Salt is removed from seawater by different varieties of two basic processes: distillation, which is also referred to as "thermal," and reverse osmosis, which is also called "membrane." Distillation involves boiling seawater until it evaporates, leaving the salt behind, and then allowing the water vapor to condense and collect elsewhere. Reverse osmosis uses pressure to force seawater against a specialized filter or semipermeable membrane with openings too small to allow the dissolved salt to pass through.
The distillation processes used for desalination include the multi-stage flash method, multi-effect distillation and vapor compression distillation. Membrane processes include reverse osmosis, electrodialysis and electrodialysis reversal. Multi-stage flash distillation and reverse osmosis account for the majority of the desalination plant methods in use worldwide. The world's largest desalination plant, an integrated power and water facility, is in Saudi Arabia. The largest portion of a domestic water supply produced by seawater desalination, at about 40 percent, is in Israel.
Both the distillation and reverse osmosis desalination methods are expensive. A great deal of electrical power is used, with distillation ranking as the greater energy consumer. Cogeneration is a possible solution to the cost issue which applies the excess heat produced by an electrical power plant to an additional process, such as desalination, in a dual-purpose facility. The majority of cogeneration power plants producing potable water from seawater are in North Africa or in the Middle East where abundant petroleum reserves help offset the limited accessibility of drinking water.