Acetogenic bacteria use the volatile fatty acids and form acetic, propionic, and lactic acids. In addition, hydrogen and carbon dioxide gas can be released by the acetogenic bacteria. Other bacteria use hydrogen and carbon dioxide produced in stage 1 to form methane. The anaerobic digestion process is carried out by a delicately balanced population of various bacteria. These bacteria can be very sensitive to changes in their environment.
Temperature is a prime example.
As the temperature falls, bacteria activity decreases and biogas production decreases. As the temperature increases some bacteria begin to die, once again biogas production decreases. Insulation, heat exchangers, heating elements, water baths, and steam injection are all means which have been used to control digester temperature. Temperature control is an important consideration when designing digesters. The materials list includes two potential heat sources.