Anaerobic digestion is a biological process in which a consortium of microorganisms breaks down organic matter in the absence of oxygen and generates two valuable products: biogas (a methane-rich mixture) and digestate (an organic fertiliser). It happens in four linked phases (hydrolysis, acidogenesis, acetogenesis and methanogenesis) and needs controlled conditions of temperature, pH and nutrients. It is the go-to technology for treating organic waste and turning it into renewable energy. This article explains what it is, how it works step by step, which microorganisms take part, what conditions it needs and what it is used for in industry.
Anaerobic digestion is one of the oldest biological processes on the planet and, at the same time, one of the most relevant technologies of the circular economy. It is the mechanism by which organic matter breaks down without oxygen and turns into biogas and digestate.
It occurs naturally in marshes, sediments and in the digestive tract of ruminants. Biogas engineering does no more than reproduce and control that process inside a sealed reactor (the digester) to maximise methane production.
This is the reference guide: what it is exactly, the four biochemical phases that make it up, the microorganisms involved, the conditions it needs and its industrial applications.
Technical definition of anaerobic digestion
Anaerobic digestion is the biological degradation of organic matter in the absence of oxygen, carried out by a consortium of microorganisms that work in a linked chain. The result is two products: biogas (energy) and digestate (fertiliser).
The word «anaerobic» is the key: the process only works without oxygen. Oxygen inhibits the methanogenic archaea, the microorganisms responsible for producing methane. That is why the digester is a sealed, airtight enclosure.
Unlike aerobic decomposition (with oxygen), which releases the energy of the organic matter as heat and CO₂, anaerobic digestion keeps most of that energy in the methane, which can be used as a fuel.
The four biochemical phases of anaerobic digestion
Anaerobic digestion is not a single reaction, but a chain of four phases that occur simultaneously inside the digester, each handled by a different group of microorganisms. If one phase fails, the following ones suffer. You can go deeper into each stage in the post on the phases of digestion.
| Phase | What happens | Main product |
|---|---|---|
| 1 · Hydrolysis | Large molecules (proteins, fats, carbohydrates) are broken into simpler, soluble ones (amino acids, fatty acids, sugars). It is the slowest phase with fibrous substrates. | Soluble monomers |
| 2 · Acidogenesis | The simple molecules are fermented. It is the fastest phase; an excess without balance acidifies the digester. | Volatile fatty acids (VFA), H₂, CO₂ |
| 3 · Acetogenesis | The VFA are transformed into acetic acid. It depends on a syntrophic relationship: it only works if the archaea remove the hydrogen. | Acetic acid, H₂, CO₂ |
| 4 · Methanogenesis | The archaea produce methane from acetic acid or from H₂+CO₂. It is the phase that adds value and the most sensitive. | Methane (CH₄) |
Methanogenesis is the bottleneck of the process: the archaea are slow and vulnerable to changes in pH, temperature and inhibitors. When something fails in a digester, it is almost always this phase that suffers first.
Microorganisms involved
Anaerobic digestion is a team effort of four microbial groups that depend on one another:
- Hydrolytic bacteria: break the polymers into monomers with extracellular enzymes.
- Acidogenic bacteria: ferment the monomers into VFA, alcohols and hydrogen. They are the fastest and most robust.
- Acetogenic bacteria: convert the VFA into acetate, H₂ and CO₂, in syntrophy with the archaea.
- Methanogenic archaea: produce the methane. They are the slowest (doubling time of 3 to 7 days) and the most sensitive. They set the pace of the whole process.
That mismatch in speeds (fast acidogens, slow methanogens) is the cause of most operational imbalances.
If you feed too fast, the acids accumulate before the archaea can consume them.
Operating conditions the process needs
For the consortium to work, the digester maintains a controlled environment. Four variables are critical:
| Condition | Optimal range | Why it matters |
|---|---|---|
| Temperature | Mesophilic 35-40 °C / Thermophilic 50-57 °C | Determines the speed and stability of the process |
| pH | 6.8-7.4 | The methanogens only work in a neutral range; below 6.5 they are inhibited |
| Oxygen | Absent | Oxygen is toxic to the methanogenic archaea |
| Trace nutrients | Ni, Co, Fe, Mo (small doses) | Cofactors of the methanogenic enzymes |
There are two usual temperature regimes: the mesophilic (35-40 °C), more stable and robust, and the thermophilic (50-57 °C), faster and with greater pathogen destruction, but more sensitive. The choice between mesophilic vs thermophilic depends on the substrate and the plant’s objective.
