12 53
  13. 13. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting Handbook Table 5 Estimated unit costs for standard supplies used in RWH Glossary systems in the Caribbean (2008 prices) Source: Daniel and Daniel Engineering Inc Aquifer Underground bed or layer yielding ground water for wells and springs Catchment The component of a RWH system from which rainwater is Item Retail cost US$ collected (especially a natural drainage area). Typical catch- 25.20 ment surfaces include building rooftops, roads and pave- 6” down-pipe (13 foot long) and guttering ments. 90o bends for guttering 14.80 32o bend for guttering 11.90 Chlorine The amount of chlorine that remains in water at the end of a Guttering to down pipe fitting 7.40 residual period of time following chlorination 90o bend for down pipe 4.40 Cistern A receptacle for holding water typically integrated as a struc- 45o bend for down pipe 4.40 tural part of a building 200 gallon plastic tank 166.70 Conveyance The component of the RWH systems that includes the gut- 400 gallon plastic tank 185.20 system ters and pipes that carry the water from the catchment to 600 gallon plastic tank 351.90 the storage tank. The filter screens and first-flush diverters 800 gallon plastic tank 463.00 are also included in this component 1,000 gallon plastic tank 555.60 Desalination The removal of salt from sea water to convert to potable water Disinfection Treatment process to destroy harmful microorganismsTable 6 Typical cost of rainwater system installations Down-pipe The pipe that carries water from the gutter down to theSource: Daniel and Daniel Engineering, Inc, 2006. Based on data collected by Peters (2006) in the Grenadine Islands of Bequia andUnion Island, (St. Vincent and the Grenadines) and Carriacou, (Grenada). storage Approx. cost Approx. cost First-flush An installation typically on the downpipe (from the roof gut- Size of diverter ter) that diverts the initial ‘dirty’ wash off the roof when it No. of Contributing of RWH of RWH cistern starts to rain from entering the tank permanent roof/ catch- system using system using required household ment area polyethylene reinforced con- Groundwater Water beneath the surface of the earth which saturates the (imp. members (m2)(sq ft) tanks as cis- crete tanks as pores and fractures of sand, gravel, and rock formations gallons) terns ($US) cisterns ($US) Gutter A channel installed along the edge of a catchment surface 1 (Single) 50 (538) 400-500 556 741 that carries collected rainwater toward the storage 2 100 (1,076) 1,000 963 1,148 Municipal Of or relating to a town or city or its local government 3 150 (1,614) 1,500 1,185 1,556 Potable Fit to drink Rational A calculation procedure to estimate the amount of rain that 4 180 (1,937) 2,000 1,593 1,963 Method can be captured off a roof surface per year 5 200 (2,152) 2,500 1,926 2,482 Runoff Excess water that runs over a surface 6 220 (2,367) 3,000 2,296 2,852 Runoff The amount of water that drains from the surface relative to coefficient the amount the falls on the surface as rain 7 250 (2,690) 4,000 2,889 3,630 Surface Water collecting on the ground, in a stream, river, lake, 8 280 (3,012) 4,500 3,111 4,037 water wetland, or ocean Vector An organism, such as a mosquito that carries disease-causing microorganisms from one host to another 52 13
  14. 14. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting Handbook 1. Background 5. Cost of installation of RWH systems 1.1 Water is Life! Rainwater harvesting system costs vary considerably de- pending on location, type of materials used, and RWH sys- Water is our most vital natural resource, sup- tem capacity desired. In the Caribbean the cost of a plastic porting life and life support processes. While tank varies between US$0.53/gallon (imp.) to over US$0.74/ there is as much as 1,400 million km3 of water gallon (imp.), depending on size, quality and manufacturer on earth, only one-hundredth of 1% of this (2007 prices). Catchment area costs (e.g. rooftops) and the amount is easily available for human use. The most economical solution for a RWH system will depend amount of water available for each person will on layout, topography, and elevation of the catchment areas with respect to continue to decrease as the world’s population the area allocated for cisterns or storage tanks. The cost for the conveyance expands. Unfortunately our present and future system (guttering and down-pipes) and the first-flush system will vary, based water supplies in many parts of the world are on the configuration or physical layout of the development/household. Thebeing degraded by pollution from domestic waste water, solid waste, indus- most expensive component is always the cistern or storage tank. If incorpo-trial effluent and agricultural drainage to name a few. As our natural waters rating an underground cistern, the cost depends on the size of the cistern,become more polluted, less water is available for our needs and the needs of and the volume and difficulty of excavation. For rocky foundations the exca-the natural environment. Every year, approximately 25 million people die, by vation requirements can inflate installation costs to US$1.11/gallon (imp.)either drinking polluted water or because they do not have enough water to (Daniel and Daniel Engineering Inc., 2006).meet their daily needs. Based on experience in Grenada where construction