The organisms responsible for methane production in anaerobic digesters

Note this abstract has arrows reversed on homoacetogenesis and SAO. Graphical abstract from https://www.sciencedirect.com/science/article/abs/pii/S0043135420313075

The methanogens found in anaerobic digesters are actually not bacteria but from the kingdom archaea. Archaea are very different from bacteria and are their own unique life forms found in a variety of environments. In anaerobic systems, it is the methanogens that produce methane and maintaining a stable population of methanogens is key to successful digester operation. But, methanogens also have multiple metabolic pathways that impact digester performance. 

Archaea Methanogenic Pathways:
Acetoclastic                   CH3COOH  CH4 + CO2
Hydrogenotrophic       CO2 + 4H2  CH4 + 2H2O
Methylotrophic             4CH3OH  3CH4 + CO2 + 2H2O
 
Bacterial Pathways in Digesters:
            Fermentation              Produces organic acids from complex biological polymers
            Acetogenesis               Continued fermentation to CH3COOH
            Homoacetogeneis       4H2 + 2CO2  CH3COOH + 2H2O
                                                   Metabolic rate at 3 to 10x faster than HM Archaea
             SAO                             CH3COOH  4H2 + 2CO2
                                                  Syntrophic with Hydrogenotrophic methanogens
 
Causes of increases CO2 (lower methane %) in produced gas

• Acetoclastic and methylotrophic methanogens both produce CO2 along with methane. With the acetoclastic making equal parts CO2 and methane.  In digesters dominated by acetoclastic organisms (especially Methanothrix from Aster Bio's data), we see lower rates of methane production.
• Overgrowth by homoacetogenic bacteria - these organisms convert H2 and CO2 back to acetate which favors acetoclastic methanogen growth. These organisms deprive desirable hydrogenotrophic methanogens of needed inputs for growth.
• Need for SAO bacterial populations that work directly with hydrogenotrophic methanogens in a syntrophic relationship where the bacteria provide the archaea with H2 and CO2 - the result is improved methane yields. 

Good articles on the SAO & Homoacetogenesis bacteria in digester performance.

• Deep insights into the network of acetate metabolism in anaerobic digestion: focusing on syntrophic acetate oxidation and homoacetogenesis​ - ​www.sciencedirect.com/science/article/abs/pii/S0043135420313075
• Syntrophy mechanism, microbial population, and process optimization for volatile fatty acids metabolism in anaerobic digestion - www.sciencedirect.com/science/article/abs/pii/S1385894722046162
• Novel syntrophic bacteria in full-scale anaerobic digesters revealed by genome-centric metatranscriptomics   www.nature.com/articles/s41396-019-0571-0/