AQA GCSE Combined Science (8464), Higher tier · Chemistry › Using resources › Using the Earth's resources and obtaining potable water
Practise Waste water treatment. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How sewage and industrial waste water are treated before they are released into the environment, and how the ease of getting potable water from waste water, ground water and salt water compares. Expect 'describe the stages' questions and short comparisons on Paper 2.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Say why waste water must be treatedTowns and industry produce large amounts of waste water that must be treated before it is released into the environment.
4
State what sewage treatment must removeSewage and agricultural waste water need organic matter and harmful microbes removed.
5
List the stages of sewage treatment in orderScreening and grit removal, sedimentation, anaerobic digestion of the sludge, and aerobic biological treatment of the effluent.
6
Describe what happens at each stageFor example, in sedimentation solids settle to form sludge, leaving the liquid effluent on top.
6
State what industrial waste water needsRemoval of organic matter and harmful chemicals.
7
Compare treating waste, ground and salt waterGround water needs the least treatment; waste water needs many stages; salt water needs desalination, which uses large amounts of energy.
Notes
Why waste water is treated
Urban lifestyles and industrial processes produce large amounts of waste water. It must be treated before it is released into rivers or the sea.
Sewage and agricultural waste water need organic matter and harmful microbes removed.
Industrial waste water may need organic matter and harmful chemicals (e.g. toxic metal compounds) removed, which can need extra stages.
The stages of sewage treatment
1. Screening and grit removal: screens remove large objects (e.g. rags, plastic, twigs) and grit is allowed to settle out.
2. Sedimentation: in settlement tanks, heavier solids sink to form sewage sludge. The liquid above is called effluent.
3. Anaerobic digestion of the sludge: bacteria break down the organic matter without oxygen. This produces biogas (mainly methane), which can be used as a fuel, and digested sludge, which can be used as a fertiliser.
4. Aerobic biological treatment of the effluent: air is bubbled through it, so bacteria break down the remaining organic matter using oxygen. The treated water can then be released safely.
Comparing sources of potable water
Ground water is fresh water with low levels of dissolved substances, so it usually needs only filtering and sterilising.
Waste water needs many stages (screening, sedimentation, biological treatment) to remove organic matter and microbes, and would still need sterilising before people could drink it.
Salt water must be desalinated by distillation or reverse osmosis, which needs large amounts of energy.
So in the UK most potable water comes from fresh water, because it is the easiest and cheapest to treat.
Cheatsheet
Sewage and agricultural waste water: remove organic matter and harmful microbes
Industrial waste water: remove organic matter and harmful chemicals
Sludge = solids that settle at the bottom; effluent = the liquid above
Anaerobic = without oxygen (produces biogas, mainly methane)
Aerobic = with oxygen (air bubbled through the effluent)
Ground water: least treatment. Waste water: many stages. Salt water: desalination (large energy use)
How to answer each type of question
Describe how sewage is treated
4 to 6 marks6
Write the stages in the correct order.
For each stage, say what happens and what is removed.
Use the key words: screening, sedimentation, sludge, effluent, anaerobic, aerobic.
Say which part (sludge or effluent) each biological treatment is used on.
Example. Describe how sewage is treated so that the water can be released into a river. (4 marks)
Show the model answer
Screening removes large objects, and grit is removed (1). In sedimentation tanks the solids settle to form sludge, leaving liquid effluent (1). The sludge is broken down by anaerobic digestion (bacteria, without oxygen) (1). The effluent is treated aerobically: air is bubbled through so that bacteria break down the remaining organic matter (1).
Explain why industrial waste water needs different treatment
2 marks5
Say what sewage treatment removes: organic matter and harmful microbes.
Say that industrial waste water may also contain harmful chemicals, which must be removed too.
Example. Waste water from a factory contains toxic metal compounds. Explain why this waste water may need extra treatment compared with sewage. (2 marks)
Show the model answer
Sewage treatment is designed to remove organic matter and harmful microbes (1). The factory waste water also contains harmful chemicals (the toxic metal compounds), which must be removed before the water is released (1).
