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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation
in Small Island Developing States>


PART B - ALTERNATIVE TECHNOLOGIES

1. TECHNOLOGIES GENERALLY APPLICABLE TO ISLAND STATES

1.1 Freshwater Augmentation Technologies

1.1.1 Rainwater Harvesting

Technical Description

A rainwater harvesting system consists of a rainwater catchment surface, conveyance system, and water storage tank(s). Figure 8 shows a schematic of a typical rainwater catchment system.

Rainwater harvesting systems can serve households or communities of various sizes. Household systems generally catch rain from the rooftops of homes and store it in tanks adjacent to the homes. Water is drawn from the tanks by means of taps at the base of the tanks. In some cases rainwater may be reticulated within a house using a pump/pressure system. Alternatively the tank may be partly buried and a handpump used to withdraw water. In cases where the majority of homes have thatched roofs, community systems are popular. The roofs of large community buildings, such as churches and schools, are used as catchment surfaces and the water is stored in large tanks adjacent to these buildings (Figure 8). Alternatively, if no suitable catchment surface is available, a separate catchment surface is built adjacent to, or directly over, the water storage tank. Residents of the community walk to these tanks, draw water from a tap at the base of the tank, and transport it back to their homes for drinking or cooking. In some cases, individual homes with thatched roofs have also built separate catchment surfaces serving household storage tanks.

Materials commonly used in the construction of the roofs are corrugated aluminium and galvanized iron, concrete, asphalt or fibreglass shingles, tiles with a neoprene-based coating, and mud, which is used primarily in rural areas. Roofs are generally sloped to avoid ponding and roof coatings are required to be non-toxic. The effective roof area and the material used in constructing the roof influence the collection efficiency and water quality. The growing number of permanent-material roofs, as opposed to straw, grass, or palm leaves, is encouraging rainwater harvesting in many countries. Because natural roofing materials attract rodents and insects, and often yield contaminated and coloured water, most people find them objectionable for use as a collecting surface. In such cases, specially-constructed ground surfaces (concrete, paving stones, or some kind of liner) or paved runways can also be used to collect and convey rainwater to storage tanks or reservoirs. These surfaces should be fenced to prevent the entry of people and animals.

Conveyance systems usually consist of gutters and drain pipes that deliver rainwater from the catchment area into the storage tanks. The conveyance systems should be of inert material to avoid adverse affects on water quality. Ground catchments would normally use pipes and/or open channels to convey rainwater to the storage tanks/reservoirs.

The rainwater ultimately is stored in a storage tank, which should also be constructed of inert material. Reinforced concrete, ferrocement, fibreglass, polyethylene, or stainless steel have been found to be suitable. Storage tanks may be constructed as part of a building or may be a separate unit some distance away. In Bermuda and the United States Virgin Islands, where most buildings are designed with rooftop catchments, storage tanks are usually constructed under the building. These tanks are built in two sections to facilitate cleaning. On a large number of islands in the Pacific Ocean and the South China Sea, ferrocement tanks are becoming increasingly popular.

Figure 8
Figure 8. Schematic of a rainwater catchment system.

Important factors to incorporate into the design of a storage tank include adequate capacity; division of tank into two sections or dual tanks to facilitate cleaning (when possible); provision of a sloped bottom and provision for collection of settled grit and sediment; overflow protection; inclusion of a manhole for easy access for cleaning; provision of a vent for air circulation (often the overflow pipe); and, protection against insects and rodents. A water-level indicator is an optional refinement of this technology.

The quantity of water available from a rainwater harvesting system depends on the size of the catchment surface, the percentage catchment surface area that is guttered, the efficiency of the gutters in transporting the water, and the size of the storage tank. If a catchment surface is too small, it may not provide sufficient water to fill the tank. Furthermore, the rainfall pattern and user-demand are also factors that must be taken into account.

Extent of Use

Rainwater collection systems are extensively used by most SIDS, especially those low-lying islands where rainwater catchments constitute the major part of the water supply for the inhabitants. This supply will often be supplemented by groundwater. In St. Lucia, the type of tank varies from 200 l, used oil drums, to varying sizes of polyethylene plastic tanks (usually 1 300 to 2 300 l), to underground concrete tanks with capacities of 100 m3 to 150 m3. In the Turks and Caicos Islands, there are a number of Government-built, public rainwater catchment systems. Government regulations make it mandatory that all developers construct a water tank large enough to store 400 l/m2 of roof area.

