VIII.

Water Desalinization

To meet the ever-increasing demands for fresh water, especially in arid and semiarid areas, much research has gone into finding efficient methods of removing salt from seawater and brackish waters. In the U.S., desalinization research is directed by the Bureau of Reclamation, Department of the Interior. Several processes are being developed to produce fresh water cheaply.

Three of the processes involve evaporation followed by condensation of the resultant steam and are known as multiple-effect evaporation, vapor-compression distillation, and flash evaporation. The last-named method, the most widely used, involves heating seawater and pumping it into lower-pressure tanks, where the water abruptly vaporizes (flashes) into steam. The steam then condenses and is drawn off as pure water. In 1967, Key West, Florida, opened a flash-evaporation plant and thus became the first city in the U.S. to draw its fresh water from the sea.

Freezing is an alternate method, based on the different freezing points of fresh and salt water. The ice crystals are separated from the brine, washed free of salt, and melted into fresh water. In another process, called reverse osmosis, pressure is used to force fresh water through a thin membrane that does not allow the minerals to pass. Reverse osmosis is still undergoing intensive development. Electrodialysis is being used to desalt brackish waters. When salt dissolves in water, it splits into positive and negative ions, which are then removed by electric current through anion and cation membranes, thus depleting the salt in the product water. Although developmental work on electrodialysis is continuing, a number of commercial plants are in operation. In 1962 Buckeye, Arizona, became the first town to have all its water supplied by its own electrodialysis-desalting plant, which provides about 2,460,000 liters (about 650,000 gallons) of water daily at a cost of about $1 per 6300 liters (1670 gallons).

One major problem in desalinization projects is the cost of producing fresh water. Using conventional fuels, plants with a capacity of 3.8 million liters (1 million gallons) per day or less produce water at a cost of $1 or more per 3800 liters (1000 gallons). More than 500 such plants are in operation, with a total capacity of nearly 473 million liters (nearly 125 million gallons) a day; however, their high costs limit their use to areas of great water scarcity. Water from conventional sources, such as wells and reservoirs, is sold for less than 30 cents per 3800 liters delivered to the home, and water for irrigation is usually priced at less than 5 cents per 3800 liters. The dual-purpose atomic power and water-desalting plants now being planned are designed to produce fresh water for between 20 and 30 cents per 3800 liters.

Most experts expect more immediate results from efforts to purify mildly brackish water that contains between 1000 and 4500 parts per million of minerals, compared to 35,000 parts per million for ocean water. Because water is potable if it contains fewer than 500 parts per million of salt, the cost of desalting brackish water is correspondingly less than it is for desalting seawater. See Solar Energy; See also Waterpower; Water Supply and Waterworks.

For other functions of water, see Erosion; Geology; Metabolism.