Polymeric ferric sulfate (PFS) has been proved effective for improving membrane filterability of the mixed liquor in the membrane bioreactor (MBR) by previous investigations. In order to confirm the effect and clarify the behavior of PFS in a long-term running MBR, two MBRs with PFS and without PFS addition were operated for 60 d in parallel. Gel chromatography, field emission scanning electronic Microscope and energy dispersive spectrometer were applied for characterization of foulants. The mixed liquor properties were compared in terms of mixed liquor suspended solid (MLSS) concentration, average floc size, distribution of supernatant organic molecular weight and so on. Additionally, removal of pollutants according to influent and effluent quality was monitored. It was found that membrane fouling was effectively retarded by PFS addition in the long-term running MBR system. The suitable dosage and dosing interval were 1.0 mM Fe and 15-30 d for the MBR with MLSS of 7-10 g/L, respectively. The MBR-PFS featured slow formation of gel layer on membrane surface, which was resulted from the effect of PFS of removing organics with high molecular weight from supernatant by coagulation. PFS also enlarged suspended flocs, because PFS could supply positive charges for organic particles and enhance charge neutralization. After PFS addition, exotic Fe did not deposit on the membrane surface and most of the PFS transferred into suspended solids but did not lead to a large amount of the residual sludge as supposed. No severe inorganic fouling occurred on the membrane. pH decreased after PFS addition which slightly influenced microbial activity but did not affect the effluent quality of MBR. The feasibility of PFS application to mitigate membrane fouling in a long-term running MBR was proved by this study. (C) 2008 Elsevier B.V. All rights reserved.