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:
· Pre-filtration of the raw water using disposable 5-10 micron polypropylene cartridge filter elements.· Pressurization of the raw water to a pressure of about 1 000 psi, utilizing either positive-displacement or multistage centrifugal pumps.
· Separation of the raw water (approximately 40% of the seawater and 73% of the brackish water) into product water and brine, utilizing spiral-wound membrane elements contained in FRP pressure vessels.
· Recovery of the pressure in the brine, by means of a work-exchanger energy recovery system that significantly reduces energy use.
· Disposal of the spent brine.
· Post-treatment of the product water by chlorination, pH adjustment, and corrosion inhibition so that the final water meets the WHO standards for drinking water supply.
· Distribution of the product water, including metering at the exit of the plant and monitoring of the production process through instrumentation and control of automated plant operations.
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).· Backwashing and flushing of the media filters.
· Replacing cartridge filter elements (approximately every 8 weeks).
· Cleaning the membrane elements (approximately every 4 months).
· Repairing and calibrating instruments.
· Replenishing the pre- and post-treatment chemicals.
· Controlling inventories and ordering spare parts.
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 me 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.· Desalinated seawater = $ 16.50.
· Desalinated brackish water = $9.10Ocean Conversion (BVI) Ltd.
· Desalinated seawater = $ 15.80
Virgin Gorda
Aqua Design (BVI) Ltd.· Desalinated seawater = $ 13.10
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
· Desalinated water is a reliable source of water that is not subjected to seasonal changes in, or locally extreme, weather events.· There is generally minimal usage of chemicals in the process.
· There is minimal environmental impact.
· The plants can be modular in design and easily expanded to meet changing demands.
· Delivery times for modular units and spares are short, typically 3 to 12 months, depending on the location and size of plant.
· If private contractors are used to supply the water, minimal capital investment by the government is required.
· Water price can be fixed and/or linked to inflation for the duration of the agreement.
Disadvantages
· Great care and staff expertise is required to minimize the rate of membrane replacement.· In the case of open-sea intakes, there is the chance of interruptions during stormy weather.
· The sophistication of plant and the high pressures involved require materials and equipment of a very high standard, not usually available locally, which may result in high importation costs.
· There is usually a need for foreign expertise, with a concomitant commitment of foreign exchange.
· There are many dissimilar components used in the plants, so a highly varied spare parts inventory is required.
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.
