Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for
in Small Island Developing States>
PART B - ALTERNATIVE TECHNOLOGIES
4. TECHNOLOGIES APPLICABLE TO SMALL ISLANDS WITH SPECIFIC PROBLEMS
4.4 Water Conservation
4.4.1 Dual Distribution Systems
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.
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
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.
Use of lesser quality waters for non-potable purposes reduces the use of
limited freshwater resources for such purposes.
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.
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.
Rogers, M. 1989. The Public Water Supply in Antigua. APUA Review,
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,
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.