S2 Science: The Water Cycle And Sewage Treatment (Paper 2 (intermediate))

S2 Science: The Water Cycle and Sewage Treatment (Paper 2 (intermediate))

2026-11-14

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Tutor Prep Guide · The processes of the water cycle, the ratio of fresh to salt water, the order of water and sewage treatment, and BOD with water saving figures

S2 Science: The Water Cycle and Sewage Treatment (Paper 2 (intermediate))

This paper covers the order of drinking water treatment, the division of labour between primary and secondary sewage treatment, and BOD with water use figures.

Direct answers　The order of treatment cannot be swapped, primary removes solids while secondary removes organic matter, and percentages and units must be shown.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S2 Science: The Water Cycle and Sewage Treatment
Level	S2

02　Questions (10 items)

FoundationWhat is usually the first stage of treating drinking water?
FoundationWhat is the purpose of adding chlorine or using ultraviolet light at the end of water treatment?
FoundationWhich kind of method does primary sewage treatment mainly use?
IntermediateWhat removes organic matter in secondary sewage treatment?
IntermediateInfluent BOD is 300 mg/L and secondary treatment removes nine tenths. What is the effluent BOD?
IntermediateIf that effluent then passes tertiary treatment removing another nine tenths, what are the final BOD and the overall removal?
ChallengeA shower uses 9 L a minute for 8 minutes while a bath uses 150 L. How much water and what share does the shower save?
ChallengeFour people each use 150 L a day. How many cubic metres does the household use in a 30 day month?
ChallengeWhat is BOD and what does a high value mean?
Error redoA student says the more chlorine added the safer the drinking water. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Screening and sedimentation to remove large solids such as twigs and grit	Removing large solids first keeps later filters from clogging and disinfectant from being used up.
2	Disinfection, killing disease causing microbes	Disinfection kills germs and leaves a chlorine residual in the pipes to stop recontamination.
3	Physical methods: screening, grit removal and sedimentation	Primary treatment only separates solids from water, leaving dissolved organic matter untouched.
4	Microbes break it down while the tank is aerated	Aeration supplies oxygen to aerobic microbes, which break organic matter down into carbon dioxide and water.
5	30 mg/L	300 times 0.1 is 30, so one tenth leaves with the treated water.
6	3 mg/L, an overall removal of 99 per cent	30 times 0.1 is 3, so BOD falls from 300 to 3, a removal of 99 per cent.
7	78 L, about 52 per cent	9 times 8 is 72 L, 150 minus 72 is 78 L, and 78 over 150 is about fifty two per cent.
8	18 m³	150 times 4 is 600, times 30 is 18000 L, which is 18 m³.
9	The oxygen microbes use to break down organic matter, and a higher value means heavier pollution	More organic matter means more oxygen used by microbes, so dissolved oxygen falls and fish struggle.
10	Excess chlorine forms disinfection by products and spoils taste, so the dose must be controlled	Disinfection must be adequate but not excessive, as residual chlorine is capped and overdose brings new risks.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

For water cycle questions ask three things: which way the water changes state, whether the question asks about a quantity or a concentration, and whether the treatment is physical, biological or chemical.Tutor Registration →

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