NCERT grounding
This subtopic is drawn verbatim from Section 8.4, Microbes in Production of Biogas, of the NCERT Class XII Biology chapter Microbes in Human Welfare. The chapter opens by reminding the reader that not all microbes are harmful — many are useful — and biogas is one of the clearest demonstrations of microbes converting waste into a usable resource. The NCERT text defines the term, names the responsible microbes, links them to the cattle rumen, and describes the plant that harvests the gas. Every fact on this page is anchored to those few paragraphs; nothing has been added beyond them.
"Biogas is a mixture of gases (containing predominantly methane) produced by the microbial activity and which may be used as fuel." — NCERT Class XII Biology, Section 8.4.
The key shift NCERT makes here is one of gas chemistry. In the earlier examples — fermentation of dough, cheese making, production of beverages — the main gas produced by the microbes was carbon dioxide. In biogas, the dominant gas is methane. The type of gas produced depends on the microbes involved and the organic substrate they utilise, and that single contrast is the conceptual hinge of this subtopic.
Methanogens and the making of biogas
Microbes release different gaseous end-products during growth and metabolism. The species of microbe and the organic substrate together decide which gas dominates. Certain bacteria that grow anaerobically — that is, in the complete absence of oxygen — on cellulosic material produce a large amount of methane along with carbon dioxide and hydrogen. These bacteria are collectively called methanogens, and one common, frequently named example is Methanobacterium.
Two conditions are non-negotiable for methanogens to function: the environment must be anaerobic, and the feedstock must be cellulosic, meaning rich in plant-derived organic matter such as dung, crop residue and other biowastes. Where both conditions are met — the anaerobic sludge of a sewage plant, the rumen of a cow, the sealed slurry tank of a biogas plant — methanogens thrive and convert organic matter into a combustible gas mixture. The methane in that mixture is inflammable, which is precisely what makes biogas usable as fuel for cooking and lighting.
Three things define a methanogen for NEET: the conditions it needs, the substrate it eats, and the gases it produces. Memorise the trio and most fact-recall questions become trivial.
Condition required
Strictly anaerobic. Methanogens grow only in the absence of oxygen — sealed slurry, sludge digesters, the rumen.
Substrate used
Cellulosic material. Plant-rich organic waste — cattle dung, crop residue, biowaste — supplies the cellulose they break down.
Gases produced
Methane (predominant) plus carbon dioxide and hydrogen. Methane being inflammable makes the mixture a fuel.
It is worth noting where methanogens fit in the larger story of the chapter. In the subtopic on sewage treatment, the leftover activated sludge is pumped into large tanks called anaerobic sludge digesters. There, bacteria that grow anaerobically digest the bacteria and fungi in the sludge, and during this digestion they produce a mixture of gases — methane, hydrogen sulphide and carbon dioxide — which together form biogas. The very same kind of bacteria, the methanogens, are therefore commonly found in the anaerobic sludge during sewage treatment. Biogas is not a separate, isolated technology; it is the energy-bearing output of the same anaerobic microbial digestion that finishes off sewage.
The predominant gas
Biogas is predominantly methane, mixed with carbon dioxide and hydrogen. The earlier chapter examples — dough, cheese, beverages — produced mainly CO2; biogas is the case where methane takes over.
Why methane and not carbon dioxide
The distinction NEET tests most often is the gas. Dough rising, the holes in Swiss cheese, the fizz of fermenting juice — all are driven by carbon dioxide released by yeasts and other bacteria. Biogas is different because methanogens, working anaerobically on cellulose, route their metabolism toward methane production. This is not a minor footnote: an examiner can swap "methane" for "carbon dioxide" in a single-line statement and turn a correct fact into a wrong one. The safe mental model is that methanogens are methane producers — the name itself encodes the answer.
Figure 1. Methanogens such as Methanobacterium digest cellulosic biowaste under strictly anaerobic conditions, releasing biogas — a mixture in which methane predominates, accompanied by carbon dioxide and hydrogen.
The rumen connection and gobar gas
Methanogens are not confined to engineered tanks. They occur naturally inside cattle, in a part of the stomach called the rumen. The food of cattle contains a large amount of cellulosic material, and that cellulose ends up in the rumen. There, methanogens carry out exactly what they do best — they help in the breakdown of cellulose. Because cattle, like humans, cannot digest cellulose on their own, this microbial breakdown is essential. The methanogens therefore play an important role in the nutrition of cattle.
This biological detail leads directly to the practical one. Since methanogens live and multiply in the rumen, they pass out of the animal in its excreta. The dung of cattle — commonly called gobar — is consequently rich in these bacteria. That is why dung is the ideal raw material for a biogas plant: it supplies both the cellulosic feedstock and a ready, built-in population of methanogens to act on it. The biogas generated from cattle dung is therefore commonly called gobar gas. Because cattle dung is available in large quantities in rural areas, biogas plants are more often built in villages, and the biogas produced is used for cooking and lighting.
From cattle food to gobar gas
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Step 1
Cellulose in feed
Cattle feed carries large amounts of cellulosic plant material.
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Step 2
Rumen digestion
Methanogens in the rumen break down cellulose, aiding cattle nutrition.
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Step 3
Dung is rich in microbes
Methanogens pass out in the excreta — gobar is loaded with them.
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Step 4
Biogas generation
Dung fed to a biogas plant yields gobar gas for cooking and lighting.
