Anaerobic digestion requires a certain process temperature. Heat in the digesters is what makes the breakdown of the feedstock possible. There are two main temperature ranges to break down the feedstock effectively. These are:
- Mesophilic
- Thermophilic
Mesophilic temperatures in a digester are usually between 30-38˚c. This is the most common temperature range as it promotes a stable environment for the feedstock to be broken down, with minimal risk of toxicity, and minimal heat losses within the system.
The digesters for a mesophilic process are usually larger as the heat does not need to be intense for the reaction to take place.
One disadvantage to the mesophilic process is that an extra stage of pasteurisation needs to be added to adequately kill off pathogens within the feedstock. Also, due to the lower heat, the process itself takes longer than other methods.
Thermophilic temperatures are commonly upwards of 50˚c. Although this type of process is much more seldom than a mesophilic process, it does have many advantages.
In some cases, a thermophilic reaction is said to be up to ten times faster than a mesophilic process. The mesophilic process can often take 20-30 days at around 35˚c, whereas it potentially could only take 10-12 days at temperatures above 50˚c.
The advantages of a thermophilic system is that hydraulic retention time (HRT) is greatly reduced, meaning that you can get more processed through the digester. Also, you don’t have the need to pasteurise the feedstock as the thermophilic temperature is enough to kill pathogens.
One disadvantage with a thermophilic process is that it can be much more unstable compared to a mesophilic system. Another disadvantage is that it requires a large amount of energy to get up to temperature. With the higher required temperature inside the digesters, it also means there is much more heat loss in a thermophilic process.
