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How to drink the Mediterranean

The ancient dream of producing vast quantities of fresh water from the sea is closer to becoming an economic reality. Several large plants, based on a new, less costly method of removing salt from sea water, are either being built or are at an advanced stage of planning.

The new "reverse osmosis" technology is still expensive. But further technological innovations are likely to make it cheaper and more widely accessible. Hence it holds out the promise of significant new supplies of pollution-free fresh water within the foreseeable future.

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Reverse osmosis uses pressure literally to reverse the usual tendency of water molecules in a solution to flow selectively through certain kinds of membrane from a dilute solution on one side to a more concentrated solution on the other.

Modern reverse-osmosis plants employ man-made materials for filters including millions of hair-thin tubes. The sea water molecules are forced through these at pressures of several dozen atmospheres. The dissolved materials in the water , including the salts, are simply left behind.

The process still demands a heavy initial investment. And it involves an operating cost of something like $4 per 1,000 gallons of water. This limits the scope of the process to the capital-rich world.

But the reverse-osmosis process now has reached the threshold of economic feasibility on a large scale. And it is a timely development.

The United Nations has declared the 1980s as the International Water Supply and Sanitation Decade -- a decade in which to transform radically the lives of many millions of people. Universal access to clean water and sanitation would substantially reduce public health problems in the developing regions, encourage food production and industrial development, and end the drudgery of women and children who often walk several miles daily to fetch water that may be unsafe.

The reverse-osmosis process is not limited to sea water. It can be used to purify water by removing various dissolved materials. One obvious early application, therefore, is the re-purification of water resources in industrially developed regions where the local ground water supply has become polluted or inadequate.

The traditional approach to de-salting or "desalination" is simple distillation involving the boiling and subsequent condensation of water -- an energy-intensive process. Reverse osmosis needs about half the energy investment of distillation to produce the same quantity of water. And new developments in membrane materials and engineering techniques are about to reduce the present energy input by 50 percent. Also, water purification installations based on the new process can be built in something like half the space and two-thirds of the time it takes to erect comparable distillation plants.

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Given the rapid development of desalination technology and the world's need for fresh water, reverse osmosis may become widely available before the end of the UN's water decade.

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