Anaerobic digestion (AD) is a biological process in which microorganisms break down organic material without oxygen. The process takes place inside a sealed digester and produces two main outputs: methane-rich biogas and digestate.
The biogas can be used in a combined heat and power unit to generate electricity and heat, or it can be upgraded to biomethane. The digestate still contains much of the nitrogen, phosphorus and potassium that entered the plant with the original feedstock.
How does anaerobic digestion work?
An AD plant is a continuous biological process rather than a system that simply burns organic waste. The main stages are:
- feedstock reception and preparation,
- feeding material into the digester,
- anaerobic digestion,
- collection and treatment of the biogas,
- use of the gas for CHP or biomethane production,
- storage and use of the resulting digestate.
Feedstock is normally added regularly rather than all at once. As the organic material is broken down, part of its carbon is converted into methane and carbon dioxide. The remaining material leaves the process as digestate.
What goes into an AD plant?
Feedstocks can include cattle or pig slurry, manure, food waste, crop residues, silage and other biodegradable materials. The exact mix depends on the design of the plant and the materials available locally.
Not all feedstocks have the same energy potential. A tonne of dilute slurry can contain far less biodegradable organic matter than a tonne of food-processing residue or silage.
A tonne of feedstock is not a tonne of energy potential. Biogas yield depends much more on dry matter, organic matter and how readily that material can be digested.
Why does the feedstock need preparation?
A digester works best with a reasonably consistent feed. Depending on the material, preparation may include shredding, mixing, removing packaging or contaminants, and adjusting the solids content before the feedstock enters the digester.
Sand and other heavy contaminants can settle in tanks and reduce their working volume. Long fibrous material can interfere with pumps and mixers. Sudden changes in feed composition can also upset the biological process.
What happens inside the digester?
The digester is a sealed vessel with no free oxygen. Different groups of microorganisms progressively break down the organic material through four main stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis.
Most farm-scale mesophilic systems operate at roughly 35–40°C. The contents are usually mixed and the digester is heated to keep conditions as stable as possible.
Temperature is only one part of the process. Feed rate, retention time, pH, the composition of the feedstock and the balance between different groups of microorganisms all affect performance.
This is why an AD plant is better understood as a controlled biological reactor than as a storage tank for organic material.
What is biogas made of?
Raw biogas typically contains around 50–70% methane. Most of the remainder is carbon dioxide, with smaller amounts of water vapour, hydrogen sulphide and other trace gases.
| Component | Why it matters |
|---|---|
| Methane (CH₄) | Main energy-carrying component |
| Carbon dioxide (CO₂) | Reduces the energy content of raw biogas |
| Hydrogen sulphide (H₂S) | Corrosive and normally needs to be reduced |
| Water vapour | Can condense in pipework and equipment |
The methane is the valuable fuel component. A higher methane concentration means more energy per unit volume of gas.
Why does biogas need cleaning?
Raw biogas is not normally sent straight from the digester into an engine or the gas grid. Water and hydrogen sulphide need to be controlled because they can cause corrosion and reliability problems.
The level of treatment depends on what happens next. Gas intended for a CHP engine needs a different level of cleaning from gas that will be upgraded to biomethane.
What is the difference between biogas and biomethane?
Biogas and biomethane are related, but they are not the same product.
| Property | Biogas | Biomethane |
|---|---|---|
| Methane content | Typically around 50–70% | Much higher after upgrading |
| CO₂ | Large proportion remains | Most is removed |
| Water and H₂S | Need treatment | Reduced to meet the required gas specification |
| Typical use | CHP, boilers | Gas grid, transport fuel, gas storage |
Biomethane is produced by upgrading biogas. This removes much of the carbon dioxide and reduces other unwanted components until the gas reaches the required specification.
Once upgraded, biomethane can be injected into the gas grid where the necessary connection and gas-quality requirements are met.
How does an AD plant generate electricity and heat?
One common route is to feed treated biogas into a gas engine connected to a generator. The generator produces electricity, while heat is recovered from the engine cooling system and exhaust.
