Osmotic equilibrium in the forward osmosis process: Modelling, experiments and implications for process performance

•Net water flux in an FO process occurs until the point osmotic equilibrium (OE).•OE in the FO process was simulated using established mass transport models.•Without external intervention, draw solution (DS) dilution beyond OE is impossible.•Counter-current crossflow has higher average water flux and final DS dilution.•The OE concept can be applied to estimate water extraction capacity of draw solutes.

Forward osmosis (FO) has gained significant research interest due to the wide range of potential applications in desalination and wastewater reuse. However, the FO process being concentration (osmosis) driven has its own intrinsic limitations. Net transfer of water across the membrane occurs until the point of osmotic equilibrium between the draw solution (DS) and the feed solution (FS). Without external intervention, it is impossible to dilute the DS beyond the point of osmotic equilibrium. In this study, the concept of osmotic equilibrium in the FO process is introduced by simulating conditions in a plate-and-frame FO membrane module using established mass transport models. The simulations evaluated the influence of various operating parameters on process performance, assessed in terms of water flux, feed recovery rate and the final concentration of the diluted DS. The counter-current crossflow mode of operation has been observed to be advantageous because it can achieve higher module average water flux, higher feed water recovery rates and higher DS final dilution. Based on the osmotic equilibrium concept and mass balance analysis, a modified equation for the water extraction capacity of a draw solute has been proposed. This study underscores the need for process optimisation for large-scale FO operations.

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