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Methods of Desalination:
At least three principle methods of desalination exist: thermal, electrical, and pressure.
The oldest method, thermal distillation, has been around for thousands of years. In thermal distillation, the water is boiled and then the steam is collected, leaving the salt behind. However, the vaporization phase change requires significant amounts of energy. More modern methods of distillation make use of various techniques such as low-pressure vessels to reduce the boiling temperature of the water and thus reduce the amount of energy required to desalinate.
A second major type of desalination utilizes electric current to separate the water and salt. Typically, electric current will be used to drive ions across a selectively permeable membrane, carrying the dissociated salt ions with it. A key characteristic of this method is that the energy requirement depends on how much salt is initially present in the water. Consequently, it is suitable for water with initial salt concentrations but too energy intensive for sea water.
A third principle method of desalination is reverse osmosis, in which pressure is used to drive water through a selectively permeable membrane, leaving the salt behind.Similarly to electrically-driven separation, the amount of energy required for desalination depends on the initial salt content of the water. Again, this renders reverse osmosis unsuitable for sea water purification.
It depends on the scale you need, on the personal scale solar thermal desalination is the cheapest it done by using sunlight to evaporate the salty water then condensing the generated vapor on a cold surface but the problem in this method is the low productivity and this problem can be solved by applying vacuum to evaporating vessel.
While on the industrial scale, if you have less salty water it is economical to use membrane separation such as nano-filteration or reverse osmosis while if the salt concentration is high it is economical to use multi-effect desalination
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