Transport mechanisms behind enhanced solute rejection in forward osmosis compared to reverse osmosis mode

Forward osmosis (FO) as emerging technology for water treatment was reported to exhibit higher solute rejections compared to reverse osmosis (RO). In this work, the rejection of two uncharged organic compounds (nicotinamide, caffeine) were investigated the first time in low-pressure reverse osmosis (LPRO) mode and FO under similar process conditions utilizing the same hollow fiber forward osmosis membrane modules. FO consistently showed higher rejection than LPRO mode for both nicotinamide (FO: 94.8%, LPRO: 85.5%) and caffeine (FO: 99.9%, LPRO: 99.2%). To elucidate the influence of draw solute on the rejection, different salts (NaCl, MgCl2, MgSO4), concentrations, and draw flow rates were investigated. A rise in draw concentration increased rejection in FO for all salts (RNaCl:94.1→97.2%,RMgCl2:94.5→97.5%,RMgSO4:90.0→94.7%) indicating an enhancement of rejection due to the presence of salt on the permeate side of the membrane. Two theories are proposed: While the resistance for feed solute transport could be increased due to the counter-directed reverse solute flux and the presence of salt in the support layer, the driving force for feed solute diffusion might be lowered due the presence of an ionic compound on the permeate side of the membrane.