A novel approach based on distinction of actual and pseudo resistances in membrane fouling: “Pseudo resistance” concept and its implementation in nanofiltration of single solutions

This study proposes an integrated modeling approach based on distinction of actual and pseudo resistances to elucidate the relationships between flux decline and actual fouling in nanoscale membrane processes. The approach was based mainly upon joint solution of gel-modified concentration polarization and gel-concentration polarization-modified osmotic pressure models proposed herein. Both models were developed by a novel mass transport model in accordance with simultaneous transport of mass along sequential layers comprising concentration polarization, gel and membrane. Actual resistances of the fouling in each layer and the membrane itself could be estimated by taking into account viscosity differences of bulk solution and pure water in the pores compared to the feed solution. The implementation of the approach was carried out for nanofiltration of single solutions containing anionic dye (tartrazine), monovalent salt (NaCl) and anionic surfactant (linear alkyl benzene sulfonate, LABS). The surface properties and layer thicknesses of the membranes studied were investigated by scanning electron microscope, atomic force microscope and contact angle measurements. It was determined that flux decline increased with increasing of gel thickness regardless of solution type as directly dependent on the decrease in solvent amount penetrating from membrane–gel interface toward the membrane. External fouling accounted for the most of all actual membrane fouling with a ratio between 90.0 and 97.7% in which the gel resistance performed dominant effect with a ratio between 81.0 and 92.5%. As flux decline increased in an order of dye, NaCl and LABS, portion of actual fouling in all fouling increased by a decrease in proportional total pseudo resistance as a function of decreasing of proportional pseudo intrinsic membrane resistance. Proportional total pseudo resistances were calculated as much as 25.6, 37.3 and 60.9% for LABS, NaCl and dye solutions, respectively. These levels of total pseudo resistances put forward that the pseudo resistance approach needs to be included in mass transport domains through nanoporous membrane structures in order to eliminate both erroneous predictions of resistances in series and false interpretations of process dynamics on membrane fouling and flux decline.