The deterioration in Experiment 1 may be partly related to the
higher suspended solids (SS) content of the CSTR effluent. Prior to
utilisation by methanogen, particulate organic matter must be
decomposed by extracellular enzymes and then be further hydrolysed and acidified. However, for high-rate anaerobic reactors
(such as an IC reactor) that operate with a higher organic load
and a lower HRT, the particulate organic matter may be difficult
to fully decompose. The influent SCOD/TCOD ratio was employed
to reflect the impact of the SS in this study. However, the SCOD/
TCOD ratio of the influent was relatively stable at 0.50–0.60 in
stage III of Experiment 1, while the SCOD/TCOD ratio of the effluent
continuously rose after 90 d (Fig. 4a). Moreover, the initial TCOD
removal efficiency in Experiment 2 was only 20–30% even though
the influent SCOD/TCOD ratio was approximately 0.60–0.75
throughout this experiment (Fig. 4b). Therefore, the continual increase in the TAN content, which had been above 1000 mg/L in
the corresponding period of Experiment 1 (Fig. 2e), probably led
to ammonia inhibition in the methanogenesis system.