Products of anaerobic digestion: biogas and digestate
The process generates two products, both with economic value:
Biogas
A gaseous mixture composed mainly of methane (50-65%) and CO₂ (35-45%), with traces of H₂S (200-3,000 ppmv), water vapour and other compounds.
Its energy content is 5-7 kWh/Nm³ depending on the methane percentage. It is used to generate electricity and heat, or purified to biomethane for injection into the grid. More detail in the post on biogas composition.
Digestate
The liquid or semi-solid residue that remains after digestion. It keeps the nitrogen, phosphorus and potassium of the substrate in forms more assimilable by plants, so it is valorised as an organic fertiliser. Managing the digestate well closes the circular-economy loop.
Industrial applications of anaerobic digestion
Anaerobic digestion solves two problems at once: it treats waste and produces renewable energy. Its main applications are:
- Waste treatment: livestock slurry and manure, sewage sludge (WWTP), the organic fraction of municipal waste (OFMSW) and agro-industrial by-products.
- Energy production: electricity and heat by cogeneration, or biomethane for the natural gas grid and transport.
- Agronomic valorisation: the digestate replaces mineral fertilisers and reduces emissions.
A single plant can combine several substrates (co-digestion) to balance the diet and maximise production. The choice of digester type depends on the substrate, the scale and the objective.
Differences from other treatment technologies
Compared with other ways of managing organic matter, anaerobic digestion has a key advantage: it recovers energy instead of spending it.
| Technology | Needs oxygen? | Recovers energy? | Product |
|---|---|---|---|
| Anaerobic digestion | No | Yes (biogas) | Energy + fertiliser |
| Composting (aerobic) | Yes | No (spends energy) | Compost |
| Incineration | Yes | Partial (heat) | Ash |
| Landfill | — | No (emits methane uncaptured) | Leachate and emissions |
Composting is aerobic and releases the energy as heat; incineration destroys the matter and emits CO₂; landfill generates methane uncontrolled (a potent greenhouse gas).
Anaerobic digestion is the only one that captures that methane and turns it into usable energy.
Frequently asked questions about anaerobic digestion
What exactly is anaerobic digestion?
It is a biological process in which a consortium of microorganisms breaks down organic matter in the absence of oxygen. The result is two products: biogas (a methane-rich mixture used as renewable energy) and digestate (an organic fertiliser). It occurs naturally in marshes and ruminants, and engineering reproduces it in a controlled way inside a digester.
How does anaerobic digestion differ from aerobic digestion?
The difference is oxygen. Aerobic digestion (like composting) occurs with oxygen and releases the energy of the organic matter as heat and CO₂. Anaerobic digestion occurs without oxygen and keeps most of that energy as methane, usable as a fuel. In addition, the anaerobic route generates much less sludge and allows energy to be recovered rather than consumed.
What waste can be treated by anaerobic digestion?
Almost any biodegradable organic waste: livestock slurry and manure, sewage sludge, the organic fraction of municipal waste (OFMSW), food-industry residues, glycerine, fats and oils. Very lignocellulosic materials (wood, straw) digest poorly without pre-treatment, because the hydrolysis phase is very slow.
How much biogas does a kilo of organic waste produce?
It depends on the substrate. The biochemical methane potential (BMP) of common waste is between 200 and 500 NmL of CH₄ per gram of volatile solids. Fatty substrates (FOG, oils) produce much more (up to 1,000 NmL CH₄/g VS) and very fibrous ones much less. As a reference, a kilo of easily biodegradable volatile solids can yield on the order of 0.3-0.5 m³ of methane.
Smallops and anaerobic digestion
Understanding anaerobic digestion is the first step; operating it efficiently is another. At Smallops we turn these fundamentals into real performance: an Operational Excellence Diagnosis measures what is limiting your digester and how to unblock it.
Do you want to get more from your digester?
Request a Smallops Operational Excellence Diagnosis: we characterise your process across 14 variables and tell you where the bottleneck is and how to solve it.
References and standards
Angelidaki, I. & Sanders, W. (2004). Assessment of the anaerobic biodegradability of macropollutants. Reviews in Environmental Science and Bio/Technology, 3, 117-129. → doi.org/10.1007/s11157-004-2502-3
Appels, L. et al. (2008). Principles and potential of the anaerobic digestion of waste-activated sludge. Progress in Energy and Combustion Science, 34 (6), 755-781. → doi.org/10.1016/j.pecs.2008.06.002
Weiland, P. (2010). Biogas production: current state and perspectives. Applied Microbiology and Biotechnology, 85, 849-860. → doi.org/10.1007/s00253-009-2246-7
McCarty, P.L. (1964). Anaerobic waste treatment fundamentals. Public Works, 95 (9-12).