The effect of ammonia inhibition was evaluated during the enhanced anaerobic treatment of digested
effluent from a 700 m3 chicken-manure continuous stirred tank reactor (CSTR). A 12.3 L internal circulation (IC) reactor inoculated with an anaerobic granular sludge and operated at 35 ± 1 C was employed for
the investigation. With a corresponding organic loading rate of 1.5–3.5 kg-COD/m3 d over a hydraulic
retention time of 1.5 d, a maximum volumetric biogas production rate of 1.2 m3/m3 d and TCOD (total
COD) removal efficiency ranging from 70% to 80% was achieved. However, the continual increase in
the influent TAN content led to ammonia inhibition in the methanogenesis system. The SCOD/TAN (soluble COD/total ammonia nitrogen) ratio was presented to be the key controlling factor for the anaerobic
treatment of semi-digested chicken manure, and further validation through shock loading and ammonia
inhibition experiments was conducted. The threshold value of the SCOD/TAN ratio was determined to be
2.4 (corresponding to a TAN of 1250 mg/L) at an influent pH of 8.5–9.