A biogas plant is a sealed, oxygen-free tank (the digester) in which bacteria break down cattle dung and other wet organic waste into biogas, a fuel gas of about 50 to 70% methane and 30 to 40% carbon dioxide, plus a digested slurry that is used as manure. Family plants in India are rated by how much gas they make per day, from 1 to 6 m3 for most households, and a 2 m3 plant fed with about 50 kg of dung a day covers the cooking needs of a family of five to eight.
This page explains how biogas is made, compares the main plant types and shows how to pick the right size. How the digester and dome are actually built is covered on our page on constructional details of biogas plants, and first filling and start-up on filling a biogas digester for starting.
How does a biogas plant work? The four stages of anaerobic digestion
Dung is mixed with water into a slurry and fed into the digester every day. Inside, with no air, different groups of microbes pass the material down a chain, each group living on the waste products of the one before:
- Hydrolysis: enzymes released by bacteria (cellulase, amylase, protease, lipase) split large molecules into small ones. Carbohydrates become simple sugars, proteins become amino acids, fats become fatty acids.
- Acidogenesis: acid-forming bacteria ferment those small molecules into volatile fatty acids (such as propionic and butyric acid), alcohols, hydrogen and CO2.
- Acetogenesis: other bacteria convert the longer acids and alcohols into acetic acid, hydrogen and CO2.
- Methanogenesis: methanogens, which are archaea rather than true bacteria, turn acetic acid into methane and CO2, and combine hydrogen with CO2 to make more methane.
The first three stages are fast and forgiving. The last one is slow and fussy, because methanogens multiply slowly and dislike acid, oxygen and sudden temperature changes. Most operating problems (a sour digester, no gas in winter) are really methanogen problems. That is also why dung has to stay in the digester for weeks: the hydraulic retention time (HRT) must be long enough for the methanogens to keep up.
Conditions the bacteria need
| Factor | Good range | Why it matters |
|---|---|---|
| Temperature | Mesophilic range, 20 to 40 °C; about 35 °C is the optimum | Gas production virtually stops when the slurry cools to about 10 °C |
| pH | Feed at 6 to 7; a stable digester sits at about 7.2 to 8.2 | Methanogens do not thrive below pH 6.5 |
| Total solids | 7 to 10% in the slurry | Too thin and solids settle; too thick and gas cannot escape. Fresh dung mixed 1:1 with water lands here. |
| C/N ratio | 20 to 30; cattle dung is about 24 | Too much nitrogen gives ammonia, which is toxic above pH 8.5 |
| Oxygen and toxins | None | Detergents, disinfectants, antibiotics and heavy metals kill the bacteria |
Figures are from the FAO biogas training manual and the GTZ/ISAT Biogas Digest.
What is biogas made of?
| Gas | Share by volume |
|---|---|
| Methane (CH4) | 50 to 70% |
| Carbon dioxide (CO2) | 30 to 40% |
| Hydrogen, nitrogen, water vapour | A few percent |
| Hydrogen sulphide (H2S) | Traces; gives the rotten-egg smell and corrodes metal |
Only the methane burns. Biogas has a calorific value of about 20 MJ/m3 and burns with a clear blue flame. Mixed with air at about 5 to 12% biogas it is explosive, so leaks are checked with soap solution, never a flame. Scrubbed of CO2 and H2S and compressed, it becomes compressed biogas (CBG), a vehicle fuel similar to CNG.
