Water recovery and solute rejection in forward osmosis modules: Modeling and bench-scale experiments

• Developed model for water recovery and solute leakage in forward osmosis modules.
• Analytical model compares well with numerical solutions and bench-scale experiments.
• Current membranes achieve >99.3% rejection of feed solute at high draw concentrations.
• More selective membranes are needed for 99.3% rejection at low draw concentrations.
• Identified a simple heuristic for designing the flow ratio in counter-current modules.

Forward osmosis (FO) processes have potential applications in the treatment of complex and highly impaired sources of water. In this study, the performance of a FO module is evaluated in terms of its ability to recover water from a feed solution and its ability to minimize solute leakage. These performance metrics are expressed in terms of the recovery rate and separation factors, respectively. An analytical model that quantifies the effects of membrane properties (e.g., selectivity) and operating conditions (e.g., the ratio of the draw solution flow rate to the feed solution flow rate) on the recovery rate and separation factors is developed. Numerical solutions of the non-linear governing equations and bench-scale experiments with ionic and non-ionic solutes (i.e., NaCl, KCl, MgCl2, CaCl2, and urea) were used to test the validity of the derived model. Both the numerical solutions and experimental results corroborated the ability of the derived model to predict the performance of a FO module. Electrostatic and other multicomponent interactions, which were not included in the model, were observed in some experiments resulting in poor agreement with the predicted performance. Analysis of the model establishes membrane selectivity and draw solute concentration as the dominant factors that determine the extent of water recovery and solute rejection in a FO module. Further analysis of the model suggests that current membranes could achieve acceptable levels of solute rejection for desalination applications (> 99.3%) at relatively high draw solute concentrations (2.5 M draw solution for a 0.6 M feed solution at 50% recovery), but more selective membranes are necessary to reduce the draw solute concentration closer to the thermodynamic limit established by osmotic equilibrium. Finally, a simple heuristic for selecting the flow ratio based on the entering concentrations of the feed solution and draw solution and the desired recovery rate is identified.

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