Activated sludge should have the ability to settle quickly so that it can

Options

A. be rapidly pumped back from sedimen- tation tank to aeration tank
B. absorb pathogenic bacteria present in waste water while sinking to the bottom of the settling tank
C. be discarded & anaerobically digested
D. absorb colloidal organic matter

Answer

### Step-by-Step Solution:

1. **Understanding Activated Sludge**: Activated sludge is a key component in the wastewater treatment process, specifically in the secondary treatment phase. It consists of a mixture of microorganisms, primarily bacteria and protozoa, that help in breaking down organic matter in wastewater.

2. **Importance of Quick Settling**: The ability of activated sludge to settle quickly is crucial for efficient wastewater treatment. When wastewater undergoes treatment, it is aerated to promote the growth of microbial flocs, which then need to settle in a sedimentation tank.

3. **Function of Settling**: In the sedimentation tank, the microbial flocs (activated sludge) settle to the bottom. This process allows for the separation of treated water from the settled sludge. The settled sludge can then be partially recycled back to the aeration tank as inoculum, which is essential for maintaining the microbial population that continues to treat the incoming wastewater.

4. **Consequences of Slow Settling**: If the activated sludge does not settle quickly, it can lead to several issues:
- Inefficient separation of treated water and sludge.
- Difficulty in recycling the sludge back to the aeration tank.
- Potential overloading of the treatment system, which can compromise the treatment efficiency.

5. **Conclusion**: Therefore, the primary reason for the activated sludge needing to settle quickly is to allow for its rapid pumping back from the sedimentation tank to the aeration tank, ensuring a continuous and efficient wastewater treatment process.

### Final Answer:
Activated sludge should have the ability to settle quickly so that it can be rapidly pumped back from the sedimentation tank to the aeration tank.

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