Biogas Production Process Steps: From Feedstock to Usable Biogas

The biogas production process converts biodegradable organic material into a methane-rich gas that can be used as a renewable source of energy.

But there is an important distinction that is often missed when the process is explained.

The four stages commonly described as the “4 stages of biogas production” — hydrolysis, acidogenesis, acetogenesis and methanogenesis — are more precisely the four biochemical stages of anaerobic digestion.

They are at the heart of biogas production, but they are not the whole process.

A working biogas plant must also receive and prepare its feedstock, maintain suitable conditions inside the digester, monitor and control the process, collect the raw biogas, remove contaminants where necessary, use or upgrade the gas, and manage the digestate remaining after digestion.

The complete sequence can therefore be summarised as:

Feedstock → Reception and Preparation → Anaerobic Digestion → Monitoring and Process Control → Biogas Collection → Gas Cleaning and Conditioning → Energy Use or Biomethane Upgrading → Digestate Management

This guide explains each of these biogas production process steps, while also showing what happens biochemically inside the anaerobic digester.

Key Takeaways

• Anaerobic digestion is only part of the complete biogas production process. Feedstock reception and preparation, process monitoring and control, gas treatment, utilisation and digestate management are also important steps.
• The commonly cited 4 stages of biogas production are more accurately described as the four biochemical stages of anaerobic digestion: hydrolysis, acidogenesis, acetogenesis and methanogenesis.
• These biochemical stages interact and can occur concurrently within a conventional digester. They should not be imagined as four separate pieces of equipment.
• Biogas contains mainly methane and carbon dioxide. Its treatment depends on whether it will be used for heat, combined heat and power (CHP), or upgraded to biomethane.
• Biogas and biomethane are not interchangeable terms. Biomethane is produced by upgrading biogas to remove much of its carbon dioxide and other unwanted constituents.
• Digestate is the other major output of anaerobic digestion and its quality, storage and beneficial use should be considered as part of the overall AD process.

What Is the Biogas Production Process?

Biogas is produced when microorganisms break down biodegradable organic matter in an environment where oxygen is absent. This process is called anaerobic digestion (AD).

Suitable feedstocks can include:

• animal manure and slurry;
• food and beverage processing residues;
• separately collected food waste;
• sewage sludge and wastewater solids;
• crop residues;
• fats, oils and greases;
• suitable industrial organic residues; and
• in some circumstances, purpose-grown crops.

Different materials can also be digested together in a process known as co-digestion.

For more about the materials that can be used, see our guide to anaerobic digestion feedstocks.

The two principal outputs from the anaerobic digester itself are:

1. biogas, containing principally methane and carbon dioxide; and
2. digestate, containing much of the water, nutrients and less readily degradable organic material remaining after digestion.

The precise plant configuration varies enormously according to feedstock, scale, climate, regulation and the intended use of the biogas.

Nevertheless, the fundamental production sequence is broadly similar.

Step 1: Select and Receive the Feedstock

Successful biogas production starts before material enters the digester.

The characteristics of the feedstock affect:

• potential biogas and methane yield;
• solids concentration;
• nutrient balance;
• digester loading;
• retention time;
• mixing requirements;
• contamination risk;
• process stability; and
• digestate quality.

A manure-fed farm digester therefore has very different feedstock-handling requirements from a commercial plant accepting packaged food waste.

Similarly, sewage sludge digestion is integrated into wastewater treatment processes and should not be regarded as simply another version of a food-waste AD plant.

Feedstock selection is consequently both a biological and an engineering decision.

Where several materials are co-digested, their combined characteristics also need to be considered. A high-energy feedstock can increase gas production, for example, but excessive loading or an unsuitable nutrient balance can destabilise the digestion process.

Step 2: Prepare the Feedstock for Digestion

Many feedstocks require preparation before entering the anaerobic digester.

Depending on the material and plant design, preparation may include:

• removal of contaminants;
• depackaging;
• screening;
• maceration or size reduction;
• removal of grit and heavy material;
• blending;
• dilution or adjustment of dry-matter content;
• homogenisation; and
• regulated heat treatment where required.

The purpose is not simply to make the feedstock pumpable. Preparation should deliver material to the digester in a form appropriate to the particular digestion system while minimising contamination and unnecessary energy consumption.

