Popular in the 1960s and '70s, desalination technology stagnated for the next decade. Now, dwindling water supplies are forcing cities and counties to take another look.

"Water, water, everywhere, but not a drop to drink," cried the Ancient Mariner. Indeed, more than 95 percent of the world's water is sea or brackish water, unsuitable for drinking. The amount of fresh water on the planet essentially has not changed in millions of years, but the number of users has, and in some places that has put a tremendous strain on water resources.

Ron Linsky of the National Water Research Institute in California says the country needs to improve the reliability of its supply and suggests that water could be on the commodities market in the next century. "We may ultimately face rationing or importing," he says.

There are a number of ways to forestall this, however. One of those ways - desalination - is already being used across the country to stretch water supplies, clean up polluted water and provide protection for aquifers. The technique is ancient, dating back to the 4th century B.C. when, according to the National Water Supply Improvement Association, Greek sailors used simple evaporation to desalinate seawater.

The technology, however, is far more modern. Desalination - separating saline water into fresh water and water containing the concentrated salts - is accomplished in two main ways: through distillation or use of membranes.

Nearly 60 percent of the world's desalted water is produced via the first method by heating salty water to produce water vapor that is then condensed to form fresh water.

The second process uses membranes to separate the salts from the water. In reverse osmosis (RO) facilities, water is forced through bundles of membranes under pressure, leaving behind impurities. In electrodialysis reversal (EDR) plants, an electrical current transfers ions through membranes, resulting in desalted water and concentrates.

Worldwide, desalting plants have the capacity to produce 3.5 billion gallons of water a day, nearly enough to provide 15 gallons a day for every American. Some nations, such as Saudi Arabia and Malta, desalt ocean water to produce fresh water for public and industrial consumption. In the United States, most desalting plants treat brackish water, a process that costs one-third to one-fourth as much as the treat involved in desalting ocean water.

The product water is used for direct supply, reserves or groundwater recharge. According to the American Desalting Association (ADA), other uses include irrigation, wastewater treatment and water purification. Hospitals, resorts, manufacturing plants, oil rigs and pleasure boats also employ desalination technology. During the Persian Gulf War, the Army had mobile desalination units that could produce 3,000 gallons per hour of potable water from brackish pools.

In a 1988 report, the Congressional Office of Technology Assessment suggested desalination could find application in treating contaminated groundwater, be it runoff from mines, agriculture, landfills or storage tanks. Of desalting in general, the report noted, "Desalination should be included as a viable option in any evaluation of water-supply alternatives."

STRETCHING CALIFORNIA'S WATER

In Southern California, which in large part owes its growth to water drawn from the northern part of the state and from the Colorado River, water supply has long been an issue. The Orange County Water District, with some 2 million people in its service area, has an ambitious goal - 90 percent independence from external water supplies by 2010.

One means of reaching that objective is running secondary effluent from municipal and industrial sources through its Water Factory 21.

That RO plant, built about 20 years ago, produces 15 mgd for injection into the basin where it serves as a hydraulic barrier to salt water intrusion. It also becomes mixed with the groundwater and reused. Reportedly, Water Factory 21 already produces 17 percent of the water supplies of Fountain Valley and Huntington Beach. The product water of the plant surpasses water quality standards and is no longer required to be blended with deep-well water.