costs are reflective ofA single person needs at least half a litre (0.11 gallons) per day to meet basic the majority of the Caribbean islands, the average cost (inclusive of materialsurvival needs and two litres (0.44 gallons) per day to avoid thirst. Some 27 and labour) for the installation of reinforced concrete tanks approximatesto 200 litres (6 to 44 gallons¹) are needed per person per day for drinking, US$0.74/gallon (imp.). In addition to material and construction costs, materi-sanitation, bathing and cooking. Household water needs vary depending on als transportation must be factored in. The figures cited above, that is thethe type of dwelling, number of residents and type of plumbing fixtures. US$0.74 to US$1.11/gallon (imp.) range can generally be used as a guideline in estimating the cost for installation of a RWH system. Budgeting at the upperTraditional sources of water to meet our needs typically include surface wa- end of the range should be considered to take into account difficulties thatters (rivers and lakes), ground water (water stored below-ground in aquifers) may likely be experienced during construction, including costly transportationand rainwater. Direct capture, storage and use of rainwater, called Rainwater of construction materials, elevated labour and equipment costs. Tables 5 andHarvesting (RWH) is the oldest method of securing water, having been prac- 6 provide indicative cost for installation of RWH systems.tised by ancient civilisations for more than 4,000 years. This technique con-tinues to be an important means of supplying water in many communities,especially those located far away from municipal potable water supplies.RWH continues to be among the most simple and low-cost means of watersupply, employing technologies that are generally easy to install and maintain. 1 Gallons means Imperial gallons in this document 14 51
  15. 15. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting Handbookprinted on chlorine or bleach containers. Proper hand and eye protection 1.2 Rainwater harvesting in the Caribbeanshould be worn when handling or preparing chlorine solutions to avoid burn- The Caribbean region has a sub-tropical climate with approximately 80% ofing skin and damaging eyes. the annual rainfall concentrated between May to December. Rainfall ranges 4.2.1 Chlorine disinfection procedure from 1,500 mm to in excess of 3,000 mm per year, depending on island size and relief 1. Calculate the volume of water in your tank. 2 . Rainfall accumulations are higher on the 2. Add ½ bottle (125 ml) of plain household-grade unscented and larger, more mountainous islands often in uncoloured bleach (with 4% active chlorine) to every 1,000 litres excess of 2,500 mm per annum in the inte- (220 gallons) of water currently in your tank. Different bleaches rior, with surface and ground waters in rela- have different levels of active ingredient, which is usually marked tively good supply. The smaller, low-lying on the container. islands on the other hand, tend to be rela- 3. Wait 24 hours after putting in the chlorine to allow enough time tively arid with annual rainfall generally less Figure 1 Typical household rainwater to disinfect the water before you drink it. Any chlorine smell and than 1,500 mm. On these islands, free- harvesting in the Caribbean taste in the water will go away after a short time. If you find the flowing surface water is scarce. taste of chlorine unacceptable, an alternative is to boil the water for at least 5 minutes before drinking it. The amount of bleach to Before the advent of centralized conventional potable water supply systems, add, based on a 4% active ingredient, is given in Table 4 below rela- RWH was the only means of water security in all the Caribbean islands. The tive to the amount of water in the tank. practice continues to remain the mainstay of water supply in many of the drier islands such as the Grenadines, the Virgin Islands and the Bahamas, Table 4 Chlorine disinfection mixing ratios for stored water although in some of these islands in recent times, there has been a move away Adapted from SOPAC (2004) from traditional RWH methods to alternative technologies such as desalina- Volume of water in tank Approximate amount of tion and deep-well abstraction. However, these alternative technologies bleach (with 4% active Imp. gallons Litres ingredient) (cups) come at a higher cost, and the sustainability of these operations depends on consumers’ ability and willingness to pay for these services. The practice is 200 909 ½ also declining due to a lack of awareness, knowledge and skills to set up sys- 400 1,818 1 tems. Where investment in relatively expensive, water supply options are not 800 3,637 2 viable, and where appropriate knowledge and skills exist, RWH nonetheless 1,000 4,546 2½ remains an attractive low-cost option to meet shortfalls in water supplies 2,500 11,365 5½ during dry months. 