Once BOD from the sewage is reduced significantly, the 'flocs' are allowed to sediment and it is known as ........

Options

A. primary sludge
B. secondary sludge
C. activated sludge
D. inactivated sludge

Answer

To solve the question, we need to understand the process of sewage treatment and the terminology associated with it. Here's a step-by-step breakdown:

### Step-by-Step Solution:

1. **Understanding BOD**:
- BOD stands for Biochemical Oxygen Demand. It measures the amount of oxygen that microorganisms will consume while decomposing organic matter in water. A higher BOD indicates a higher level of organic pollution.

2. **Sewage Treatment Process**:
- The sewage treatment process typically involves three main steps: primary treatment, secondary treatment, and tertiary treatment.
- In the primary treatment, physical processes like filtration and sedimentation are used to remove large particles from sewage.

3. **Secondary Treatment**:
- In the secondary treatment, biological processes are employed. This involves the use of microorganisms to further reduce BOD by consuming the organic matter in the sewage.
- During this stage, flocs (masses of bacteria and fungi) are formed, which help in the breakdown of organic matter.

4. **Sedimentation of Flocs**:
- Once the BOD is significantly reduced, the effluent containing these flocs is passed into a settling tank.
- In the settling tank, the flocs are allowed to sediment, meaning they settle down to form a solid mass.

5. **Naming the Sediment**:
- The sediment formed from the flocs after the BOD reduction is referred to as "activated sludge." This is because the sludge contains active microorganisms that have been involved in the decomposition process.

6. **Conclusion**:
- Therefore, the correct answer to the question is that once BOD from the sewage is reduced significantly, the flocs are allowed to sediment and it is known as **activated sludge**.

### Final Answer:
**Activated Sludge**

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