Methanogenesis is the last of the four phases of anaerobic digestion and the one that produces methane. It is carried out by methanogenic archaea, of two types: acetoclastic (from acetic acid) and hydrogenotrophic (from H₂ and CO₂). They are slow microorganisms, sensitive to pH, temperature, redox potential and free ammonia, which is why methanogenesis is the rate-limiting phase of the process: when a digester fails, it almost always fails here. This article explains what it is, its two pathways, its critical conditions and how to diagnose a failure.
Methanogenesis is the final step of anaerobic digestion and, in practical terms, the most important one: it is where methane is produced, the gas that gives value to the whole process.
This stage is carried out by the methanogenic archaea, a group of very ancient, strictly anaerobic and surprisingly delicate microorganisms. Understanding how they work is key to operating a digester.
This is the “what is” of methanogenesis: its two biochemical pathways, the microorganisms involved, the conditions it needs and why it sets the pace of the entire digestion process.
Definition of methanogenesis
Methanogenesis is the biochemical process by which methanogenic archaea produce methane (CH₄) as the final product of their metabolism, in the absence of oxygen. It is the last of the four phases of anaerobic digestion, after hydrolysis, acidogenesis and acetogenesis.
Unlike the microorganisms of the previous phases, methanogens are not bacteria: they are archaea, a different domain of life. And they obtain energy only by producing methane, which makes them highly specialised and vulnerable.
The two groups of methanogenic archaea: acetoclastic and hydrogenotrophic
Practically all the methane in a digester comes from two pathways:
- Acetoclastic methanogenesis: from acetic acid (CH₃COOH → CH₄ + CO₂). It provides most of the methane in conventional digesters, although it releases little energy (≈31 kJ/mol), which explains why it is so slow.
- Hydrogenotrophic methanogenesis: from hydrogen and CO₂ (4 H₂ + CO₂ → CH₄ + 2 H₂O). It is a minority in volume but critical: it keeps the H₂ pressure low and releases more energy (≈135 kJ/mol).
Telling them apart matters because they have very different speeds and sensitivities, and they react differently when something goes wrong.
Acetoclastic pathway: Methanosaeta and Methanosarcina
The two dominant genera are Methanosaeta and Methanosarcina. Methanosaeta is very efficient at low acetate concentrations but slow and fragile; Methanosarcina is more robust and faster, tolerates more load and stress, but needs more acetate.
Their doubling rate is low: between 3 and 7 days. That slowness is the reason why a digester takes weeks to start up and reach peak production, and also to recover from an imbalance.
Hydrogenotrophic pathway: Methanobacterium and Methanospirillum
Genera such as Methanobacterium or Methanospirillum consume H₂ and CO₂. They are much faster (doubling in 6 to 12 hours) and more resistant than the acetoclastic ones.
Their role goes beyond producing methane: by removing hydrogen, they make acetogenesis thermodynamically viable, a relationship of mutual dependence known as syntrophy. Without them, the previous phase blocks and propionic acid accumulates.
Critical operating conditions: temperature, pH, redox and free ammonia
Methanogens only work well within a narrow window of conditions:
| Variable | Optimal range | Effect if it goes outside |
|---|---|---|
| pH | 6.8-7.4 | Below 6.5 they are inhibited rapidly |
| Redox potential | -350 to -250 mV | They need a strongly reducing environment |
| Temperature | Mesophilic 35-40 °C / Thermophilic 50-57 °C | Abrupt changes stress them |
| Free ammonia (NH₃) | < 150 mg N/L (non-acclimatised) | Above that, progressive inhibition |
Free ammonia is one of the most frequent inhibitors in co-digestion with protein-rich substrates. An acclimatised consortium tolerates considerably more, but an abrupt jump always takes its toll.
Why methanogenesis is the rate-limiting phase of anaerobic digestion
The four phases of the process run at very different speeds. The acidogenic ones are fast and robust; the methanogenic ones are slow and fragile. That mismatch is behind almost every operational problem.
If the digester is fed faster than the methanogens can consume, acids accumulate, the pH drops and inhibits them even further: a vicious circle. This is why digester stability is decided in this phase.
How a methanogenesis failure is diagnosed in the plant
A methanogenesis failure is detected by a characteristic pattern, not by a single reading:
- Methane production falls and the CH₄ percentage in the biogas drops.
- Volatile fatty acids (VFA) rise, especially propionic acid.
- FOS/TAC rises and, later, the pH falls.
- Relative CO₂ increases in the biogas.
The key is to act early: reduce the load and allow time, before acidification becomes irreversible. A timely diagnosis avoids expensive shutdowns.
Frequently asked questions about methanogenesis
What is methanogenesis?
It is the final phase of anaerobic digestion, in which methanogenic archaea produce methane (CH₄) in the absence of oxygen. It happens through two pathways: the acetoclastic one (from acetic acid) and the hydrogenotrophic one (from hydrogen and CO₂). It is the step that gives energy value to the process and, at the same time, the most sensitive one.
What is the difference between acetoclastic and hydrogenotrophic archaea?
Acetoclastic archaea produce methane from acetic acid and provide most of the methane, but they are slow (doubling in 3-7 days) and fragile. Hydrogenotrophic archaea produce methane from H₂ and CO₂, are much faster (6-12 hours) and robust, and their key function is to remove hydrogen so that the preceding acetogenesis remains viable.
Why is methanogenesis the most sensitive phase of anaerobic digestion?
Because methanogenic archaea are slow (they take days to double) and only work within a narrow window of pH (6.8-7.4), a strongly reducing redox potential and a stable temperature, on top of being inhibited by free ammonia. As the previous phases are much faster, any overload accumulates acids faster than the methanogens can consume them, and that blocks them.
How do you know if there is a methanogenesis problem in a plant?
Through a combined pattern: production and methane percentage fall, volatile fatty acids rise (propionic above all), FOS/TAC increases and, later, the pH drops. No single reading confirms it; it is the combination that shows that methanogenesis cannot keep up with the previous phases. Detecting it early makes it possible to correct by reducing the load before the digester acidifies.
Smallops and the control of methanogenesis
Methanogenesis is where a biogas plant is won or lost. At Smallops we diagnose the real state of this phase and what is limiting it with an Operational Excellence Diagnosis.
Is your methanogenesis running at the pace it should?
Request a Smallops Operational Excellence Diagnosis: we measure the sentinel variables of your digester and detect whether the methanogenic phase is your bottleneck.
References and standards
Thauer, R.K. et al. (2008). Methanogenic archaea: ecologically relevant differences in energy conservation. Nature Reviews Microbiology, 6, 579-591. → doi.org/10.1038/nrmicro1931
Demirel, B. & Scherer, P. (2008). The roles of acetotrophic and hydrogenotrophic methanogens during anaerobic conversion of biomass to methane. Reviews in Environmental Science and Bio/Technology, 7, 173-190. → doi.org/10.1007/s11157-008-9131-1
Conrad, R. (2020). Importance of hydrogenotrophic, aceticlastic and methylotrophic methanogenesis. Microorganisms, 8 (5), 739. → doi.org/10.3390/microorganisms8050739
Ferry, J.G. (2010). The chemical biology of methanogenesis. Planetary and Space Science, 58 (14-15), 1775-1783.