How Does Desalination Occur?

Seawater is rich in dissolved salt.

"Water, water everywhere, nor any drop to drink," lamented the ancient mariner in Coleridge's famous poem. Coastal cities in arid regions sometimes face the same dilemma: seawater is abundant but undrinkable. Desalination is a process that removes salt from seawater to make it potable.

Types

There are two major methods for removal of salt from seawater---distillation and reverse osmosis. While there are other processes, according to "Desalination: A National Perspective," reverse osmosis and thermal distillation account for over 99 percent of current worldwide desalination capacity.

Function

In the distillation technique, saltwater is heated to evaporate the water while the salt residues remain behind. Reverse osmosis uses high pressure to force pretreated water through semipermeable membranes that extract salts and contaminants to leave behind purified potable water.

Considerations

According to "Essential Environment," one of the primary drawbacks to desalination at present is its cost; for many municipalities, desalination is one of the more costly sources of drinking water. Current research focuses on finding new techniques that will be more energy-efficient or reduce cost in comparison with alternatives.

References

• "Essential Environment, The Science Behind the Stories, 2nd Edition"; Jay Withgott and Scott Brennan; 2007
• The National Academies Press: Desalination, A National Perspective