Background Information
Digesters

Digesters utilize human wastes and convert them to biogas and biosolids through anaerobic bacterial processes. Biogas is approximately forty percent carbon dioxide and sixty percent methane, with trace amounts of other gases. The bacteria use the organic material in the municipal waste to produce the carbon dioxide and methane, as well as material for the proliferation of the bacteria.  There are two types of bacteria that carry out this process: mesophilic and thermophilic. Mesophilic bacteria require operating temperatures at approximately 35� C, while thermophilic bacteria require temperatures at 55� C and higher. (EIDN Pty Ltd.)

Used with permission of Roediger

   Temperature is an extremely important component of the digestion process due to its direct proportionality to methane production.  If the temperature falls below the ideal level, the production of methane is reduced. Although higher temperatures can amplify methane production, beyond a threshold temperature, additional elevations in temperature do not constitute an improvement in productivity.  Maintaining digester temperature constantly near 35� C will enhance digester performance and create stability. (Atlas Internet Corporate Solutions)(Cornell University, Composting; Science and Engineering)

Anaerobic digestion occurs through two processes: acidification and methane production. During acidification, acid-forming bacteria break down complex organic wastes into fatty acids. Proteins are broken down into amino acids, which are then broken down into fatty acids. Carbohydrates are broken down into simple sugars and then into fatty acids. Acetogenic bacteria then use the fatty acids to form acetic, propionic, and lactic acids.  During methane production, methane forming bacteria, methanogens, use the acids formed during acidification to produce methane.  In addition, other bacteria use the hydrogen and carbon dioxide produced during acidification to produce more methane.  As acid-forming bacteria introduce more acid,

Used with permission of Roediger

the methanogens continue to populate and balance the narrow pH range of 6.5 to 8.0 in the environment.  If the pH level drops below 6.5, the methanogens begin to die off and the biogas produced is greatly reduced. (University of Florida, Civil Engineering, Waste Digester Design)

   Anaerobic digestion occurs through two processes: acidification and methane production. During acidification, acid-forming bacteria break down complex organic wastes into fatty acids. Proteins are broken down into amino acids, which are then broken down into fatty acids. Carbohydrates are broken down into simple sugars and then into fatty acids. Acetogenic bacteria then use the fatty acids to form acetic, propionic, and lactic acids.  During methane production, methane forming bacteria, methanogens, use the acids formed during acidification to produce methane.  In addition, other bacteria use the hydrogen and carbon dioxide produced during acidification to produce more methane.  As acid-forming bacteria introduce more acid, the methanogens continue to populate and balance the narrow pH range of 6.5 to 8.0 in the environment.  If the pH level drops below 6.5, the methanogens begin to die off and the biogas produced is greatly reduced. (University of Florida, Civil Engineering, Waste Digester Design)