The improved sludge compressibility and settleability of the A2O-B process compared to the A2O-C process were probably due to greater microbial richness and abundance (i.e., Bacteroidetes) (see Section 3.5). Li et al.(2013) previously reported that the members of Bacteroidetes can effectively degrade complex organic materials and hydrolyze sludge flocs that are closely associated with the reduction of the excess sludge and improvement of sludge compressibility and settleability.