Which of the following is mainly produced by the activity of anaerobic bacteria on sewage

Options

A. Laughing gas
B. Propane
C. Mustard gas
D. Marsh gas

Answer

### Step-by-Step Solution:

1. **Understanding Anaerobic Bacteria**:
- Anaerobic bacteria are microorganisms that thrive in environments devoid of oxygen. They play a crucial role in the decomposition of organic matter, especially in sewage treatment.

2. **Anaerobic Digestion Process**:
- When sewage is treated anaerobically, organic matter is broken down by these bacteria. This process generates various gases as by-products.

3. **Identifying the Gases Produced**:
- The primary gas produced by anaerobic bacteria during the breakdown of organic matter is methane (CH4), which is commonly referred to as marsh gas or biogas.

4. **Evaluating the Options**:
- **Option 1: Laughing gas** - This is nitrous oxide (N2O) and is not produced by anaerobic bacteria.
- **Option 2: Propane** - This is a hydrocarbon gas used primarily as fuel and is not a product of anaerobic digestion.
- **Option 3: Mustard gas** - This is a chemical warfare agent and is not related to the activity of anaerobic bacteria.
- **Option 4: Marsh gas** - This is another name for biogas, which is primarily composed of methane produced by anaerobic bacteria.

5. **Conclusion**:
- Based on the breakdown of organic matter by anaerobic bacteria in sewage, the gas that is mainly produced is marsh gas (biogas).

### Final Answer:
**Marsh gas**

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