5,000 22,730 11½ 10,000 45,461 22½ RWH has proven utility during times when conventional water supplies have 20,000 90,922 45½ become polluted or unavailable after natural disasters like hurricanes where the municipal water distribution system is damaged and out of commission forDepending on the percentage of active ingredient, the amount to be added extended periods.will need to be adjusted accordingly. For example, using bleach with 8% ac-tive ingredient would halve the amount of bleach listed in the above tables for Rainwater harvesting is not only useful for domestic purposes, but can also bea particular water volume (based on use of 4% active ingredient). The recom- used for agricultural and industrial/commercial applications that have heavymended bleach dosing seeks to ensure that enough chlorine be added to pro- water requirements. RWH will likely see heightened importance as a watervide a free chlorine residual of approximately 0.5 parts per million (0.5 mg/l) security measure in the context of climate change, with the likelihood ofafter 30 minutes. As a general guide, an initial dose of 5 parts per million (5 changing rainfall regimes, prolonged droughts and extreme storm events.mg/l) of chlorine will provide this residual. If desired, the chlorine residual canbe tested using a swimming pool test kit or dip strips, which may be locallyavailable at a pool supplies or hardware store. 2 In Dominica, mean annual rainfall in the interior in in excess of 7000mm 50 15
  16. 16. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting Handbook 4. Managing water quality In general, when the system is in a good shape, and adequately protected from contamination, it is not necessary to disinfect rainwater. Where rain- water harvesting systems are not well constructed or maintained, disinfection is required to safeguard human health. In cases where the water has a bad odour or colour, the tank should be emptied and disinfected. Disinfecting the water before con- sumption is also recommended for Figure 32 A typical ultra-violet lamp on a persons with weakened or compro- RWH installation (before entry to building) mised immune systems, such as the ill, the very young or very old, and recovering patients. Boiling and chlorination are common means of disinfection. Disinfection by ultraviolet (UV) light expo- sure is very effective as well but requires an energy supply. A UV lamp is rela- tively expensive, ranging in cost from US$300 to over US$1,000. Figure 32 Figure 2 Map of the Caribbean shows a typical ultra-violet lamp on a RWH installation. 4.1 Disinfection by boilingThe United Nations Commission on Sustainable Development has called forthe use of rainwater harvesting to supplement water supplies in countries Boiling is the best way to kill potentially harmful agents without the use ofaround the world. chemical additives. It is recommended to boil the water for 3 minutes at 100°C. 4.2 Disinfection by chlorinating Adding small quantities of chlorine to the water tank is the cheapest and most effective means of disinfection. Chlorine is an effective agent against most bacteria and viruses and provides residual protection. However, some bacte- ria can survive chlorination by forming resistant colonies that settle on the tank wall hence the necessity to clean the tank on an annual basis. You should disinfect your water if one or more of the following situations occur: • A known bacterial risk has been identified through water testing; • Individuals are getting sick (sore stomachs; diarrhoea) as a result Figure 3 Old public water supply cistern and a hillside catchment surface both on St. of drinking the water; Thomas, USVI (photos courtesy Henry Smith) • An animal or faecal material has entered a tank; • Inability to completely empty the tank for cleaning. Instructions for disinfecting a rainwater storage tank using household bleach are detailed below. Be sure to read and follow safety and handling instruc- tions. 16 49
  17. 17. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting Handbook• Tree roots can intrude underground masonry including water 1.3 Rainwater harvesting – the pros and cons tanks causing them to leak. Trees in close proximity to the water tanks should be pruned or cut to restrict advancement of As with all water supply options, RWH has many advantages but there a few encroaching tree roots; disadvantages that should be noted. The disadvantages may be minimized• If mosquito breeding is observed in the tank (larvae present), it is with the adoption of a few simple water safety measures. best to seek advice from your environmental health department for assistance on control measures. Advantages:• In the event when the water is contaminated by a dead animal, the tank should be drained immediately, cleaned, and disinfected with chlorine; ☺ Rainwater harvesting provides a source of water at the point where it is• Monitor tanks for leaks and repair as needed; needed. It is owner-operated and managed;• Hygienically store and dispense water within the household. Usu- ally water collected from the storage tank is stored for short ☺ It provides an essential periods in small containers in the home. To protect the quality of reserve in times of emer- this water: gency and/or breakdown of ○ Place storage containers out of reach of small children public water supply systems, and animals particularly following natural ○ Keep containers covered or sealed disasters; ○ Ensure containers are clean ○ Draw water from containers in a hygienic manner. ☺ The construction of a rooftop rainwater catchment system is simple, and these systems can be built to meet almost any requirement; Figure 4 Domestic RWH system with cistern ☺ Households can start with a in St. Thomas (Source: H. Solomon and H. Smith, Univer- single small tank and add sity of the Virgin Islands) more when they can afford them; ☺ Installation of concrete cisterns as part of the building foundation can improve the structural integrity of the building substructure; ☺ The physical and chemical properties of rainwater are often superior to those of groundwater or surface waters; ☺ Operating costs are low; ☺ The construction, operation, and maintenance does not have to be labour-intensive; ☺ Reduces runoff. 48 17
