There are at least three principle methods of sea water desalination: thermal, electrical, and pressure.
Thermal Distillation
This is the oldest and simplest method that has been used for thousands of years. Water from the ocean is first drawn in and filtered to remove larger impurities and particles. It is then heated or boiled to produce steam which is collected, leaving the salt residue behind. The salt-free steam is condensed to create mineral-free water.
But like most processes that require heat, the vaporization stage uses a significant amount of energy. Modern methods of thermal distillation uses various techniques such as low pressure vessels to reduce the boiling temperature of seawater, thereby reducing the amount of energy needed for desalination.
Electrodialysis
This is a membrane-based processes that uses an electric current to separate water and salt. An electric current is used to drive salt ions across a membrane. One issue with this method is that the amount of energy required depends on how much salt is present in the water. Therefore it is more ideal for the treatment of low-salinity water. Applying this process to seawater would be too energy intensive.
Reverse Osmosis
In RO filtration, pressure is used to drive seawater through a membrane, leaving salt behind. Seawater is first filtered through sand or charcoal filters to eliminate larger particles and algae. High pressure is then used to feed the salt water through an RO membrane, allowing water through but trapping the salt. RO membranes typically have pores that measure one millionth of a millimeter, making it an expensive product. It's not surprising that some plants opt to use pre-filters for their RO membranes to extend it's lifespan.
The world's largest RO plant is located is Isreal, with nearly 50,000 membranes producing 600,000 cubic meters of water daily. RO is similar to Electrodialysis, where the amount of energy required depends on the salt content in the water. Electricity is used to operate the pumps and compressors needed to push seawater through the membranes.
Energy Costs
For these reasons, thermal distillation has traditionally been adopted in most regions with an abundance of cheap energy where desalination plants are built near thermal power plants since large quantities of heat is needed to evaporate seawater. This way, the heat produced in the generation of electricity wouldn't go to waste as it can be recovered by the desalination plant.
However, the rise of cheap renewable energy has opened up new opportunities for RO and electrodialysis options in seawater desalination. New developments include combining solar farms with desalination plants.
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