Methanosarcina species with a high maximum specific growth rate (l max ) and high half-saturation coefficient (K S ) and Methanosaeta species with a low l max and low K S are the only known aceticlastic methanogens. Because of Methanosaeta's low K S , the low acetate concentrations in conventional, mesophilic anaerobic digestion yield Methanosaeta dominance. However, Methanosarcina absorbs increases in acetate more efficiently and thus promotes more stable digestion. This paper tests the hypothesis that decreasing digester feeding frequencies can increase Methanosarcina predominance. Two acetate-fed reactors were established at a 17-day solids retention time. 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. Water Environ. Res., 78, 486 (2006).

No abstract

The ADM1 was employed to assess the effect of variations in solids hydrolysis and acetoclastic methanogen process characterizations on municipal digester stability relating to excess acetate utilization capacity. First-order single- and dual-pathway hydrolysis rate functions and single and competitive acetoclastic methanogen rate functions were implemented in the ADM1. The acetate capacity number (ACN), defined as the ratio between the maximum acetate utilization rate and the average acetate production rate, was used to index digester instability. Simulations of a single CSTR at steady state indicate a similar ACN can be obtained with a 12-day SRT digester dominated by Methanosarcina sp and a 24-day SRT digester dominated by Methanosaeta sp. An increase in ACN with a decrease in SRT representing Methanosarcina sp. selection was observed for particulate feed loadings from 40 g COD/L to 90 g COD/L. Feeding frequency and dual-pathway hydrolysis were found to have less effect on the ACN than the competitive acetoclastic model structure.

No abstract

Recuperative thickening of anaerobic digester sludge (thickening with solids return) can increase digester capacity, but common methods used to thicken digester sludge involve exposure to oxygen. This study monitored the effects of oxygen exposure on acetoclastic methanogens and found no significant difference between the acetate utilization rate for the digesting sludge samples with and without oxygen exposure on the gravity belt. However, continuous oxygen exposure over the course of seven days decreased the acetoclastic methanogen biomass in digester sludge. Finally, the long-term effect of slight oxygen exposure on lab-scale digesters did not harm digester volatile solids destruction but did decrease the acetate utilization rate, suggesting that the oxygen exposure did affect the health of the methanogens. This research suggests that slight oxygen exposure by recuperative thickening may not affect digester performance but may change the acetoclastic methanogen population structure and the health of the anaerobic community.