Water for drinking

Waste water and groundwater must be treated to make them potable. Potable means safe to drink.

Treating fresh water

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:

  • large objects are removed by screening using sieves
  • 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
Water is purified by filtration, sedimentation and the addition of chlorine.The main stages in treating fresh water to make it potable

Treating waste water

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 waste water is screened to remove large objects such as nappies, paper towels
  • the water then passes through a sedimentation and filtering process

Any solid materials are then treated in a digester. Bacteria digest the sludge to form methane gas and digested sludge, which is used as fertiliser.

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.

Treating seawater

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.

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