Select the statement that correctly relates to biogas and sewage treatment process.

Options

A. Activated sludge sediment in settlement tanks of the sewage treatment plants 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's
D. Biogas, commonly called gobar gas is pure methane.

Answer

To solve the question regarding the statements related to biogas and sewage treatment processes, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Activated Sludge**:
- Activated sludge is a mixture of microorganisms, including bacteria, that are used in the treatment of sewage.
- It is introduced into a settling tank where it helps in the breakdown of organic matter in wastewater.

**Hint**: Remember that activated sludge is crucial in sewage treatment and is rich in aerobic bacteria.

2. **Evaluating Statement A**:
- The statement claims that activated sludge sediment in a settlement tank is a rich source of aerobic bacteria.
- This statement is **correct** because the aerobic bacteria in the activated sludge play a vital role in breaking down organic materials in sewage.

**Hint**: Focus on the role of aerobic bacteria in treating sewage and their presence in activated sludge.

3. **Evaluating Statement B**:
- This statement claims that biogas is produced by the activity of aerobic bacteria on animal waste.
- This statement is **incorrect** because biogas is produced by anaerobic bacteria, not aerobic bacteria. Anaerobic digestion occurs in the absence of oxygen.

**Hint**: Recall the difference between aerobic and anaerobic processes, especially in the context of biogas production.

4. **Evaluating Statement C**:
- The statement mentions methanobacterium as an anaerobic bacterium found in the rumen of cattle.
- While methanobacterium is indeed an anaerobic bacterium, the statement implies that it is related to the production of biogas, which can be misleading. However, it is technically correct that methanobacterium is anaerobic.

**Hint**: Understand the role of methanobacterium in digestion and its anaerobic nature.

5. **Evaluating Statement D**:
- This statement claims that biogas is commonly known as a "cobalt gas" and is pure methane.
- This statement is **incorrect** because biogas is not pure methane; it is a mixture of gases, primarily methane and carbon dioxide, along with other gases like hydrogen sulfide.

**Hint**: Remember that biogas is a mixture and not a single pure substance.

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

### Final Answer:
The correct statement is **Option A**.