Chlorinating a Turbid Field Water Tank
A field water point serves 400 personnel at 5 L per person per day for drinking and cooking. The source is a 5,000 L open tank filled from a turbid river; measured turbidity is 25 NTU. You hold 4 L of sodium hypochlorite solution labelled 5 percent available chlorine, stored in a hot shed for eight months; you estimate it has lost about half its strength. Standing procedures require a free chlorine residual of 0.5 mg/L after 30 minutes of contact and at least 1.0 mg/L at dosing for turbid water, with a target dose of 2 mg/L for water below 10 NTU. Above 10 NTU, the procedure says to settle or filter first, then dose, and to verify residual before release. A sergeant proposes adding the whole 4 L to the tank and distributing immediately.
1. Evaluate the sergeant's proposal numerically and explain why it should be rejected.
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The label says 5 percent available chlorine, which is 50 g/L. Four litres therefore contain 4 × 50 = 200 g of available chlorine if fresh. With the estimated 50 percent loss, about 100 g remains.
In 5,000 L, 100 g gives 100,000 mg ÷ 5,000 L = 20 mg/L, which is ten times the 2 mg/L target. Even if the bleach were fresh, the dose would be 40 mg/L. The 25 NTU turbidity will consume some chlorine, but the amount consumed is unknown and cannot justify an uncontrolled overdose. Distributing immediately also gives no 30-minute contact time and no residual check.
Reject the proposal: it risks an unpleasant or unsafe chlorine concentration and provides no verification.
2. Calculate the correct dose after pretreatment, allowing for bleach degradation.
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Because turbidity is 25 NTU, above the 10 NTU threshold, first settle or filter the water and recheck turbidity. Then dose at the target 2 mg/L for the 5,000 L batch: 5,000 × 2 = 10,000 mg = 10 g of available chlorine.
With degraded bleach at roughly 25 g/L, the required volume is 10 g ÷ 25 g/L = 0.4 L, or 400 mL. With fresh 50 g/L bleach it would be 200 mL. Add the 400 mL with stirring, wait 30 minutes, and measure free residual. The acceptance criterion is at least 0.5 mg/L free residual; if it is below that, add further 100 mL increments and retest rather than guessing a large dose.
3. The residual after 30 minutes is only 0.1 mg/L. What do you do, and what daily management issue remains?
Read the worked answer
A residual of 0.1 mg/L means the chlorine demand of the turbid water consumed the dose. Do not release the water. Improve pretreatment first, since adding more chlorine to turbid water wastes chemical and creates taste problems. Then re-dose in increments, for example another 200 mL of the degraded solution, stir, wait 30 minutes and retest. If residual still fails, increase settling time or filter through sand or cloth before the next batch.
Daily management: 400 people × 5 L = 2,000 L per day, so the 5,000 L tank is about two and a half days of supply. Chlorine residual decays with time, heat and sunlight, so the tank should be covered, batches should be dosed and checked individually, and residual should be retested before issue rather than assumed from the morning reading.