Step 1: Understanding the Concept:
Methanogens are anaerobic bacteria commonly found in anaerobic sludge during sewage treatment and in the rumen of cattle.
Step 2: Detailed Explanation:
1. Anaerobic Digestion: These bacteria grow anaerobically on cellulosic materials present in organic waste (like cow dung/gobar).
2. Production of Gases: During their metabolic activity, they produce large amounts of methane ($CH_4$) along with carbon dioxide ($CO_2$) and hydrogen ($H_2$).
3. Biogas Generation: This mixture of gases is highly inflammable and is collectively called biogas.
4. Example: Methanobacterium is a common genus involved in this process.
Step 3: Final Answer:
Methanogens decompose organic matter in the absence of oxygen to release methane-rich biogas, used as a renewable energy source.