The supernatant left after sedimentation of sewage water during primary treatment is known as

Options

A. affluent
B. flocs
C. sludge
D. effluent

Answer

To solve the question regarding the supernatant left after sedimentation of sewage water during primary treatment, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Sewage Treatment**:
- Sewage treatment involves multiple stages to clean wastewater. The primary treatment is the first step in this process.

**Hint**: Remember that sewage treatment has multiple stages, and primary treatment is the first one.

2. **Identifying Primary Treatment**:
- The primary treatment consists of physical processes such as filtration and sedimentation. During sedimentation, heavier particles settle at the bottom of the treatment tank.

**Hint**: Focus on the processes involved in primary treatment, especially sedimentation.

3. **Sedimentation Process**:
- In the sedimentation tank, when sewage water is allowed to stand, solids like sand, stones, and grit settle down to form a layer at the bottom. This settled material is known as "sludge."

**Hint**: Think about what happens to solids during sedimentation and what remains above them.

4. **Understanding the Supernatant**:
- The liquid that remains above the settled solids after sedimentation is referred to as "effluent." This effluent is usually clearer and is sent for further treatment in the secondary treatment phase.

**Hint**: Identify the terminology used for the liquid above the settled solids.

5. **Conclusion**:
- The question specifically asks for the supernatant after sedimentation. The correct term for the settled solids is "sludge," while the liquid above is called "effluent." Therefore, the answer to the question is "effluent."

**Hint**: Distinguish between sludge (the settled solids) and effluent (the liquid above).

### Final Answer:
The supernatant left after sedimentation of sewage water during primary treatment is known as **effluent**.