Sustainable operation of an anaerobic sewage sludge digester requires the effective shuttling of carbon from complex organic material to methane gas. The accumulation of intermediates and metabolic products such as volatile fatty acids and hydrogen gas not only reveal inefficiency within the digestion process, but can be detrimental to reactor operation at sufficiently high levels. Eight anaerobic digesters (1 mesophilic and 7 thermophilic) were operated in order to determine the effect of steady-state digestion temperature on the operational stability and performance of the digestion process. Replicate reactors operated at 57.5 °C, the highest temperature studied, were prone to accumulation of volatile fatty acids (4052 and 3411 mg/L as acetate) and gaseous hydrogen. Reactors operated at or below 55 °C showed no such accumulation of intermediate metabolites. Overall methanogenesis was also greatly reduced at 57.5 °C (0.09 L CH4/g VS fed) versus optimal methane formation at 53 °C (0.40 L CH4/g VS fed). Microbial community assessment and free energy calculations suggest that the accumulation of fatty acids and hydrogen, and relatively poor methanogenic performance at 57.5 °C are likely due to temperature limitations of thermophilic aceticlastic methanogens.

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