Study the following statements and select the incorrect ones.
(i) Physical removal of large and small particles through filtration and sedimentation is called primary sewage treatment.
(ii) Secondary sewage treatment is mainly a mechanical process.
(iii) Activated sludge sediment in a sewage treatment plant is a rich source of aeroic bacteria.
(iv) Biogas, commonly called as gobar gas, is pure methane.

Options

A. (i) and (ii)
B. (ii) and (iv)
C. (ii) and (iii)
D. (iii) and (iv)

Explanation

To determine the incorrect statements from the provided options, let's analyze each statement step by step.

### Step-by-Step Solution:

1. **Statement (i)**: "Physical removal of large and small particles through filtration and sedimentation is called primary sewage treatment."
- **Analysis**: This statement is correct. Primary sewage treatment involves the physical processes of filtration and sedimentation to remove solid particles from sewage.

2. **Statement (ii)**: "Secondary sewage treatment is mainly a mechanical process."
- **Analysis**: This statement is incorrect. Secondary sewage treatment is primarily a biological process, not mechanical. It involves the use of microorganisms to degrade organic matter in the sewage.

3. **Statement (iii)**: "Activated sludge sediment in a sewage treatment plant is a rich source of aerobic bacteria."
- **Analysis**: This statement is correct. Activated sludge contains a high concentration of aerobic bacteria, which are essential for the biological treatment of sewage.

4. **Statement (iv)**: "Biogas, commonly called as gobar gas, is pure methane."
- **Analysis**: This statement is incorrect. While biogas contains a significant amount of methane (about 60-70%), it is not pure methane. Biogas also contains carbon dioxide, hydrogen sulfide, and traces of other gases.

### Conclusion:
The incorrect statements are:
- **Statement (ii)**: Secondary sewage treatment is mainly a mechanical process. (Incorrect)
- **Statement (iv)**: Biogas, commonly called as gobar gas, is pure methane. (Incorrect)

### Final Answer:
The incorrect statements are (ii) and (iv).

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