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

The correlation between NH₃ and methanogenesis metabolism was evaluated by investigating CH₄ production pathways and their specific rates at 0.14-9 g/L total ammonium nitrogen (TAN) concentrations in three methanogenic sludges fed with acetate at both mesophilic and thermophilic conditions. Results showed that high levels of TAN had substantial inhibition on methanogenesis however this could be recovered via syntrophic acetate oxidation (SAO) combined 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 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. CH₄ production rate via SAO-HM at 7-9 g/L TAN was about 5-9-fold lower than that of AM at 0.14 g/L TAN which was also lower than the rate of AM pathway recovered at 7 g/L TAN 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. Thus the operational strategy is suggested to be adjusted when NH₃ exceeds 200 mg/L.