Rooftop and purpose-built catchments also are common place in the Bahamas. One settlement (Whale Cay) has a piped distribution system based on rooftop-collected water. On New Providence, most of the older houses collect rooftop rainwater and store it in tanks averaging 70 000 l capacity. Industrial use of rooftop-collected rainwater is also practised, and a preliminary assessment of rainwater harvesting from the impervious surfaces of Nassau International Airport has been carried out. In multi-storied apartment buildings and other areas serving large concentrations of people (such as hotels and restaurants), water augmentation using rainwater from rooftop storage tanks has been supplemented.

The Islas de la Bahia, off the coast of Honduras, meet a substantial proportion of their potable-water needs from rooftop catchments. Rainwater catchment systems are practically the sole water supply source for a small group of islands north of Venezuela; these arid islands experience only 500 mm to 700 mm of rainfall per year, and have largely saline groundwater reserves which cannot be used for potable purposes.

Rainwater harvesting is widely used in Male, as well as on the other islands of the Maldives. Rainwater is harvested and stored in both public and private tanks. In Male, the public rainwater storage tank is 8 500 m3, which provides an average water supply of 0.14 m3/person. The private rainwater storage capacity is estimated to be approximately 10 000 m3, or 0.16 m3/person

In the Federated States of Micronesia, in response to a 1982 cholera epidemic on Chuuk, the State government constructed 14 m3 ferrocement tanks for all of the households on several islands. On Majuro, Marshall Islands, a rainfall catchment system was installed at the airstrip to serve about 14 000 people. Adjacent reservoirs, into which the rainwater is pumped from the catchments, have capacities of about 57 000 m3 of raw water and 7 600 m3 of treated water. On the high islands, rainwater harvesting is often used to supplement drinking and cooking water.

Operation and Maintenance

Rainwater harvesting systems require minimal attention with respect to their operation. Contamination of water as a result of contact with certain materials can be avoided by using appropriate materials during construction, or selecting tanks made from acceptable materials. The major concern is to prevent the entry of contaminants into the tank while it is being replenished during a rainstorm. Bacterial contamination can be minimized by keeping the rooftop surfaces and drains clean. The main causes of bacterial pollution are from debris, bird and animal droppings, and insects that enter the tank. The following maintenance guidelines should be considered in the operation of rainwater harvesting systems:

- Use an installed first flush (or foul flush) device, which directs the initial batch of rainwater away from the tank to avoid the entry of debris from the catchment area into the tank.

- Check and clean the storage tank periodically. All tanks need cleaning, and their designs should allow for the thorough scrubbing of the inner walls and floors using a chlorine solution (followed by thorough rinsing). This sometimes can be difficult to accomplish without emptying the tank.

- Cover and ventilate the tank to avoid mosquito breeding, prevent insects and rodents from entering the tank, and minimise the growth of algae.

- Chlorinate the storage tanks as necessary if the stored water becomes contaminated using slow-release chlorine tablets. (Most times the rainwater is used without treatment.)

- Maintain gutters and downpipes. A good time to inspect gutters and downpipes is while it is raining, so that leaks can easily be detected. Regular cleaning is necessary to avoid contamination.

Community systems require community involvement and organisation for effective maintenance, while household systems require a correspondingly smaller scale involvement by residents. In some cases, where the water is pumped, periodic, preventive maintenance is required on the small pumps that lift water to selected areas of a house or building, or provide public supply from underground storage tanks; more-often-than-not, however, only repairs after breakdowns are done. Additional requirements for ground catchments include fencing the paved catchment to keep out trespassing animals (primarily small livestock such as goats, cows, donkeys and pigs) that can affect water quality, cleaning the paved catchment of leaves and other debris, and repairing large cracks in the paved catchment that result from soil movements, earthquakes, and/or exposure to the elements

Problems commonly encountered in maintaining the system at an efficient operating level are the lack of availability of chemicals required for appropriate treatment and the lack of adequate funding.

