Relationship between microbial community, operational factors and ammonia inhibition resilience in anaerobic digesters at low and moderate ammonia background concentrations

•Ammonia inhibition resilience was studied for 13 inocula from distinct digesters.•Digesters substrate and temperature influenced microbial community composition.•Ammonia inhibition KI50 varied moderately among the inocula 32–175 mgNH3-N·L−1.•No microbial or operational factors correlated with ammonia inhibition resilience.•Methanogenic activity was significantly correlated with archaeal relative abundance. The relationship between anaerobic digestion operational conditions and (i) microbial community, (ii) acetoclastic methanogenic activity and (iii) free ammonia (NH3) inhibition resilience was investigated. Thirteen inocula were obtained from full and pilot scale digesters fed with different substrates, digester configurations, operating temperatures and NH3 concentrations (0.1–241 mgN·L−1). Substrate type and temperature were the primary factors influencing microbial community composition. Methanogenic activity ranged from 0.04 to 0.14 gCOD-CH4·g−1VS·day−1, and was significantly correlated with archaeal relative abundance and archaeal community PC2. The variability of NH3 resilience among inocula was moderate, with inhibition threshold values (KI50) ranging between 32 and 175 mgNH3-N·L−1. No microbial or operational factors correlated with NH3 resilience. However, the slopes of inhibition threshold curves were influenced by some environmental factors, namely substrate type, digester temperature and NH3 concentration. Overall, these results indicate that low and moderate background NH3 concentrations is not a key determinant of microbial community nor NH3 resilience.