Making safe drinking water from a canal with limited chlorine and fuel
A 10-soldier section must produce drinking water for 3 days from a turbid canal. Planning requirement is 4.5 L per person per day. Available: two 20 L jerry cans, one 1-micron ceramic filter, 500 mL of 2% w/v sodium hypochlorite solution, 40 chlorine tablets (each releasing 67 mg available chlorine and treating 1 L), 30 mL iodine tincture, and fuel sufficient to boil only 30 L of water. There is no electricity and no way to measure chlorine residual. The canal water is visibly brown after recent rain. Last week a neighbouring unit recorded four cases of diarrhoea after drinking filtered canal water.
1. Explain why the filter alone and boiling alone each fail as a complete plan, and give the treatment sequence you would actually use for each can, with the chemistry behind the sequence.
Read the worked answer
Filter alone fails because a 1-micron ceramic element physically removes protozoan cysts and most bacteria but not viruses (hepatitis A, norovirus, rotavirus are far smaller than 1 micron). It also does nothing about dissolved contamination, and a loaded, rarely cleaned filter becomes a reservoir. The neighbouring unit's four diarrhoea cases are consistent with exactly this gap, or with recontamination during storage.
Boiling alone fails on logistics, not on microbiology. A rolling boil kills all waterborne pathogens, but 10 soldiers × 4.5 L × 3 days = 135 L, and the fuel will boil only 30 L - about 22% of the requirement. Boiling also leaves the brown turbidity in place.
Sequence for each can: settle and coarse-strain the water → pass it through the ceramic filter (clarification) → chlorinate → 30 minutes contact → store covered and decant without touching the inside of the lid or the rim.
The chemistry is the reason clarification comes first. Chlorine is consumed by organic matter and reduced compounds in the water - this is the chlorine demand. Dosing brown, turbid water spends most of its hypochlorous acid on the sediment and leaves too little free residual to inactivate pathogens, and the residual cannot be measured here. Clarifying first reduces demand so the dose you add is the dose that disinfection actually uses. Hypochlorous acid (HOCl) is the active species and works best in water that is clear and not heavily loaded with organic matter.
Note also that 40 tablets treat only 40 L - under a third of the requirement - so tablets are a reserve for small parties, not the main method.
2. Calculate how much of the 2% hypochlorite to add to a 20 L jerry can, showing your working, and explain why 'add extra to be safe' is a bad decision here.
Read the worked answer
Working:
- 2% w/v = 20 g of sodium hypochlorite per litre = 20,000 mg/L of product.
- Available chlorine is about 95% of the product mass, so roughly 19,000 mg/L of available chlorine.
- Target dose for clarified water: about 2 mg/L.
- Chlorine needed for 20 L: 20 × 2 = 40 mg.
- Volume of stock: 40 mg ÷ 19,000 mg/L = 0.0021 L = about 2 mL per 20 L can (roughly 40 drops at 20 drops/mL).
- If the water is still noticeably cloudy, double to about 4 mL - but only after clarifying, because the extra would otherwise be consumed by sediment.
Why not simply add more:
- Above roughly 4–5 mg/L free chlorine the water tastes and smells of swimming pool, and soldiers in heat reduce intake of unpalatable water - which causes the dehydration you were trying to prevent. Non-compliance is a real dosing error.
- In water still containing organic matter, excess chlorine forms chloramines and trihalomethanes, which taste worse and carry long-term health concerns.
- With no residual test kit, an over-dose is a guess that hides the actual problem (turbidity), and it teaches the section that dosing is not a calculation.
- Excess chlorine does not fix what the filter was supposed to fix.
The judgement point is that in field water treatment the dose is a calculation with a target, not a gesture of caution.
3. Decide how to allocate the 30 L of boiling fuel and justify the allocation in one or two sentences that you could give as an order.
Read the worked answer
Allocation: boil the water destined for the people at highest risk - anyone with diarrhoea, the medic's own supply, and water used for cleaning a wound or dressing - and treat the bulk requirement with filtration plus chlorination. Do not boil water that will be chlorinated anyway if the filter is serviceable.
Order: 'Boiling is our only virus-proof method and we have fuel for 30 litres out of the 135 we need. It goes to the sick, the medic and wound care. Everything else is filtered, settled and chlorinated at 2 mL per can, 30 minutes' contact. Nobody drinks untreated water, and nobody uses the iodine except on the medic's instruction.'
Reasoning behind the order: the scarce resource should buy the capability nothing else provides. Filtration plus chlorination covers bacteria and protozoa adequately for healthy troops; it is weakest against viruses and against anything going into a wound, so boiling is spent there. Iodine is held back because it is a poor primary disinfectant for turbid water, is unpalatable, and is unsuitable for anyone with thyroid disease or pregnancy - a decision for the medic, not for individuals.