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Effect of spacer configuration on the characteristics of FO membranes: Alteration of permeation characteristics by membrane deformation and concentration polarization.

Membrane deformation is a significant problem in osmotically driven membrane processes, as it restricts practical operating conditions and reduces overall process performance due to unfavorable alteration of membrane permeation characteristics. In this respect, a spacer plays a crucial role, as it dictates the form and extent of membrane deformation in association with concentration polarization (CP), which is also influenced by spacer-induced hydrodynamic behavior near the membrane surface. These two roles of spacers on membrane permeation characteristics are inherently inseparable with the coexistence of hydraulic and osmotic pressures. Here, we suggest a novel analytical method to differentially quantify the proportions of effective osmotic pressure drop caused by membrane deformation and CP. Furthermore, we tested two different FO membranes with three different spacer configurations to define and discuss different forms of membrane deformation and their effects on membrane permeation characteristics. The differential analysis revealed the effect of spacer configuration on effective osmotic pressure drop in membrane deformation (up to ~ 201% of variation) is much greater than that in CP (up to ~20.1% of variation). In addition, a combined configuration of a feed spacer and tricot spacer demonstrated its ability of mitigating membrane deformation with lower selectivity loss and channel pressure drop under pressurization.