Types of biogas plant compared
Indian household plants come in three families. The Ministry of New and Renewable Energy (MNRE) lists approved models of each for 1 to 25 m3 per day.
| Point | Floating drum (KVIC) | Fixed dome (Deenbandhu, Janata) | Bag / balloon (e.g. Flexi) |
|---|---|---|---|
| Gas holder | Steel, FRP or HDPE drum floating on the slurry | Masonry dome, part of the digester | Plastic or rubber bag that is both digester and gas holder |
| Gas pressure | Constant; drum height shows gas stored | Rises as gas is stored, falls as it is used | Low; may need weights or a pump |
| Cost | Higher (the drum) | Lower; local brick, cement and sand | Lowest, prefabricated |
| Life | Masonry lasts; steel drum up to about 15 years if repainted, much less in coastal areas | 20 years or more if built gas-tight | Usually 2 to 5 years |
| Weak point | Rust; drum can stick in scum with fibrous feed | Leaks through poor masonry are hard to find and fix | Punctures, UV damage, few local repairers |
| Suits | Users who want steady pressure and easy operation | Most rural households with a skilled mason nearby | High water table, rocky ground, portable or trial use |
Life and pressure figures for each type are from the GTZ/ISAT Biogas Digest. Dimensions, materials and construction steps for the KVIC and Deenbandhu models are on our construction page.
How to choose a biogas plant
| If your situation is… | Lean towards… |
|---|---|
| Tight budget, trained mason available, stable dry soil | Fixed dome (Deenbandhu) |
| Want constant gas pressure, or will run an engine | Floating drum, or a fixed dome with a pressure regulator |
| High water table or hard rock, so a deep pit is impractical | Bag plant or a shallow prefabricated unit |
| Coastal, humid site | Fixed dome or an FRP/HDPE drum rather than mild steel |
| Cold winters (hills) | Larger digester (longer retention time), buried and insulated; expect less gas in winter |
| Fewer than two or three cattle | A 1 m3 plant at most, or add kitchen waste and a toilet connection |
Biogas plant sizes: the IS 9478 guide
In India the “size” of a plant means its rated gas output in m3 per day, not the tank volume. The Bureau of Indian Standards code of practice IS 9478:1989 links each size to the dung it needs and the family it serves:
| Plant size (m3 gas/day) | Dung needed (kg/day) | Cattle | Family members |
|---|---|---|---|
| 1 | 25 | 2-3 | 3-4 |
| 2 | 50 | 4-5 | 5-8 |
| 3 | 75 | 6-7 | 9-12 |
| 4 | 100 | 8-9 | 13-16 |
| 6 | 150 | 10-15 | 17-20 |
| 8 | 200 | 16-20 | 21-26 |
| 10 | 250 | 20-25 | 27-32 |
The same standard sets the retention time by climate: 30 days where the average winter temperature is 30 °C or above, 40 days for 20 to 30 °C, and 55 days for 10 to 20 °C. A plant for Punjab’s winters therefore needs a much bigger digester than a plant of the same rating in coastal Tamil Nadu.
Worked example: sizing a plant for a family
This example starts from the kitchen, not the cowshed. (Our construction page works the other way, from the dung available.)
Given: a family of 6 in a district where the average winter temperature is about 25 °C. Gas use for cooking is taken as 0.33 m3 per person per day (FAO manual figure for Indian and Nepali conditions). Fresh dung is about 10 kg per cow and 15 kg per buffalo per day; gas yield is 0.023 to 0.040 m3 per kg of fresh cattle dung (FAO).
Step 1, daily gas need
- 6 × 0.33 = 1.98 m3/day, so choose a 2 m3 plant. IS 9478 agrees: 2 m3 serves 5 to 8 people.
Step 2, dung needed
- IS 9478 figure: 50 kg/day, which assumes 0.04 m3 of gas per kg (2 / 0.04 = 50).
- At a more cautious 0.036 m3/kg: 2 / 0.036 = 55.6 kg/day.
- So plan for 50 to 56 kg of dung a day: about 5 to 6 cows (55.6 / 10), or about 4 buffaloes (55.6 / 15 = 3.7).
Step 3, daily slurry
- Dung mixed 1:1 with water: 50 kg dung + 50 L water ≈ 100 L = 0.1 m3/day (taking slurry density close to that of water).
Step 4, digester volume
- Winter average 25 °C falls in the 40-day class of IS 9478.
