Published: 5 April, 2006

Distillation, reverse osmosis can remove salt from water

How does the desalination of water work?

Desalination or literally desalting of seawater is a process used to change seawater into a source for drinking water. There are several ways to remove salt from seawater. The two methods described here are distillation and reverse osmosis.

Distillation is the heating of seawater into steam and condensing it (by cooling) into a new container. In this way the salt will be left behind, while the desalted water is separated. This method is not very practical for large amounts of water because of the large amount of heat needed, but it is a nice experiment to try during science class. Reverse osmosis is the technique most in use today. The seawater, after pretreatment to remove solids, is put under very high pressure while flowing around a special type of filter called a semi-permeable membrane. The membrane has pores (holes) that allow water molecules, but not other dissolved contaminants, to be pushed through. The clean water can flow to a collection point while the salt remains on the original side of the membrane and is collected as waste, which is actually very concentrated salt water. The higher the pressure applied, the more clean water produced. The clean water is then treated further to turn it into drinking water.

Desalination has been used since the 1950s to produce desalted drinking water, and it is becoming more widely used in the US now. California has a few small plants and Florida is considering building one. The big problems remain that it is quite expensive, much more so than using ground water or surface water. The other problem is the amount of salt water (brine) that is generated as waste. It is deposited back in the sea or diluted with non drinking quality water and sprayed on big grassy open spaces.