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The dominant acetate degradation pathway/methanogenic composition in full-scale anaerobic digesters operating under different ammonia levels Fotidis, I.A.; Karakashev, D. & Angelidaki, I. Abstract Ammonia is a major environmental factor influencing biomethanation in full-scale anaerobic digesters. In this study, the effect of different ammonia levels on methanogenic pathways and methanogenic community composition of full-scale biogas plants was investigated. Eight full-scale digesters operating under different ammonia levels were sampled, and the residual biogas production was followed in fed-batch reactors. Acetate, labelled in the methyl group, was used to determine the methanogenic pathway by following the 14CH4 and 14CO2 production. Fluorescence in situ hybridisation was used to determine the methanogenic communities’ composition. Results obtained clearly demonstrated that syntrophic acetate oxidation coupled with hydrogenotrophic methanogenesis was the dominant pathway in all digesters with high ammonia levels (2.8–4.57 g NH4+-N L-1), while acetoclastic methanogenic pathway dominated at low ammonia (<1.21 g NH4+-N L-1). Thermophilic Methanomicrobiales spp. and mesophilic Methanobacteriales spp. were the most abundant methanogens at free ammonia concentrations above 0.44 g NH3-N L-1 and total ammonia concentrations above 2.8 g NH4+-N L-1, respectively. Meanwhile, in anaerobic digesters with low ammonia (<1.21 g NH4+-N L-1) and free ammonia (<0.07 g NH3- N L-1) levels, mesophilic and thermophilic Methanosaetaceae spp. were the most abundant methanogens. Keywords Acetoclastic pathway; Ammonia toxicity; Anaerobic digestion; Hydrogenotrophic pathway

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