Compare obtaining potable water from different sources
4 to 6 marks7
Cover every source named in the question.
For each source, say what treatment it needs.
Compare the number of stages and the energy needed.
Finish with a judgement about which is easiest, with a reason.
Example. A town could obtain potable water from ground water, from treated waste water or from sea water. Compare the treatment needed for each source. (4 marks)
Show the model answer
Any four from: ground water has low levels of dissolved substances, so it only needs filtering and sterilising (1); waste water must first go through screening, sedimentation and biological treatment to remove organic matter and microbes (1); it would then still need to be sterilised (1); sea water must be desalinated by distillation or reverse osmosis, which needs large amounts of energy (1); so ground water is the easiest and cheapest source (1).
Shortcuts and memory tricks
Order of stages: 'screen, settle, then bacteria'.
Sludge sinks; the liquid on top is the effluent.
An-aerobic means 'no air'. Sludge goes to the sealed anaerobic tank; the effluent gets the air bubbles.
Biogas from sludge is mainly methane, so a sewage works can make some of its own fuel.
Where marks are lost
Mixing up sludge (the settled solids) and effluent (the liquid).
Swapping anaerobic and aerobic: sludge is digested anaerobically; effluent is treated aerobically.
Describing potable water treatment (filter beds and chlorine) when the question asks about sewage treatment.
Forgetting that industrial waste water needs harmful chemicals removed, not just organic matter and microbes.
Exam technique
Keep the stages in order: screening and grit removal come first, then sedimentation, then the biological stages.
Use the exact terms sludge, effluent, anaerobic and aerobic. Vague words such as 'dirty bits' do not score.
In 'compare' questions, make a point about each source and use comparative words such as 'fewer stages' or 'more energy'.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the process used to break down organic matter in sewage sludge, which also produces methane gas.
anaerobic digestion
Which stage happens first: screening, or sedimentation?
screening
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Sewage must be treated before the water can be released into the environment.
(a) Which stage of sewage treatment produces sewage sludge and effluent? Tick (✓) one box.[1]
Anaerobic digestion
Screening
Sedimentation
Sterilisation
(b) What is removed from sewage during screening?[1]
(c) Name two things that must be removed from sewage and agricultural waste water.[2]
Show the answer and mark scheme
(a)Answer: Sedimentation
(b)
large objects (e.g. rags, plastics) / grit
(c)
organic matter
harmful microbes
Question 2Medium5 marks
After screening and grit removal, sewage is left to settle in sedimentation tanks. This produces sewage sludge and effluent.
(a) Describe how the sewage sludge and the effluent are then treated.[2]
(b) Industrial waste water may need extra treatment compared with sewage. Explain why.[2]
(c) Anaerobic digestion of sewage sludge produces methane. Suggest how the sewage works could use this methane.[1]
Show the answer and mark scheme
(a)
sewage sludge: anaerobic digestion (by bacteria)
effluent: aerobic biological treatment (air is bubbled through so bacteria break down organic matter)
(b)
it may contain harmful chemicals (e.g. toxic metal compounds)
which must be removed as well as organic matter before the water is released
(c)
as a fuel, e.g. to generate electricity / heat for the sewage works
Question 3Hard6 marks
Urban lifestyles produce large amounts of sewage that must be treated before the water is released into a river.
Describe how sewage is treated so that the water can be released into a river. Give reasons for the stages used.[6]
Show the answer and mark scheme
screening removes large objects such as rags and plastics, and grit is removed
sedimentation in tanks: solids settle to form sewage sludge, leaving liquid effluent
sewage sludge undergoes anaerobic digestion by bacteria, breaking down organic matter
anaerobic digestion produces methane that can be used as a fuel; digested sludge can be used as fertiliser
effluent undergoes aerobic biological treatment: air is bubbled through so bacteria break down organic matter
organic matter and harmful microbes must be removed before the water is released
industrial waste water may also need harmful chemicals to be removed
Marked with levels of response: the full level descriptors are in the app.