Two lab scale membrane-based biological nutrient removal systems were operated using municipal wastewater at a hydraulic retention time of 6 h and a solids retention time of 10 days. Membrane fouling parameters i.e. fouling rate, extracellular polymeric substance (EPS), soluble microbial product (SMP), particle size and sludge volume index (SVI) were monitored. It appeared that both systems showed similar membrane fouling trends with close fouling rate of 4.4 x 10(-2) LMH/kPa h. Fouling rate was more influenced by SMP than bound FPS. The higher protein/carbohydrate ratio in SMP was related with higher fouling rate. SVI and floc size were closely linked to each other and SVI was affected More by the bound protein/total protein ratio than the amount of bound protein. A statistical analysis confirmed that SMP impacts membrane fouling more significantly than floc size, the bound protein/total protein ratio and bound EPS. Biofilm layer deposited on the membrane played a beneficial role in denitrification, removing as much as 1.5 mg N/L, and was primarily impacted by dissolved oxygen (DO) and transmembrane pressure (TMP) rather than chemical oxygen demand rejection and attached biomass. The amount of nitrate reduction increased at lower DO condition (less than 2 mg/L) and the denitrification increased membrane fouling. (C) 2009 Elsevier B.V. All rights reserved.