Select the correct statement from the following :

Options

A. Activated sludge-sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria
B. Biogas is produced by the activity of aerobic bacteria on animal waste
C. Methanobacterium is an aerobic bacterium found in rumen of cattle
D. Biogas commonly called gobar gas, is pure methane

Answer

To solve the question of selecting the correct statement from the provided options, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Analyze Statement 1:**
- "Activated sludge sediment in settlement tank of sewage treatment plant is a rich source of aerobic bacteria."
- **Explanation:** Activated sludge is indeed a mixture of microorganisms, primarily aerobic bacteria, that are used in the treatment of sewage. These bacteria help in breaking down organic matter in the sewage. This statement is correct.

2. **Analyze Statement 2:**
- "Biogas or Methane is produced by the activity of aerobic bacteria on animal waste."
- **Explanation:** Biogas is primarily produced by anaerobic bacteria, not aerobic bacteria. Anaerobic bacteria decompose organic matter in the absence of oxygen, leading to the production of biogas, which contains methane. This statement is incorrect.

3. **Analyze Statement 3:**
- "Methanobacterium is an aerobic organism."
- **Explanation:** Methanobacterium is a type of methanogenic bacteria that functions in anaerobic conditions and is responsible for producing methane. Therefore, this statement is also incorrect.

4. **Analyze Statement 4:**
- "Biogas commonly called Gobber gas is pure methane."
- **Explanation:** Biogas is a mixture of gases, primarily methane, but it is not pure methane. It contains other gases such as carbon dioxide, hydrogen sulfide, and traces of other gases. Hence, this statement is incorrect.

### Conclusion:
After analyzing all the statements, the only correct statement is:
- **Statement 1:** "Activated sludge sediment in settlement tank of sewage treatment plant is a rich source of aerobic bacteria."

### Final Answer:
The correct statement is **Statement 1**.

---