Membrane fouling was compared in two submerged membrane bioreactors (MBRs): conventional and simultaneous nitrification and denitrification (SND) MBRs operated at a membrane flux of 14 L/m(2) h and an aeration rate of 4 L/min. The mixing rate was adjusted to maintain a DO level of 1-1.2 mg/L to achieve SND. Steady state membrane permeability and fouling rates as well as the mixed liquor filterability and characteristics such as protein, carbohydrate, and humic acids in extracellular polymeric substances (EPS) and soluble microbial products (SMP) were determined. Higher membrane fouling rates were observed in the SND MBR despite the larger floc size. The increased concentrations of SMP and EPS for the SND reactor resulted in higher fouling rate as well as higher modified fouling index (MFI) both for suspended solids and soluble components. Higher EPS concentration and relative hydrophobicity in the SND MBR increased the attachment on the membrane surface. Also, higher rejection of carbohydrate SMP and the increased concentration of this fraction retained in the membrane pores indicated that carbohydrate SMP was the major foulant indicator. (C) 2009 Elsevier B.V. All rights reserved.