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 bac- terium found in rumen of cattle
D. Biogas, commonly called gobar gas, is pure methane

Explanation

**Step-by-Step Solution:**

1. **Understand the Question**: The question asks to select the correct statement from a list of statements related to microbes and their role in human welfare, particularly in water purification.

2. **Analyze Each Statement**:
- **Statement 1**: "Activated sludge sediment is used in the water purification plants."
- This statement is correct. Activated sludge is indeed used in sewage treatment plants.

- **Statement 2**: "Activated sludge sediments in the settlement tank of sewage treatment plant is a rich source of aerobic bacteria."
- This statement is also correct. Activated sludge contains a large number of aerobic bacteria that help in the breakdown of organic matter.

- **Statement 3**: "They digest organic matter in the presence of oxygen and decrease BOD."
- This statement is partially correct. Aerobic bacteria digest organic matter in the presence of oxygen, which initially increases Biological Oxygen Demand (BOD) as they consume oxygen, but eventually leads to a decrease in BOD as organic matter is broken down.

- **Statement 4**: "Biogas is produced by the activity of aerobic bacteria."
- This statement is incorrect. Biogas is primarily produced by anaerobic bacteria, not aerobic bacteria.

- **Statement 5**: "Methanobacterium is an aerobic bacteria found in the rumen of cattle."
- This statement is incorrect. Methanobacterium is an anaerobic bacterium found in the rumen of cattle.

- **Statement 6**: "Biogas mainly contains pure methane."
- This statement is incorrect. Biogas is a mixture of gases, including methane, carbon dioxide, and other gases, but it is not pure methane.

3. **Identify the Correct Statement**:
- After analyzing all the statements, the correct statement is that "Activated sludge sediments in the settlement tank of sewage treatment plant is a rich source of aerobic bacteria."

4. **Select the Answer**:
- Based on the analysis, the correct option is A.

**Final Answer**: The correct statement is: "Activated sludge sediments in the settlement tank of sewage treatment plant is a rich source of aerobic bacteria."

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