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<Sourcebook of Alternative Technologies for Freshwater Augumentation in Latin America and The Caribbean> PART C - CASE STUDIES 5.4 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 in the British Virgin Islands on the islands of Tortola and Virgin Gorda. The operations on the island of Tortola may be classified into two different types, based on the source of the feedwater, which is brackish water either from shoreline wells or from alluvial well fields. Of the three plants on the island, the main plant is operated by Ocean Conversion (BVI) Ltd. and obtains its feedwater from wells sunk at the shoreline to a depth of roughly 75 feet. A sanitary seal from ground level to a depth of about 40 feet has been installed. Water is pumped by submersible pumps to the intake of the plant. The two other plants, operated by Aqua Design (BVI) Ltd., obtain their feedwater from either shallow wells dug in the alluvial deposits of the nearby valleys or wells drilled at the shoreline (in the case of the westernmost plant). The two plants operated by Aqua Design (BVI) Ltd. on Virgin Gorda obtain their feedwater from an open-sea intake system. In each case, however, the process of desalination at the plants is generally the same and can be divided into the following stages:
This technology is described in Part B, Chapter 2, "Water Quality Improvement Technologies." Extent of Use All of the public water supply on Tortola, and approximately 90% on Virgin Gorda, is desalinated water. The distribution system covers all areas on the islands below the 300 ft. contour. On Tortola, most of the southern side of the island, from East End, including Beef Island, to Pockwood Pond, is supplied. In the northwest, at Cappoons Bay, desalination plants cover the West End, Carrot Bay, and Cane Garden Bay areas. There are about 4 000 water connections on Tortola, serving a population of 13 500 residents and approximately 256 000 visitors annually. In 1994, the government bought 260.6 million gallons of desalinated water from the two private companies for distribution on Tortola. On Virgin Gorda, the two plants have open-sea water intakes extending about 1 500 feet from the shoreline. One is in the Valley, and the other is in the North Sound. These plants serve a resident population of 2 500 and a visitor population of 49 000 annually. There are 675 connections to the public water supply system. In 1994, the government purchased 20.8 million gallons of water for distribution in Virgin Gorda. Operation and Maintenance On both seawater and brackish water reverse osmosis plants the major maintenance work consists of the following: Maintaining and repairing the equipment, which, in the case of the High School Plant located in Road Town, where the wells and well pumps are operated by the Water and Sewerage Department, consists of weekly routine maintenance to ensure a continuous and adequate flow of water to the plant, and general maintenance (e.g., cleaning, painting, leak repair, cleaning around wells).
The staff required is approximately 1 person for a 200 m3/day plant, and 3 persons for a 4 000 m3/day plant. Level of Involvement Currently, all plants are operated on a BOOT (build, own, operate, transfer) basis by private (generally foreign) companies which finance, operate, and maintain the plants for a fixed period. The price of the product water is fixed for the period of the agreement, although provision is made for adjustment for inflation, and there are penalties for non-performance. The 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, through the Water and Sewerage Department (WSD), is responsible for the disinfection of the final product water. On Tortola, the WSD is also responsible for the operation and maintenance of the product water pumps at the exit of the plants, and it owns and operates the well fields that serve the westernmost plant. At the Ocean Conversion plant, located close to the Water and Sewerage Department head office, two technicians from the Department have been involved in the operation of the plant from the time of its commissioning. They are paid a monthly stipend as part of the contractual arrangement, and are called in to assist with repair work as and when required. The government also provides the land, tax relief and custom exemptions; buys the bulk water; and monitors the product water quality. 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 20 000 gpd is approximately $200 000. For a plant of 1.0 mgd, the cost is approximately $4 500 000. The major operating costs consist of energy (primarily), labor, replacement membranes, and spare parts. Energy consumption ranges 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 per 1 000 gallons: Tortola Aqua Design (BVI) Ltd.
Ocean Conversion (BVI) Ltd.
Virgin Gorda Aqua Design (BVI) Ltd.
In 1994, the Government of the BVI bought 260.6 million gallons of water from the desalination companies for distribution on the island of Tortola at a cost of $3 611 000. On Virgin Gorda, after desalinated water became available to the public during February 1994, the Government of the BVI bought 20.8 million gallons at a cost of $485 000. Effectiveness of the Technology The seawater/brackish water reverse osmosis technology is very effective at converting Caribbean Sea water to potable water, meeting the WHO standards for drinking water, with a total dissolved solids level of less than 500 mg/l. Suitability The technology is suitable for use throughout the Caribbean Basin, provided there is a source of clean raw water, either from boreholes or from open-sea water intakes. The technology is particularly suitable for use in areas where the freshwater resources are inadequate to meet growing demands and the centers of population are concentrated close to the coast. In considering the use of seawater in desalination, the seawater should be free from pollutants, 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 brine effluent should be carefully considered as this can have adverse effects on sea life. Advantages
Disadvantages
Further Development of the Technology The seawater/brackish water reverse osmosis technology would be further improved by the development of membrane elements that are less prone to fouling, operate at lower pressures, and require less pre-filtration, and by the introduction of highly efficient energy recovery technologies that are simpler to operate than the existing work-exchanger technologies. Information Sources Contacts Rajkumar Roopchand, Head Engineer, Operations and Maintenance Division, Water and Sewerage Department, Ministry of Communications and Works, Road Town, Tortola, 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) Limited, Post Office Box 845, Road Town, Tortola, British Virgin Islands. Bibliography Government of the British Virgin Islands. 1995. Development Planning Unit Weekly Bulletins, vols. 1, 2, 27, 28, 29.
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