Is Pasteurisation Always Part of Biogas Production?

No.

Pasteurisation is sometimes shown on simplified biogas process diagrams as though every AD plant includes an obligatory pasteurisation stage. That is misleading.

Whether heat treatment or pasteurisation is required depends on factors including the feedstock, jurisdiction, intended digestate use and applicable animal by-product or waste regulations.

For example, plants treating certain animal by-products and catering wastes can be subject to specified treatment requirements, including time, temperature and particle-size criteria.

A manure-only farm digester, sewage sludge digester and commercial food-waste AD plant should therefore not automatically be assumed to require identical pre-treatment.

Step 3: Feed the Anaerobic Digester

Prepared feedstock enters a sealed reactor or digester designed to maintain conditions suitable for anaerobic microorganisms.

There are many digester configurations. These include continuously stirred tank reactors, plug-flow digesters, high-solids systems, covered lagoons and various specialist reactor designs.

Operating temperature is particularly important. Many commercial digesters operate within either mesophilic or thermophilic temperature ranges, although the appropriate conditions depend on the process and feedstock.

Other important operating variables can include:

• organic loading rate;
• hydraulic retention time;
• solids retention time;
• pH;
• alkalinity;
• volatile fatty acid concentration;
• nutrient and trace-element availability;
• mixing;
• ammonia concentration; and
• the presence of inhibitory or toxic compounds.

There is therefore no single correct retention time or loading rate that applies to every biogas plant.

The objective is to maintain a stable microbial community while processing the required quantity of biodegradable material.

What Are the 4 Biochemical Stages of Biogas Production?

People searching for the 4 stages of biogas production are usually referring to:

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

More precisely, these are the four principal biochemical stages of anaerobic digestion.

They describe the linked microbial and biochemical transformations through which complex organic matter is progressively converted into compounds from which methane can be produced.

They should not be confused with four separate pieces of equipment or four physically isolated stages within every AD plant.

In many digesters these biochemical processes occur concurrently, with products formed by one microbial group becoming substrates for another.

Stage 1: Hydrolysis

Much of the organic material entering a digester consists of relatively complex compounds such as carbohydrates, proteins and lipids.

These large molecules cannot necessarily be taken up directly by the microorganisms responsible for the later stages of digestion.

During hydrolysis, extracellular enzymes help break complex organic materials into smaller soluble compounds.

For example:

• carbohydrates can be broken down towards simpler sugars;
• proteins towards peptides and amino acids; and
• lipids towards glycerol and long-chain fatty acids.

Hydrolysis is particularly important when digesting solid and fibrous materials and can be a rate-limiting part of the overall process for some feedstocks.

Stage 2: Acidogenesis

The products of hydrolysis are then metabolised by acidogenic microorganisms.

During acidogenesis, these soluble organic compounds are fermented into a mixture that can include:

• volatile fatty acids;
• alcohols;
• hydrogen;
• carbon dioxide; and
• other intermediate compounds.

This is an important transition because the original complex organic matter is now being transformed into relatively simple intermediates that can feed subsequent microbial processes.

However, excessive accumulation of volatile fatty acids can reduce digester pH and may indicate that acid production is occurring faster than subsequent microbial communities can consume the products.

Process balance therefore matters.

Stage 3: Acetogenesis

During acetogenesis, various products of acidogenesis are further converted into compounds that can be used by methanogens.

Important products include:

• acetate;
• hydrogen; and
• carbon dioxide.

This stage illustrates why anaerobic digestion is better understood as an interacting microbial ecosystem rather than four completely independent reactions.

The organisms involved depend upon each other. Maintaining sufficiently low hydrogen partial pressures, for example, can be important to the energetics of some acetogenic reactions.

Stage 4: Methanogenesis

Methanogenesis is the stage in which methane is formed.

The microorganisms responsible are known as methanogens. Importantly, methanogens are members of the domain Archaea, rather than bacteria.

Different methanogenic pathways exist. Two particularly important routes are:

• conversion of acetate to methane and carbon dioxide; and
• conversion of hydrogen and carbon dioxide to methane.

