Optimizing Water Treatment: Reverse Osmosis and Nanofiltration Membrane Performance with a Focus on the Influence of Active Layer Thickness and Porosity

Reverse osmosis (RO) and nanofiltration (NF) membranes are widely used to remove ions from water, but the interdependencies among active layer pore size, thickness, surface charge, and membrane porosity and their influence on performance are not clear. In this study, we evaluated these parameters for two commercial RO and eight commercial NF membranes and demonstrated that membrane porosity can serve as a critical linkage between these membrane characteristics and membrane performance. Porosity was mainly determined by pore size (as opposed to permeability or thickness), and thicker, more porous membranes with smaller pores (including RO) had higher sodium chloride removal and less permeability. Membrane volume charge density increased with increasing porosity. Higher porosity membranes have greater inner pore surface area available to support a greater number of functional groups, which increases counterions and reduces co-ions in the membrane, thereby reducing ion flux and increasing ion removal. The Donnan Steric Pore Model was used to determine that the contribution of diffusion and electromigration flux to the total counterion flux declined with increasing porosity, whereas the contribution of convection flux increased. Co-ion flux was dominated by diffusion flux, with electromigration and convection flux contributing about 3% and 2% of the total co-ion flux, respectively. Accordingly, calculation of just the diffusion flux for the co-ion can yield a good rough estimate of ion flux using high-porosity (small pore) membranes. Controlling membrane porosity through pore size and thickness can help to tune permeate water flux and ion removal to targeted water treatment goals using the trends reported in this study.