Filtration performances and fouling behaviours of polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF) membranes were studied in a pilot-scale membrane bioreactor. Results showed that removable fouling was dominant for both membranes while irremovable fouling of the PVDF membrane was severer than that of the PAN membrane. The PAN membrane could reject more soluble microbial products (SMP) than the PVDF membrane due to different pore sizes. The PVDF membrane rejected more carbohydrates and fewer proteins than the PAN membrane. The mean particle size of surface foulants on the PAN membrane was smaller than that on the PVDF membrane. Gel filtration chromatography analysis indicated that the surface foulants of the PVDF membrane were comprised of larger molecular weight molecules than those of the PAN membrane. Three-dimensional excitation-emission matrix fluorescence spectroscopy analysis showed that soluble microbial by-product-like substances were the key factor causing membrane fouling of the two membranes. Hydrophobic humic-like substances played a more important role in forming irremovable fouling of the PAN membrane whereas protein-like substances were the main contributor to the irremovable fouling of the PVDF membrane. Hydrophobic fraction had the largest fouling propensity for the PAN membrane, while neutral hydrophilic fraction demonstrated the strongest fouling potential for the PVDF membrane. (C) 2011 Elsevier B.V. All rights reserved.