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Newsletter and Technical Publications

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


PART C - CASE STUDIES

5.2 Seawater/Brackish Water Desalination by Reverse Osmosis in the British Virgin Islands

Technical Description

Desalination by reverse osmosis for public water supply is carried out on the islands of Tortola and Virgin Gorda, British Virgin Islands (Figure 39). The operations on the island of Tortola can be classified into two types based solely on the way the source water or feedwater is obtained. There are three plants on the island of Tortola. At the main plant, operated by Ocean Conversion (BVI) Ltd, the feedwater is obtained from wells sunk at the shoreline to a depth of roughly 25 m, with a sanitary seal from ground level to about 12 m. Water is pumped via submersible units to the inlet of the plant. The two other plants are operated by Aqua Design (BVI) Ltd. One uses brackish water obtained from shallow wells dug in the

Figure 39
(larger image)
Figure 39. Map of the British Virgin Islands.

alluvial deposits of the nearby valleys as feedwater, while the second, on the western end of Tortola, uses feedwater obtained from wells drilled at the shoreline. The two plants operating in Virgin Gorda are seawater desalination plants and are of an open-sea intake design. In each case the process of desalination at the plants is generally the same and can be divided into the following elements: pre-filtration using disposable 5 æm to 10 æm polypropylene cartridge filter elements; pressurisation to about 7 000 kPa; separation (at an efficiency of approximately 40% for the seawater-fed systems and 73% for the brackish water-fed systems) utilizing spiral-wound membrane elements contained in FRP pressure vessels; pressure recovery from the waste brine solution by means of work-exchange energy recovery system (which significantly reduces energy usage); brine disposal; post-treatment of the product water by means of chlorination, pH adjustment, corrosion inhibition, such that the final water meets all WHO standards for drinking water; metering of the product water at the exit of the plant; and, distribution of the product water.

Extent of Use

All of the water supply areas on Tortola, and approximately 90% of the water supply areas on Virgin Gorda, are supplied with desalinated water. In general, the water supply covers all areas below the 90 m contour. On Tortola, most of the south side of the Island, starting from East End and including Beef Island to Pockwood Pond on the South West, is supplied. In the North-West of Tortola, at Cappoons Bay, where one of the seawater desalination plants is located, the supply covers the West End, Carrot Bay and Cane Garden Bay areas. On the island of Tortola, there are about 4 000 water connections serving a population of 13 500 residents and a visitor population of approximately 256 000 annually. In 1994, the Government bought 1.2 million m3 of desalinated water from the two private companies for distribution on the island of Tortola. On Virgin Gorda, where are two seawater desalination plants operated by Aqua Design (BVI) Ltd., both plants have open seawater intakes extending about 1 500 m from the shoreline. One is in the Valley, and the other is in North Sound. These plants serve a resident population of 2 500 and a visitor population of 49 000 annually. There are 675 water connections to the public water supply system in Virgin Gorda. In 1994, the Government purchased 95 000 m3 of water for distribution in Virgin Goda.

Operation and Maintenance

The major maintenance work on seawater reverse osmosis and brackish water reverse osmosis plants consists of maintenance and repair of the rotating equipment (consisting primarily of pumps); backwashing and flushing of the media filters; replacement of the cartridge filter elements (approximately every 8 weeks); cleaning of the membrane elements (approximately every 4 months); instrument repair and calibration; general maintenance (e.g., cleaning, painting, leak repair, cleaning around wells, etc); replenishment of the pre- and post-treatment chemicals; and inventory control and ordering of spare parts. In the case of the High School plant, located in Road Town, the wells and well pumps are operated by the Water and Sewerage Department, who conducts routine, weekly maintenance to ensure a continuous and adequate flow of water to the plant. Staffing levels are approximately 1 person for a 200 m3/day plant, and up to 3 persons for a 4 000 m3/day capacity plant

Level of Involvement

Currently, all plants are operated on a BOOT (build, own, operate, transfer) basis by private companies (generally foreign), which provide financing, operation, and maintenance for a fixed period. The price of water is fixed for that period by agreement with periodic adjustment for inflation. There are penalties for non-performance. Contracts prescribe a minimum quantity of water which the Government is obligated to buy. At two of the five plants operating on the islands, the government Water and Sewerage Department (WSD) is responsible for the disinfection of the final product water. At the plants on Tortola, the WSD is responsible for the operation and maintenance of the product water pumps at the exit of the plants. At the brackish water plant, the WSD owns and operates the wells that serve the plant and ensures an adequate and continuous flow of feedwater. The ocean conversion plant, located close to the WSD head office, has employed two technicians from the Department in the plant operation since its commissioning. A monthly stipend is paid as part of the contractual arrangements and these individuals are called in to assist with repair work as and when required. The government also provides the land, tax and custom exemptions, cost of the bulk water received, and monitoring of the quality of the product water. The WSD distributes the water.

