The effect of ozonation followed by biological activated carbon (BAC) filtration on the characteristics and ultrafiltration (UF) performance of a highly coloured activated sludge (AS) effluent was investigated. The major dissolved organic membrane foulants were identified as proteins, polysaccharides, protein-like and polysaccharide-like materials, and humic substances (HS). Ozonation at a dose of 10 mg L(-1) reduced the true colour of the raw AS effluent by 74%, thereby greatly improving its aesthetic appearance. Fouling of the UF membrane by the AS effluent was reduced after ozonation due to partial oxidation of the membrane foulants to give building block molecules, low molecular weight acids, and low molecular weight neutrals, some of which were then utilised by the micro-organisms in the BAC filtration process. Further improvement in UF flux performance after BAC filtration was mainly due to the reduction in the suspended solids level in the ozonated effluent. The unified membrane fouling index was used to evaluate the relative effectiveness of the two pre-treatment methods on fouling reduction. (C) 2010 Elsevier B.V. All rights reserved.