Question
Explain the process of biogas production. What is the specific role of methanogenic bacteria? Where do these bacteria naturally occur?
Solution — Step by Step
Biogas is a mixture of gases produced when organic matter is broken down by microorganisms in the absence of oxygen (anaerobic conditions). It consists primarily of:
- Methane (CH₄): 55–70% — the combustible component that provides energy
- Carbon dioxide (CO₂): 30–45%
- Traces of hydrogen sulphide, nitrogen, water vapour
Biogas is used for cooking, lighting, and electricity generation. India has one of the world’s largest biogas programmes, especially in rural areas through the National Biogas and Manure Management Programme (NBMMP).
Biogas production is not a single-step process. It involves a consortium of different microorganisms working in sequence:
Stage 1 — Hydrolysis: Complex organic molecules (cellulose, proteins, fats) are broken down into simpler soluble compounds (sugars, amino acids, fatty acids) by hydrolytic bacteria (e.g., Clostridium, Bacteroides).
Stage 2 — Acidogenesis (Fermentation): Simple molecules from Stage 1 are converted into volatile fatty acids (acetic, propionic, butyric acid), alcohols, CO₂, and H₂ by acidogenic bacteria.
Stage 3 — Acetogenesis: The fatty acids and alcohols from Stage 2 are converted into acetic acid, CO₂, and H₂ by acetogenic bacteria (e.g., Syntrophobacter). This is required because methanogenic bacteria can only use specific substrates.
Stage 4 — Methanogenesis: Methanogenic bacteria (methanogens) convert acetic acid, CO₂, and H₂ into methane gas — this is the final and most important step.
Methanogens (e.g., Methanobacterium, Methanosarcina, Methanobrevibacter) are strict anaerobes — they die in the presence of oxygen. They belong to the domain Archaea (not bacteria in the strict modern taxonomy, though commonly called methanogenic bacteria).
They perform methanogenesis through two main reactions:
Reaction 1 (Acetoclastic):
(Acetic acid → Methane + Carbon dioxide)
Reaction 2 (Hydrogenotrophic):
About 70% of methane in biogas comes from the acetoclastic reaction.
Key characteristics of methanogens:
- Strictly anaerobic — oxygen is toxic to them
- Slow-growing (slow doubling time) — why biogas plants need 20–40 days of retention time
- Sensitive to pH — optimal pH is 6.8–7.2 (slightly neutral)
- Sensitive to temperature — mesophilic methanogens work best at 30–40°C
Methanogenic bacteria are found wherever organic matter decomposes in anaerobic conditions:
- Rumen of cattle: The rumen is an anaerobic fermentation chamber. Methanogens break down cellulose from grass. Cattle belching is a source of atmospheric methane — a significant contributor to greenhouse gas emissions.
- Marshy/swampy areas (wetlands): Decomposition of plant material at the bottom of marshes produces “marsh gas” (mostly methane). This is why you see flames sometimes over marshes.
- Bottom of lakes and ponds (sediments): Anaerobic sediments are rich in methanogens.
- Human and animal gut: Small amounts in the large intestine.
- Sewage sludge: The basis for biogas generation in wastewater treatment plants.
A typical biogas plant (gobar gas plant in India) has:
- Mixing tank: Cattle dung + water (1:1 ratio) mixed here
- Digester (anaerobic tank): Underground sealed tank where all four stages occur
- Gas holder: Collects the methane produced
- Slurry outlet: The spent material (slurry) is an excellent nitrogen-rich fertiliser
The KVIC (Khadi and Village Industries Commission) model and the Janata model are the two main designs used in India.
Why This Works
Methanogenesis is the terminal step in anaerobic food chains. The methanogens depend on the earlier microorganisms (hydrolytic bacteria, acidogens, acetogens) to prepare suitable substrates. This interdependence is a syntrophic relationship — the success of the entire process depends on all groups working efficiently.
The strict anaerobic requirement of methanogens is why biogas digesters must be sealed. Even trace oxygen inhibits methanogens, reducing biogas yield.
Alternative Method
In biogas plants, the process can be monitored by checking pH: if it drops below 6.5, acids are accumulating faster than methanogens can consume them. Adding lime or increasing retention time can correct this.
Common Mistake
Students often write that methanogens “break down cellulose” to produce methane. This is wrong. Methanogens cannot hydrolyse cellulose — they only act on simple substrates like acetic acid, CO₂, and H₂ that are produced by earlier-stage bacteria. Cellulose breakdown is done by hydrolytic bacteria in Stage 1. Always present biogas production as a multi-stage, multi-microorganism process.