Choosing a Drinking-Water Source After a Flood
After a flood, a cluster of 12 people in four households must choose a drinking-water source for at least three days. They need 3 litres per person per day. Three sources are available. Source A is a community tap: pH 7.1, turbidity 1 NTU, and zero E. coli per 100 mL in the last test, but it runs for only two hours a day and feeds a rusty overhead tank. Source B is a hand pump: pH 6.8, turbidity 12 NTU, and 25 E. coli per 100 mL. Source C is a canal: pH 8.2, turbidity 60 NTU, and 300 E. coli per 100 mL. The family has four 20-litre jerrycans, a clean cloth, a pot, and chlorine tablets labelled one tablet per 20 litres with 30 minutes contact time. Fuel for boiling is limited. A neighbour advises putting three tablets into each jerrycan of canal water to 'make it safe'.
1. Select the safest source and describe the treatment chain before drinking, justifying each step with the test values.
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Source A is the best first choice. Its turbidity of 1 NTU is low, its pH of 7.1 is near neutral, and its last test showed zero E. coli. Those values matter because chlorine works best in clear water at pH below about 8; low turbidity means less organic matter shielding microbes. The weakness is the intermittent supply and the rusty tank, which could contaminate water after the test, so A should still be chlorinated or boiled and stored in covered jerrycans. If A is unavailable, use B: turbidity 12 NTU is high enough to reduce chlorine efficacy, so filter through the clean cloth, let solids settle, then chlorinate. Source C should be a last resort: turbidity 60 NTU and pH 8.2 both undermine chlorination, and its E. coli count is very high.
2. Calculate the daily and three-day water requirement, and state how many chlorine tablets are needed per treated batch using the 20-litre jerrycans.
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The group needs 12 people × 3 litres = 36 litres per day. For three days that is 36 × 3 = 108 litres. The four jerrycans hold 80 litres in total, so the group cannot carry all three days' water in one trip; it will need refills or a storage drum. For chlorination, the label allows one tablet per 20 litres. A full 20-litre jerrycan takes one tablet. To treat 36 litres, fill two jerrycans - for example 18 litres each, or one full 20-litre can and one 16-litre can - and use two tablets, one per container, because a partial container is still dosed as a container and tablets cannot be split reliably. Wait 30 minutes before drinking. If Source B is used, filter before adding the tablets, since turbidity consumes chlorine.
3. Evaluate the neighbour's advice to put three tablets into each jerrycan of canal water.
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The advice is unsafe and scientifically flawed. Adding extra chlorine does not simply 'make it safer'. Canal water at 60 NTU contains suspended silt and organic matter that reacts with chlorine, so much of the dose is consumed before it can disinfect; the correct response is to settle and filter the water first, not to triple the dose. The pH of 8.2 also shifts chlorine toward a less effective form. Meanwhile, exceeding the label dose raises the risk of unpleasant taste, eye and stomach irritation, and disinfection by-products. The label dose of one tablet per 20 litres should be followed after filtration, with 30 minutes contact time. If fuel allows, boiling is a more reliable pathogen-control step for turbid water because it does not depend on pH or clarity.