The rumen link is also the reason a 2016 NEET question described methanogens as the prokaryotes responsible for producing biogas "from the dung of ruminant animals." The phrase "ruminant animals" is a direct nod to the rumen — cattle are ruminants, and the rumen houses the methanogens. Recognising that the rumen, the dung, and the biogas plant are three stages of one continuous microbial story is what separates a confident answer from a guessed one.
Methanogens are classified as primitive prokaryotes
A 2016 NEET stem called methanogens "primitive prokaryotes" and asked students to choose them over thermoacidophiles and halophiles. All three are archaea-like extremophile groups, and the distractors are deliberately similar. Only methanogens produce methane.
Rule: Among thermoacidophiles, halophiles and methanogens, only methanogens generate biogas — the name carries the answer.
Structure of a biogas plant
The biogas plant is a deceptively simple structure, and NEET expects you to be able to describe its parts and their functions. According to NCERT, the biogas plant consists of a concrete tank, 10 to 15 feet deep, in which biowastes are collected and a slurry of dung is fed. The concrete tank is the digestion chamber where methanogens act on the dung slurry in the absence of oxygen.
Over the slurry sits a floating cover. As microbial activity generates gas inside the tank, the gas accumulates beneath this cover and lifts it — so the floating cover keeps on rising as the gas is produced. The rising cover both stores the gas and gives a visible indication of how much has been produced. The plant has an outlet connected to a pipe, and this pipe carries the biogas to nearby houses where it is used as fuel. Finally, the digestion leaves behind a residue: the spent slurry is removed through another outlet, and because it is rich in nutrients it may be used as fertiliser. The biogas plant thus delivers two useful outputs at once — a clean-burning fuel and an organic fertiliser.
Figure 2. A typical biogas plant: dung slurry is fed into a concrete tank 10–15 feet deep, methanogens digest it anaerobically, the floating cover rises as gas accumulates, an outlet pipe delivers biogas to houses, and the spent slurry leaves through a separate outlet to be used as fertiliser.
The four working parts
For an exam answer, a biogas plant reduces to four parts and what each does. The concrete tank is the sealed, 10–15 feet deep digestion chamber. The inlet admits the slurry of dung and biowaste. The floating cover stores the gas and rises with it. The two outlets do different jobs: one outlet feeds a pipe carrying biogas to houses, while the second outlet drains the spent slurry for use as fertiliser. Keeping the two outlets distinct — gas versus spent slurry — is a common point of confusion that the table below resolves.
Gas outlet
Pipe
carries the useful fuel
- Connected to a pipe at the top of the tank
- Supplies biogas to nearby houses
- Output is the fuel — used for cooking and lighting
Spent-slurry outlet
Drain
removes the residue
- A separate outlet from the gas pipe
- Removes the digested, spent slurry
- Output is the residue — used as fertiliser
One credit-line completes the syllabus picture. The technology of biogas production was developed in India mainly due to the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC). This is a pure fact-recall point: NEET can ask which institutions developed biogas technology, and the safe answer is the pair IARI and KVIC. Both names reflect the rural, agricultural setting in which biogas plants are typically built.
"The technology of biogas production was developed in India mainly due to the efforts of IARI and KVIC."
NCERT Class XII Biology · Section 8.4
Pulling the strands together: biogas is a microbial product in the truest sense. Methanogens — exemplified by Methanobacterium — grow anaerobically on cellulose and produce a methane-rich gas. They live in the rumen of cattle, where they aid digestion, and so cattle dung carries them in abundance. Fed into a simple concrete-tank plant with a rising floating cover and two outlets, that dung yields both a clean fuel and a fertiliser. The whole subtopic is a tight chain of cause and effect, and NEET tests the links one at a time.
Worked examples
A bacterium grows anaerobically on cellulosic material and produces a gas mixture that is predominantly methane. Name the group it belongs to and give one common example.
The group is the methanogens — bacteria that grow anaerobically on cellulosic material and produce a large amount of methane along with carbon dioxide and hydrogen. A common example is Methanobacterium. The methane-rich mixture they produce is biogas.
Why is cattle dung, rather than ordinary plant litter, the preferred raw material for a biogas plant?
Methanogens live in the rumen of cattle, where they break down the cellulose in the animal's food. Because these bacteria multiply in the rumen, they pass out in the excreta, so cattle dung (gobar) is naturally rich in methanogens. Dung therefore supplies both the cellulosic feedstock and a ready inoculum of the right microbes — which is why the biogas produced is called gobar gas.
Describe the function of the floating cover in a biogas plant and state what is done with the spent slurry.
The floating cover sits over the dung slurry inside the concrete tank. As microbial activity produces gas, the gas collects beneath the cover, so the cover keeps rising — it stores the biogas and indicates the quantity produced. The spent slurry left after digestion is removed through a separate outlet and is used as fertiliser, because it is rich in nutrients.
In sewage treatment, where do methanogens act, and how is this connected to biogas?
During sewage treatment, the major part of the activated sludge is pumped into anaerobic sludge digesters. There, anaerobically growing bacteria — methanogens among them — digest the bacteria and fungi in the sludge and produce a mixture of gases (methane, hydrogen sulphide, carbon dioxide). This gas mixture is biogas, so methanogens are commonly found in the anaerobic sludge during sewage treatment.
Common confusion & NEET traps
Biogas questions are almost always fact-recall, so the traps are precise word-swaps rather than conceptual puzzles. The most frequent confusions cluster around the dominant gas, the identity of the microbes, and the institutions credited with the technology. The callouts below isolate each one.