SignificanceThe strategies for reducing SMP-caused membrane fouling are essential for MBR sustainable operation. Comparing the bioprocesses with and without denitrification, we found that the specific yield coefficients of polysaccharides (PS) and proteins (PN) in SMP existing denitrification were significantly lower than those without denitrification. In addition, the freshly produced PS and PN from extracelluar polymeric substances (EPS) could be biodegraded and utilized by denitrifers. These findings indicated that biological denitrification was an important mechanism to reduce SMP and MBR fouling. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 3569-3573, 2013