Methanogens can be sensitive to adverse environmental conditions. Temperature, pH, ammonia, toxic compounds and rapid changes in loading can therefore affect methane production.

The health of the methanogenic population is one reason stable digester operation is so important.

For a more detailed explanation of these biochemical processes, see our guide to the anaerobic digestion process.

Step 4: Monitor and Control the Digestion Process

Producing biogas reliably requires more than filling a sealed tank with organic material. The process must be monitored so that operators have the information needed to control it and maintain suitable conditions for the interacting microbial communities.

Depending on the plant, operators may monitor parameters such as feed rate, gas production, methane concentration, digester temperature, pH, alkalinity, volatile fatty acids, ammonia and solids concentration.

A change in gas production is not necessarily caused by one factor. Changes in feedstock composition, overloading, temperature variation, inhibition, mixing problems and equipment failures can all affect performance.

This is why optimisation should be based on understanding and monitoring the process rather than simply attempting to maximise the quantity of feed entering the digester.

For more on improving plant performance, see How to Increase Biogas Production.

Step 5: Collect the Raw Biogas

As anaerobic digestion proceeds, biogas accumulates and is collected from the digester gas space or gas-storage system.

Raw biogas normally contains principally:

• methane (CH₄);
• carbon dioxide (CO₂);
• water vapour; and
• smaller quantities of other gases and contaminants.

Trace constituents can include hydrogen sulphide and, depending on the feedstock, compounds such as siloxanes and volatile organic compounds.

The composition varies according to feedstock and operating conditions. The methane fraction is the principal source of the gas's energy value.

Before utilisation, the gas normally passes through equipment appropriate to its intended use. This may include condensate management, pressure control, monitoring and safety equipment as well as treatment systems.

Step 6: Clean and Condition the Biogas

The degree of gas treatment required depends heavily on what happens next.

Biogas burned in a boiler may require relatively modest treatment compared with gas supplied to a reciprocating CHP engine. Biogas intended for upgrading to biomethane requires much more extensive processing.

Treatment can include:

• removal of water and condensate;
• hydrogen sulphide reduction;
• particulate removal;
• siloxane removal where necessary; and
• other contaminant control appropriate to the feedstock and gas-use equipment.

Gas cleaning should therefore be designed around the required gas specification, rather than regarded as a standard treatment train that is identical at every plant.

Step 7: Use the Biogas — or Upgrade It to Biomethane

Once appropriately conditioned, biogas has several possible uses.

Heat

One of the simplest uses is combustion in a boiler to produce hot water or steam.

Where there is a dependable local heat demand, direct thermal use can provide an efficient way to use the energy contained in the gas.

Combined Heat and Power

Biogas can fuel a combined heat and power (CHP) unit that produces both electricity and useful heat.

The electricity may be consumed on site or exported, depending on the installation and local arrangements. Recovering useful heat from the engine can substantially improve overall energy utilisation compared with generating electricity while rejecting the heat.

See our detailed guide to biogas CHP systems, benefits and cost savings.

Biomethane

Instead of burning the raw biogas on site, it can be upgraded.

Biogas upgrading separates carbon dioxide and removes or reduces other unwanted constituents to produce methane-rich biomethane meeting the specification required for its intended use.

Depending on local infrastructure, regulation and markets, biomethane can be:

• injected into a gas grid;
• compressed for use as a vehicle fuel;
• used as a renewable compressed gaseous fuel in suitable industrial, commercial and other applications that might otherwise rely on fossil fuels such as LPG;
• liquefied for transport or other applications requiring liquefied biomethane (LBM or bio-LNG); or
• supplied for other uses that would otherwise rely on fossil natural gas.

It is important to distinguish the two products:

Anaerobic digestion produces biogas. Upgrading biogas produces biomethane.

The two terms should not be used interchangeably.

Note: At many biomethane plants, the biogenic carbon dioxide separated during upgrading is currently released unless it is recovered for another use. As the need to reduce greenhouse gas emissions increases, recovery of this CO₂ as a useful product, or its capture for permanent storage, may increasingly become an additional process step in biomethane production.

For more information, see our guide to biogas upgrading technologies.

Step 8: Manage the Digestate

Biogas is only one major output of anaerobic digestion. The other is digestate.

