Waste Water Treatment — AQA GCSE Chemistry

Waste water from homes, agriculture and industry must be treated before it is returned to the environment.

Sources of waste water

Domestic (sewage) — from homes, containing organic matter and harmful microbes.
Agricultural — containing nutrients (from fertilisers) and animal waste.
Industrial — which may contain harmful chemicals and so needs additional treatment.

Screening and grit removal — removing large solids (twigs, grit) and rubbish.
Sedimentation — the waste water is allowed to settle. Heavy solids sink to form sludge; the liquid effluent remains on top.
Anaerobic digestion of sludge — microorganisms break down the sludge without oxygen, producing biogas and a fertiliser.
Aerobic biological treatment of effluent — air is bubbled through so aerobic bacteria break down remaining organic matter and harmful microbes.

The treated water can then be safely released.

Treating sewage needs more steps than treating fresh ground water (which mainly needs filtering and sterilising).
It uses less energy than desalinating sea water (which needs a lot of energy).

Learn the waste-water steps: screening → sedimentation → anaerobic digestion of sludge + aerobic treatment of effluent.
Aerobic bacteria treat the effluent; anaerobic digestion treats the sludge (making biogas).
Sewage treatment needs more steps than ground water but less energy than desalination.
Industrial waste may need extra treatment for harmful chemicals.