To assess the relationship between membrane hydrophobicity and fouling properties, polysaccharide and protein rejection characteristics in a membrane bioreactor (MBR) system, two ultrafiltration membranes with different hydrophilic/hydrophobic properties were tested using activated sludge and soluble microbial products (SMPs). The initial and pseudo steady-state fouling rates for sludge filtration by both membranes were higher than those of SMP filtration suggesting that the suspended solids exerted a negative impact on membrane fouling rate. A modification of the resistance-in-series model was applied and the results showed that cake resistance was the main fouling mechanism for both membranes. The hydrophobic membrane (polyethersulfone) showed greater polysaccharide and protein rejection and consequently lower flux than the hydrophilic membrane (regenerated cellulose). The relatively higher cake resistance of the polyethersulfone membrane rationalizes the increased solute rejection in the hydrophobic membrane which suggests that the deposited cake layer plays an important role in solute rejection. The comparable observed pseudo steady-state fouling rates and permeability with the two membranes appears to dispel any advantages of hydrophilic membranes for sludge filtration. (C) 2009 Elsevier B.V. All rights reserved.