Desalination (WJEC GCSE Chemistry: Combined Science)

• Desalination is the removal of salt from seawater 
• This process is useful in countries that have limited fresh water sources such as rivers and steams, little rainfall but have a coast line such as Saudi Arabia 
• Desalination involves removing the salt from sea water by distillation or reverse osmosis, a process that involves the use of membranes
• Reverse osmosis involves pushing salt water through a semi-permeable membrane at high pressure, so only water molecules can pass through it.
  • This happens as the membrane stops larger molecules and ions passing through

Diagram to show reverse osmosis 

Only water molecules can get through the semi-permeable membrane 

• There are a few issues with using desalination to produce drinking water:
  • Desalination is an expensive process as it consumes large amounts of energy and is not ideal when producing large quantities of fresh water
  • The cost of desalinsation means it is not available to poorer countries
  • The large amounts of energy required mean that desalination produces more greenhouse gases in comparison to traditional water treatment methods 
  • Due to the nature of the process, desalination plants can be far from populated areas meaning large lengths of water pipes must be built to get the water to where it is needed, increasing costs further 
  • The salty water left behind when the fresh water has been extracted is a pollutant and would need to be disposed of carefully