How Is Biogas Produced? A Simple Explanation

Biogas is produced when microorganisms break down biodegradable organic matter in the absence of oxygen, a process known as anaerobic digestion. Organic materials such as food waste, animal manure, sewage sludge and agricultural residues are placed in a sealed digester, where a community of microorganisms progressively converts part of the organic matter into a gas containing mainly methane and carbon dioxide.

The process also leaves a nutrient-containing material called digestate.

In simple terms:

Organic material → Anaerobic digester → Microbial breakdown → Biogas + Digestate

Inside the digester, the biological conversion is usually described in four biochemical stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis.

Key Takeaways

• Biogas is produced by anaerobic digestion, which means the breakdown of biodegradable organic material in the absence of oxygen.
• Common feedstocks include manure, food waste, sewage sludge, agricultural residues and suitable industrial organic wastes.
• The four principal biochemical stages are hydrolysis, acidogenesis, acetogenesis and methanogenesis.
• Methane is produced primarily during methanogenesis by microorganisms known as methanogenic Archaea.
• Raw biogas consists mainly of methane and carbon dioxide, together with water vapour and smaller quantities of other gases.
• Biogas can be used for heat and combined heat and power (CHP), or upgraded to produce biomethane.
• The material remaining after digestion is called digestate and can have beneficial uses when its quality and regulatory status allow.

What Materials Can Be Used to Produce Biogas?

Biogas can be made from many types of biodegradable organic material.

Common feedstocks include:

• animal manure and slurry;
• food waste;
• food and beverage processing residues;
• sewage sludge and wastewater solids;
• crop residues;
• fats, oils and greases;
• agricultural residues; and
• suitable biodegradable industrial wastes.

Some anaerobic digestion plants use one principal feedstock, while others digest a mixture of materials. The digestion of two or more suitable feedstocks together is known as co-digestion.

Not every organic material is equally suitable. Its biodegradability, solids content, nutrient balance, potential contaminants and possible inhibitory substances all affect how successfully it can be digested.

For more on this subject, see our guide to anaerobic digestion feedstocks.

How Is Biogas Produced in an Anaerobic Digester?

The prepared organic material is fed into an anaerobic digester. This is a sealed reactor designed to prevent the entry of air and maintain conditions in which anaerobic microorganisms can thrive.

The microorganisms do not simply turn a piece of food waste or a quantity of manure directly into methane.

Instead, a complex microbial community progressively breaks down the organic material. Products created by one group of microorganisms become substrates for other groups.

This is conventionally explained as four linked biochemical stages:

1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis

These stages describe what happens biochemically inside the digester. They are not normally four separate tanks or pieces of equipment, and in many anaerobic digesters the processes occur concurrently and interact with one another.

The 4 Biochemical Stages of Biogas Production

1. Hydrolysis

Much of the organic material entering an anaerobic digester consists of relatively large and complex molecules, including carbohydrates, proteins and fats.

During hydrolysis, these materials are broken down into smaller, more soluble compounds that can be used by microorganisms in the next stages.

For example, carbohydrates can be broken down towards simpler sugars, proteins towards amino acids, and fats towards glycerol and fatty acids.

2. Acidogenesis

During acidogenesis, microorganisms ferment the products of hydrolysis.

This produces a mixture of intermediate compounds including volatile fatty acids, together with substances such as hydrogen and carbon dioxide.

The balance between the microorganisms producing these intermediates and those consuming them later in the process is important to stable digester operation.

3. Acetogenesis

During acetogenesis, various products formed during acidogenesis are converted further.

Important products include:

• acetate;
• hydrogen; and
• carbon dioxide.

These provide important substrates for the methane-producing microorganisms involved in the final stage.

4. Methanogenesis

The final stage is methanogenesis.

This is when much of the methane in biogas is formed.

The microorganisms responsible are known as methanogens. Although they were historically sometimes referred to as methane-forming bacteria, methanogens belong to the domain Archaea, not Bacteria.

Different groups of methanogens use different substrates. Important methane-forming routes include the conversion of acetate and the use of hydrogen together with carbon dioxide.

The resulting methane mixes with carbon dioxide and other gases to form the raw biogas collected from the digester.

For a more detailed explanation of the microbiology and chemistry, see The Anaerobic Digestion Process.

What Is Biogas Made Of?

Raw biogas is composed principally of:

• methane (CH₄); and
• carbon dioxide (CO₂).

It also normally contains water vapour and smaller quantities of other gases and contaminants. Depending on the feedstock, these can include hydrogen sulphide and trace compounds such as siloxanes or volatile organic compounds.

The methane provides most of the useful energy value of the gas.

According to the US Environmental Protection Agency, typical raw biogas can contain approximately 50% to 75% methane, although actual composition varies according to feedstock and operating conditions.

How Is the Biogas Collected?

Because the digester is sealed, the gas produced by the microorganisms can be collected rather than escaping to the atmosphere.

Biogas accumulates in the digester gas space or in a connected gas-storage system and is then conveyed through pipework for treatment and use.

Before use, the gas may require:

• condensate removal;
• hydrogen sulphide reduction;
• particulate removal;
• siloxane removal where relevant; and
• other conditioning appropriate to the equipment that will use the gas.

The amount of treatment needed therefore depends greatly on what happens to the biogas next.

What Happens to Biogas After It Is Produced?

Biogas can be used in several ways.

Heat

Biogas can be burned in a suitable boiler to provide heat, hot water or steam.

Combined Heat and Power

It can also fuel a combined heat and power (CHP) unit, producing electricity while recovering useful heat.

For more information, see our guide to biogas CHP systems.

Upgrading to Biomethane

Instead of using raw biogas directly, it can be processed to remove much of the carbon dioxide and reduce other unwanted constituents.

