Membrane fouling has been the main obstacle to the wide application of membrane bioreactors (MBRs). The aim of this study was to investigate the effects of ferric chloride on retarding membrane fouling in MBRs. Changes in the modified fouling index (MFI) and the zeta potential of sludge flocs were used to determine the optimal concentration of Fe(III). The optimal Fe(III) concentration of 1.2 mM significantly improved the filterability of mixed liquor. Fe(Ill) supplied positive charges for soluble macromolecular substances and sludge flocs, and enhanced the function of charge neutralization. The effects of the optimal concentration of Fe(III) on the molecular weight (MW) distributions of soluble microbial products (SMP) and the particle size distributions of the flocs in the hybrid MBR (HMBR) were also tested. The addition of Fe(III) at the optimal concentration reduced both SMP with MW> 10kDa in the supernatant and the fraction of small particles in the range 1-10 mu m in the flocs. Elemental analysis of the extracellular polymeric substances (EPS) in the activated sludge indicated that Fe(Ill) added to the HMBR system interacted with the negatively charged groups on the EPS and enhanced the bioflocculation of small particles in the activated sludge. The cleaning efficiency achieved by physically and chemically washing the membrane in the HMBR was compared with that in the conventional MBR. Cleaning with deionized water followed by a metal-chelating agent (ethylenediaminetetraacetic acid) was highly effective for the fouled membrane of the HMBR. (C) 2008 Elsevier B.V. All rights reserved.