Mesophilic and thermophilic regimes
There is a doctrine about mesophilic (35–40 °C) and thermophilic (50–55 °C) regimes. Many still think that mesophilic is a more stable biological process and the thermophilic is faster and more productive but less stable. 10-15 years ago, we believed this ourselves and even taught our Customers and students.
Now we know that the mesophilic/thermophilic doctrine is wrong. Bacteria adapts to temperature between 40 °C and 50 °C. We have been operating biogas plants 15 years with digester temperatures between 30 °C and 55 °C and can detect no differences in methane yield and process stability.
In the practical operation of biogas plants, the differences between the biogas outputs at different temperature ranges can all be explained physically. The higher water vapor content in biogas is most important. At a higher operational temperature there is seemingly more biogas. But the matter is only water vapor which condenses during the cooling.
The absolute digester temperature is not important. The more important is how the digester temperature is reached and holding a stable temperature. An increase by 3–4 °C within two weeks can kill the whole biology. Especially vulnerable are biogas plants suffering from a lack of micronutrients.
Cooling is less harmful for the bacteria than warming. They are merely getting slower and lazier. If a permanent change to a lower temperature is planned, the lowering should again not amount to more than 1 °C per week. This ensures that the digester biology can adapt and no drop in performance will occur.
But if the digester has suddenly turned cold from 52 °C to 44 °C, for example, due to a damage of the CHP, and the target temperature was 52 °C, the heating back to 52 °C should be as soon as possible.
The optimal digester temperature is defined from
- region weather conditions,
- substrate incoming temperature,
- natural substrate temperature.
Two extremes can be distinguished: liquid manure mono-fermentation in a cold climate and grass/maize silage mono-fermentation in the tropics.
In a liquid manure mono-fermentation at least half of the CHP waste heat is to warm the manure, for example, to 37 °C. Also pay attention to the natural temperature of animals 37–38 °C. So, the preferable temperature is 37–38 °C. But if during severe winter temperature can’t be reached plant would work well under temperature 30–32 °C if adapted slowly.
If only grass silage is processed the required heat is almost zero. Due to the agitators and the self-heating sufficient heat is produced to maintain the fermentation temperature. Depending on the outside temperature heating is only needed on very cold days. The uncontrolled self-heating in the digester occurring in summer is more critical here. So, for grass in the tropics, we prefer regimes 44–55 °C with a cooling option.