Costs

The unit cost of production of desalinated water decreases as the plant capacity increases. The turnkey capital cost of a plant of 90 m3/day is approximately $4.5 million. The major operating costs consist of energy, labour, membrane replacement and spare parts. Energy is the primary operating cost, with consumption ranging from 3 to 6 KWh/m3 of potable water produced, depending on the size of the plant and the technology employed. Under the current purchase agreements, the companies maintain and operate the plants at their own cost and sell water in bulk to the government at the following rates: on Tortola, water is purchased from Aqua Design (BVI) Ltd at a rate of $3.60/m3 for desalinated seawater and $2.00/m3 for desalinated brackish water, and from Ocean Conversion (BVI) Ltd at a rate of $3.50/.m3 for desalinated seawater; and, on Virgin Gorda, water is purchased from Aqua Design (BVI) Ltd at a rate of $2.90/m3 for desalinated seawater. The government, through the Water and Sewerage Department, disinfects and distributes the product water. In 1994, the government bought 1.2 million m3 of water from the desalination companies for distribution on the island of Tortola at a cost of $3,611,000. On Virgin Gorda, desalinated water became available to the public from February 1994 and 95 000 m3 was bought at a cost of $485 000.

Effectiveness of the Technology

Seawater/brackish water reverse osmosis is very effective in reducing Caribbean seawater to potable water quality. The product water has a total dissolved solids level of less than 500 mg/l and meets all other requirements of the World Health Organization for drinking water. Suitability The technology is suitable for water supply throughout the Caribbean Basin, provided that there is a source of clean seawater, either by means of vertical boreholes or open-sea intakes. The technology is particularly suitable where the fresh water resources are inadequate to meet growing demands and the centres of population are concentrated close to the coast. In considering the end use of the desalinated water for potable purposes, the raw water should be free from pollution, especially from land-based industries, and the intake should be located in an area with little chance of pollution by ocean-going vessels. The disposal of the effluent brine should be carefully considered as this can have adverse effects on sea life.

Advantages

Desalination is a reliable source of water that is not subjected to the seasonal changes or locally extreme weather events associated with freshwater sources. There is minimal use of chemicals in the process, and the plants are modular in design and can be easily expanded. If properly operated, there is minimal environmental impact. Delivery periods are short, typically 3 to 12 months, depending on the location and size of plant, and, if private contractors are used to supply the water, there is minimum capital investment required by the government. Under such arrangements, there is an additional advantage of a fixed water price, linked only to inflation, for the duration of the agreement.

Disadvantages

Reverse osmosis systems require care and expertise to minimize the rate of membrane replacement; the sophistication of plant operation require materials and equipment of very high standard not usually available locally resulting in high importation costs. There is usually a need for foreign expertise and resulting loss of foreign exchange. There are also many dissimilar components on the plants, so a highly varied spare parts inventory is required. In the case of open-sea intakes, there is the chance of interruptions during stormy weather.

Further Development of the Technology

The seawater/brackish water reverse osmosis would be further improved through the development of membrane elements that are less prone to fouling, operate at lower pressures, and require less pre-filtration; and, of more highly efficient, energy recovery technologies that are simpler to operate than the existing work-exchanger technology.

Information Sources

Contacts

Rajkumar Rooppchand, Engineer, Water and Sewerage Department, Ministry of Communications and Works, British Virgin Islands.

William T. Andrews, Managing Director, Ocean Conversion (BVI) Ltd, Post Office Box 122, Road Town, Tortola, British Virgin Islands.

Dean Bedford, General Manager, Aqua Design (BVI) Ltd, Post Office Box 845, Road Town, Tortola, British Virgin Islands.

Bibliography

WEDC s.d. Developing World Water. Grosvenor Press International, London.

Government of the British Virgin Islands 1995. Development Planning Unit Weekly Bulletins, Vol. 1, 21, 27, 28, 29, January-July, 1995. Developing Planning Unit, British Virgin Islands.

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