Question

The amount of residual chlorine left in public water supply for safety against pathogenic bacteria is about:-
1. 0.01 to 0.05 ppm
2. 0.05 to 0.50 ppm
3. 0.50 to 1.00 ppm
4. 1.00 to 5.00 ppm

Answer 1

Correct Answer - Option 2 : 0.05 to 0.50 ppm

Explanation:

• Doses of chlorine should be sufficient so as to leave a residue of 0.2 mg/l or 0.2 ppm after 10 min of the contact period.
• The residue chlorine is tested by the Diethyl-paraphenylene diamine (DPD) test.
• The higher the residual chlorine levels in the supply, the better and longer the chemical will be able to protect the system from contamination.
• However, high levels of chlorine make the water smell and give it a bad taste, which will discourage people from drinking it.
• For normal domestic use, residual chlorine levels at the point where the consumer collects water should be between 0.2 and 0.5 mg/l.
• The higher level will be close to the disinfection point and the lower level at the far extremities of the supply network.