The rejection of trace organic contaminants (TrOCs) by an osmotically driven membrane filtration process was investigated. A set of 40 compounds representing major groups of TrOCs of concern was selected for this study. The rejection of the TrOCs by a commercial cellulose acetate asymmetric forward osmosis membrane, as well as a ‘‘tight’’ commercial thin-film composite nanofiltration (NF) membrane, was systematically investigated and compared under three different operating modes: forward osmosis (FO), pressure retarded osmosis (PRO) and reverse osmosis (RO). Results revealed that the cellulose acetate membrane had considerably smaller water and salt permeabilities as well as less negative surface charge compared to the NF membrane. However, the cellulose acetate membrane resulted in considerably higher water flux than the NF when operated in FO and PRO modes. Nevertheless, the NF membrane displayed consistently better TrOC rejection than the HTI membrane. In RO mode, electrostatic interactions played a dominant role in governing the rejection of charged TrOCs. In FO and PRO modes, the rejection of
charged TrOCs was governed by both electrostatic interaction and size exclusion, while rejection of neutral compounds was dominated by size exclusion, with rejection increasing with TrOC molecular weight. Operating in PRO mode resulted in a higher water flux but a notably lower TrOC rejection as compared with FO mode, because of more severe internal concentration polarization (ICP) phenomenon. Another important observation from this study is that rejection of neutral TrOCs in FO mode was higher than that in RO mode. This could be attributed to the retarded forward diffusion of TrOCs resulting from reverse salt flux of the NaCl draw solution, a phenomenon that takes place in FO mode but is not possible in RO mode.