Primary General Studies | Hong Kong water treatment and public health teaching guide | Four treatment steps and four common errors

2026-11-13

Tutor lesson-prep guides · Primary General Studies

Water arrives at the tap, so the answer to where drinking water comes from is always the reservoir. The reservoir is only the start: that water carries silt, algae and bacteria and must pass four treatment steps before it is safe. This lesson follows one chain from reservoir to tap, names the purpose of each step, and then uses the three links of infection, agent, route and host, to explain why hand washing works: it breaks one link, not because hands look clean.

Objectives — Students can (1) name coagulation, sedimentation, filtration and disinfection in order and give the purpose of each; (2) explain that chlorine kills germs and leaves a residual in the pipes, and that more chlorine is not better; (3) use agent, route and host to explain hand washing, and estimate household use with multiplication such as 150 litres times 4 people.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
The three states of water and the water cycle	What is the change from liquid to gas called?	Evaporation; the reverse is condensation
Mixtures can be separated by filtration	How do you separate sand from water?	Filter paper or a sand layer
Germs are living things too small to see	If water looks clear, is it free of germs?	No; clear does not mean sterile

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Opening judgement	Ask where tap water comes from and keep answers for the end	Most say reservoir only; do not correct yet
6-18 min	Sources	Reservoirs capture rain, plus Dongjiang water; Hong Kong has no natural lake	Establish that reservoir water is not drinkable
18-34 min	Four steps	Coagulation, sedimentation, filtration, disinfection, with a purpose for each	Reorder on the spot if the sequence slips
34-44 min	Chlorine and use	Explain residual chlorine, work out 150 times 4 and 9 times 5	Say clearly that the figures are examples
44-56 min	Three links	Agent, route, host, with one way to break each link	Hands: soap, running water, 20 seconds
56-60 min	Exit check	Order the four steps and answer one explanation item	See the exit section

03 Teaching script (read it out)

Scene 1: The reservoir is only the start

Hong Kong has no natural lake, so drinking water comes from two sources: local reservoirs capturing rain, and Dongjiang water piped from Guangdong. Ask first whether reservoir water, which looks clear, is safe to drink; most students say yes. Then make the point explicit: reservoir water carries silt, leaves, algae and bacteria, and clear does not mean sterile. Write that sentence on the board, because it is the premise of the whole lesson. A camping comparison helps: stream water in the hills looks clear, yet nobody drinks it straight.

Close by asking how many steps reservoir water passes before it reaches the kitchen tap.

Scene 2: Four steps, one job each

Draw a horizontal flow chart and write the purpose after each step. Coagulation adds alum so that tiny clay particles clump into larger ones, because fine particles will not settle. Sedimentation lets the water stand so the clumps sink as sludge. Filtration passes the water through a sand layer to trap what is left. Disinfection adds chlorine to kill bacteria and viruses and leaves a small residual in the pipes so water is not recontaminated on the way to the tap. Have students put the four steps in order themselves, then ask what happens if chlorine is added before filtration: the sand layer traps organic matter and grows bacteria, so the chlorine is wasted. That question is the heart of the ordering item.

Close by asking students to point at the sand layer on the flow chart; a wrong finger means reorder the steps.

Scene 3: Public health: three links, break one and the chain stops

Act three connects hand washing to everything before it. An infection needs three links at once: an agent, a route such as hands, food, water or droplets, and a host. Break any one and the chain stops. Hand washing breaks the route: soap and running water for 20 seconds lift oils and the germs attached to them, and the water carries them away. Hot water is not needed and neither is antibacterial soap. Chlorination of drinking water also breaks the route, while vaccination lowers susceptibility in the host. Close with arithmetic: about 150 litres per person per day makes 600 litres for a family of four, which is three 200-litre baths; an older toilet at 9 litres flushed five times uses 45 litres, while a 4.5-litre flush uses 22.5 litres, saving 22.5 litres a day.

Close by asking what 22.5 litres a day saves in a year: 8,212.5 litres.

04 Question types and worked steps

Type	Example	Steps	Answer
Ordering	Put coagulation, disinfection, filtration and sedimentation in the correct order	Coagulate, settle, filter, disinfect	Disinfection in the middle scores nothing
Calculation	At 150 litres per person, how much does a family of four use, in 200-litre baths?	Multiply 150 by 4, then divide by 200	Units needed; answer 600 litres and 3 baths
Explanation	Why is chlorine added last?	It kills germs and leaves a residual against recontamination	Saying disinfection only, with no residual, loses a mark
Data judgement	Compare 9-litre and 4.5-litre flushes at five flushes a day	Multiply each by 5, then subtract	22.5 litres; saying half with no number does not count

Sanity check (teach this): Sanity check: one chain, source to four steps to tap, holds the chapter together, and each step carries exactly one purpose so students do not memorise four nouns. No chemical formulae are needed, and chlorine is discussed only as killing germs and leaving a residual, not as forming by-products. All water figures are examples and are computed on the board (150 times 4 is 600, 9 times 5 is 45, 4.5 times 5 is 22.5). Public health stops at three links and 20 seconds of hand washing, with no disease-specific treatment. Boiling at home is deliberately separated from treatment works disinfection.

05 Independent practice (3 questions)

Question	Answer	What to watch
Ordering	Four steps given out of order; reorder and give each purpose	Check disinfection ends the sequence
Calculation	A family uses 600 litres a day and saves 22.5 litres with a new flush: how much in a year?	22.5 times 365 is 8212.5 litres
Explanation	Use the three links to explain why hand washing prevents disease	The route must be named as the broken link

06 Common mistakes and what to say

Mistake	Student writes	Say this
Believing reservoir water is drinkable	Reservoir water is clear, so you can drink it	Clear is not sterile. Under a microscope that water has algae and bacteria, which is why it goes through four treatment steps.
Swapping filtration and disinfection	Add chlorine first, then filter	Chlorine is the last step. Chlorinate first and the sand layer collects organic matter and grows bacteria, so the chlorine is wasted.
Thinking more chlorine is always better	More chlorine means cleaner water	Too much chlorine smells and forms by-products. The plant adds only enough to leave a residual in the pipes.
Thinking hand washing needs hot water or antibacterial soap	You need hot water and antibacterial soap to be clean	Soap and running water for 20 seconds is enough. Hot water damages skin, and antibacterial soap is no better than ordinary soap.

07 Homework (paper, section and question numbers)

Independent practice in this chapter, with answers — Use practice items 1 to 3, plus two written items: order the four steps, estimate household use
P5 General Studies paper for mixed practice — For revision week mixed practice; this paper has no water treatment items, so treat it as question-reading training

08 Exit check (last 5 minutes)

Question	Answer	Decision
Name the four treatment steps	Coagulation, sedimentation, filtration, disinfection	Full order needed to pass
Why is chlorine added last	It kills germs and leaves a residual against recontamination	Both points needed
Which link does hand washing break	The route, using soap and running water for 20 seconds	The word route must appear

09 Tutor FAQ

Should the Dongjiang share be taught?

No percentage is needed because it changes from year to year. Two sources, local reservoirs and Dongjiang water, are enough; the lesson belongs to the four steps, not to the split.

What if a student asks whether Hong Kong tap water is drinkable?

Yes, because it is disinfected before it leaves the plant and a residual protects the pipes; but if a building tank has not been cleaned for years, the last step at home is boiling. That answer is accurate and connects to what families actually do.

Is a treatment works visit worthwhile?

If one is available, yes, but the core of the lesson is order and purpose. Run the ordering task before the visit so students know which step they are looking at.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.