Level of Involvement

Household rainwater catchment systems can be constructed by, and maintained at, the community or individual level, with a minimum of training by skilled technicians. Construction and maintenance of ground catchment systems (i.e., airstrip, roads, and specially constructed surfaces) require skilled engineers and operators.

Costs

The cost of this technology varies considerably depending on location, type of materials used, and level of implementation. The components that need to be costed include roofing materials; gutters; conveyance pipes; and, storage tanks.

Storage tanks costs (1995 prices) average $125 for a 1 500 l plastic tank in St Lucia. In the Federated States of Micronesia, storage tank construction costs range from $126 for a 1.3 m3 ferrocement tank with corrugated metal cover to $327 for an 11.4 m3 ferrocement tank with concrete cover. Intermediate-sized tanks range in cost from $148 for a 1.9 m3 ferrocement tank with corrugated metal cover to $300 for a 5.3 m3 ferrocement tank with concrete cover. In the Maldives, a 10 m3 ferrocement tank costs $1 500 to construct, while a 2.5 m3 HDPE tanks costs $ 500. In The Philippines, the cost per m3 capacity of locally-constructed rainwater tanks is $48 for ferrocement tanks, $38 for inter-locking block tanks, and $70 for reinforced concrete tanks.

Effectiveness of the Technology

In most cases, a rainwater catchment system cannot meet demand during extended dry periods. Hence, it is often be necessary to have an alternative source to supplement the rainwater supply. Notwithstanding, rainwater may be the only available source of water in some locations.

Suitability

This technology has good potential for use in the very small islands, and, in many cases, could be the only option for a low-tech, economical alternative to meeting freshwater demand. It also has good potential for community-based system management. This technology is best suited to islands that have evenly-distributed rainfall throughout the year.

Advantages

Rainwater harvesting systems provide water at the point where it is needed, and the systems may be owner operated and managed. The systems also provide a water supply buffer for use in times of emergency or breakdown of the public water supply systems, particularly during natural disasters. The systems also provide a good supplement to other water sources, and relieves pressures on other water sources. Construction, operation, and maintenance are not labour intensive, and the physical and chemical properties of the rainwater may be superior to those of groundwater or desalinated water that may have been subjected to contamination.

Disadvantages

Rainwater harvesting systems are completely dependent upon the frequency and amount of rainfall. There will be shortages during dry spells or prolonged droughts, which can be exacerbated by low storage capacities. If greater storage capacities are provided, the additional construction and operation costs may be too expensive for some households. Hence, drought management may be required. Also the water may become contaminated if the storage tanks are not adequately covered, and uncovered or poorly covered storage tanks can be unsafe for small children. Contamination can also occur from dirty catchment areas. Water treatment is infrequent in many countries due to lack of adequate resources, and lack of treatment may lead to health risks. In the case of community systems, people have to walk significant distances for water if a distribution system is not in place. All systems require maintenance to minimise wastage through broken gutters, drainpipes, leaking storage tanks or outlet taps.

Cultural Acceptability

Rainwater-catchment systems encourage users to conserve water, as the responsibility of the operation and maintenance rests with the individual user.

Further Development of the Technology

Rainwater harvesting is a very well-developed technology, with only minor improvements being considered necessary. There are however a large number of factors which could contribute to the optimisation of this technology. In order to optimise the rainwater catchment systems, there are various analyses techniques that can be used that require daily rainfall data. Where rainfall data are not available, the establishment and operation of a rain gauge should be encouraged. If possible, evaporation information should also be collected. In addition, the transfer of data and designs between planners and designers in the region should be encouraged.

In the design-phase, detailed investigations should be conducted to determine the actual demand for rainwater. This can be done through questionnaires and surveys. It should be noted that it may be impossible to fully meet demand, especially during dry periods. Simple procedures for control the use of rainwater supplies during droughts need to be developed. A procedure where water rationing begins when the water level in the tank drops below a certain point is necessary to avoid emptying the tank during extended dry periods. Investigations should also be made into the most appropriate measures to be used for the collection of rainwater (i.e., into the type of materials used, the sizes of buildings, etc). Collection from flat roofs is discouraged as algae and dirt can collect on these surfaces and contaminate the runoff. Sometimes unpainted roofs are preferred as some paints contain toxic elements. In all cases, the concept of first flush (foul flush), where the initial batch of rainwater is directed away from the tank, should be supported and simple, easy-to-operate flushing devices provided. Identification of the various types of first-flush devices currently in use throughout the region should be undertaken, and assessments of appropriate types carried out to improve them. Likewise, more emphasis should be placed on designing cost-effective guttering. Guttering needs to be adequate and simple to collect rainwater from a sufficient area relative to the size of the tank to ensure optimal filling of the storage tank (often not enough roof area is guttered). Gutters need to be properly designed and fixed (structural and hydraulic problems are often found).

