Effect of ammonia on methane production pathways and reaction rates in acetate-fed biogas processes

In order to understand the correlation between ammonia and methanogenesis metabolism, methane production pathways and their specific rates were studied at total ammonium nitrogen (TAN) concentrations of 0.14-9 g/L in three methanogenic sludges fed with acetate, at both mesophilic and thermophilic conditions. Results showed that high levels of TAN had significant inhibition on methanogenesis; this could, however, be recovered via syntrophic acetate oxidation (SAO) coupled with Hydrogenotrophic Methanogenesis (HM) performed by acetate oxidizing syntrophs or through Acetoclastic Methanogenesis (AM) catalyzed by Methanosarcinaceae, after a long lag phase >50 d. Free ammonia (NH₃) was the active component for this inhibition, of which 200 mg/L is suggested as the threshold for the pathway shift from AM to SAO-HM. Methane production rate via SAO-HM at TAN of 7-9 g/L was about 5-9-fold lower than that of AM at TAN of 0.14 g/L, which was also lower than the rate of AM pathway recovered at TAN of 7 g/L in the incubations with a French mesophilic inoculum. Thermophilic condition favored the establishment of the SAO-catalyzing microbial community, as indicated by the higher reaction rate and shorter lag phase. The operational strategy is thus suggested to be adjusted when NH₃ exceeds 200 mg/L.