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.