There are many different tank designs, and a number of different materials are used in tank construction, such as ferrocement, HDPE (black polyethylene), galvanised iron, and fibreglass. Some standardisation of materials, at least at the village level, may be desirable from a maintenance and replacement point of view. Also, the volumes of the storage tanks need to be based on rainfall analyses and predicted water usage, although other factors such as cost, ease of construction, and cultural considerations may limit the size. It may be also appropriate to standardise the volume of the tank, at least at the village level. Greater involvement of the public health department in the monitoring of water quality should be promoted.

Ensuring adequate operation and maintenance of the rainwater systems may be a problem. Continuous and repetitive public information campaigns and training is required. Some methods for increasing the impact of the public information and training programmes are to use influential people (e.g., pastors, doctors, teachers, and health educators) in the programming; to establish village-based water committees (also useful for promoting safe and adequate sanitation); and, to include the training as part of the educational curriculum in schools. User fees may be appropriate for community rainwater systems, as the revenue could cover the cost of maintenance.

Information Sources

Anon. 1982. Proceedings of the International Conference on Rain Water Cistern Systems. Water Resources Research Center, University of Hawaii, Honolulu.

Anon. 1984. Proceedings of the Second International Conference on Rain Water Cistern Systems. American Society of Civil Engineers, Washington.

Anon. 1987. Proceedings of the Third International Conference on Rain Water Cistern Systems. Khon kaen University, Thailand.

Anon. 1989. Proceedings of the Fourth International Conference on Rain Water Cistern Systems. Canadian International Development Agency , Ottawa.

Anon. 1991. Proceedings of the Fifth International Conference on Rain Water Cistern System. National Taiwan Ocean University, Keelung, Taiwan.

Barbados Water Authority Sub-Committee on Water Resources 1994. Report on Increase in Supply - Use of Roof and Parking Lot Catchment Barbados Water Authority, Bridgetown.

Chapman, T.G. 1986. Design of Rainwater Tank Systems for the Tuvalu Islands. Unisearch Limited and Australian Development Assistance Bureau, Canberra.

Dillaha, T.A. and W.J. Zolan 1987. An Investigation of the Water Quality of Rooftop Rainwater Catchment Systems in Micronesia. Technical Report No. 45, Water and Energy Research Institute of the Western Pacific, University of Guam, Guam.

Edwards, D. and K. Keller 1984. A Workshop Design for Rainwater Roof Catchment Systems: Training Guide, and Appendix - Rainwater Harvesting for Domestic Water Supplies in Developing Countries. UNESCO/UNICEF Water and Sanitation Technical Report No. 27, La Paz, Bolivia.

ESCAP [Economic and Social Commission for Asia and the Pacific] 1989. Rainwater Harvesting Techniques and Prospects for Their Application in Developing Island Countries. United Nations Water Resources Series No. 63, 101-118.

Edwards, D., K. Keller, and D. Yohalem 1984. A Workshop Design for Rainwater Roof Catchment Systems: A Training Guide. UNESCO/UNICEF Water and Sanitation Technical Report No. 27, La Paz, Bolivia.

Fleming, S. 1987. Basic Water Supply and Sanitation. Women's Development Training Programme, University of South Pacific, Tonga.

Gonguez, P. 1980. Water Supplies in Off-shore Islands and Coastal Communities. In: Proceedings of the United Nations Seminar on Small Island Water Problems. Commonwealth Science Council, London.

Hadwen, P. (Ed.) 1980. Proceedings of the United Nations Seminar on Small Island Water Problems. Commonwealth Science Council, London.

Hadwen, P. 1987. Caribbean Islands: A Review of Roof and Purpose Built Catchments, In: Non-Conventional Water Resources Use in Developing Countries. Natural Resources/Water Series No. 22.

