Chlorinating a Flood-Relief Water Tank
After a flood, a relief party fills a 400-litre plastic tank from a slow-moving stream. The water is visibly turbid with fine silt. The party holds one 500 mL bottle of 5 per cent sodium hypochlorite solution, meaning 5 g of available chlorine in every 100 mL of solution. Standing instructions give a dose of 2 mg of free chlorine per litre for clear water and 5 mg per litre for turbid water, a minimum contact time of 30 minutes, and a target free residual chlorine of 0.2–0.5 mg/L at the point where water is drawn. The party has cloth filters, fuel for boiling, and test strips that read free residual chlorine. A soldier suggests adding double the recommended dose "to be safe" and issuing the water at once, because the queue is long and people are already thirsty. Nobody has measured the turbidity.
1. Calculate how many millilitres of the bleach solution are needed to dose the 400-litre tank at the turbid-water rate. Show every conversion.
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Step 1 - total chlorine required. 5 mg/L × 400 L = 2 000 mg, i.e. 2 g of available chlorine.
Step 2 - strength of the solution in mg/mL. 5 g per 100 mL = 5 000 mg per 100 mL = 50 mg/mL (equivalently 50 g/L).
Step 3 - volume of solution. 2 000 mg ÷ 50 mg/mL = 40 mL.
Check by proportion: 40 mL of solution contains 40 × 50 mg = 2 000 mg chlorine; spread over 400 L that is 5 mg/L. Correct.
For reference, the same bottle holds 500 × 50 mg = 25 g of chlorine, enough for 25 ÷ 2 = 12.5 tanks at this dose. Note the unit trap: the dose is in milligrams, the solution is quoted in grams per 100 mL; converting both to mg before dividing is what prevents a tenfold error.
2. Explain why the clear-water dose is wrong for this tank, and what must happen before and after dosing. What is wrong with the soldier's "double it and issue it now" proposal?
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Why 2 mg/L is wrong: silt and organic matter exert a chlorine demand - they consume chlorine before it can act on germs. In turbid water a 2 mg/L dose is largely used up by the silt, leaving no free residual and no reliable disinfection; particles also shield micro-organisms from the chlorine.
Before dosing: let the tank settle and pass the water through cloth filters to cut turbidity. After dosing: mix thoroughly, wait at least 30 minutes, then test with the strip; the free residual should read 0.2–0.5 mg/L. If below 0.2 mg/L, re-dose and wait again; if well above, allow it to stand or blend with cleaner water. Where fuel is available, bringing water to a rolling boil is the more certain option.
The soldier's proposal fails on three counts: (1) doubling a dose does not remove the need to filter - turbidity is the actual problem; (2) issuing immediately defeats the 30-minute contact time, so the water may still be unsafe; (3) an unmeasured overdose gives a strong chlorine taste, and people then reject the water or drink from the stream instead, which is worse for health. Dose by calculation, then verify by test strip.
3. After 30 minutes the strip reads 0.05 mg/L. What do you conclude, and what do you do next?
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Conclusion: a residual of 0.05 mg/L is below the 0.2 mg/L minimum, so the chlorine demand of the water was not satisfied. The water cannot be assumed safe and must not be issued.
Action, in order: (1) keep the tank closed off and tell the queue water is not ready; (2) re-dose at the full turbid-water rate - another 40 mL for 400 L - mix and wait a further 30 minutes; (3) retest. If the residual now sits in the 0.2–0.5 mg/L band, the water may be issued.
If it fails again, the source is heavily loaded with organic matter and dosing alone will not fix it: settle and filter first, then dose, or switch to boiling (rolling boil, then cool and store covered), or move to a cleaner source.
The reasoning to state: the test strip is the check on the calculation, not a decoration. A measured residual close to zero is evidence that the dose was consumed, so the correct response is to treat the water again - not to argue that chlorine "does not work" or to hand it out because people are thirsty.