Elucidating the kinetics of ammonia inhibition to anaerobic digestion through extended batch experiments and stimulation-inhibition modeling.

Ammonia can be accumulated to a level inhibitory to methanogenesis. There are large discrepancies in the reported inhibition thresholds. Through extended batch digestion experiments (up to 110 d) at 6 ammonia concentrations (0.70-13 g N/L), this study discovered sequential occurrence of adaptable and unadaptable inhibition that reveals the discrepancies. Lag phase length representing adaptable inhibition increased exponentially with ammonia concentration. The kinetics of specific biogas yield that reveals unadaptable inhibition was best simulated with the Han & Levenspiel model. The 50% unadaptable inhibition thresholds were 10.7 g N/L with active inoculum and 6.8 g/L with stressed inoculum. The digesters with stressed inoculum had faster adaptation to adaptable inhibition though less resistance to unadaptable inhibition. The inhibition sequence was evidenced by microbial population shifts and confirmed by earlier studies employing short (2-65 d) and long (80-198 d) batch experiments. Distinguishing adaptable from unadaptable inhibition provides precise guidance for mitigating ammonia inhibition.