For an island nation weary of summer hosepipe bans, the prospect of using our greatest natural resource to provide unlimited drinking water seemed an obvious solution.

But as weather forecasters predicted another hot, dry summer, plans to convert the sea into drinking water were abandoned after a two-year trial showed the scheme to be unfeasible.

A pilot desalination scheme at Newhaven harbour, East Sussex, investigated the potential for a full-scale plant capable of producing 9.5 million litres (two million gallons) of drinking water a day. It was watched closely by water companies across the South of England, which have experienced supply shortages in recent summers.

The trial found that producing drinking water through desalination is up to ten times more expensive than from traditional sources. The cost of producing desalinated water during shortages, or at time of peak demand, would cost £450 per million litres, compared with £35 from ground water supplies and £50 from rivers.

It also found that the power required for the two-stage purification process — in which water is first filtered through sand before being forced at high pressure through a semi-permeable membrane — is environmentally unacceptable. The investigation concluded that it would require 10,000 square metres of solar panels — 1½ times the size of a football pitch — to produce one million litres of water a day, enough to supply about 6,000 people. There would also be environmental impact in disposing of the highly concentrated brine left over from purification.

David Shore, operations director at South East Water, which serves Sussex and Kent, said: “Our trial has demonstrated that desalination is not yet the right solution for delivering water at peak times or during extended dry periods such as droughts.

“Desalination remains an expensive option in terms of operating and environmental costs when compared to developing additional resources, or through other ways of managing customer demand. Many technological advances have been made in desalination and so it will remain an option to consider as part of long-term water resources plans.”

Desalination is used widely in other parts of the world, but mainly in countries with relatively low energy costs or those with no alternative supplies from rivers or ground water.

Robin Wiseman, editor of Desalination & Water Reuse, said that scientists in such locations were making significant advances in technology, which could make plants viable in the future. “I am sure that that by 2020 the technology will have advanced so that there will be some plants on the south coast of England,” he said.

A report by the Water Services Regulation Authority (Ofwat), which approved the South East Water trial, said: “Desalination is becoming more efficient but still uses substantial amounts of energy . . . [and] creates a waste stream of highly concentrated salts. Because of this, in most cases desalination is not cost-effec-tive as there are lower cost alternative sources of water available.”

Thames Water has appealed to ministers for permission to build a desalination plant after plans were blocked by Ken Liv-ingstone, the Mayor of London, on environmental grounds. The plant, at Beckton, would produce 140 million litres a day, enough for about 900,000 people, from the “brackish” waters of the Thames estuary, which have about a third of the salt content of the sea.

Low salt

— The the first recorded desalination was in the 17th century, when Japanese sailors used earthenware pots to boil seawater

— In 1791 Thomas Jefferson was asked to investigate claims that a new “desalting process” (by chemical addition /distillation) could provide fresh water to the infant US Navy

— Malta claims credit for the world’s first “commercial” desalination plant, which opened at Sliema in 1881

— In 1907 the Ottoman Turks installed Saudi Arabia’s first desalination plant in Jedda

Sources: Rensselaer Polytechnic Institute; National Environmental Services Center, West Virginia University; Halcrow Water Services

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