Anaerobic digestion can operate in a mesophilic regime (35-40 °C) or a thermophilic one (50-57 °C). The thermophilic regime is faster (hydrolysis runs 2-3 times quicker, with retention times of 12-20 days versus 20-40) and hygienises the digestate, but it is more sensitive to ammonia and consumes 30-60 % more heating. The mesophilic regime is slower but far more stable and cheaper, which is why it is the industrial standard. This article compares both and explains when each one pays off.
Choosing the thermal regime is one of the first and most decisive decisions when designing a biogas plant. It conditions the size of the reactor, the stability of the process and the operating cost for the whole life of the plant.
The comparison boils down, in essence, to a trade-off: speed versus stability. And that trade-off is decided before building.
This is the technical and operational comparison between mesophilic and thermophilic: what sets them apart, their advantages and drawbacks and in which cases each one pays off. It builds on the fundamentals of what anaerobic digestion is.
The thermal regimes: psychrophilic, mesophilic and thermophilic
Anaerobic digestion can operate in three temperature ranges, depending on the dominant microorganisms:
- Psychrophilic (below 25 °C): very slow, hardly used at industrial scale.
- Mesophilic (35-40 °C): the most common regime in biogas plants.
- Thermophilic (50-57 °C): faster, but more demanding.
The choice of regime conditions the speed, the stability and the cost of the plant. In practice, the decision comes down to comparing the first two: mesophilic and thermophilic.
Mesophilic (35-40 °C): the industrial standard
The mesophilic regime works at 35-40 °C, with an optimum around 37 °C. It is the industry standard: the vast majority of biogas plants are mesophilic.
Its advantage is not speed but robustness. The mesophilic consortium better tolerates variations in diet, temperature and load; it is easier to operate and more forgiving of mistakes.
Thermophilic (50-57 °C): more speed, more risk
The thermophilic regime works at 50-57 °C, with an optimum around 55 °C. At that temperature, biochemical reactions run much faster: hydrolysis can be 2-3 times quicker than in mesophilic conditions.
That allows shorter retention times (12-20 days versus 20-40 for mesophilic) and, therefore, smaller reactors or greater treatment capacity. But that speed comes at a price: the thermophilic consortium is more sensitive and less stable. In addition, the operating cost and the heat losses are higher.
Technical comparison at a glance
Summarised, the comparison between both regimes is as follows:
| Parameter | Mesophilic | Thermophilic |
|---|---|---|
| Temperature | 35-40 °C (optimum 37) | 50-57 °C (optimum 55) |
| Hydrolysis rate | Reference | 2-3 times higher |
| Typical HRT | 20-40 days | 12-20 days |
| Stability | High | Lower (more sensitive) |
| Hygienisation | Not guaranteed | Yes (class A) |
| Heating cost | Reference | +30-60 % |
The thermophilic regime wins on speed; the mesophilic one, on stability and cost. There is no better regime in the abstract: it depends on the objective.
Operational stability: why mesophilic is preferred
The reason mesophilic dominates the industry is stability. The thermophilic consortium is more sensitive to changes and, above all, to free ammonia: its inhibition threshold is lower (≈600 mg N/L versus ≈700 in mesophilic conditions).
This means that a nitrogen-rich substrate (slurries, protein-rich waste) inhibits a thermophilic digester sooner. In plants with diets that vary both in the share of substrates in the feed and in the composition of the substrates over the year, that fragility comes at a high price.
Hygienisation: the specific advantage of thermophilic
The advantage specific to the thermophilic regime is hygienisation. At 55 °C, the process destroys pathogens and seeds, something the mesophilic regime does not guarantee.
This is key when the digestate must meet sanitary requirements, for example to be used as fertiliser with animal by-products (class A under Regulation EU 1069/2009). It is an important factor in the digestate regulation (SANDACH).
Energy costs compared
The trade-off of the thermophilic regime is the energy cost. Keeping the digester at 55 °C instead of 37 °C consumes much more energy: between 30 and 60 % more, depending on the climate and the insulation.
In plants with abundant waste heat (from their own cogeneration) that extra cost is diluted; in others, it can completely wipe out the advantage of the greater capacity.
When each regime pays off
As a general rule:
- Mesophilic: the default option for most plants, especially with variable or livestock-based diets. It prioritises stability and cost.
- Thermophilic: it pays off when maximum capacity is needed in little volume, or when hygienisation is mandatory (waste containing animal by-products, WWTP sludge).
The decision is taken at the design stage and is hard to reverse, so it should be analysed carefully before building. The different types of digesters can operate in either regime.
Frequently asked questions about mesophilic and thermophilic
What is the difference between mesophilic and thermophilic digestion?
The main difference is the operating temperature and its consequences. Mesophilic digestion works at 35-40 °C and thermophilic at 50-57 °C. Thermophilic is faster (hydrolysis 2-3 times higher, shorter retention times) and hygienises the digestate, but it is more sensitive and consumes more energy. Mesophilic is slower but far more stable and cheaper to operate, which is why it is the industry standard.
When is a thermophilic plant worth it?
It pays off when a lot of substrate needs to be treated in little reactor volume (thanks to its higher speed) or when hygienisation of the digestate is mandatory, for example with waste containing animal by-products or sewage sludge that must meet sanitary requirements (class A). Outside those cases, the extra energy cost and the lower stability usually tip the balance towards mesophilic.
Why is mesophilic more stable?
Because the mesophilic microbial consortium is more robust and tolerant. It copes better with variations in diet, temperature and load, and its free ammonia inhibition threshold is higher (≈700 mg N/L versus ≈600 for thermophilic). The thermophilic consortium, working closer to its limits and with more specialised microorganisms, is unbalanced more easily by any disturbance.
How much extra biogas does a thermophilic digester produce?
A thermophilic digester does not necessarily produce much more total biogas per kilo of substrate: the difference in specific production is usually moderate. Its advantage is speed: it degrades the substrate faster, which allows treating more flow in the same volume or using smaller reactors. With fibrous, difficult substrates it can also somewhat improve degradation. The decision is driven less by the extra biogas than by capacity and hygienisation.
Smallops and the choice of thermal regime
Choosing between mesophilic and thermophilic conditions the whole life of the plant. At Smallops we assess which regime fits your substrates and objectives with an Operational Excellence Diagnosis.
Mesophilic or thermophilic for your plant?
Request a Smallops Operational Excellence Diagnosis: we analyse your substrates, your production target and your hygienisation requirements to recommend the optimal thermal regime.
References and standards
Kim, M. et al. (2002). Comparative process stability and efficiency of anaerobic digestion. Water Research, 36 (17), 4369-4385.
Bouallagui, H. et al. (2004). Mesophilic and thermophilic anaerobic digestion of fruit and vegetable wastes. Process Biochemistry, 39 (12), 2143-2148.
Gao, S. et al. (2015). Thermophilic and mesophilic anaerobic digestion of solid wastes. Renewable and Sustainable Energy Reviews, 50, 1014-1022.
Labatut, R.A. et al. (2014). Anaerobic digestion of solid waste in a thermophilic and mesophilic regime. Bioresource Technology, 169, 537-545.