Issues of fouling and effective cleaning limit the adoption of UF in pulp and paper mill applications The choice of an optimal membrane for a specific filtration application is a challenging task due to the fact that fouling is a complex phenomenon This study compares regenerated cellulose (RC) and polyethersulphone (PES) membranes in UF of two chemithermomechanical pulp mill process waters The process waters originated from hardwood and softwood pulping Based on their flux recovery PES membranes sustained greater fouling than RC membranes even though the hydrophobicity of RC membranes was increased remarkably by adsorptive fouling The process water affected the performance of the membranes The membrane characteristics were more Important in determining fouling when softwood process water was used than when hardwood process water was used Hydrophilicity and morphology of the membranes were seen to have a clear influence on fouling (C) 2010 Elsevier BV All rights reserved