As the name implies, dual distribution systems involve the use of water supplies from two different sources in two separate distribution networks. The two systems work independently of each other within the same service area. Dual distribution systems are usually used to supply potable water through one distribution network and non-potable water through the other. The systems would be used to augment public water supplies by providing untreated, or poorly treated, water for purposes other than drinking. Such purposes could include fire-fighting, sanitary flushing, street cleaning, or irrigation of ornamental gardens or lawns. This system has been used in some Caribbean islands like Saint Lucia and the U.S. Virgin Islands.
The systems are designed as two separate pipe networks: a potable water distribution system, and a system capable of distributing sea water or other non-potable waters. The system includes distribution pipes, valves, hydrants, standpipes, and a pumping system, if required. Pipes in the systems are generally cast iron or ductile iron, although fiberglass has also been used.
In seawater-supplied systems, pumps are required to lift the seawater to higher elevation storage tanks. Likewise, pumps may be required to lift wastewaters from wastewater sumps or other collection points. The pumping systems consist of a pumping station containing the water intake, a pumping well, and an elevated storage tank for emergency use. The pumps require foot valves, or one-way valves, in order to retain their charge of water. The water is pumped through a manifold into the secondary or alternative distribution system.
The potable-water, or primary, system operates like any other potable-water supply and distribution system, requiring a water source, treatment plant, storage facility, and distribution system. Pumps are generally required to lift potable water from the treatment plant to storage tanks, from which it is distributed by gravity to the point of use.
Extent of Use
This technology is rarely used. Seawater-based systems have been used in Castries, Saint Lucia, for fire-fighting purposes and in Charlotte Amalie, U.S. Virgin Islands. U.S. Navy bases have installed and operated similar systems in the past. Wastewater-based systems are discussed in Chapter 3, "Wastewater Treatment Technologies and Reuse."
Operation and Maintenance
Depending on the use (i.e., intermittent use in the case of fire-fighting supplies or regular in the case of irrigation supplies) and water source used (e.g., seawater or wastewater), in the dual distribution system, regular testing of the system is recommended. The seawater-based system used in the U.S. Virgin Islands was tested daily in the past, but is now tested once a week. The pumps are turned on and a by-pass is used to allow the return of seawater to the sea to avoid pressurizing the distribution system. The pumps and engines are routinely serviced according to manufacturers' specifications.
Problems experienced in the operation and maintenance of this system include accidental damage to foot valves and standpipes. In the case of seawater systems, ships have been known to damage foot valves located in the harbor, and, in the case of the distribution systems, vehicles frequently damage hydrants and standpipes, which then have to be replaced. In addition, foot valves require frequent servicing to remove fungal and other growths which can prevent their proper opening and closing and can make it impossible for the pumps to maintain their charge. On the landward side, regular inspection and maintenance of the standpipes and hydrants is required to remove debris from the openings of the hydrants and standpipes, which become clogged as a result of vandalism (persons pushing debris into the hydrant openings). It is also necessary to ensure that the pump engines arc supplied with adequate reserves of oil and fuel, and that the pumps and motors are properly lubricated for optimal operation.
Level of Involvement
The systems are entirely a government-run operation in most cases. In Saint Lucia, the fire department had direct involvement in the implementation of this technology, which supplies non-potable water for fire-fighting purposes. Variations on this system, involving the reuse of process water, have been implemented by specific industries as a means of reducing their use of raw water.
The cost of constructing a new distribution system for seawater (capital costs) would be similar to that for laying regular distribution pipelines (approximately $4/ft of pipe). In effect, the installation of a dual distribution system approximately doubles the cost of construction of the distribution system, although some savings may be achieved if the two systems are installed at the same time (instead of in series, with the non-potable system retrofitted into an existing distribution system).
Pumping costs (operation and maintenance costs) are also similar to those incurred by a typical water utility. For systems that are used intermittently, these costs would only be incurred on the few occasions when fire necessitates pumping and/or when pumps are being tested.
Effectiveness of the Technology
This technology is highly effective. Seawater is as effective as potable water when used for fire-fighting purposes, but does not result in the drawdown of potable supplies. The system installed in Castries provides sufficient urban coverage and adequate supplies of water to fight most fires in the city. In contrast, public support for the dual distribution system in the U.S. Virgin Islands has diminished, making the system more prone to vandalism and less effective overall.
The technology is suitable only in areas where a supply of raw water is available. This type of system is generally used near the coast where seawater is abundant, or in places where wastewater is readily available as a source of supply. It can also be utilized in areas that have rivers, streams, or other water sources but lack treatment facilities; in other words, in areas supplied with public water but having access to additional water sources that would otherwise go unutilized or underutilized.
· This technology allows the use of cheaper sources of water for non-consumptive purposes, which may currently be served from more expensive, and limited, potable water supplies.
· If used to augment the regular distribution system, it makes more potable water available to the general public.
· A dual distribution system requires that two distribution systems have to be installed, at essentially double the cost of a single system.
· Having non-potable water in a distribution system creates a potential to cross-contaminate the potable water system (while this is of limited concern in seawater systems, accidental consumption of non-potable water from wastewater-based systems could have serious consequences).
· Use of untreated seawater or wastewater to irrigate leafy vegetables could also threaten human health.
· Seawater can be highly corrosive to metal pipes, fittings, and appurtenances; it increases maintenance costs associated with distribution lines and affects toilet and other metal fixtures that come into contact with the water.
· If return flows enter the wastewater stream, the introduction of large volumes of seawater to treatment plants make sewage treatment more difficult since the salts can impair the effectiveness of activated sludge reactors or rotating biofilters, for example.
This technology is accepted as a alternative for the supply of non-potable water for use in firefighting, street cleaning, etc. It is generally best suited to areas having a plentiful alternative source of water such as seawater or wastewater. In the latter case, concerns about possible human health effects may arise.
Further Development of the Technology
Development and use of non-corrosive materials, such as fiberglass, may make this technology more attractive, especially in cases where seawater is the principal source of non-potable water used in the dual distribution system. The use of alternative materials such as PVC in components such as foot valves might reduce potential for fungal growth and other growths that clog or damage the valves. There is also a great need for public awareness, among users, plumbers, and others, to minimize cross-connections and other potential sources of cross-contamination of the potable water supply.
Vincent Sweeney, do Caribbean Environmental Health Institute (CEHI), Post Office Box 1111, Castries, Saint Lucia, Tel. (809)452-2501. Fax (809)453-2721. E-mail: email@example.com.
Primus Duplessis, Chief Fire Officer, Fire Service, Ministry of Home Affairs, Castries, Saint Lucia. Tel/Fax (809)452-3064.
Henry H. Smith, Director, Water Resources Research Institute, University of the Virgin Islands, #2 John Brewers Bay, St. Thomas, US/Virgin Islands 00802-9990. Tel. (809)693-1063. Fax (809)693-1074. E-mail: firstname.lastname@example.org.