10.1.4Using Resources
Waste Water Treatment — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 8
Part 1 of 4Where waste water comes from
Waste water
Homes
- Homes Water from toilets, showers, baths, washing machines and kitchens flows into sewers.
Agriculture
- Agriculture Animal waste and fertiliser run-off from fields add organic matter and nutrients.
Industry
- Industry Factories produce waste water that may contain toxic chemicals, e.g. from processes such as the Haber process.
Why treat it?
- Why treat it? Untreated waste water holds microbes, organic matter and toxins that would harm health and wildlife in rivers and lakes.
Part 2 of 4Stages of sewage treatment
1Screening
- Screening Grids remove large objects such as plastic and twigs, and grit is allowed to settle.
2Sedimentation
- Sedimentation Water stands in a settlement tank.
- Heavy solids sink to form sewage sludge and the liquid on top is the effluent.
3Aerobic digestion
- Aerobic digestion Air is pumped through the effluent so aerobic bacteria break down organic matter and harmful microbes.
4Return to rivers
- Return to rivers The treated water is safe to release into the environment.
Part 3 of 4What happens to the sludge?
1
Sludge is removed from the bottom of the settlement tank.
2
It is placed in large closed tanks without air.
3
Anaerobic bacteria break down the organic matter.
4
Methane gas is released and can be burned as a fuel; the remaining digested waste is used as fertiliser.
Part 4 of 4Removing toxic substances
Extra treatment
- Extra treatment Industrial waste water needs additional stages before release.
Methods
- Methods Membranes can filter out unwanted ions, chemicals can be added to precipitate metal ions out of solution,
- and ultraviolet radiation can kill microbes.
Cost versus desalination
- Cost versus desalination Treating sewage takes longer than treating fresh water but uses less energy than desalinating seawater,
- so it is useful where fresh water is scarce.
Key terms
| Term | Meaning |
|---|---|
| Effluent | The liquid that remains after solids have settled out in sewage treatment. |
| Sewage sludge | The solid material that sinks to the bottom of the settlement tank. |
| Aerobic digestion | Breakdown of waste by bacteria that need oxygen. |
| Anaerobic digestion | Breakdown of waste by bacteria in the absence of oxygen, producing methane. |
| Precipitate | An insoluble solid formed when a chemical reaction takes place in solution. |
Avoid these mistakes
1
Mixing up aerobic and anaerobic digestion
- aerobic acts on the effluent with air;
- anaerobic acts on the sludge without air.
2
Forgetting that screening comes first and removes large debris.
3
Describing the methane as a waste product. It is captured and used as fuel.
4
Saying that sewage treatment makes water drinkable. It makes it safe to return to the environment; drinking water needs further treatment.
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