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. During hydrolysis, complex materials such as carbohydrates, proteins and fats are broken into smaller compounds. Acidogenesis converts these into organic acids and other intermediates, followed by acetogenesis, which produces acetate, hydrogen and carbon dioxide. Finally, methane-producing microorganisms carry out methanogenesis, converting these products into methane. In a single-stage digester, these linked reactions take place alongside one another within the same vessel.

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.

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.