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

PART B - ALTERNATIVE TECHNOLOGIES

4. TECHNOLOGIES APPLICABLE TO SMALL ISLANDS WITH SPECIFIC PROBLEMS OR CIRCUMSTANCES

4.4 Water Conservation

4.4.1 Dual Distribution Systems

Technical Description

Dual distribution systems involve the use of water supplies from two different sources supplied through two separate distribution networks. The two systems work independently of each other within the same service area. Dual distribution systems are normally used to supply potable water (in one distribution network) and non-potable water (in the other network). The non-potable water system would be used to augment public water supplies by providing seawater, untreated or poorly-treated water, and other wastewaters for non-drinking purposes such as fire-fighting, sanitation (flushing toilets), and irrigation. The potable water system would supply freshwater for household use. This type of technology would generally be used near the coast where seawater or brackish water is abundant, although feasibility studies should always be undertaken, and alternative water sources and conservation of existing water sources should be investigated prior to implementing a dual distribution system. Reticulation design standards need special consideration. In particular, rigorous standards must be applied to the selection of construction materials used for a seawater system (due to corrosion problems) and the colour-coding or other identification of pipelines used for a wastewater system (due to potential cross-contamination problems).

Extent of Use

Seawater or brackish water is used for toilet flushing and firefighting on a number of SIDS. These include the US Virgin Islands, St. Lucia, and the Bahamas in the Caribbean; Tarawa in the Republic of Kiribati; and, Majuro in the Marshall Islands. Many islands, particularly the less-developed ones in the Pacific Ocean, also make use of seawater or brackish well water for bathing and some washing purposes (UNESCO, 1991).

Operation and Maintenance

The operation and maintenance of dual reticulation systems is similar to the operation and maintenance of a normal piped reticulation system but double the amount of work. Corrosion-resistant materials must be used for the reticulation of seawater and/or brackish water to minimize maintenance problems. In such systems, precautions should be taken to prevent marine organism growth in the pipes. Prevention of cross-connections and backflows by careful design and automatic valves is essential.

Level of Involvement

Dual water systems in SIDS are usually operated by public utilities corporations with trained engineers and technicians.

Costs

Dual water systems tend to be expensive due to the duplication of distribution networks. Furthermore, because corrosion-resistant materials are required to counter the corrosive effects of seawater or brackish water, the initial capital cost is higher as such materials are generally more expensive than conventional materials (except for some items, common to both applications, such as PVC or polyethylene pipes and fittings). The operation and maintenance of a dual system will cost more than a conventional system.

Suitability

This technology is suitable where a supply of non-potable water such as seawater is available. Rivers, streams, or other water sources not fit for drinking could also be utilised.

Advantages

Use of lesser quality waters for non-potable purposes reduces the use of limited freshwater resources for such purposes.

Disadvantages

Two distribution systems need to be installed, resulting in high capital, operating, and maintenance costs. There are risks of polluting the groundwater when leaks occur, and risks of cross connections. There is also a risk of misuse, especially by children.

Cultural Acceptability

No cultural barriers are immediately apparent, although some cultures have restrictions regarding the reuse of wastewater.

Further Development of the Technology

This is a well known technology, where further development would consist of optimizing the design and the materials.

Information Sources

Rogers, M. 1989. The Public Water Supply in Antigua. APUA Review, 1(3).

Smith, H.H. 1987. Dual Water Systems in the United States Virgin Islands. In: Non-Conventional Water Resources Use in Developing Countries, United Nations Natural Resources/Water Series No. 22, 321-325.

UNDESD [United Nations Department of Economic and Social Development] 1992. Water Resources Management Techniques for Small Islands. United Nations, New York.

UNESCO [United Nations Education Scientific and Cultural Organization] 1991. Hydrology and Water Resources of Small Islands, A Practical Guide. Studies and Reports on Hydrology No. 49, UNESCO, Paris.

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