3.2.1. Effect of HRT
Regardless of the operating SRT, some common phenomena
were observed: (1) Very high total COD removal efficiencies
(>97%) were achieved under all investigated HRTs with
insignificant differences among the three SRTs. This was
attributed to the MF membrane that can completely retain
biomass present in the mixed liquor to produce a high quality
effluent with non-detectable solids; (2) Rt was higher than Rs.
This observation indicated that although the membrane pore
size was 0.45 mm, the MF membrane could further remove
soluble COD present after biological treatment. This additional
removal was attributed to internal and external
membrane adhesion, interception, as well as possible organic
degradation by the biofilm which was developed on the
membrane surface; (3) Higher MLSS or MLVSS concentrations
and more methane production were observed when HRT was
successively reduced from 12 to 8 h. Higher organic loading
rate (OLR) in shorter HRT induced more biomass multiplication
and more carbon conversion from organic compounds to
methane gas.