  18. 18. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookDisadvantages: • Refill the tank with water. • Leave the water to settle overnight before use. • Disinfect as outlined in Section 4 to ensure that any contamination Rainwater harvesting depends upon the frequency and amount of rainfall; from entry into the tank is minimized. therefore, it is not a dependable water source in times of prolonged drought; 3.2 Summary: Protecting collected rainwater from Low storage capacity limits the volume of contamination rainwater that can be harvested so that RWH systems may not be able to provide Good system design and proper operation and maintenance will reduce the the required water during prolonged dry possibility of contamination of the water supply. The following are summary periods. Increasing the storage capacity best-practice guidelines in maintaining water quality: raises the construction and operating costs making it expensive (particularly for lower- • Use an appropriate roofing material and ensure that it is kept clear of income households), unless subsidized by dirt and soot. When this material accumulates, it promotes the government; growth of moss, lichen or other vegetation. Use a clean brush to sweep roofs and gutters especially before the start of the rainy Leakage from cisterns can cause the dete- season and at other times as necessary; rioration of load-bearing slopes that • Replace rusted roofing as needed. Fix any holes to realize maximum support the building foundation; runoff. If minor rusting is present, paint using lead-free paint; Figure 5 Cistern serving apart- • Remove branches from overhanging trees to prevent leaf debris from Cisterns and storage tanks can be unsafe ment building in St. Thomas, falling on and accumulating on the catchment area. Branches also for small children if manholes (access USVI. (Photo courtesy Henry Smith) provide roosting for birds (with the increased opportunity for points) are not adequately secured; defecation), and access to the roof by rodents and other animals; • Keep gutters clear. If the gutters sag or leak, they need to be Water can become contaminated by animal waste (bird, bat, rodent repaired. Sagging gutter systems will retain water, providing breeding droppings) and vegetable matter (rotting leaves, fruit); sites for mosquitoes. Leaking gutters will also cause valuable water to be wasted. Ensure guttering is slanted toward the down-pipes to Health risks may result where water treatment is not practised prior to ensure steady flow; use (drinking); • Install a coarse filter and/or first-flush device to prevent dirt and debris entering the tank. Inspect and clean/drain these devices Cisterns and other storage facilities can be breeding grounds for mosqui- periodically; toes where not adequately sealed; • Cover all openings to tanks with mosquito mesh to prevent insects, frogs, toads, snakes, small mammals or birds entering the tank. You Rainwater harvesting systems increase home construction costs and may must inspect and clean the mesh periodically; add significantly to the cost of a building; • Install taps or draw-off pipes above the tank floor to avoid entraining any settled material into the water flow that is to be used; Rainwater harvesting systems may reduce revenues to public utilities and • If the tank is exposed to sunlight, make sure that it is covered or may cause variable demands on public supplies; made of opaque materials or painted with opaque paint. This will exclude light and prevent the growth of algae and micro-organisms; Rainwater is mineral-free, and may cause nutrition deficiencies in people • Clean and disinfect the tank annually (See Section 4). A tank floor who are already on mineral-deficient diets. sloping towards a sump and washout pipe can greatly aid tank clean- ing; 18 47
  19. 19. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting HandbookAdditional risks exist in the case of below-ground storage facilities located inareas that may be prone to flooding. Should polluted flood waters inundate 2. Rainwater harvesting system designunderground storage, this will render the water unusable and will need to bedrained and the tank disinfected. Precautionary structures to exclude flood 2.1 Main RWH componentswaters from contaminating ground storage tanks should be installed in high The typical RWH system has four main parts:flood-risk zones. 