Inside a digester live hundreds of species of microorganisms organised into four large groups: hydrolytic, acidogenic and acetogenic bacteria, and methanogenic archaea. A stable digester harbours between 150 and 500 different species and up to 10¹⁰ cells per millilitre. Knowing that community, now accessible with molecular techniques (qPCR, 16S, metagenomics), makes it possible to understand why a digester fails and to anticipate it. This article describes the key communities, their dynamics and how they translate into operational diagnosis.
A biogas digester is not a chemical tank: it is a living ecosystem. Inside live hundreds of species of microorganisms that take over from one another to convert organic matter into methane.
For decades, that microbiology was a black box: it was known to work, but not what was inside. Today, molecular techniques have opened up that uncertainty.
This is the what is inside of the digester: the key microbial communities, how they change under disturbances and how that knowledge translates into diagnosis. It is the deepest level of the phases of anaerobic digestion.
The four large communities of the digester
The microbial community of the digester is organised into four groups, each responsible for one phase of the process:
| Community | Function | Typical groups |
|---|---|---|
| Hydrolytic bacteria | Break down macromolecules | Bacteroidetes, Firmicutes |
| Acidogenic bacteria | Ferment to VFA, H₂ and CO₂ | Clostridium, Lactobacillus |
| Acetogenic bacteria | Produce acetate and hydrogen | Syntrophomonas, Syntrophobacter |
| Methanogenic archaea | Produce the methane | Methanosaeta, Methanosarcina, Methanobacterium |
Each group depends on the previous one. If one weakens, the whole chain feels it.
A stable digester maintains between 150 and 500 different species and of the order of 10⁸-10¹⁰ cells per millilitre.
Hydrolytic bacteria: Bacteroidetes and Firmicutes
They are the first line. They secrete enzymes that break macromolecules (proteins, carbohydrates, lipids) into simple molecules.
The Bacteroidetes and Firmicutes phyla dominate. They are very versatile and abundant, and their activity sets the start-up speed with fibrous substrates.
Acidogenic and acetogenic bacteria
The acidogenic bacteria ferment the products of hydrolysis into volatile fatty acids (VFA), hydrogen and CO₂. They are fast and robust.
The acetogenic bacteria (such as Syntrophomonas or Syntrophobacter) transform those VFA into acetate and hydrogen. They are slow and depend on the methanogens removing the H₂: this is syntrophy, one of the most fragile balances in the system.
Methanogenic archaea: Methanosarcina, Methanosaeta, Methanobacterium
They are the ones that produce the methane and, because of their slowness, the most delicate. The key genera are:
- Methanosaeta: very efficient at low acetate concentrations, but fragile. It dominates in stable, lightly loaded digesters.
- Methanosarcina: more robust and faster, it tolerates more load and stress, but needs more acetate. It appears when the load rises.
- Methanobacterium: hydrogenotrophic, it produces methane from H₂ and CO₂.
That is why the Methanosaeta/Methanosarcina ratio is a good indicator of the digester load level. It is covered in detail in the post on methanogenesis.
Sulphate-reducing bacteria (SRB) and their competition with methanogens
When the substrate contains sulphur (slurries, vinasses, poultry manure), sulphate-reducing bacteria (SRB) appear. They compete with the methanogens for hydrogen and acetate, and produce H₂S.
The result is a double penalty: less methane and a more corrosive biogas. Controlling the sulphur in the diet is, in part, controlling this microbial competition.
Modern characterisation techniques: qPCR, 16S and metagenomics
Today the microbiology of the digester can be measured. The main techniques are:
| Technique | What it provides | Approx. cost / turnaround |
|---|---|---|
| qPCR | Quantifies specific groups (e.g. methanogens) | Low, days |
| 16S rRNA sequencing | Identifies which genera are present and in what proportion | Medium, 2-3 weeks |
| Metagenomics | Full profile: who is there and what functions they can perform | €500-1,500/sample, weeks |
The most expensive technique is not always needed: for routine monitoring, qPCR is enough; metagenomics is reserved for complex problems.
How microbiology explains operational problems
Many failures regarded as operational have, deep down, a microbiological explanation:
- An acidification is a mismatch between acidogenic bacteria (fast) and methanogens (slow).
- A digester that does not start up with fibre usually has few active hydrolytic bacteria.
- An excess of H₂S points to an SRB community favoured by the sulphur.
The community also changes with the season: in co-digestion, diversity can vary by 20-40 % over the year.
Practical applications in diagnosis
Microbiological analysis does not replace the operational variables (FOS/TAC, VFA, production), but it complements them: it explains the why behind the symptoms.
It is especially useful for diagnosing recurring problems, validating acclimation to a new substrate or deciding on a bioaugmentation. All of this requires well-taken, well-traced samples (see biogas sample traceability).
Frequently asked questions about digester microbiology
What microorganisms are there in an anaerobic digester?
Four large groups coexist in a digester: hydrolytic bacteria (which break down macromolecules), acidogenic bacteria (which ferment to volatile fatty acids), acetogenic bacteria (which produce acetate and hydrogen) and methanogenic archaea (which produce the methane). Together they are usually between 150 and 500 different species and of the order of 10⁸-10¹⁰ cells per millilitre. Each group depends on the previous one.
What is microbiology analysis for in biogas?
It helps understand the why behind the operational symptoms. The classic variables (FOS/TAC, VFA, production) say something is wrong; microbiology says which community is failing. It is useful for diagnosing recurring problems, validating adaptation to a new substrate, controlling the competition from sulphate-reducing bacteria or deciding whether a bioaugmentation makes sense.
What is 16S analysis and what information does it provide?
16S rRNA gene sequencing is the standard technique for knowing which microorganisms are in the digester and in what proportion. It identifies the genera present (for example, which methanogenic archaea dominate) and allows comparing communities between plants or over time. Its turnaround is usually 2-3 weeks and its cost, intermediate. It does not directly measure activity, but composition.
When is a metagenomic analysis of the digester worth it?
Full metagenomics (€500-1,500 per sample) is worth it when there is a complex or recurring problem that the operational variables do not explain, when a digester needs to be characterised in depth before a major change, or in R&D projects. For routine monitoring, cheaper techniques such as qPCR, which quantify specific groups in a few days, are usually enough.
Smallops and microbiological diagnosis
Microbiology explains what the operational variables only hint at. At Smallops we integrate process analysis and, when needed, microbiological characterisation into an Operational Excellence Diagnosis.
Do you know which microbial community sustains your digester?
Request a Smallops Operational Excellence Diagnosis: we combine operational variables and, if the case requires it, microbiological analysis to explain and correct what is limiting your plant.
References and standards
De Vrieze, J. et al. (2018). The full-scale anaerobic digestion microbiome. Microbial Biotechnology, 11 (6), 1006-1016. doi.org/10.1111/1751-7915.13280
Vanwonterghem, I. et al. (2014). Linking microbial community structure, interactions and function in anaerobic digestion. Current Opinion in Biotechnology, 27, 55-64.
Werner, J.J. et al. (2011). Bacterial community structures are unique and resilient in full-scale bioenergy systems. PNAS, 108 (10), 4158-4163.
Calusinska, M. et al. (2018). A year of monitoring 20 mesophilic full-scale bioreactors. Biotechnology for Biofuels, 11, 196.