The removal of salt from water. This is an energy-intensive process. Also known as desalinisation.
It is preferable to make potable water from fresh water reserves rather than from sea water
Desalination by distillation
Sea water is heated until it boils. The salt remains in the liquid, and the steam is pure water. The steam is cooled and condensed to make potable water.
Distillation requires a lot of energy to boil the water, and also to cool the steam down to condense it
The waste water from distillation is very salty and can be difficult to dispose of in a sustainable way which does not harm marine ecosystems
Desalination by reverse osmosis
Water is put under high pressure and passed through a membrane which has tiny pores (holes) in it. The pores allow water molecules through, but prevent most ions and molecules from passing through.
Reverse osmosis requires expensive membranes and also produces a large volume of waste water, so its efficiency is often quite low
