Question

Chlorine is sometimes used in sewage treatment:
1. To avoid flocculation
2. To increase biological activated sludge
3. To avoid bulking of activated sludge
4. To help in grease separation

Answer 1

Correct Answer - Option 3 : To avoid bulking of activated sludge

Explanation:

Sludge bulking:

(i) A Sludge that exhibits poor settling characteristics is called a bulking sludge. Filamentous micro-organisms (e.g. fungi) have been found to be responsible for a bulked sludge. Filamentous micro-organisms are observed due to low nutrient content, high sludge age, and low F/M ratio.

(ii) The most successful methods to control these organisms, as per the latest research, are:

• Reduction of the sludge age to less than 6 days
• Chlorination of return activated sludge
• Addition of nutrient if it is less (BOD5 : N2 : P ratio recommends = 100 : 5 : 1)