- V = daily slurry × HRT = 0.1 × 40 = 4 m3 of slurry space.
- For comparison, the same plant needs 0.1 × 30 = 3 m3 in a warm coastal district, and 0.1 × 55 = 5.5 m3 in a cold one.
Step 5, gas storage
- IS 9478 gives a gas holder of 1.10 m3 for a 2 m3 KVIC plant: 1.10 / 2 × 24 = about 13 hours of production, enough to carry gas from the night over to the morning cooking.
Step 6, energy check
- 2 m3 × 20 MJ/m3 = 40 MJ/day in the gas. At the 60% efficiency the FAO manual gives for a biogas stove, about 24 MJ/day reaches the pots.
Two lessons follow. First, size the plant from the dung you can collect every day, all year: a plant too big for its feed gives little gas. Second, the climate changes the digester volume by almost a factor of two for the same gas output, which is why cold-region plants cost more.
Biogas plants in India
- Numbers: MNRE’s RE-Statistics 2025-26 reports 51,20,527 small biogas plants installed under its schemes as on 31 March 2026, with Maharashtra, Karnataka and Uttar Pradesh leading.
- Programme: MNRE has supported family biogas plants since 1981-82 under the National Biogas Programme. From 2021-22 to 2025-26 this ran as the Biogas Programme, a sub-scheme of the National Bioenergy Programme (Phase I, 1 April 2021 to 31 March 2026), covering plants of 1 to 2,500 m3/day. Under Phase I the central financial assistance for a 2 to 4 m3 plant was ₹14,350 in most states and ₹22,000 in hilly and north-eastern states, islands and for SC/ST beneficiaries, with ₹1,600 extra if the plant is linked to a toilet. Phase I has ended; check MNRE or your state nodal agency for the terms now in force.
- Beyond the household: larger plants run on dairy, poultry, press mud and municipal waste to make power or CBG. See biomass energy for the national picture and biomass conversion technologies for how anaerobic digestion compares with gasification, pyrolysis and fermentation.
Advantages and limitations
- For: clean, smoke-free cooking fuel from waste; digested slurry is a better manure than raw dung; less firewood cutting; toilets can be connected; low running cost.
- Against: needs a steady supply of dung and water every day; output drops sharply in cold weather; fixed domes must be built carefully to stay gas-tight; the gas cannot easily be bottled at household scale.
Biogas plants appear in the non-conventional energy sources unit of most B.Tech mechanical and electrical syllabi. For background see biomass, the Ministry of New & Renewable Energy (MNRE), Government of India, and IEA – Bioenergy.
FAQs
What are the stages of anaerobic digestion in a biogas plant?
Four: hydrolysis (large molecules broken into sugars, amino acids and fatty acids), acidogenesis (these fermented into volatile fatty acids), acetogenesis (converted to acetic acid, hydrogen and CO2) and methanogenesis (methanogens turn acetic acid and hydrogen into methane).
What percentage of biogas is methane?
About 50 to 70% methane and 30 to 40% carbon dioxide, with small amounts of hydrogen, nitrogen, water vapour and traces of hydrogen sulphide. The methane is the part that burns.
Which biogas plant is best for a home: floating drum or fixed dome?
A fixed dome (Deenbandhu) plant is cheaper and lasts longer, so it suits most rural homes with a trained mason nearby. A floating drum (KVIC) plant costs more and its steel drum needs repainting, but it gives steady gas pressure and is easier to check for leaks.
What size biogas plant do I need for a family of five?
A 2 m3 per day plant, which needs about 50 kg of fresh dung a day, from four or five cattle. IS 9478 matches 2 m3 plants to families of five to eight.
How long does dung stay in a biogas digester?
IS 9478 designs plants for a retention time of 30, 40 or 55 days depending on how cold the winter is. Colder places need a longer retention time and therefore a bigger digester for the same gas output.