As HRT was reduced from 12 to 8 h (resulting in OLR
increase from 1.10 to 1.65 kg COD m3 d1), average MLVSS
concentration increased and eventually stabilized at 7074,
8698 and 10,452 mg L1 for R30, R60 and RN, respectively.
Statistical tests showed that at HRTs of 10 and 8 h, at 5%
significance level, R30 had the highest Rs, the lowest MLVSS
concentration and the lowest methane yield rate. This
phenomenon was similar at HRT of 12 h due to the same
reasons mentioned earlier.