Thermophilic vs. Mesophilic System
Production of biogas is based on anaerobic fermentation of organic waste. Fermentation is done by different bacterias, operating in different temperature ranges, which are psichrofil range (10-20°C), mesophilic range (20-45°C), and thermphilic range (50-67°C). In biogas production mesophilic and thermophilic temperatures are commonly in use. Temperature has a direct effect on the speed of the bacterial digestion. The optimal temperature for a thermophilic system is 55°C. To keep the process temperature at the target is important, and this can be done by good process design and good process control.
With the higher temperature, the thermophilic system is two to four times faster than the mesophilic system. The biomass retention time in thermophilic system is only 15-20 days, which leads to a considerably smaller reactor size compared to mesophilic process.
According to the literature, the thermophilic system produces 8-12% more biogas compared to mesophilic system, also methane concentration is 10-15 %-units higher compared to the mesophilic system.
A continuous thermophilic process can digest efficiently from the start of the process with return flow of digestate fluid from the end of the process. This ensures microbial population, and the fermentation starts rapidly. The return flow of digestate is needed since the thermophilic bacterias are rare in the nature.
Higher temperatures in the thermophilic digestion kills pathogens effectively, thus contamination risk is lower. Thermal sanitation or pasteurizing is needed for biomass or digestate in order to use the digestate as fertilizer. The temperature difference between the thermophilic process and sanitation phase is low, so the energy is not been wasted.
Benefits of a thermophilic system:
- faster digestion
- lower retention time
- higher yield of biogas
- higher concentration of methane/li>
- low contamination risk
- better energy balance when used with thermal sanitation
Sources and links:
- Book: The Microbiology of Anaerobic Digesters, Michael H. GerardiDRY DIGESTION THERMOPHILIC FAQS