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Desalination

water seawater osmosis distillation

Desalination, also called desalting, is the removal of salt from seawater. It provides essential water for drinking and industry in desert regions or wherever the local water supply is brackish. In 1991, about 3.5 billion gallons of desalinated water were produced in about 4,000 desalination plants worldwide. Most of this water was produced through distillation. However, other methods, including reverse osmosis and electrodialysis, are becoming increasingly important.

At its simplest, distillation consists of boiling the seawater to separate it from dissolved salt. The water vapor rises to a cooler region where it condenses as pure liquid water. Heat for distillation usually comes from burning fossil fuels. To reduce costs and pollution, desalination plants are designed to use as little fuel as possible. Many employ flash distillation, in which heated seawater is pumped into a low pressure chamber. The low pressure causes the water to vaporize, or "flash," even though it is below its boiling temperature. Therefore, less heat is required. Multi-stage flashing passes the seawater through a series of chambers at successively lower pressures. For even greater efficiency, desalination plants can be linked with electrical power plants. Heat from the hot gasses that turn the generators is recycled to warm the incoming seawater. Distillation is widely used in the Middle East, where fossil fuel is plentiful but fresh water is scarce.

Reverse osmosis uses high pressure to force pure water out of saltwater. Normal osmosis occurs when pure water and saltwater are separated by a semi-permeable membrane, which permits only water to flow through. Under these conditions, the pure water will move into the saltwater side, but if the saltwater is squeezed under high enough pressure, freshwater moves out of it. Pressures on the order of 60 atmospheres (800-1,200 psi) are required to push pure water out of seawater. Reverse osmosis is widely used to desalinate brackish water, which is less salty than sea-water and therefore requires pressures only about half as great.

Like reverse osmosis, electrodialysis is presently best suited for desalinating brackish water. Salts consist of ions, which are atoms that have acquired electrical charge by losing or gaining electrons. Because of their charge, ions are attracted to oppositely charged electrodes immersed in the saltwater. They move toward the electrodes, leaving a region of pure water behind. Special membranes prevent the ions from drifting back into the purified water as it is pumped out.

Ongoing research seeks to improve existing desalination methods and develop new ones. The costs of distillation could be greatly reduced if clean, renewable energy were used to heat the water. Solar, geothermal, and oceanic temperature differences are among the energy sources being studied. Reverse osmosis could be used on a larger scale, and with saltier water, through development of semi-permeable membranes able to withstand higher pressures for longer times. All desalination methods leave extremely salty residues. New methods for disposing of these must be developed as the world's use of desalination grows.

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