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Last Updated: Tuesday, 8 June 2004, 10:51 GMT 11:51 UK
Turning sea-water into tap water
By Jonathan Duffy
BBC News Online Magazine

As temperatures push upwards with the onset of summer, some water firms are looking to the Middle East in a bid to stop their supplies from drying up.

At first glance there's little that binds the urban sprawl of industrialised east London with the sun-baked scenery of the Canary Islands.

But if Thames Water gets its way, the borough of Newham in east London will host Britain's first large-scale project to turn salty sea-water into fresh drinking water.

Desalination plants are widespread in places such as Tenerife and Gran Canaria, where minimal rainfall means natural water resources are scarce.

Israel and Saudi Arabia already rely heavily on such plants to supply their populations, while the American states of Florida and California have also ventured down this road.

The Thames plant could supply 150m litres of water a day
Average daily water used a head is up 23 litres in 20 years
Lifestyle changes such as home swimming pools have contributed
Now, concerns about fresh water resources in Britain have prompted drastic action.

According to Thames Water, in the south and east of England there is less water per person in an average year than in Ethiopia or Sudan, although tapping that resource in Africa has always proved a problem.

Earlier this year Folkestone and Dover Water Services said it planned to apply for "water scarcity status". If granted by the government, the firm would be the first in the UK to be able to impose water meters on its customers.

At the same time, South East Water is planning to pilot a small desalination plant in Newhaven, East Sussex. Plans were reportedly brought forward after the record-breaking nine-month dry spell in 2003.

The need for new sources in Britain is put down to two points in particular:

  • Lifestyle changes which mean, as individuals, we use more water than ever
  • A predicted explosion in new homes - more than a million are expected to be built over the next 12 years in the South East alone

    Tight balance

    "Our lifestyle expectations have grown. People wash more than they used to," says Barrie Clarke of the industry body Water UK.

    "In the South East and East Anglia the balance between supply and demand is pretty tight."

    In the old days, the answer would have been to build more reservoirs. But the hike in land prices has put this option beyond the reach of commercial water companies.

    Desalination is a relatively recent solution, coming of age in the Middle East in the 1980s and 1990s. Thames Water plans to use a process known as "reverse osmosis" to draw salt from the brackish flow of the Thames estuary.

    HOW REVERSE OSMOSIS DESALINATION WORKS
    1. Water flows in from the estuary or sea
    2. Salt water contains sodium and chloride ions
    3. Pressure is applied to force salt water through membrane
    4. Semi-permeable membrane with millions of microscopic holes
    5. Clean water fit for drinking
    6. Saline concentrate flows out

    It works by pushing salty water through a series of ultra-fine membranes which draw out the tiny sodium and chloride ions.

    The process is eased by the fact that water in the tidal Thames is less salty than that in the sea. In addition, Thames Water plans to draw water only from the ebb of the tide, which is less salty again.

    "We expect the salt content of the water to be no more than 11,000 parts per million - less than a third as salty as ordinary sea water," says a spokesman for the company.

    The process is expected to yield 85 litres of drinking water per 100 litres processed.

    But it comes at a cost - financial and environmental. Water prices are expected to rise towards the end of the decade, says Barrie Clarke, to reflect the growing demand.

    At the same time, eco activists are expected to oppose both the Thames and South East Water desalination projects. The process consumes a lot of energy and can be damaging to seawater habitats.




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