The HRT for the UASB and I-BAFs is gradually reduced from 40
(start-up), 20 (127–160 d), and 15 (161–183 d) to 12 h (184–252 d)
during the entire operation period. However, the COD removal is
observed to be stable in the proposed system in spite of shorting
HRT, indicating that the HRT reduction has no significant effect on
COD removal efficiency in this study. This observation is in agreement
with other reports. The HRT (only 12 h) required in our
study is shorter than the 32 h used in hydrolysis acidification/biocontact
oxidation (HA/BCO) system and the 60 h required in
anaerobic baffled reactor (ABR) system.