Question

How many kg of bleaching powder is needed per day to chlorinate 4 MLD of water so than, after 40 minutes of contact, there remains residual chlorine of 0.25 mg/l. The input water has a chlorine demand of 1.25 mg/l. And that the bleaching powder has only 25% available chlorine. 
1. 8 kg
2. 20 kg 
3. 24 kg 
4. 6.6 kg

Answer 1

Correct Answer - Option 3 : 24 kg 

Concept:

Chlorination: It is the process of adding chlorine or chlorine compounds such as sodium hypochlorite (or bleaching powder) to water. This method is used to kill certain bacteria and other microbes present in water. In particular, chlorination is used to prevent the spread of waterborne diseases such as cholera, dysentery, and typhoid.

Calculations:

Given, chlorine demand for the water = 1.25 mg/l and residual chlorine demand = 0.25 mg/l.

So, total chlorine concentration required = 1.25 + 0.25 = 1.5 mg/l.

Total water is to be treated = 4MLD = 4 × 10⁶ litres/day.

∴ Total chlorine required per day = (4 × 10⁶ × 1.5) mg = 6 kg.

Now, this amount of chlorine is to be produced by bleaching powder which has only 25% available chlorine.

∴ Bleaching powder required per day = 6/0.25 kg = 24 kg.