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Waste water from homes, industry and agriculture must be treated before being released into the environment. Here are some examples of pollutants which might be present in waste water:

• sewage contains organic matter and harmful bacteria
• industrial waste water may contain organic matter and harmful chemicals such as toxic metal compounds
• agricultural waste water may contain fertilisers or pesticides which can disrupt sensitive ecosystems

Sewage treatment involves the following steps:

1. Screening and grit removal to remove large particles.
2. Sedimentation allows tiny particles to settle out from still water, which produces sewage sludge and effluent (the liquid which remains on top).
3. The effluent enters an aeration tank, where oxygen is pumped into the water. Here aerobic bacteria break down the remaining waste.
4. The sewage sludge is digested anaerobically by bacteria. It can be recycled to be used as a fertiliser on farms.