Effect of some physical and chemical parameters on the fermentation of cellulose to methane by a coculture system

In the fermentation of cellulose to methane by the triculture of Acetivibrio cellulolyticus – Desulfovibrio sp. – Methanosarcina barkeri, methanogenesis was the rate-limiting step. The optimal temperature was 35 °C. In the presence of initially added hydrogen, initiation of cellulose hydrolysis was delayed until most of the hydrogen was metabolized, and then the fermentation which followed was comparable with the N2: CO2 control. Increased CH4 yields and rates of formation were stoichiometrically related to the utilization of the hydrogen initially present. Added acetate had no effect on cellulolysis. The increased yields of CH4 observed could be accounted for by utilization of the added acetate. When Methanobrevibacter sp. was included in the coculture (without added H2 or acetate), no hydrogen accumulated and lower rates of acetate utilization and subsequent CH4 evolution were observed. These results suggest a requirement for hydrogen by M. barkeri for efficient acetate utilization. Controlling the pH at 6.8 increased the amount of cellulose degraded, but CH4 yields were lower and no acetate was used. Increasing sulfate levels altered the ratio of fermentation products but had no effect on cellulose degradation. Lower CH4 yields were obtained at elevated sulfate concentrations.