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Various methods from evaporation to distillation and chemical treatment may be employed for producing fresh water from sea water and other salty water sources. The different types of technology employed include multi-stage flash distillation,
thermo-compression distillation, reverse osmosis and reverse electrodialysis.

A multi-stage flash distillation plant typically consists of a series of twenty or more chambers, each operating at a lower pressure from the last. As heated brine flows from one chamber to the next, some of it flashes off into water vapour. This
passes through moisture separators that remove any entrained droplets of brine. The vapour condenses on colder condensation tubes, and drips as distillate into trays from which it is led away to storage.

A thermo-compression distillation plant usually has two to six evaporator stages and uses a multistage thermo-compression process operating at quite low temperatures and sub-atmospheric pressures. In a typical four-stage unit, source vapour
introduced at, say 62ºC and 0.22 bar, into the condenser tubes of the first stage is condensed by externally sprayed raw water and the condensate is drawn off as product. In cooling the condenser tubes, the raw water in stage one is heated and
part of it vaporizes at a lower temperature and pressure, say 58ºC and 0.19 bar. This vapour enters the tubes of the second stage, is condensed by raw water as in stage one and is also drawn off as product. Part of the raw water in the second stage
is vaporized at still lower conditions, say 54ºC and 0.15 bar, and enters the tubes of the third stage. The process is repeated, and again in stage four.

Part of the vapour produced in the fourth stage is drawn up at 46ºC and 0.10 bar by the thermo-compressor, which compresses it with high pressure steam at 0.22 bar to feed the first stage. The condensed vapour from each evaporator stage
and from the condenser stage is extracted by a pump to form the product water. Thermo-compression distillation plants can produce very pure water from any sea water or brackish source without a complex pretreatment or filtration.

For many years salt water was purified by distillation processes, in relatively small quantities (such as on board a passenger ship, or in a desert town) because of its energy intensity. The development in the early 1960s of desalination by reverse osmosis, using a membrane as the separating medium, completely changed the desalination picture, making it much more accessible, even if still not as cheap as deep-bed filtration.

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