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Home > Different Processes Used In Different Countries > Seawater

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

Reverse osmosis or RO is a filtration or separation method, wherein many types of large molecules and ions are separated or removed from a solution by exerting pressure on the solution. This is done by keeping the solution on one side of a selective or a semipermeable membrane. This way the solute is retained on the pressurized side of the porous membrane, whereas the pure solvent is allowed to freely pass to the other side of the same membrane through small pores. Using reverse osmosis or RO treatment to desalinate public water supply is a common water purification process implemented in the British Virgin Islands on the islands of Tortola and Virgin Gorda.

On the islands of Tortola, feedwater is either brackish water from shoreline wells or brackish water from alluvial well fields. There are three plants operating in the island, one under Ocean Conversion (BVI) Ltd., and the remaining two are operated under Aqua Design (BVI) Ltd. Ocean Conversion (BVI) Ltd. gets its feedwater from wells sunk at the shoreline to a depth of roughly 75 ft, whereas feedwater of the plants under Aqua Design (BVI) Ltd., comes either from wells drilled in the alluvial deposits of the nearby valleys or wells installed at the shoreline. Aqua Design (BVI) Ltd. manages two other plants on Virgin Gorda, where feedwater is exclusively derived from an open-sea intake system. Though feedwater obtained is different for different plants of British Virgin Islands, the process of desalination employed in all of them is more or less the same.

The Desalination Process - Stages Involved

Generally, seawater/brackish water desalination is carried out at the plants in the following manner:
  • First of all, disposable 5-10 micron polypropylene cartridge filter elements are used for pre-filtering the raw water.
  • Next, raw water is subjected to a pressure of about 1 000 psi, with the help of positive-displacement or multistage centrifugal pumps.
  • The raw water is then separated into product water and brine using spiral-wound membrane elements contained in FRP pressure vessels.
  • A work-exchanger energy recovery system is then used to recover the pressure in the brine.
  • The spent brine is disposed off and the product water undergoes chlorination, pH adjustment, and corrosion inhibition.
  • Finally, the distribution of the product water is done. Metering at the plant exit and strict evaluating of the production process through instrumentation and control of automated plant operations also form a part of the distribution process.
Advantages of the Desalination Process
  • Good quality water can be obtained throughout the year.
  • Use of chemicals is very less, which results in minimum environmental degradation.
  • Plants are compact and can be easily installed and upgraded.
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