Characterization of nanofiltration and reverse osmosis membrane performance for aqueous salt solutions using irreversible thermodynamics

A novel protocol is developed to characterize nanofiltration (NF) and reverse osmosis (RO) membranes that employ the Spiegler-Kedem membrane-transport model based on irreversible thermodynamics coupled with a Film Theory description of the concentration polarization. The novel aspect of this characterization protocol is extracting the concentration polarization boundary-layer thickness as well the three membrane transport parameters from permeation data. This methodology is applied to four dilute aqueous salt systems for a Dow Filmtech NF-255 membrane and one dilute aqueous salt system for a Dow Filmtech BS-30 membrane that were studied using a well-mixed flat sheet membrane permeation cell. Interestingly, the total volumetric flux and intrinsic rejection can be described quite well over the entire range of studied feed concentration, flow rates, and transmembrane pressures using constant membrane transport coefficients in the Spiegler-Kedem model. However, it should be emphasized that the conclusions drawn from this study are restricted to the dilute aqueous salt solutions that were employed. Some prior studies of aqueous salt systems have found it necessary to employ concentration-dependent transport coefficients to describe the permeation behavior. It is possible that the more exacting method for determining the thickness of the concentration boundary-layer thickness that is used in this study properly accounts for volumetric flow rate effects that have been attributed to nonconstant membrane transport coefficients. A design correlation for the intrinsic rejection of an NF or RO membrane as a function of the total volumetric flux is developed that encompasses the full range of concentrations used in the experiments, flow rates, and transmembrane pressures for a specified binary aqueous salt system. This design correlation formalism in principle can be used to characterize any NF or RO membrane irrespective of the type of membrane contactor employed.