Question
Describe the process of biogas production. Which microbes are involved, and how does a biogas plant work?
(NCERT Class 12, Chapter 10 — Microbes in Human Welfare)
Solution — Step by Step
Biogas is produced by methanogenic bacteria (methanogens). The most important genus is Methanobacterium. These are strict anaerobes — they work only in the absence of oxygen.
Methanogens belong to the domain Archaea, not typical bacteria. They thrive in anaerobic environments like the rumen of cattle, marshy areas, and biogas digesters.
The raw material is cattle dung (gobar), which is rich in cellulose-degrading bacteria from the cow’s gut. Agricultural waste, sewage, and other biodegradable matter can also be used.
The cattle dung already contains methanogens — this is why cow dung is the preferred feedstock. No external inoculation is needed.
The biogas plant has a concrete tank (digester) with a floating gas holder on top.
- Slurry preparation — cattle dung is mixed with water to form slurry and fed into the digester through the inlet.
- Anaerobic digestion — inside the sealed digester, methanogens break down cellulosic material. The process occurs in stages: first, acidogenic bacteria convert complex organics to organic acids; then, methanogens convert these acids to methane (CH₄) and CO₂.
- Gas collection — the biogas (mainly CH₄ + CO₂) collects in the gas holder, which rises as gas accumulates. A pipe carries the gas for use as fuel.
- Spent slurry — the digested material exits from the outlet and is used as organic manure.
Biogas is a mixture: 50-70% methane, 30-40% CO₂, and traces of H₂S and H₂. The high methane content makes it an excellent fuel — it burns with a clean, blue flame.
Why This Works
The entire process depends on anaerobic conditions. Methanogens cannot survive in the presence of oxygen — that’s why the digester is sealed. The multi-step breakdown (complex organics → organic acids → methane) involves a consortium of bacteria working together, but the final methane-producing step is exclusively done by methanogens.
The technology was popularised in India through the Gobar Gas (KVIC) model and the IARI model developed at the Indian Agricultural Research Institute, New Delhi.
Alternative Method — Flow Summary
Cattle dung + water → Slurry → Sealed digester (anaerobic) → Methanogens act → Biogas (CH₄ + CO₂) rises → Collected via gas holder → Used as fuel
For NEET, remember two things: the microbe is Methanobacterium and the condition is anaerobic. Most MCQs test whether you know these two facts. Also remember that methanogens are Archaea, not eubacteria — this is a favourite trick question.
Common Mistake
Students often write that biogas contains “only methane.” This is wrong — biogas is a mixture of methane and CO₂ (with traces of other gases). The question “What is the main component of biogas?” has the answer methane, but “What is biogas composed of?” requires you to mention both CH₄ and CO₂. Read the question carefully.