Digestate contains the material remaining after microorganisms have degraded part of the biodegradable organic matter. It generally contains water, nutrients, residual organic matter and mineral material derived from the feedstock.

Depending on the process, digestate may be used whole or separated into liquid and solid fractions.

Potential beneficial uses depend on factors including:

• feedstock origin;
• nutrient content;
• contamination;
• regulatory status;
• quality standards;
• local agricultural demand; and
• transport and storage requirements.

Digestate should therefore not be treated as an afterthought.

Its storage, handling, quality and eventual use can materially affect both the environmental performance and economics of an AD project.

See our detailed guide to digestate and its uses for further information.

What Determines How Much Biogas Is Produced?

There is no universal quantity of biogas that can be expected from a tonne of “organic waste”.

Gas yield depends strongly on what is being digested and how the plant is operated.

Important factors include:

• the proportion of biodegradable volatile solids;
• carbohydrate, protein and lipid content;
• lignin and other poorly degradable fractions;
• feedstock preparation;
• temperature;
• organic loading;
• retention time;
• microbial health;
• inhibition;
• mixing; and
• losses through incomplete digestion or poor operation.

Fats and readily degradable food residues can have very different methane potentials from dilute manure or sewage sludge, but theoretical methane potential alone does not determine whether a feedstock is suitable.

A high-energy material that destabilises a digester when added too rapidly is not being used effectively.

For that reason, good biogas production is about stable conversion of appropriate feedstocks, rather than simply pursuing the highest theoretical gas yield.

How Long Does the Biogas Production Process Take?

There is no single correct answer.

The required retention time varies with:

• feedstock;
• digester design;
• operating temperature;
• solids concentration;
• loading rate; and
• the degree of degradation required.

Some high-rate anaerobic wastewater processes retain active biomass while allowing liquid to pass through relatively quickly. Digesters treating slurries or solid organic feedstocks may require much longer residence periods.

Statements such as “anaerobic digestion takes three weeks” should therefore be treated only as examples for particular systems, not as a general rule for biogas production.

Is Biogas Produced Continuously?

It can be.

Many commercial AD plants operate as continuous or semi-continuous processes in which feedstock is added regularly and digestate is removed correspondingly.

Biogas production then continues throughout operation, although the production rate varies.

Batch digesters operate differently: material is loaded and digested for a period before being removed.

The appropriate arrangement depends on the feedstock, scale and process design.

From Biogas Production to a Successful AD Plant

Understanding the biogas production process steps is essential, but a commercially and environmentally successful anaerobic digestion plant depends on much more than the biochemical reactions occurring inside the digester.

Feedstock availability, contamination, storage, process selection, monitoring and control, equipment reliability, gas utilisation, digestate outlets, planning, permitting, regulation and project economics can all determine whether a project succeeds.

This is why the best AD systems are designed as integrated processes.

The digester cannot sensibly be considered in isolation from what comes before it or what happens to the biogas and digestate afterwards.

Biogas Production Process Steps — Summary

The complete process can be reduced to eight practical steps:

1. Select and receive suitable organic feedstocks.
2. Prepare the feedstock for the chosen digestion process.
3. Feed it into an anaerobic digester.
4. Monitor and control the biochemical digestion process.
5. Collect the raw biogas.
6. Clean and condition the gas for its intended use.
7. Use the biogas for heat or CHP, or upgrade it to biomethane.
8. Manage and beneficially use the digestate where appropriate.

Within the digester itself, organic matter passes through the interacting biochemical processes of hydrolysis, acidogenesis, acetogenesis and methanogenesis.

Understanding that distinction removes much of the confusion found in simplified descriptions of biogas production.

The four biochemical stages explain how microorganisms transform organic matter into biogas.

The eight practical process steps explain how a biogas plant turns a feedstock into useful energy products and digestate.

Frequently Asked Questions About the Biogas Production Process

What are the 4 stages of biogas production?

The four stages commonly referred to as the “4 stages of biogas production” are hydrolysis, acidogenesis, acetogenesis and methanogenesis. More precisely, these are the four principal biochemical stages of anaerobic digestion. They describe the transformations by which complex organic matter is progressively broken down and ultimately converted into methane-rich biogas.

What is the process of biogas production?

