Fresh water contains objects that must be removed to make it potable. These include large objects such as branches and leaves, insoluble particles such as grit, and harmful microorganisms. Different separation methods and treatments are used to deal with them:
Waste water from homes includes sewage, as well as water from washing machines and dishwashers. The treatment process starts in the same way as the treatment of fresh water:
The water is then passed to tanks that are kept oxygenated. Microorganisms in the water digest any remaining organic waste and also consume harmful bacteria. The treated water is usually released into a river.
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 led away and cooled. It condenses to form pure water, leaving the salt behind.
It is expensive to produce drinking water this way in the UK 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.