Chlorination Failure at a Field Water Point
A camp of 300 soldiers draws drinking water from a hill stream into a 5,000-litre ground tank. The medical officer orders chlorination to give 2 mg of available chlorine per litre, using bleaching powder solution labelled '5% available chlorine'. The stream has run turbid since the rain began and the tank is fed directly, without settling. Within 48 hours 20 soldiers report watery diarrhoea; all of them drank from the tank, but the cookhouse staff, who drink boiled tea, are unaffected. The quartermaster proposes simply doubling the chlorine dose. The camp has no water-testing laboratory, but does hold chlorine test strips, boiling sets, cloth-and-candle filters and jerrycans.
1. How much of the 5% solution is required to dose a full 5,000-litre tank at 2 mg of available chlorine per litre?
Read the worked answer
Step 1 - mass of chlorine needed 5,000 L × 2 mg/L = 10,000 mg = 10 g of available chlorine.
Step 2 - strength of the stock solution 5% available chlorine means 5 g per 100 mL, i.e. 0.05 g per mL (equivalently 50 g per litre).
Step 3 - volume to add 10 g ÷ 0.05 g/mL = 200 mL of the solution for a full tank.
Step 4 - the check that matters Adding the calculated dose is not the end of the task. After 30 minutes' contact, test the free residual with the strips; the target is roughly 0.2–0.5 mg/L. If the residual reads near zero, the dose has been consumed by the turbidity, not by the bacteria - which is exactly what the strips are there to reveal.
2. Is doubling the chlorine dose the right first response to the outbreak? Justify your answer and state the order of actions you would actually take.
Read the worked answer
No - doubling the dose treats the symptom and not the cause. Three reasons:
- Chlorine demand. Turbid water contains silt and organic matter that consumes chlorine before it can act on pathogens. With heavy turbidity, even a doubled dose may leave no free residual, so the water is still unsafe while everyone believes it has been treated.
- Particles shield organisms. Bacteria adsorbed on suspended particles are protected from disinfection; the particle must be removed, not just dosed around.
- Some pathogens resist routine chlorination. Cryptosporidium oocysts survive the free-chlorine concentrations used in field water treatment, so chlorine alone cannot be the only barrier at a suspect source.
Order of actions:
- Stop drawing from the tank for drinking; issue boiled water (rolling boil, 1 minute) in the interim.
- Settle and/or filter the raw water to cut turbidity before chlorination - cloth plus candle filters are already held.
- Then chlorinate to the calculated dose, and verify free residual 0.2–0.5 mg/L after 30 minutes.
- Protect the tank: cover it, draw from a tap, ban dipping of utensils.
- The pattern supports a water route: those who drank tank water fell ill, while the cookhouse, drinking boiled tea, did not. Food is an unlikely source, so do not divert effort to the kitchen first.
3. A post at about 3,000 m reports that water boils at roughly 90°C. A sepoy argues that a one-minute boil is therefore useless there and a pressure cooker is essential. Assess this claim.
Read the worked answer
The claim is wrong. At 3,000 m the atmospheric pressure is roughly 70 kPa and water boils near 90°C - lower than at sea level, but still far above the temperature at which enteric pathogens are destroyed.
- The organisms of concern - E. coli, Salmonella, Shigella, V. cholerae, Giardia cysts, Entamoeba and hepatitis A virus - are inactivated at around 70°C given a few minutes of contact, and almost instantly at 90°C.
- Bringing water to a rolling boil for about 1 minute (3 minutes is often advised above 2,000 m) is therefore fully effective. A pressure cooker raises the boiling point but is a convenience for fuel and cooking time, not a necessity for safety.
Two cautions for the report:
- Boiling kills organisms but does not remove turbidity, salt or chemical contamination.
- At altitude the water takes longer to reach the boil, so fuel expenditure per litre rises; that is a logistics problem, not a microbiological one.