A Turbid Water Supply, a Cluster of Diarrhoea Cases and Three Proposals
A boys' higher secondary school in a flood-affected union council has 600 students and one overhead tank of 2,000 litres, refilled each morning from a shallow borehole. Over the past ten days, 23 students and two teachers have reported diarrhoea; the local health centre has recorded similar but smaller numbers in the surrounding village. The tank was last cleaned in March, roughly eight months ago, and its lid is missing. Water from the tank is visibly cloudy; a field kit shows turbidity of about 25 NTU and a faint smell.
Three proposals are on the table: (a) boil all drinking water for the whole school; (b) add bleaching powder to the overhead tank each morning; (c) fit a sediment filter before the tank outlet and chlorinate the filtered water, while cleaning and covering the tank. The school has one LPG stove, two large pots, and a budget of PKR 60,000 for the term; the caretaker, who already handles several other duties, would have to run whatever is chosen.
1. Which proposal best fits the evidence and the constraints? Explain what each option achieves and where it fails.
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Proposal (c). The reasoning follows the order in which the evidence presents itself.
- Turbidity must be reduced before disinfection. At about 25 NTU the water carries suspended organic matter that consumes chlorine before it can act on pathogens. Option (b), dosing the raw tank, either leaves no free residual at the tap or demands a dose heavy enough to cause taste complaints and disinfection by-products - and a caretaker with several other duties has no reliable way to measure residual. Clarify first, then disinfect.
- Storage is the likely amplifier, not the source. A missing lid for months, combined with no cleaning for eight months, is a textbook route for recontamination after any treatment. Cleaning and covering the tank is cheap, permanent, and does not depend on daily correct behaviour by one person.
- Boiling does not scale. 2,000 litres a day on one LPG stove with two pots cannot be done within a school day, and water boiled and then poured back into an open, contaminated tank is no safer. Option (a) is only viable for drinking water, in small volumes.
- Budget and ownership fit. Filters, a lid and a chlorine drum sit inside PKR 60,000; the caretaker's task becomes weekly backwashing and a residual check, not a daily production process.
Caveat: none of the three options touches contamination inside the borehole or the service pipe, and the village-level cases suggest a wider source. A sanitary inspection is still required before the money is spent.
2. What is the single most important action for the next 48 hours, and what should be deferred? Justify the split.
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Next 48 hours - remove the immediate hazard, do not yet build the system.
- Stop drinking from the tank taps. Provide safe drinking water only: boiled, or filtered and chlorinated. Drinking needs are a few litres per child, not 2,000 litres, so this is achievable with one stove and two pots - unlike boiling the whole supply.
- Clean and cover the tank now. Drain, scrub, disinfect and fit a lid. This is the cheapest single intervention available and it closes the pathway the evidence points to (open tank, sediment, smell).
- Manage the cases. Refer the 23 symptomatic students to the health centre, keep cases away from food and water handling, and report the cluster to the union council health authorities: 23 cases in a school plus cases in the village is a surveillance signal, not a school problem alone. Request bacteriological testing of the supply at the same time.
- Handwashing with soap before meals, and check that the toilets have water and soap.
Deferred: filter procurement, staff retraining, and the full chlorination protocol - all worth doing, none of which changes today's risk.
The principle behind the split: separate the action that removes the hazard today from the action that prevents recurrence. A partial chlorination of a turbid, uncovered tank would be the worst outcome - it would look like action, produce no residual, and give the school a false sense of safety.
3. What evidence would you gather in the next week to identify the source before committing the PKR 60,000? State what result would change the recommendation.
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Evidence, cheapest and most decisive first:
- Bacteriological testing (faecal coliform / E. coli) at the tank outlet and at the borehole before the tank. This is the pivotal test. Outlet positive with borehole negative implicates storage and confirms proposal (c). Borehole positive implicates the source, and no amount of tank cleaning will fix it.
- Free and total chlorine residual at the tap and in the tank, to show whether any chlorine survives at all.
- Physical tank inspection: sediment depth, cracks, screening of the overflow pipe, signs of bird or rodent access.
- Sanitary inspection of the borehole: distance to latrines, soakage pits and the open drain, casing condition, depth relative to the water table.
- Case interviews: did the 23 drink from the school tank, and did the two teachers (who may drink boiled water at home) have a different exposure? Crude, but informative.
- Stool testing of a few cases to identify the organism, which distinguishes a water route from food or person-to-person spread.
- Attendance records for the previous fortnight, since 23 reported cases probably undercount the true attack rate.
What would change the recommendation: a positive borehole sample would shift spending from tank repair and point-of-use treatment to source protection, and would justify asking the union council to close or relocate the supply. A negative outlet sample with no cases among tank drinkers would suggest the route is food or hygiene, and the PKR 60,000 should go to handwashing points and kitchen practice instead of filters.