1. Catchment area: Commonly a roof surface or pavement. Concrete 3.1.4 Contamination during maintenance (mainly for and other impervious pavements may be used for multiple-user commu- nity systems and can have applications for agricultural or commercial uses masonry cisterns) with large water requirements. Figure 6 illustrates a community catch-When performing maintenance tasks such as de-silting, tank cleaning, cleaning ment surface that provides water to the Ashton community in Unionscreens, catchment surfaces and sediment traps, it is possible to inadvertently Island, St. Vincent and theintroduce these accumulated residues into the system, contaminating it in the Grenadines.process. In cleaning out cisterns, there is a possibility of introducing noxious 2. C o n ve ya n c e sy s tem :substances and faecal matter that may be carried on the soles of ones feet or Network of guttering andon footwear if not careful. Washing feet or footwear with soap is strongly pipes to transfer the rain-recommended before entering the cistern. water from the catchment to the storage tank. This con-The water tank should be cleaned once a year. The following is required: sists of connections to one or • Liquid chlorine/bleach such as Sodium Hypochlorite 3%, 4%, 5%, or more down-pipes connected 5.5% by weight (these are most common on the market) or chlo- to the roof gutters. A key rine tablets such as Calcium hypochlorite and trichloro-s- component of the convey- triazinetrione (40%, 70%, up to 90% active chlorine). If household ance system to improve the bleach is used, it should be the un-scented and un-coloured type. cleanliness of the harvested • Bucket water is a ‘first-flush device’ Figure 6 Surface catchment for communal • Scrub-brush that diverts the dirtiest roof- RWH system at Ashton on Union Island, St. water away from the storage Vincent and the Grenadines • Eye and hand protection (glasses, rubber gloves) tank. • A helper to monitor the person inside the tank (to guard against 3. Storage device: A tank situated above, underneath or partially below accidents) the ground. 4. Distribution system: In the most basic case, this can be simply a con-Cleaning procedure: tainer to extract the water from the storage tank or a pipe functioning • Drain any water in the tank to the level at the tap. If this is being solely as an outlet. For a household, this will be the piping network that done during the dry season to avoid loss, transfer water to a clean, supplies the building with the harvested water. For a community RWH contaminant-free storage or temporary vessel. If tanks are cleaned system, this could be a single outlet pipe or a complex network of pipes during the rainy season, any lost water will soon be replaced. For serving multiple users. A pump may be used to transmit the water two-chambered cisterns, there is the advantage of cleaning one throughout the distribution system. chamber at a time. • Once inside the tank use the scrub brush to thoroughly scrub the A typical RWH system can be complemented by a host of other devices and bottom and sides of the tank. Make sure that ventilation is ade- measures to maintain and improve water quality. These include filters, quate and that the helper is watching should any emergency arise. screens, first-flush diverters and storage facilities with special tank inlet and • Remove the water and bleach solution remaining below the tap outlet configurations. Disinfection, vector control and overflow management with the bucket. measures can also be installed. 46 19
  20. 20. Caribbean Rainwater Harvesting Handbook Caribbean Rainwater Harvesting Handbook 3.1.2 Contamination during rainwater capture(a) Catchment contamination can be chemi- cal or biological. Chemical contamina- tion may occur if the catchment is manufactured from, and/or coated with materials that may leach or dissolve into water flowing across the catchment surface. A common concern relates to the use of roofing sheets coated with Figure 30 An example of a rat guard toxic paints (containing lead and other along a gutter to a storage tank in the compounds). Although the manufacture Grenadines of paints using lead-based pigments was banned several years ago, many older buildings may still have roofs coated with these paints. As noted previously, noxious chemical compounds originating from nearby industrial areas as air-borne emissions may become deposited on catchment(b) surface, posing a contamination hazard. Biological contamination is often associated with either the transmission of harmful pathogens directly from faecal deposits on the catchment surface by birds, bats, rodents, lizards, etc., or from contact by these animals with human faecal matter and subsequent transmission of associated pathogens on to the catchment surfaces. In general, since most faecal pollution from man and animals occurs at or below ground level, water collected from raised roof catchments is likely to be of better bacteriological quality than that collected from ground-level catchments. 3.1.3 Contamination during storage It is important to prevent mosquitoes from breeding inside the storage tank, so as to reduce the likelihood of diseases spread by these vectors such as dengue and malaria. In addition, other animals need to be excluded from ac-Figure 7 Typical domestic RWH systems with (a) above-ground and (b) below-ground cess to the water storage such as vermin (rodents, other insects), frogs andstorage toads, as they too can transmit potentially harmful pathogens. Once storage tanks are fitted with properly fitting lids, all openings are screened, and water extracted using a tap or pump, the likelihood of contamination can be reasonably reduced. With control on the introduction of new contamination (through use of screens and first-flush diverters), any bacteria and other pathogens that are in the tank will eventually die-off resulting in improvement of stored water over time. 20 45
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