An experimental and computational fluid dynamic (CFD) study is reported for two forward osmosis (FO) module configurations, i.e. spiral wound (SW) and plate and frame (PF), featuring two types of draw channel spacers. Modules were operated both in FO and pressure assisted osmosis (PAO) modes. By varying the cross flow velocities (CFV), inlet feed pressures (PFeed-in) and draw solution concentrations, FO module performance, reported in terms of pressure drop and water flux, was evaluated. The experiment results showed that the increase of PFeed-in raised the draw channel pressure drop by 37% for SW module and around 4 folds for PF module. Additional feed side pressure also unexpectedly decreased the water flux by 28% for PF module with 35 g L− 1 red sea salt draw solution. Such results indicated significant changes of both feed and draw flow channels height. This phenomenon was defined as relative displacement of membrane, which was quantified by validated CFD simulation. The results suggested that PFeed-in can induce the displacement of membrane in the PAO mode. Based on CFD simulation, 100 and 200 μm membrane displacements were predicted for SW and PF modules, respectively operated at 4 bar feed pressure, resulting in significant impact in process performance.