Waste water treatment
Waste water
Waste water must be treated before it is released into rivers or the sea.
- from homes and farms contains organic matter and harmful microbes.
- Industrial waste water may also contain harmful chemicals, which need extra treatment.
Untreated, these would pollute rivers, harm wildlife and spread disease.
Key terms
Sewage treatment
- removes large objects such as rags and plastics, plus grit.
- in a settlement tank, heavier solids sink to form , and the liquid on top is .
- Aerobic digestion of the effluent: air is bubbled through, and bacteria break down the organic matter and kill harmful microbes. The treated water can then be released safely.
- bacteria break it down without oxygen, producing biogas (methane), used as a fuel, and digested waste used as .
Key terms
Which water is easiest to treat?
(ground water, rivers, reservoirs) is the easiest to make potable: it needs only filtering and sterilising.
Waste water needs more processes to remove organic matter, microbes and sometimes harmful chemicals.
needs , which uses the most energy and is the most expensive.
Aerobic and anaerobic
Sewage treatment uses bacteria in two different ways:
| Effluent | Sludge | |
|---|---|---|
| Treatment | Aerobic digestion | Anaerobic digestion |
| Oxygen | Air bubbled through | No oxygen |
| What it gives | Water safe to release into rivers | Biogas and fertiliser |
Common mistake
Don't swap them: aerobic means with oxygen and is used on the effluent; anaerobic means without oxygen and is used on the sludge.
A sustainable process
Sewage treatment turns waste into useful products:
- biogas (methane) is burned as a fuel, often to power the treatment works itself
- the digested sludge is spread on farmland as a
- the treated water returns safely to rivers and the sea
It still needs energy, for pumping and for bubbling air through the effluent, and industrial waste water may need extra chemical treatment to remove harmful substances before it is released.