Course 26.2Lesson 3 of 3

Biogas and the gobar gas plant

Biogas is a mixture of gases, mostly methane, that microbes make from organic waste without oxygen. The microbes are methanogens such as Methanobacterium. They live in the rumen of cattle, so dung is rich in them. A gobar gas plant turns dung into fuel, and the spent slurry is used as fertiliser.

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A fuel from dung

In many villages, the cooking is done with a gas that comes from a pit of cattle dung. The gas is called biogas, or gobar gas. Gobar is the Hindi word for cow dung.

Biogas is a mixture of gases that microbes make. It is mostly methane, and it burns, so it can be used as a fuel.

The microbes of biogas

The microbes are bacteria called methanogens. One example is Methanobacterium. They grow without oxygen, which means that they are anaerobic. They feed on cellulose rich material and make methane, with carbon dioxide and hydrogen.

You have already met them. They are found in the anaerobic sludge of a sewage treatment plant. They are also found in the rumen of cattle. The rumen is a part of the stomach. The food of cattle has a great deal of cellulose. In the rumen the bacteria help to break the cellulose down, and so they are important for the nutrition of cattle.

So the dung of cattle is rich in these bacteria. This is why dung can be used for biogas.

A flow chart in three boxes down the left. Dung and plant waste, cellulose, proteins and fats, are broken by bacteria into smaller molecules, sugars and amino acids. Other bacteria turn these into acids, acetic acid, hydrogen and carbon dioxide. An arrow marked methanogens leads from the acids to a box on the right, biogas, methane and carbon dioxide. A note says that no oxygen is used at any step and that methanogens are anaerobic.

More about the stages. NCERT does not describe the stages inside the digester. A-Level and IB books do. Different bacteria work one after another. Some break the large molecules down, some make acids, and the methanogens make methane at the end.

The gobar gas plant

Much of what is known about biogas technology in India came from the work of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

A biogas plant has these parts.

A concrete tank, 10 to 15 feet deep, which is about 3 to 4.5 m. In it, the waste is collected and a slurry of dung is fed.
A floating cover over the slurry. It rises as the gas is made in the tank by the microbes.
An outlet with a pipe that carries the biogas to nearby houses.
A second outlet for the spent slurry. It is used as a fertiliser.

A cross section of a gobar gas plant. A deep tank in the ground, the digester, is filled with brown slurry of dung. Over the slurry floats an inverted drum, the floating gas holder, with a pale yellow space of gas under its grey cap. A mixing tank for dung and water stands at the left above the ground, and a sloping inlet pipe leads from it into the digester. A gas pipe leaves the top of the gas holder, rises, and runs to a small house with a stove, labelled biogas to the house. On the right, an outlet pipe leads from the digester up to a small tank, labelled outlet for the spent slurry and fertiliser.

Cattle dung is available in large amounts in villages. So biogas plants are common in rural areas. The gas is used for cooking and lighting.

Some numbers

The numbers are round teaching values. A family of 4 cattle might make 40 kg of fresh dung a day.

Each kg of dung gives about 0.04 m³ of biogas.
About 60 per cent of the biogas is methane.
Methane gives 35.8 MJ of energy for each m³ that burns.
One kg of LPG gives about 46 MJ.

So 40 kg of dung gives $40 \times 0.04 = 1.6$ m³ of biogas, which holds $1.6 \times 0.6 = 0.96$ m³ of methane. The energy is $0.96 \times 35.8 = 34.4$ MJ, as much as 0.75 kg of LPG.

You should find that the gas is in direct proportion to the dung, and that the methane share changes the energy and not the volume of the biogas. A real plant is also slower in a cold season, when the microbes grow more slowly.

Try it in Python

The program works out the gas, the methane and the energy for a day.

Practise with new numbers

Common mistakes

Saying that the methanogens are aerobic. They are anaerobic.
Saying that biogas is only methane. It is a mixture, mostly methane, with carbon dioxide and other gases.
Saying that the spent slurry is a waste with no use. It is used as a fertiliser.
Mixing up the two Indian bodies with other bodies. The biogas work is that of IARI and KVIC.
Saying that the floating cover sinks. It rises as the gas collects under it.

Remember these

Biogas: a mixture of gases, mostly methane, made by microbes without oxygen. It is used as a fuel.
Methanogens, for example Methanobacterium, are anaerobic. They live in anaerobic sludge and in the rumen of cattle, so dung is rich in them.
Gobar gas plant: a tank 10 to 15 feet deep, a slurry of dung, a floating cover, a gas outlet to the houses and an outlet for the spent slurry, which is a fertiliser.
IARI and KVIC developed biogas technology in India.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

What is biogas, and which gas is its main part?

Name the microbes that make methane, and give an example.

Why is cattle dung a good material for a biogas plant?

Name the parts of a gobar gas plant.

What is the use of the spent slurry?

References

NCERT (2023). Biology, Textbook for Class XII, chapter 8, Microbes in Human Welfare.
Gunaseelan V. N. (1997). Anaerobic digestion of biomass for methane production: a review. Biomass and Bioenergy, 13, 83 to 114.
Tchobanoglous G., Stensel H. D., Tsuchihashi R. and Burton F. (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th edition, McGraw Hill, the chapter on anaerobic suspended growth processes.