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Desalination is the removal of salt from seawater. This produces clean drinking water and is particularly useful in countries that have coastlines but no readily available fresh water sources, such as rivers and streams.

Desalination uses one of two main methods. The first is reverse osmosis, where seawater is forced through a membrane at high pressure. The membrane allows water molecules to pass through but prevents any other chemicals dissolved in the water from passing through.

The second is thermal desalination. Older (thermal) desalination plants may work using a principle similar to distillation:

• the salt water is heated or the water is allowed to evaporate
• the water vapour is collected rather than being lost
• the water vapour is condensed to form pure water/fresh water
• the salt is left behind and can be used for other purposes

There are a number of disadvantages to using desalination:

• Both desalination processes use a lot more energy than traditional water treatment methods.
• The increased energy demand means that desalination is more expensive than traditional water treatment methods. This means some poorer countries that would benefit from desalination cannot afford it.
• The increased energy demand means that desalination produces more greenhouse gases than traditional water treatment methods, which produce very little in comparison.
• The very salty water produced by membrane desalination is a pollutant and must be disposed of carefully.
• Desalination plants may be a very long distance from some populated areas and large lengths of water pipes must be built to get the water to the people who need it.

Desalination is most commonly used in the Middle East because:

• rainfall is low
• many of the countries have coastlines
• many of the countries have access to cheap oil for energy
• many of the countries are quite wealthy