Methanogenic bacteria are not found in

Options

A. rumen of cattle
B. gobar gas plant
C. bottom of water-logged paddy fields
D. activated sludge

Explanation

### Step-by-Step Solution:

1. **Understanding Methanogenic Bacteria**:
- Methanogenic bacteria are a type of archaea that produce methane as a metabolic byproduct in anoxic conditions (absence of oxygen). They play a crucial role in the anaerobic digestion process.

2. **Identifying Habitats of Methanogenic Bacteria**:
- Methanogenic bacteria are typically found in environments where anaerobic conditions prevail. Common habitats include:
- The rumen of cattle (a part of their digestive system).
- Gobar gas plants (where organic waste is decomposed anaerobically).
- Bottom of waterlogged paddy fields (where water creates an anaerobic environment).

3. **Analyzing the Options**:
- The question provides four options:
1. Rumen of the cattle
2. Gobar gas plant
3. Bottom of waterlogged paddy fields
4. Activated sludge

4. **Evaluating Each Option**:
- **Rumen of the cattle**: Contains methanogenic bacteria that help in digestion.
- **Gobar gas plant**: Specifically designed for anaerobic digestion, thus contains methanogenic bacteria.
- **Bottom of waterlogged paddy fields**: An anaerobic environment conducive to methanogenic bacteria.
- **Activated sludge**: This is a process used in wastewater treatment that primarily involves aerobic bacteria, which thrive in the presence of oxygen.

5. **Conclusion**:
- Methanogenic bacteria are not found in activated sludge because this environment is dominated by aerobic bacteria. Therefore, the correct answer is option 4: Activated sludge.

### Final Answer:
Methanogenic bacteria are not found in **activated sludge**.

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