Today there are following methods of desalination: distillation, electrodialysis, reverse osmosis, ion exchange and freezing.
During distillation because of instant boiling seawater evaporates in several stages, in each of which different pressure is used with its gradual reduction.
During membrane distillation seawater is heated by hydrophobic membrane on one side. This membrane passes only those vapor that condensate on the back side.
Multicolumn distillation method involves heating water in the first column, and the resulting vapor is moved to the next heating column.
Under compression distillation seawater is heated in the first column, using a partially compressed vapor. It reduces the power consumption of the process, but this method is not suitable for great amount of water.
Seawater can be purified by freezing. The water is freezed till crystallization and then the crystals are separated to produce potable water.
At the present time they more often use distillation units for desalination and water purification filters. Electrolysis and freezing are used much less frequently. The combination of the distillation plant with a thermal power plant that uses fossil or nuclear fuel (so-called multi-purpose power plant), can provide industrial districts with all kinds of energy services at the lowest cost within the most rational use of fuel.
Cogeneration is the process of using excess heat from electricity generation for another task: in this case the production of potable water from seawater or brackish groundwater in an integrated, or "dual-purpose", facility where a power plant provides the energy for desalination. Alternatively, the facility's energy production may be dedicated to the production of potable water (a stand-alone facility), or excess energy may be produced and incorporated into the energy grid (a true cogeneration facility). Cogeneration takes various forms, and theoretically any form of energy production could be used. However, the majority of current and planned cogeneration desalination plants use either fossil fuels or nuclear power as their source of energy. Most plants are located in the Middle East or North Africa, which use their petroleum resources to offset limited water resources. The advantage of dual-purpose facilities is they can be more efficient in energy consumption, thus making desalination a more viable option for drinking water.
The best economic, environmental and technological indexes belongs to the combined schemes of water treatment where the first stage of the desalination is non-reagent method - reverse osmosis or evaporation , and deep purification of water is made by means of ion exchange. In comparison with "pure" ion exchange, such scheme allows to reduce reagent consumption and the volume of saline effluent approximately 10 times at the maximal quality of water treatment. This method is most commonly used in all developed and under construction schemes of production of high purity water for power engineering, electronics andmedicine in Russia and abroad.