This study compares fouling propensities of reverse osmosis (RO) membranes in two parallel MBR-RO systems. Two MBRs were operated at different food to microorganism (F/M) ratios and the permeate was fed to the respective RO membrane. The results show that greater amounts of organic substances in the high F/M (0.50 g/g day(-1))-MBR permeate led to higher RO fouling rates (>4.5-fold) compared to the low F/M ratio (0.17 g/g day(-1))-MBR permeate. The presence of filters (similar to 5 mu m) in the feed line and recycled RO concentrate line significantly alleviated RO fouling. Chemical analysis of RO foulants indicated that the soluble polysaccharides and transparent exopolymer particles (TEP) accumulated on the RO membranes were strongly associated with RO fouling. However, propagation of bacterial cells on the membranes did not determine RO fouling development. This finding was further confirmed by confocal laser scanning microscopy images. Furthermore, excitation-emission matrix (EEM) fluorescence spectroscopy was used to trace the fate and transport of the potential soluble foulants in the MBR-RO system. (C) 2012 Elsevier B.V. All rights reserved.