Hart, C. 1990. Western Samoa Taps the Sky. Source, 2 (1):9-11.

Heitz, L.F. and S.J. Winter 1996. Designing Your Rainwater Catchment and Storage System. Water Information Bulletin No. 1, Water and Energy Research Institute of the Western Pacific, University of Guam, Guam.

Lee, M.D. and J.T. Visscher 1992. Water Harvesting: A Guide for Planners and Project Managers. Technical Paper Series No. 30, International Water and Sanitation Centre, The Hague.

Michaelides, G., M. Allybokus, and R.J. Young 1986. Optimised Design and Water Quality Studies of Roof Top Rainwater Catchment Project in Mauritius. Water Supply, 4:117-122.

Mourits, L.J.M. and P.B. Kumar 1995. Rainwater Utilization in Rural Fiji. Waterlines, 14(2):8-10.

Pacey, A. and A. Cullis 1986. Rainwater Harvesting: The Collection of Rainfall and Runoff in Rural Areas. Intermediate Technology Publications, London.

Pieck, C. 1985. Catchment and Storage of Rainwater. TOOL/TWO, Amsterdam.

Reller, R. 1982. Rainwater Harvesting for Domestic Water Supplies in Developing Countries. Working Paper No. 20, US Agency for International Development Water and Sanitation for Health Project, Washington.

Rinehart, F. 1983. Water Quality of Cistern Water in St. Thomas, United States Virgin Islands. Technical Report No. 15, Caribbean Research Institute, College of the Virgin Islands.

Ruskin, R., et al. 1988. Maintenance of Cistern Water Quality in the Virgin Islands. Technical Report No. 30, Water Resources Research Institute, University of the Virgin Islands.

Stephenson, R.A. and H. Kurashina 1983. A Comparison of Water Catchment and Storage Systems in Two Micronesian Communities: Laura and Nama. Technical Report No. 50, Water and Energy Research Institute of the Western Pacific, University of Guam, Guam.

Smith, H.H. 1983. Effects of Various Factors on the Sizing of Rain Water Cistern Systems. Technical Report No. 19, Caribbean Research Institute, College of the Virgin Islands.

Thomas, E.N. 1980. The Artificial and Roof Rainwater Catches of Bermuda. In: Proceedings of the United Nations Seminar on Small Island Water Problems. Commonwealth Science Council, London.

UNDTCD [United Nations Department of Technical Cooperation and Development] 1989a. Roof Catchments, Roof Coverings, Guttering and Downpipes. United Nations Water Resources Assessment and Planning in Pacific Islands Project No. RAS/87/009, United Nations Development Programme, New York.

UNDTCD [United Nations Department of Technical Cooperation and Development] 1989b. Ferrocement Rainwater Tanks. United Nations Water Resources Assessment and Planning in Pacific Islands Project No. RAS/87/009, United Nations Development Programme, New York.

USP [University of the South Pacific] 1988. Building a Ferrocement Water Tank. Institute of Rural Development Transfer of Appropriate Technology Leaflet No. 9, University of the South Pacific, Suva.

UNEP [United Nations Environment Programme] 1983. Rain and Stormwater Harvesting in Rural Areas. United Nations Environment Programme Water Resources Series, 5:92-113.

Watt, S.B. 1978. Ferrocement Water Tanks and Their Construction. Intermediate Technology Publications, London.

Winter, S. 1988. Construction Manual for a Ferrocement Rainwater Storage Tank. Appropriate Technology Enterprises for Integrated Atoll Development Project, United Nations Development Programme, Suva.

Winter, S.J. and B.L. Campbell 1994. (Video) Water for Your Island: A Guide to Water Supply for the Remote Islands of the Federated States of Micronesia. Water and Energy Research Institute of the Western Pacific, University of Guam, Guam.

Winter, S. J. and B.L. Campbell 1995. Water Supply for Remote Tropical Islands C A High School Teaching Supplement. Water and Energy Research Institute of the Western Pacific, University of Guam, Guam. (includes a companion slide presentation).

World Health Organisation 1983. Manual on Construction of Cook Islands' Modular Water Tanks. Regional Office for the Western Pacific, World Health Organization, Geneva.

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