The still common understanding assumes the existence of a mesophilic (35 – 40 °C) as well as a thermophilic (50 – 55 °C) temperature range for the biogas production. More stable biological process conditions should be enabled by the mesophilic range, the thermophilic range is said to be faster and to produce more gas. 20 years ago I believed this also, even though I had doubts at that time already.
Is it not very illogical, that there should exist no bacteria between 40 and 50 °C, which adapted optimally to this temperature range? Meanwhile I know that the mesophilic/thermophilic doctrine is wrong, even though it is copied in the latest guidelines. I am operating biogas plants since more than 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 the plants discernible differences between the temperature ranges can all be explained physically. The higher water vapour content in the biogas is most important. This produces seemingly more biogas – but the matter is only water vapour which condenses during the cooling. Therefore the biogas cooling must be dimensioned a bit larger at higher digester temperatures.
So how shall be chosen the optimal digester temperature?
I define the digester target temperature from the conditions of the location. Thereby two extremes can be distinguished for example, liquid manure mono-fermentation and silage mono-fermentation. If I intend to ferment liquid manure exclusively and have a good heat utilization I decrease the digester temperature as far as possible. Thus a higher proportion of usable heat remains.
This additional usable heat is quite relevant. In a liquid manure mono-fermentation at least half of the CHP waste heat is needed for the warming of the liquid manure to a fermentation temperature of for example 37 °C. At a digester temperature of 30 – 32 °C and a sufficient time for the adaptation the biology works as good as with 37 °C, however the heat requirement for the digester heating decreases considerably and about 30 % more usable heat remains available.
If in a biogas plant, however, only silage is processed („dry fermentation“), the required heating energy for the digester is almost negligible, especially when the plant is operated at a higher loading rate. Due to the agitators and the self-heating sufficient heat is produced to maintain the fermentation temperature upright. Depending on the fermentation system and 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.
The absolute digester temperature is not an issue
An uncontrolled heating of the digester is harmful whenever the temperature rise is too fast and strong. An increase by 3 – 4 °C within two weeks can damage the whole biology to an extent that the plant „crashes“. An especially low resilience show plants, which for example suffer from a lack of micronutrients or which are biologically overloaded.
In my experience in the longterm practical operation of several different biogas plants the absolute digester temperatur does not matter if it is somewhere between 30 and 60 °C. Probably much higher and lower temperatures are possible. However, it is crucial how this digester temperature is reached and that this temperature is held permanently constant.
The chosen digester temperature must be set controlled. If possible in a new commissioning a substrate should be used which is adapted to a similar temperature. If such a substrate is not available or a change of the existent temperature level is necessary, the change of the temperature has to take place so slowly that the bacteria can adapt themselves well. The change of the digester temperature should thereby amount to not more than 1 °C per week.
If the digester has turned cold, for example due to a damage of the CHP – do I have to heat then so slowly again? I claim no. If the biology is adapted to a temperature level of for example 52 °C (my preferred temperature in silage mono-fermentation in Germany), the whole biology is trimmed for this. If the digester is cooling then for example to 44 °C, the target temperature of 52 °C can be set in almost any period without damaging the biology. However, it has to be ensured that the bacteria will not be cooked at too high heating areas.
A cooling is less harmful for the bacteria as a warming. In this case they are merely getting slowlier and lazier. If a permanent change of a digester 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. Short version:
- Forget mesophilic and thermophilic, this is not true.
- It does not matter if you work with 30, 38, 44 or 52 °C, as long as the temperature is held constant.
- The temperature must be held constant as far as possible.
- Specific temperature changes shall occur with 1 °C per week at a maximum.
- A fast increase of the temperature over 3 – 4 °C in two weeks is harmful.
If you have questions, please leave a comment or contact me. I will collect the questions and answere in a special article.