Choosing a Household Water Treatment in a Village With Frequent Diarrhoea
A health worker tests four ways of treating water from a hand-dug well. Each sample is one litre taken from the same bucket and tested the same day. Bacteria colonies per 100 mL are recorded.
| Method | Colonies | Cost per 20 L | Time per 20 L |
|---|---|---|---|
| Untreated | 240 | 0 | 0 |
| Boiling 5 minutes | 0 | 8 Nakfa fuel | 15 min + cooling |
| Chlorine tablet | 4 | 3 Nakfa | 30 min wait |
| Cloth filter | 165 | 0 | 5 min |
The household has six people and needs about 60 litres a day. Fuel is bought weekly and cash income is small. The nearest clinic is 9 km away, and it recorded 14 cases of diarrhoea last month among children under five.
1. Recommend a treatment plan for the household and support it with calculations from the table.
Read the worked answer
Daily volume in 20-litre units: 60 ÷ 20 = 3 units per day.
Monthly cost (30 days)
- Boiling: 3 × 8 = 24 Nakfa/day → 24 × 30 = 720 Nakfa/month, plus about 45 minutes of labour a day.
- Chlorine: 3 × 3 = 9 Nakfa/day → 270 Nakfa/month, plus waiting time.
- Cloth filter: 0 Nakfa, about 15 minutes a day.
Recommendation: filter through cloth first, then add one chlorine tablet per 20 litres, and store covered.
Why:
- Boiling removes the most bacteria (240 → 0) but costs 720 Nakfa a month, which is 450 Nakfa more than chlorine - about 5,400 Nakfa a year for a household with little cash income.
- Chlorine leaves only 4 colonies, a 98% reduction, and is the cheapest safe option at 270 Nakfa a month.
- Cloth alone is not safe: it reduces 240 to 165, only about 31%.
- Filtering before chlorinating matters because chlorine works poorly in cloudy water; removing mud first lets the tablet do its job.
Boiling remains the choice when a child has diarrhoea or when tablets run out.
2. A parent says that if the cloth filter does not make water safe, the family should not bother with it. Evaluate this claim.
Read the worked answer
The claim is mistaken, though it contains a fair warning.
- The data show the filter is not a stand-alone treatment: 165 colonies per 100 mL is still unsafe. Anyone who drinks only filtered water is still at risk, so the parent is right that filtering alone is not enough.
- But the filter removes suspended mud, leaves and some parasites, which is exactly what makes the later chlorine step work better. Chlorine is used up by dirt before it can kill microbes.
- Filtering also makes the water look and taste acceptable, so the family is more likely to treat and drink it rather than return to the unprotected well water.
- Practical benefit: it costs nothing and takes 5 minutes, and it reduces contamination by roughly a third before any chemical is added.
Conclusion: the filter is a useful first step, not a substitute for disinfection. The safe message is "filter, then chlorinate - or boil". The parent should be told plainly that filter-only water is not safe, which is what the 165 figure means.
3. Identify one weakness in this comparison and state what extra evidence you would collect before advising the whole village.
Read the worked answer
Main weakness: the test counts bacteria only. Typhoid and cholera are bacteria, but viruses and protozoa such as amoeba and Cryptosporidium are not counted by this test, and chlorine is much weaker against some protozoa. Boiling kills all of them, so the table may understate boiling's advantage for a village where those diseases occur.
Other weaknesses
- One sample per method, no repeats: a single reading could be a chance result.
- No measurement of how cloudy the water was, which changes how well chlorine works.
- The 4 colonies after chlorine could reflect too small a dose or too short a wait by the tester, not the method itself.
Evidence to collect
- Repeat tests on several days and several wells.
- Test for protozoa and viruses, or check clinic records for which diseases actually occur.
- Check whether chlorine tablets can be bought locally every month and whether families accept the taste.
- Record turbidity before and after cloth filtering.
Advice should change if the clinic confirms amoebic disease, if tablets are often out of stock, or if families abandon chlorinated water because of taste.