Question
A biogas plant uses animal dung mixed with water to produce biogas. Explain the role of methanogens in this process. What is the composition of biogas, and why is it considered a clean fuel?
Solution — Step by Step
Animal dung is mixed with water to form slurry, which is fed into an airtight digester tank. The key word here is airtight — oxygen must be absent for the right bacteria to work.
First, a group of anaerobic bacteria break down complex organic molecules — cellulose, proteins, fats — into simpler compounds like organic acids, CO₂, and H₂. These are not the methanogens; they are the preparatory crew.
Without this first stage, methanogens would have nothing to work with. The process is sequential, not simultaneous.
Methanobacterium (the main methanogen here) then acts on the organic acids and CO₂ + H₂ produced in stage one. They convert these into methane (CH₄), which collects in the gas holder above the digester.
The reaction essentially is:
The gas that accumulates is biogas. Its typical composition:
| Component | Percentage |
|---|---|
| Methane (CH₄) | 50–70% |
| Carbon dioxide (CO₂) | 25–45% |
| Traces of H₂S, H₂ | small amounts |
The high methane content is what makes it combustible and useful as fuel.
After gas production, the spent slurry is removed from the other end. It is rich in nitrogen and phosphorus — making it an excellent biofertiliser. Nothing is wasted in a well-run biogas plant.
Why This Works
Methanogens are strict anaerobes — they die in the presence of oxygen. This is why the digester is airtight. Any oxygen leak shuts down methanogenesis, which is a common practical failure in poorly built biogas plants.
The reason animal dung works so well is that it already contains a natural population of these bacteria in the gut flora of cattle. You are not adding anything artificial — the microbial community is already there, you just create the right conditions (no oxygen, correct temperature around 35°C, adequate moisture).
Biogas is called a clean fuel because burning CH₄ produces mainly CO₂ and water vapour, with no soot or sulfur dioxide. Compare this to burning cow dung cakes directly, which releases particulate matter and CO into homes — a major health issue in rural India.
Alternative Method — From the Exam Angle
NEET sometimes frames this as a diagram-based question showing the biogas plant structure. Know these components:
- Mixing tank — where dung + water slurry is prepared
- Digester (underground) — airtight tank where fermentation happens
- Gas holder (floating drum) — collects the biogas above
- Outlet — releases spent slurry (biofertiliser)
If the question asks to label the diagram, the gas holder and digester are the most commonly asked labels. The NCERT diagram is sufficient — memorise it.
Common Mistake
Students write that methanogens break down cellulose directly. They do not. Cellulose is broken down by other anaerobic bacteria first. Methanogens only act on the simpler products — organic acids, CO₂, H₂. Getting this sequence wrong costs marks in NEET.
NCERT explicitly names Methanobacterium — use this full name in answers, not just “methanogens”. One-mark questions in board exams often expect the genus name. Also remember: methanogens are found in the gut of ruminants like cattle, which is how they end up in dung naturally.
Final answer summary: Methanogens like Methanobacterium act anaerobically on organic acids and gases (CO₂, H₂) produced by other bacteria from dung, generating biogas (50–70% methane). The spent slurry serves as biofertiliser. Biogas is a clean, renewable fuel that reduces dependence on firewood and cow dung cakes in rural households.