The Growth Kinetics and Competition Between Methanosarcina and Methanosaeta in Mesophilic Anaerobic Digestion

The high µmax (maximum specific growth rate), high KS (half saturation coefficient) Methanosarcina and the low µmax low KS, Methanosaeta are the only known acetoclastic methanogens. Due to Methanosaeta's low KS, the low acetate concentrations in conventional, mesophilic anaerobic digestion yield Methanosaeta dominance. However, Methanosarcina absorbs acetate increases more efficiently and thus promotes more stable digestion. This paper develops the hypothesis that decreasing digester feeding frequencies can increase Methanosarcina predominance. Two acetate-fed reactors were established at a 17 day SRT. One reactor was fed hourly, and one was fed once-daily. Microscopic and molecular methods were used to verify that the hourly fed reactor enriched for Methanosaeta, while the daily fed reactor enriched for Methanosarcina. Growth and substrate use kinetics were measured for each reactor. A digester overload condition was simulated, and the Methanosarcina enriched reactor was found to perform better than the Methanosaeta enriched reactor. These findings indicate that Methanosarcina dominance can be achieved with infrequent feedings, leading to more stable digestion.