The resulting methane-rich gas is called biomethane.

Depending on the required specification and local infrastructure, biomethane can be injected into a gas grid, compressed for use as a vehicle or other gaseous fuel, liquefied, or used for applications that might otherwise require fossil natural gas.

Biogas and biomethane are therefore not the same thing. Anaerobic digestion produces biogas; upgrading biogas produces biomethane.

See our guide to biogas upgrading technologies for more information.

What Happens to the Material Left in the Digester?

Not all the material fed into an anaerobic digester becomes biogas.

The remaining material is known as digestate.

Digestate normally contains water, nutrients, residual organic matter and mineral material originating from the feedstock.

Depending on its quality, regulatory status and local circumstances, digestate may be used whole or separated into solid and liquid fractions for further treatment, storage or beneficial use.

Its management is important because producing useful biogas while creating a difficult or contaminated digestate would not represent a well-designed overall process.

Read more in our guide to digestate and its uses.

How Is Biogas Produced on a Large Scale?

Commercial biogas plants use the same underlying biological process described above, but the engineering surrounding the digester can be considerably more sophisticated.

A full-scale plant may include:

• feedstock reception and storage;
• contaminant removal and preparation;
• pumps and feed systems;
• one or more anaerobic digesters;
• mixing and heating systems;
• process monitoring and control;
• gas storage;
• gas cleaning;
• CHP or biomethane upgrading equipment; and
• digestate storage and treatment.

That distinction is important. The four biochemical stages explain how organic matter is converted biologically into methane-rich biogas, but they do not describe all the engineering steps needed to operate a commercial biogas plant.

For the complete engineering sequence, see Biogas Production Process Steps: From Feedstock to Usable Biogas.

Can Biogas Be Produced at Home?

Yes. Small anaerobic digesters are used in many parts of the world, particularly where manure, food residues and other suitable organic materials are readily available.

However, small does not mean risk-free. Biogas is combustible, methane can form explosive mixtures with air, and poorly managed digesters can also create biological, pressure and hydrogen sulphide hazards.

Anyone considering a household system should therefore use an appropriately designed digester and gas-utilisation system rather than treating biogas production as an improvised experiment.

For more information, see our guide to mini biogas plants and home biogas production.

Frequently Asked Questions About How Biogas Is Produced

How is biogas produced?

Biogas is produced when microorganisms break down biodegradable organic matter in an oxygen-free environment. This process is called anaerobic digestion. The organic matter is progressively converted through several biochemical stages, eventually producing a gas containing mainly methane and carbon dioxide.

What are the four stages of biogas production?

The four stages commonly referred to as the stages of biogas production are hydrolysis, acidogenesis, acetogenesis and methanogenesis. More precisely, they are the four principal biochemical stages of anaerobic digestion.

Which microorganism produces methane in a biogas digester?

Methane is produced by microorganisms known as methanogens. Methanogens belong to the domain Archaea rather than Bacteria.

What raw materials are used to make biogas?

Common raw materials include animal manure and slurry, food waste, sewage sludge, crop residues, food-processing wastes, fats, oils and greases, and other suitable biodegradable organic materials.

Does biogas contain only methane?

No. Raw biogas contains mainly methane and carbon dioxide, together with water vapour and smaller quantities of other gases and contaminants. Its precise composition depends on the feedstock and digestion process.

What is the difference between biogas and biomethane?

Biogas is the gas produced directly by anaerobic digestion. Biomethane is produced when biogas is upgraded to remove much of its carbon dioxide and other unwanted constituents, producing a methane-rich gas suitable for specified downstream uses.

Want to Understand the Complete Biogas Production Process?

This page explains the simple biological answer to the question “How is biogas produced?”

A commercial biogas plant involves additional stages before and after anaerobic digestion, including feedstock preparation, process monitoring and control, gas cleaning, energy utilisation or biomethane upgrading, and digestate management.

For the complete step-by-step explanation, read our main reference guide:

Biogas Production Process Steps: From Feedstock to Usable Biogas.

Readers who want an expanded downloadable treatment can also see the Biogas Production Process Steps PDF Ebook, which goes further into feedstocks, digester operation, process monitoring and control, gas treatment and utilisation, digestate and practical project considerations.

Further Reading

• Biogas Production Process Steps: From Feedstock to Usable Biogas
• The Anaerobic Digestion Process
• Anaerobic Digestion Feedstocks
• How to Increase Biogas Production
• 10 Uses and Applications of Biogas
• Digestate and Its Uses
• Mini Biogas Plants and Home Biogas Production

References and Authoritative Sources

• US Environmental Protection Agency — How Does Anaerobic Digestion Work? An overview of anaerobic digestion, feedstocks, biogas composition, digestate and biogas uses.
• US Environmental Protection Agency — Basic Information about Anaerobic Digestion. Covers common feedstocks, co-digestion, biogas and digestate.
• US Environmental Protection Agency — Frequent Questions about Anaerobic Digestion. Includes a simple explanation of how microorganisms generate biogas inside an oxygen-free digester.
• A Review of the Processes, Parameters, and Optimization of Anaerobic Digestion. Peer-reviewed review describing hydrolysis, acidogenesis, acetogenesis and methanogenesis and their role in anaerobic digestion.

This article provides a general explanation of how biogas is produced. Individual anaerobic digestion systems vary considerably according to feedstock, digester design, operating conditions, scale, regulation and intended use of the gas and digestate.

You can continue to read about this, or alternatively, watch our YouTube video on this subject of “How is Biogas Produced” (see below). Scroll down past the video for more answers to this question, from the web, as well.

[First published in 2018. Updated October 2024. Rewritten September 2026.]