Reverse osmosis (RO) is commonly used to concentrate solutions in food preparation and environmental applications among others. Limitations on the applied pressure, which must be higher than the osmotic pressure of the feed solution, determine the concentration that can be achieved by RO. Recently, an osmotically assisted reverse osmosis (OARO) system that can be operated even with low applied pressure has been suggested as a new concentration process. In this study, we performed concentration tests on a multistage OARO system that used two types of hollow fiber (HF) membrane modules made from cellulose triacetate with different numbers of HFs. Water permeated through the membrane and the concentration increased as it progressed through the modules, even when using concentrated NaCl solutions (1.0 and 2.0 M) with low applied pressures (10 and 15 bar). The concentration increased in each of ten stages in a multistage concentration test. Water flux and concentration ratio were strongly influenced by the internal concentration polarization in the modules. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.