Select the correct statement from the following .

Options

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

Answer

To solve the question, we need to analyze each statement provided and determine which one is correct. Here’s a step-by-step breakdown:

### Step 1: Analyze the First Statement
- **Statement**: "Biogas is produced by the activity of aerobic bacteria on animal waste."
- **Analysis**: Biogas is produced through anaerobic digestion, which means it occurs in the absence of oxygen. The statement incorrectly states that aerobic bacteria are involved, making this statement incorrect.

### Step 2: Analyze the Second Statement
- **Statement**: "Methanobacterium is an aerobic bacterium which is found in cattle's rumen."
- **Analysis**: Methanobacterium is actually an anaerobic bacterium, meaning it thrives in environments without oxygen. Therefore, this statement is also incorrect.

### Step 3: Analyze the Third Statement
- **Statement**: "Biogas is commonly called govergas and it is pure methane."
- **Analysis**: Biogas is not pure methane; it is a mixture of gases including methane (CH4), carbon dioxide (CO2), and traces of other gases. The term "govergas" is not a standard term for biogas. Thus, this statement is incorrect as well.

### Step 4: Analyze the Fourth Statement
- **Statement**: "Activated sludge is the sediment of the settling tank of the sewage treatment plant and it is a very good source of aerobic bacteria."
- **Analysis**: Activated sludge refers to the biomass that is produced during the treatment of sewage. It is indeed rich in aerobic bacteria and is formed in the aeration tank of sewage treatment plants. This statement is correct.

### Conclusion
After analyzing all the statements, the only correct statement is the fourth one: "Activated sludge is the sediment of the settling tank of the sewage treatment plant and it is a very good source of aerobic bacteria."

### Final Answer
The correct statement is option 4.

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