Select the correct statement regarding activated sludge formed during secondary sewage treatment.

Options

A. A small part of it is rapidly pumped back from sedimentation to aeration tank
B. It absorbs pathogenic bacteria present in waste water while sinking to the bottom of the settling tank
C. A major part of it is anaerobically digested
D. Both (a) and (c)

Answer

To solve the question regarding the correct statement about activated sludge formed during secondary sewage treatment, we can analyze the provided options step by step.

### Step-by-Step Solution:

1. **Understanding Activated Sludge**: Activated sludge is a mixture of microorganisms that is used in the secondary treatment of sewage. It helps in breaking down organic matter in wastewater.

2. **Option A**: "A small part of it is rapidly pumped back from sedimentation to aeration tank."
- This statement is correct. During the sewage treatment process, a portion of the activated sludge is returned to the aeration tank to maintain the microbial population needed for the treatment process.

3. **Option B**: "It absorbs pathogenic bacteria present in wastewater while sinking to the bottom of the settling tank."
- This statement is incorrect. Activated sludge does not specifically absorb pathogenic bacteria while sinking; instead, it is involved in the breakdown of organic matter and does not selectively absorb pathogens.

4. **Option C**: "A major part of it is anaerobically digested."
- This statement is correct. A significant portion of the activated sludge undergoes anaerobic digestion, which is a process where microorganisms break down organic matter in the absence of oxygen, leading to the production of biogas.

5. **Option D**: "Both A and C."
- Since both statements A and C are correct, this option is also correct.

### Conclusion:
The correct answer is **Option D: Both A and C**.