What are the four stages of anaerobic digestion?

Hydrolysis, acidogenesis, acetogenesis and methanogenesis. Complex organic matter is broken down into simple molecules, fermented into volatile fatty acids, converted to acetate and hydrogen, and finally turned into methane by methanogenic archaea. In a single-stage digester all four happen at the same time in the same vessel.

What is the difference between anaerobic digestion and composting?

Composting is aerobic: microorganisms use oxygen, most of the energy in the material is released as heat and the carbon leaves as carbon dioxide. Anaerobic digestion takes place in a sealed vessel without oxygen, so much of that energy is captured as methane-rich biogas that can be used as fuel. The remaining digestate is usually wetter than compost.

What is biogas made of?

Biogas is typically 50–75% methane and 25–50% carbon dioxide, with smaller amounts of water vapour, hydrogen sulphide, ammonia and hydrogen. The methane content depends on the feedstock: fats and proteins give gas with more methane than carbohydrates do.

What temperature does anaerobic digestion run at?

Usually mesophilic, around 35–40 °C, or thermophilic, around 50–55 °C. Mesophilic digestion is the more common and more forgiving choice; thermophilic digestion breaks material down faster and can improve pathogen reduction, but needs closer control.

How long does anaerobic digestion take?

In a full-scale plant, material typically stays in the digester for several weeks, depending on the feedstock and the design. In the laboratory, a batch biochemical methane potential (BMP) test usually runs until gas production levels off, from about two weeks for food waste to six weeks or more for fibrous material.

What can make an anaerobic digester fail?

The most common causes are feeding too much too quickly, which lets volatile fatty acids build up and the pH fall; ammonia from nitrogen-rich feedstocks; sudden temperature changes; inhibitory substances in the feed; foaming; and a shortage of trace elements. Falling gas production or methane content is often the first sign, which is why gas monitoring is so useful.

A biochemical methane potential (BMP) test measures how much methane a material can produce when it is digested in a small batch reactor with an active inoculum. It is the standard way to compare feedstocks, and is described in guidelines such as VDI 4630 and the Holliger et al. (2016) recommendations.

Why test feedstocks in the laboratory before using them at a plant?

A BMP test shows how much gas a feedstock can give, while a regularly fed laboratory digester shows how a plant would respond to it over weeks of operation, including any inhibition. Testing at small scale lets operators and researchers try new feedstocks, loading rates or additives without putting a full-scale digester at risk.

Anaerobic digestion explained

Anaerobic digestion (AD) is a biological process in which microorganisms break down organic material in the absence of oxygen. It is used to treat materials such as food waste, agricultural residues, animal manure and sewage sludge, recovering part of their energy as biogas and retaining nutrients in a material known as digestate.

In an engineered system, this process takes place inside a sealed vessel called a digester. The material entering it is known as the feedstock. Depending on its source, preparation may include removing contaminants, reducing particle size or blending different materials into a more consistent mixture. Storage and controlled feeding help regulate what enters the digester, while heated, mixed systems provide conditions that support the microorganisms carrying out the conversion.

Simplified drawing: organic feedstock enters a sealed digester, producing biogas and nutrient-rich digestate

Detailed plant layout showing feedstock preparation, the digester, gas storage and power generation, and digestate storage

The biological conversion involves several groups of microorganisms working together in four linked stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis, described below. In a single-stage digester these reactions take place alongside one another within the same vessel, so the balance between the microbial groups decides how well the whole process runs.

Keeping the microbial community in balance is central to reliable operation. Feeding rate, temperature and the time material spends in the reactor all influence performance. Monitoring gas production, gas composition and digestate characteristics helps identify changes in the process and assess whether the system is operating consistently.

Biogas is collected from the digester and contains mainly methane and carbon dioxide, together with smaller amounts of other gases and water vapour. Its methane content provides the energy. After suitable treatment, biogas can be used to generate heat and electricity, including in a combined heat and power (CHP) unit. Further cleaning and the removal of carbon dioxide can upgrade it to biomethane for other fuel applications.