Biogas production begins with the selection and preparation of biodegradable organic feedstock. The material is fed to an anaerobic digester, where microorganisms break it down in the absence of oxygen. The resulting biogas is collected and treated according to its intended use, while the remaining digestate is stored, processed or beneficially used where appropriate.

What materials can be used to produce biogas?

Common biogas feedstocks include animal manure and slurry, food waste, food-processing residues, sewage sludge, fats, oils and greases, crop residues and suitable industrial organic materials. Different feedstocks may also be combined through co-digestion.

What is the main gas produced during anaerobic digestion?

Raw biogas consists principally of methane and carbon dioxide, together with water vapour and smaller quantities of other gases and contaminants. Methane provides most of the useful energy content of biogas.

What is the difference between biogas and biomethane?

Biogas is the gas produced by anaerobic digestion and contains substantial quantities of both methane and carbon dioxide. Biomethane is produced by upgrading biogas to separate much of the carbon dioxide and remove or reduce other unwanted constituents so that the methane-rich gas meets the specification required for its intended use.

How long does biogas production take?

There is no universal digestion time. Required retention time depends on the feedstock, digester design, temperature, solids content, loading rate and process objectives. Different anaerobic digestion technologies can therefore have very different retention times.

Why must an anaerobic digester be monitored?

Monitoring provides the information needed to control the process. Changes in feedstock, temperature, loading, pH, volatile fatty acids, ammonia and other operating conditions can affect the microbial communities responsible for digestion and methane production. Appropriate monitoring allows operators to identify adverse trends and take corrective action.

What happens to the carbon dioxide removed when biogas is upgraded?

The carbon dioxide separated during biomethane upgrading is often released unless there is a system and market for recovering it. Increasingly, however, biogenic CO₂ is being considered as a potentially useful product or as a carbon stream that could be captured rather than emitted.

Want a More Detailed Guide to the Biogas Production Process?

This article provides an overview of the complete production sequence.

For a more detailed, downloadable treatment covering feedstocks, feedstock preparation, the anaerobic digestion process, digester operation and monitoring, biogas collection and cleaning, biogas utilisation and biomethane, digestate management, economics and practical considerations, see the Biogas Production Process Steps PDF Ebook.

The expanded guide provides approximately 40 pages of material for readers who want to study the subject in greater depth.

Further Reading on Anaerobic Digestion and Biogas

• The Anaerobic Digestion Process — a deeper explanation of what happens during anaerobic digestion.
• Anaerobic Digestion Feedstocks — materials that can be used as AD feedstocks.
• How to Increase Biogas Production — factors affecting digester performance and gas yield.
• Uses and Applications of Biogas — ways in which biogas and upgraded biomethane can be used.
• Biogas CHP Systems: Benefits and Cost Savings — combined production of electricity and useful heat.
• Biogas Upgrading Technologies — technologies used to produce biomethane from biogas.
• Digestate — information about the other principal output from anaerobic digestion.

References and Authoritative Sources

The following sources provide further technical and regulatory background for the processes described in this guide:

• US Environmental Protection Agency — Basic Information about Anaerobic Digestion. EPA overview of AD feedstocks, biogas, digestate, gas cleaning and utilisation.
• UK Government / APHA — Using Animal By-Products at Compost and Biogas Sites. Guidance on regulatory requirements applying to relevant animal by-products and catering wastes, including treatment and pasteurisation requirements.
• A Review of the Processes, Parameters, and Optimization of Anaerobic Digestion. Peer-reviewed review describing hydrolysis, acidogenesis, acetogenesis and methanogenesis and the interactions between the microorganisms involved.
• Anaerobic Digestion of Agri-Food Wastes for Generating Biofuels. Peer-reviewed review covering the four-stage AD process and conversion of organic feedstocks to methane-rich biogas.

This article is intended as a technical overview. Plant design, operating conditions and regulatory requirements vary according to feedstock, process, location and intended outputs, and should be assessed for each individual installation.

Our Ebook Provides Much More Information About the Biogas Production Process Steps

Written for students and professionals by Steve Last, a waste processes engineer with 30+ years of experience in designing and commissioning these plants.

[First published January 5, 2019. Updated June 2025.]