Three dimensionless parameters influencing the optimal membrane orientation for forward osmosis

In many forward osmosis applications, flux is maximised (and capital costs minimised) when the membrane is oriented such that the feed solution faces the support layer (PRO mode). Here, a framework is developed to understand the factors that determine the membrane orientation that maximises flux. In the absence of fouling, a dimensionless form of the water transport equations reveals the importance of three dimensionless groups: the ratio of draw to feed osmotic pressure, the ratio of draw to feed solute diffusivity, and the resistance to water transport of the support layer relative to the active layer. A parametric study of these parameters and an application of the dimensionless equations to three important FO processes reveal that having the draw solution face the support layer (FO mode) can maximise flux in specific instances. Interestingly, this implies that operation in FO mode can both maximise flux and minimise fouling for fertigation applications and the concentration of flowback waters from hydraulic fracturing.