Water from rivers, lakes and the sea has to be treated to make it safe to use and drink. Different methods are used to do this, such as sedimentation, filtration and chlorination. Learn also about the processes of fluoridation, desalination and distillation.
Raw water contains dissolved ions, microorganisms, pollutants and insoluble materials like sand and stones. Water is treated to remove any harmful components before being fed into the public supply. This is done in several ways.
Sedimentation – the water is stored in a large tank or a reservoir. This slows down the flow of the water and allows large, insoluble particles to settle to the bottom. The cleaner water at the top can then be extracted for the next stage.
Filtration – the water is sprayed onto specially-prepared layers of sand and gravel. As it trickles through, smaller insoluble particles are removed. The filter beds are cleaned periodically by pumping clean water backwards through the filter.
Chlorination – chlorine gas is injected into the water to sterilise it. The chlorine is poisonous and so kills microorganisms.