This is known as combined heat and power, or CHP. Some of the recovered heat is normally used to maintain the digester temperature. The remainder can be used for buildings, crop drying, process heat or other nearby heat demands.
A CHP system only achieves high overall efficiency when the heat is actually used. Where both electricity and useful heat are recovered, total efficiency can exceed 80%.
What is digestate?
Digestate is the material left after anaerobic digestion. It contains water, partially stabilised organic matter and plant nutrients that were present in the original feedstock.
Anaerobic digestion does not remove most of the nitrogen, phosphorus and potassium from the material. Instead, a large proportion remains available in the digestate.
Some organic nitrogen is converted into ammonium nitrogen during digestion. This can make the nitrogen more readily available to crops, but also increases the risk of ammonia losses if digestate is stored or spread poorly.
AD removes part of the energy stored in biodegradable carbon. It does not remove the fertiliser value of all the nitrogen, phosphorus and potassium that entered the plant.
Whole digestate, liquid and fibre
Digestate can be used as a whole material or separated into liquid and solid fractions.
The liquid fraction generally contains a large share of the readily soluble nitrogen and potassium. The fibre fraction contains more dry matter and organic material, together with part of the phosphorus.
Separation can make storage, transport and nutrient management easier, particularly where large volumes of water would otherwise have to be moved over long distances.
Where digestate is produced from waste feedstocks, it is not automatically treated as an ordinary fertiliser product. Its regulatory status depends on how it was produced and whether the relevant quality requirements are met. PAS 110 is an important quality specification for anaerobic digestate in the UK.
What determines how much biogas is produced?
Fresh weight alone is a poor way to compare feedstocks. Biogas output depends on several factors:
- dry matter content,
- volatile solids content,
- the type of organic material,
- biodegradability,
- retention time,
- digester temperature,
- process stability.
A watery feedstock may weigh a great deal but contain relatively little organic material capable of producing methane. For this reason, technical comparisons often refer to volatile solids rather than tonnes of fresh material alone.
Can an AD plant produce gas continuously?
Yes. Unlike solar or wind generation, biogas production does not depend directly on sunlight or wind speed. If feedstock is supplied regularly and the biological process remains stable, gas production continues around the clock.
That does not mean the plant runs without interruption. Pumps, mixers, gas-cleaning equipment, engines and other components require maintenance, and poor feeding or biological instability can reduce gas output.
Does an AD plant smell?
The digester itself is sealed, so the digestion process should not normally release odours directly into the surrounding air.
Odour problems are more likely to arise during feedstock reception, storage and handling, or from poorly managed digestate storage. The design of reception areas, tanks and transfer systems therefore matters as much as the digester itself.
A well-run AD plant should control odorous material rather than simply move it from one part of the site to another.
Why methane leakage matters
Methane is the fuel the plant is trying to produce. Any methane lost from tanks, pipework, gas holders or storage systems is therefore both an energy loss and an environmental problem.
Another source can be methane slip from gas engines, where a small proportion of methane passes through the engine without being fully burned.
Modern AD operation therefore places increasing emphasis on leak detection, sealed digestate storage and control of residual gases from biogas upgrading.
Producing methane is only half the job. A well-run AD plant also has to keep that methane inside the gas system until it is used.
Where is anaerobic digestion technology heading?
AD is increasingly moving beyond the simple model of producing biogas and burning it in an engine. One major direction is biogas upgrading to biomethane, allowing renewable gas to be stored, transported or injected into the gas grid.
Digestate is also being treated more as a source of recoverable nutrients. Separation, concentration and nutrient-recovery technologies can reduce the amount of water that needs to be transported and allow nitrogen or phosphorus to be managed more precisely.
Biogas upgrading creates another potentially useful stream: biogenic CO₂. Once separated and purified, this carbon dioxide can be recovered for industrial or other suitable uses instead of simply being released.
The performance of an AD plant is therefore increasingly judged not only by how much gas it produces, but by how efficiently it uses the methane, nutrients, heat and other outputs created by the process.