Tapping the Seven Seas; New ways of removing salt from water are relieving shortages.

Byline: William Underhill

Only the hardier forms of river life thrive in the salt-tainted tidal water of the River Thames. It's dirty, brown and brackish, but to Tony Rachwal it's a reservoir of limitless potential. Next month his company, Thames Water, hopes to win permission to build Northern Europe's first large-scale desalination plant, which removes salt (among other things) from water, on the banks of the river a few miles east of central London. The idea is to turn these wretched waters where river mingles with ocean into sparkling drinking water for 900,000 London residents. "We have this huge resource on our doorstep," says Rachwal, the R&D director. "And now we have the technology to use it."

Why does rainy London need a $360 million plant that taps ocean water? London, it seems, isn't quite rainy enough--per capita, it's drier than Madrid or Istanbul. As the city expands, water usage is rising, and shortages threaten. Mexico City, which has 4 million more residents than it did 20 years ago, has a similar problem: the city is sinking into the ground as it drains its underground aquifers. For their part Americans are flocking to sunbelt communities and building golf cources and swimming pools. Climate change is only expected to make things worse.

The oceans, on the other hand, hold 95 percent of the world's water, and almost half of the global population lives within 100 kilometers of the shoreline. "There just aren't enough reserves of water," says Pierre-Francois Moizan of Degremont, which runs many big desalination projects, "and seawater is always available."

Of course, it's been possible to treat seawater on an industrial scale for 40 years. The problem with many of the older plants, though, is that they work by distilling the water and leaving behind the salt residue. This process requires copious amounts of heat energy and is too expensive for all but the richest and thirstiest Middle East nations. Growing shortages have created incentives to develop cheaper technologies. Today's favored method, "reverse osmosis," involves pumping salt water through polymer strips. Microscopic pores let water pass but trap the salt.

Competition, economies of scale and better polymers have halved the cost of producing a ...