Desalination

Desalination

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. Desalination plants are active in over one hundred countries around the world. Saudi Arabia produces about one-fourth of the world’s capacity of desalinated water. Israel possesses the largest desalination plant at its reverse osmosis plant in Ashkelon. Opened in 2005, it produces 130 cubic yards (100 million cubic meters) of water each year. Most of this water was produced through distillation. However, other methods, including reverse osmosis and electrodialysis, are becoming increasingly important.

Desalination has been used for many centuries. In the fourth century BC, Aristotle (384–322 BC) told of Greek sailors desalting water using evaporation techniques. Sand filters were also used. Another technique used a wool wick to siphon the water. The salts were trapped in the wool. During the first century AD, the Romans employed clay filters to trap salt. Distillation was widely used from the fourth century on —salt water was boiled and the steam collected in sponges. The first scientific paper on desalting was published by Arab chemists in the eighth century. By the 1500s, methods included filtering water through sand, distillation, and the use of white wax bowls to absorb the salt. The techniques have become more sophisticated, but distillation and filtering are still the primary methods of desalination for most of the world. The first desalination patent was granted in 1869, and in that same year, the first land-based steam distillation plant was established in England, to replenish the fresh water supplies of the ships at anchor in the harbor.

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. Multistage 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, fresh water moves out of it. Pressures on the order of 60 atmospheres (800 to 1, 200 psi [pounds per square inch]) are required to push pure water out of seawater. Reverse osmosis is widely used to desalinate brackish water, which is less salty than seawater 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.

The desalination of seawater and brackish water is still being researched throughout the world. In the United States, desalinization research is being performed by such federal organizations as the Bureau of Reclamation within the Department of the Interior. In 2005, the Long Beach Seawater Desalination Research and Development Facility opened in California. The facility, which produces about 300, 000 gallons (1.1 million liters) of desalinated water each day, will provide the latest information and data on cost-effective and environmentally sound techniques for the desalination of seawater.

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.