Digestate is the material remaining after digestion and contains both liquid and solid fractions. Although some of the organic material has been converted into gas, useful nutrients remain. Depending on its composition, quality and treatment, digestate can be used in agriculture to return nutrients and organic matter to the soil. Anaerobic digestion therefore combines waste treatment with the recovery of energy and nutrients.

The four stages of anaerobic digestion

Each stage is carried out by a different group of microorganisms, and each depends on the one before it. Most operating problems start when one stage runs faster or slower than the others.

Hydrolysis
Enzymes released by bacteria break carbohydrates, proteins and fats into smaller molecules: sugars, amino acids and long-chain fatty acids.
Why it matters: For fibrous feedstocks such as straw, grass or manure fibre, hydrolysis is often the slowest step and limits how quickly gas is produced. Pre-treatment and longer retention times are aimed at this stage.
Acidogenesis
Fermentative bacteria turn those products into volatile fatty acids (VFAs) such as propionate and butyrate, along with alcohols, hydrogen and carbon dioxide. Ammonia is released as proteins break down.
Why it matters: This stage is fast. If feeding outpaces the later stages, VFAs accumulate and the pH can fall.
Acetogenesis
Acetogenic bacteria convert VFAs and alcohols into acetate, hydrogen and carbon dioxide.
Why it matters: These reactions only proceed while hydrogen is kept low by the methanogens that consume it, so a rise in propionate is often an early sign that the process is out of balance.
Methanogenesis
Methanogenic archaea produce methane, most of it from acetate and the rest from hydrogen and carbon dioxide.
Why it matters: Methanogens grow slowly and are sensitive to temperature changes, low pH, ammonia and other inhibitors, so they often decide how stable the whole digester is.

Conditions that keep a digester stable

Because the microbial community is a chain, a digester is only as stable as its most sensitive link. These are the main conditions operators and researchers control and monitor.

Temperature: Most digesters run at mesophilic temperatures, around 35–40 °C, or thermophilic temperatures, around 50–55 °C. Thermophilic digestion is faster but more sensitive to change. Keeping the temperature steady matters more than the exact set point.
pH and alkalinity: Methanogens work best close to neutral, roughly pH 6.8–7.5. Alkalinity buffers the acids produced in the earlier stages, and many operators track the ratio of volatile acids to alkalinity (FOS/TAC) as an early warning of trouble.
Organic loading rate: The organic loading rate (OLR) is how much organic matter is fed per cubic metre of digester per day, usually in kg of volatile solids. Raising it too quickly is the most common cause of VFA build-up and a souring digester.
Retention time: The hydraulic retention time (HRT) is how long, on average, material stays in the digester. It is typically several weeks and has to be long enough for the slow-growing methanogens to keep pace, or they are washed out.
Nutrients and inhibitors: Microorganisms need a balanced diet: a carbon-to-nitrogen ratio of roughly 20–30:1 is often recommended, with trace elements such as cobalt and nickel. Too much ammonia from nitrogen-rich feedstocks, sulphide, salt or some disinfectants can slow or stop digestion.
Mixing: Mixing keeps the microorganisms in contact with the feedstock, evens out temperature and stops solids settling or forming a floating crust.

Understanding plant performance through laboratory research

Laboratory experiments provide a controlled setting for investigating the questions that arise at full scale. A biochemical methane potential (BMP) test estimates how much methane a feedstock can produce under the test conditions. A regularly fed reactor allows researchers to examine how a digestion process responds over time. Together, these approaches help build the evidence needed to evaluate feedstocks and investigate operating strategies.

Anaero researchers working with an auto-fed reactor system in the laboratory

Laboratory-scale digestion research

An Anaero auto-fed continuously stirred tank reactor (CSTR) allows researchers to study a regularly fed digestion process under controlled conditions. This makes it possible to investigate how a feedstock or a change in operating conditions affects performance over time.

