Q.(a) Write the scientific name of methanogen bacteria. Where are these bacteria generally found ? Explain their role in biogas production.
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Start your 14-day free trial to unlock the full solution →Methanogens like Methanobacterium are archaebacteria found in anaerobic environments (rumen, sewage, wetlands) that convert organic acids and H2–CO2 into methane, the key step in biogas production. Biogas contains methane (50–70%), carbon dioxide (30–40%), and traces of hydrogen sulfide, nitrogen, and hydrogen.
Understanding Methanogens and Their Ecological Niche
Methanogens are not true bacteria — they belong to domain Archaea, a group of prokaryotes with distinct cell-wall chemistry and metabolic pathways. The confusion arises because they were historically called "methanogenic bacteria" before the three-domain classification was established. These organisms are strict anaerobes, meaning oxygen is toxic to them. They thrive in oxygen-free environments rich in organic matter: the rumen of cattle and buffalo, marshy wetlands, rice paddies, and most importantly for us, the sludge at the bottom of sewage treatment tanks and biogas digesters.
Their metabolic superpower is producing methane (CH4) as an end product of energy metabolism, which makes them indispensable in biogas technology.
(a) Scientific Name and Role in Biogas Production
Scientific name: The most commonly cited methanogen in biogas contexts is Methanobacterium (other examples include Methanococcus, Methanosarcina, and Methanospirillum).
Where they are found:
- Anaerobic sludge in biogas plants and sewage treatment facilities
- Rumen (stomach) of ruminant animals
- Waterlogged paddy fields and natural wetlands
- Bottom sediments of lakes and ponds
Role in biogas production:
Biogas generation is a multi-stage anaerobic digestion process, and methanogens perform the final, rate-limiting step:
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Hydrolysis and Acidogenesis (by other bacteria): Complex organic polymers — cellulose, proteins, fats in cattle dung, agricultural waste, sewage — are broken down by hydrolytic and fermentative bacteria into simpler molecules: sugars, amino acids, fatty acids. These are then converted into volatile fatty acids (acetic acid, propionic acid, butyric acid), alcohols, H2, and CO2.
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Acetogenesis (by acetogenic bacteria): Acetogenic bacteria convert these fatty acids and alcohols into acetic acid (CH3COOH), hydrogen (H2), and carbon dioxide (CO2).
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Methanogenesis (by methanogens): Methanogens take over in the final step, using two main pathways:
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Acetoclastic pathway:
CH3COOH -> CH4 + CO2
About 70% of methane comes from acetate splitting.
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Hydrogenotrophic pathway:
CO2 + 4H2 -> CH4 + 2H2O
The remaining 30% comes from reduction of CO2 by H2.
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Without methanogens, organic acids would accumulate, the pH would drop, and the entire digestion process would stall. Methane production also removes hydrogen, which keeps the thermodynamics favorable for the earlier fermentation steps.
Biogas plants maintain temperature around 35–37 °C (mesophilic range) or 55–60 °C (thermophilic range) because methanogens are temperature-sensitive and work best in these windows.
(b) Components of Biogas
Biogas is not pure methane — it's a mixture whose composition depends on feedstock and digestion conditions. The typical components are:
| Component | Percentage Range | Notes | …
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