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PART C - CASE STUDIES
5.4 Seawater/Brackish Water Desalination by Reverse Osmosis in the
British Virgin Islands
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
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
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 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.
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:
Aqua Design (BVI) Ltd.
- Desalinated seawater=$16.50.
- Desalinated brackish water=$9.10
Ocean Conversion (BVI) Ltd.
- Desalinated seawater=$15.80
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.
- 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
- 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.
- Great care and staff expertise is required to minimize the rate of
- 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.
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
Dean Bedford, General Manager, Aqua Design (BVI)
Limited, Post Office Box 845, Road Town, Tortola, British Virgin Islands.
Government of the British Virgin Islands. 1995. Development Planning
Unit Weekly Bulletins, vols. 1, 2, 27, 28, 29.