GCSE · Chemistry · AQA · Spec 8462
Waste water treatment
The treatment process is really a matching problem: identify what is in the water, then choose how to remove it.
Follow sewage through treatment
Remove large solids and grit.
Move stage by stage and keep sludge separate from effluent.
Why desalination is harder
Reason it through
Why is obtaining potable water from sea water generally harder than from a suitable fresh-water source?
First link · your turn
What extra substance is present in sea water?
UK EXAM PRACTICE
Build a concise comparison
A strong answer compares the treatment burden, not just the names of the water sources.
Question
Comment on the relative ease of obtaining potable water from a suitable fresh-water source and from sea water.
Student answer
A suitable fresh-water source can be filtered and sterilised, whereas sea water must be desalinated by distillation or reverse osmosis, which requires large amounts of energy.
Match the waste water to the treatment concern
Commit before revealing: which statement matches the lesson?
Which statement correctly distinguishes sewage/agricultural waste water from industrial waste water?
Relationship matrix
Tap any cell to reveal it. Tap a column header to read one property down every item.
Each cell hides a short answer and the reason behind it. Predict before you tap.
CHEMISTRY · WATER
Waste water treatment
Different water sources need different treatment: remove what is harmful, then make the water suitable for use.
What you need to know
- Sewage and agricultural waste water need organic matter and harmful microbes removed; industrial waste water may also need harmful chemicals removed.
- Sewage treatment includes screening and grit removal, sedimentation, anaerobic digestion of sludge and aerobic biological treatment of effluent.
- A suitable fresh-water source can be passed through filter beds and sterilised; chlorine, ozone and ultraviolet light are sterilising methods.
- Salt water can be desalinated by distillation or reverse osmosis, but these processes require large amounts of energy.
- You should be able to compare how easy it is to obtain potable water from waste, ground/fresh and salt water.
The big picture
Waste water from homes, agriculture and industry must be treated before release because it can contain organic matter, harmful microbes and harmful chemicals. Sewage treatment uses screening and grit removal, sedimentation, anaerobic digestion of sludge and aerobic biological treatment of effluent. Suitable fresh water can be filtered and sterilised, while salty water needs desalination by distillation or reverse osmosis. Desalination requires large amounts of energy.
Key points
Worked example
Problem
Explain why sea water is generally more difficult to turn into potable water than a suitable fresh-water source.
Memory hook
S-S-A-A for sewage: Screen, Settle, Anaerobic sludge, Aerobic effluent.
★ Exam tip
When comparing water sources, name the treatment each source needs and then explain the consequence, especially the large energy demand of desalination.
⚠ Watch out
Saying that filtration removes dissolved salt. Filter beds remove solids; dissolved salts need desalination such as distillation or reverse osmosis.
Check yourself
After sedimentation, what happens to the sludge and the effluent, and why is sea water harder to make potable?
Flashcards
(20)Why must large amounts of waste water be treated before release?
What may sewage and agricultural waste water contain that must be removed?
What may industrial waste water need removed?
What is the first sewage-treatment stage named in this lesson?
What happens during sedimentation?
What is sewage sludge?
What is effluent?
How is sewage sludge treated?
How is sewage effluent treated?
What does anaerobic mean?
What does aerobic mean?
What three broad steps can make suitable fresh water potable?
Name three sterilising methods for fresh-water treatment.
What is desalination?
Name one desalination method.
What is reverse osmosis in this topic?
What major disadvantage do desalination processes share?
When may desalination be needed?
Which is generally easier: suitable fresh water or sea water?
What should you compare when judging the ease of obtaining potable water?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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