Fresh water contains objects that must be removed to make it potable. These include large objects such as branches and leaves, insolubleparticles such as grit, and harmful microorganisms.
Different separation methods and treatments are used to deal with them:
large objects are removed by screening using grids
a coarse filter bed made from clean sand and gravel removes larger insoluble grit particles
aluminium sulfate is added to clump smaller insoluble particles together, which then settle to the bottom in a sedimentation tank
a fine filter bed removes very small insoluble particles
chlorine gas is added to kill harmful microorganisms
Seawater contains too much dissolved salt to make it suitable as drinking water. However, pure water can be produced from seawater by simple distillation.
The seawater is boiled and the water vapour is then cooled. It condenses to form pure water, leaving the salt behind.
It is expensive to produce drinking water this way because large amounts of energy are needed to heat the seawater. However, water produced by distillation is useful in the laboratory for dissolving substances. It does not contain any dissolved ions that might interfere with a chemical analysis.