The prepared feedstock is loaded into a feeding syringe, ready for measured additions to the reactor.
Automatic feeding introduces the material at selected intervals while mixing brings it into contact with the microbial culture.
Gas measurements show how the system responds throughout the experiment, helping researchers compare conditions and identify changes in performance.
Digestate is collected in dedicated bottles for sampling and further analysis alongside the gas production data.

Full-scale anaerobic digestion plant with green digester tanks and a gas storage dome

Full-scale anaerobic digestion plants

At an operational plant, the same biological principles are supported by larger systems for receiving and preparing material, feeding the digester and handling its outputs. The equipment and operating approach depend on the feedstock and the purpose of the plant.

Incoming organic material is received, checked and prepared before being held in feed storage.
The prepared feedstock is transferred into the digester through a controlled feeding system, with operating conditions managed to support the microbial community.
Biogas is collected and directed to storage, treatment or an energy system, depending on how the plant uses the gas.
Digestate is transferred to storage and may undergo separation or further treatment before an appropriate end use.

Frequently asked questions

What are the four stages of anaerobic digestion?
Hydrolysis, acidogenesis, acetogenesis and methanogenesis. Complex organic matter is broken down into simple molecules, fermented into volatile fatty acids, converted to acetate and hydrogen, and finally turned into methane by methanogenic archaea. In a single-stage digester all four happen at the same time in the same vessel.
What is the difference between anaerobic digestion and composting?
Composting is aerobic: microorganisms use oxygen, most of the energy in the material is released as heat and the carbon leaves as carbon dioxide. Anaerobic digestion takes place in a sealed vessel without oxygen, so much of that energy is captured as methane-rich biogas that can be used as fuel. The remaining digestate is usually wetter than compost.
What is biogas made of?
Biogas is typically 50–75% methane and 25–50% carbon dioxide, with smaller amounts of water vapour, hydrogen sulphide, ammonia and hydrogen. The methane content depends on the feedstock: fats and proteins give gas with more methane than carbohydrates do. Measuring biogas composition in the lab
What temperature does anaerobic digestion run at?
Usually mesophilic, around 35–40 °C, or thermophilic, around 50–55 °C. Mesophilic digestion is the more common and more forgiving choice; thermophilic digestion breaks material down faster and can improve pathogen reduction, but needs closer control.
How long does anaerobic digestion take?
In a full-scale plant, material typically stays in the digester for several weeks, depending on the feedstock and the design. In the laboratory, a batch biochemical methane potential (BMP) test usually runs until gas production levels off, from about two weeks for food waste to six weeks or more for fibrous material. How BMP tests work
What can make an anaerobic digester fail?
The most common causes are feeding too much too quickly, which lets volatile fatty acids build up and the pH fall; ammonia from nitrogen-rich feedstocks; sudden temperature changes; inhibitory substances in the feed; foaming; and a shortage of trace elements. Falling gas production or methane content is often the first sign, which is why gas monitoring is so useful.
What is a BMP test?
A biochemical methane potential (BMP) test measures how much methane a material can produce when it is digested in a small batch reactor with an active inoculum. It is the standard way to compare feedstocks, and is described in guidelines such as VDI 4630 and the Holliger et al. (2016) recommendations. The BMP Nautilus test system
Why test feedstocks in the laboratory before using them at a plant?
A BMP test shows how much gas a feedstock can give, while a regularly fed laboratory digester shows how a plant would respond to it over weeks of operation, including any inhibition. Testing at small scale lets operators and researchers try new feedstocks, loading rates or additives without putting a full-scale digester at risk. Laboratory test protocols

The four stages of anaerobic digestion

Hydrolysis

Acidogenesis

Acetogenesis

Methanogenesis

Conditions that keep a digester stable

Understanding plant performance through laboratory research

Laboratory-scale digestion research

Full-scale anaerobic digestion plants

Frequently asked questions