Activated sludge should have the ability to settle quickly so that it can (a) be rapidly pumped back from sedimentation tank to aeration tank (b) absorb pathogenic bacteria present in waste water while sinking to the bottom of the settling tank (c) be discarded and anaerobically digested (d) absorb colloidal organic matter

Options

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

Explanation

To solve the question regarding the ability of activated sludge to settle quickly, we can break down the reasoning step by step:

### Step 1: Understanding Activated Sludge
Activated sludge is a mixture of bacteria and organic matter that is formed during the secondary treatment of wastewater. It plays a crucial role in breaking down organic pollutants.

### Step 2: The Process of Wastewater Treatment
In the secondary treatment of wastewater, the biochemical oxygen demand (BOD) of the sewage is reduced. After this reduction, the wastewater is passed into settling tanks where the activated sludge can settle.

### Step 3: Formation of Activated Sludge
In the settling tanks, bacterial flocks and sediments form. These flocks settle down to the bottom, creating what we refer to as activated sludge. The ability of these flocks to settle quickly is essential for efficient wastewater treatment.

### Step 4: Purpose of Quick Settling
The quick settling of activated sludge is necessary to allow a portion of this sludge to be pumped back to the aeration tank. This is important because the sludge acts as an inoculum, providing the necessary bacteria to continue breaking down organic matter in the aeration tank.

### Step 5: Evaluating the Options
Now, let’s evaluate the given options:
- **(a)** Be rapidly pumped back from sedimentation tank to aeration tank: This is correct, as we have established that the quick settling allows for this process.
- **(b)** Absorb pathogenic bacteria present in wastewater while sinking to the bottom of the settling tank: This is incorrect; the primary function is not to absorb pathogens.
- **(c)** Be discarded and anaerobically digested: This is also incorrect; while some sludge may be digested, the focus here is on returning it to the aeration tank.
- **(d)** Absorb colloidal organic matter: This is incorrect as well; the main function is not to absorb colloidal matter.

### Conclusion
The correct answer is **(a)** be rapidly pumped back from sedimentation tank to aeration tank.

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