Chlorination Dose and Tank Filling
A Gram Panchayat receives a complaint that a 5,000-litre overhead tank serving a hamlet has not been cleaned for six months. The tank is rectangular, 2.5 m long, 2 m wide and 1 m deep; it is filled daily from a borewell pump that delivers 40 litres per minute. The caretaker proposes adding 50 g of bleaching powder containing 30% available chlorine to the full tank. The Panchayat health worker says the target dose is 2 mg of available chlorine per litre and that the water should be left for 30 minutes before use. On the same day, the pump runs for 2 hours and 5 minutes. The hamlet has 25 households and the Panchayat engineer is unavailable. The Secretary must decide whether the proposal is safe and sufficient, and how to explain it to residents.
1. Calculate the tank volume and check whether a 2-hour 5-minute run at 40 L/min fills it.
Read the worked answer
Volume = length × width × depth = 2.5 m × 2 m × 1 m = 5 cubic metres. Since 1 cubic metre is 1,000 litres, the tank holds 5,000 litres.
Pump run time = 2 hours 5 minutes = 125 minutes. At 40 litres per minute, water delivered = 125 × 40 = 5,000 litres. The run exactly fills the tank. If the tank already contains some water, the delivered volume would overflow or the actual volume treated would be less than 5,000 litres; the Secretary should confirm the tank is empty or account for the starting volume before dosing.
2. Is 50 g of bleaching powder with 30% available chlorine the correct dose for 2 mg/L? Show the calculation.
Read the worked answer
Available chlorine in 50 g = 50 g × 0.30 = 15 g = 15,000 mg. Dividing by 5,000 litres gives 15,000 ÷ 5,000 = 3 mg/L. That is 50% above the stated target of 2 mg/L.
For 2 mg/L in 5,000 litres, required available chlorine = 2 × 5,000 = 10,000 mg = 10 g. Required powder at 30% = 10 ÷ 0.30 = 33.3 g. So the caretaker's 50 g is an overdose relative to the target. The Secretary should not simply approve it. Use about 33 g, measured accurately, or follow the health worker's approved protocol. If the powder's strength is uncertain or the tank volume is not exactly 5,000 litres, the dose must be recalculated; do not guess.
3. What operational and communication steps should the Secretary take before and after chlorination?
Read the worked answer
Before dosing, clean the tank, remove silt, and confirm the actual water volume. Wear gloves, mask and eye protection; mix the powder in a small volume of water before adding it, never add dry powder directly to a full tank. Add the calculated dose, mix, and keep the tank closed for the required 30-minute contact time. Prevent residents from drawing water during this period.
After contact time, check for the approved residual chlorine if a test kit is available, and record the dose, time and persons involved. Inform all 25 households in advance through ward members and notice at the standpipe, explaining that the water may have a slight chlorine smell and should not be used until the Panchayat announces completion. If the water remains turbid or the dose cannot be verified, do not distribute it; refer to the health department. Record the complaint and action taken.