Why is the chlorine dose taken equal to the 0.2 mg/l residual?

No chlorine demand is given, so demand is assumed nil

Residual is always the same as the dose

Chlorine is not consumed in water

Residual is the only part that disinfects

2069 Asar · 8 marks

Alum and chlorine quantity, 12 MLD

Short answer

With12 MLD flow and alum dose3 mg/L, alum required=36 kg/day. Assuming negligible chlorine demand, the dose equals desired residual0.2 mg/L, giving chlorine=2.4 kg/day.

Find the quantity of alum and chlorine required in a treatment plant of capacity 12 million lit/day. If optimum dose of alum is 3 mg/l and residual chlorine is expected in the distribution pipe is at the concentration of 0.2 mg/l.

Plant capacity = 12 MLD $= 12\times10^6$ l/day. Using $1\ \text{mg/l}\times1\ \text{MLD} = 1\ \text{kg/day}$ :

Alum (optimum dose 3 mg/l):

\text{Alum} = 3\times12 = 36\ \text{kg/day}

Chlorine: the residual chlorine wanted in the distribution pipe is 0.2 mg/l. No chlorine demand is given, so the demand of water is assumed to be nil and the dose is taken as equal to the residual (if there were a demand, the dose = demand + residual):

\text{Chlorine} = 0.2\times12 = 2.4\ \text{kg/day}

If bleaching powder with 30 % available chlorine is used, the quantity would be $\dfrac{2.4}{0.30} = 8.0$ kg/day.

Answer: alum = 36 kg/day; chlorine = 2.4 kg/day.

✓ Answer checked by Claude Opus 5.5 (high effort)

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