The impact of denitrification on membrane fouling propensity was investigated using three different sludges, i.e. conventional activated sludge (CAS), ordinary heterotrophic organisms (OHO) and denitrifying phosphorous accumulating organisms (DPAO) taken from a lab-scale biological nutrient removal (BNR) system. Fouling propensity was evaluated with the modified filtration index (MFI), soluble microbial products (SMP), bound extracellular polymeric substances (bEPS), and relative hydrophobicity (RH). The tests indicated that DPAO denitrification decreased cake layer resistance, as indicated by 45% reduction in MFI as compared to an increase of 123 and 52% for CAS and OHO denitrification. Pore blocking fouling was greater in CAS and OHO than DPAO. However, SMP increased after CAS and OHO denitrification but decreased in DPAO while bEPS increased in all cases. DPAO denitrification also increased RH of bEPS but decreased carbohydrate/protein ratio, possibly due to carbohydrate utilization in EPS by OHO under substrate limiting conditions for denitrification, substantiating the reduction in cake layer resistance in DPAO denitrification. CAS fouling during denitrification was More severe than BNR sludges as reflected by higher MFI, SMP and bEPS production. (C) 2009 Elsevier B.V. All rights reserved.