10.3 Waste water treatment
5 exam questions worth 9 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is the purpose of screening in sewage treatment?
Choose one answer.
Correct Not quite: the answer is To remove large objects and grit
Mark scheme
A
Model answer
A: screening removes large objects and grit.
Describe what happens in the sedimentation tank.
Mark scheme
- heavy solids sink to form sludge (1)
- lighter liquid (effluent) stays on top (1)
Model answer
Heavy solids sink to the bottom as sludge, leaving the lighter liquid, the effluent, on top.
Name the useful products of the anaerobic digestion of sewage sludge.
Mark scheme
- biogas / methane (1)
- fertiliser (1)
Model answer
Biogas (methane), which is used as a fuel, and digested sludge, used as fertiliser.
Explain why air is bubbled through the effluent.
Mark scheme
- provides oxygen for aerobic bacteria (1)
- which break down the remaining organic matter / kill harmful microbes (1)
Model answer
Air supplies oxygen for aerobic bacteria to break down the remaining organic matter.
Suggest why it is easier to obtain potable water from groundwater than from waste water.
Mark scheme
- groundwater needs only filtration and sterilisation (1)
- waste water needs many more treatment stages / more energy (1)
Model answer
Groundwater only needs filtering and sterilising, while waste water must go through several treatment stages that use more energy.
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