Role of imperfections in transfer of organic micro-pollutants through RO/NF membranes: Numerical study

Abstract

Due to relatively low diffusivities, the rejection of organic micro-pollutants in pressure-driven membrane processes is more affected by Concentration Polarization (CP) than that of typical inorganic ions. On the other hand, it can be more sensitive to various membrane imperfections because the rejection of these solutes by intact membranes can be expected to be quite high. This makes necessary simultaneous accounting for membrane imperfections and Concentration Polarization in Reverse Osmosis and/or Nanofiltration of organic micro-pollutants. This study, for the first time, explores this problem numerically for the model of a single circular opening in a selective layer (the porous support is assumed to remain undamaged). The solute accumulated due to the CP close to the surrounding intact active layer “spills over” to the imperfection, which leads to an enhanced transmembrane solute passage as compared to the classical solution-diffusion-imperfection model assuming this passage to be controlled by the feed concentration. Our analysis reveals that the extent of this “spill-over” depends primarily on the size of imperfection (we studied the range between 0.2 μm and 20 μm) as well as on the ratio of hydraulic resistances of selective and support layers. Smaller imperfections in active layers of membranes with less permeable porous supports have larger impact. The extent of CP away from the imperfection is also important. Contrary to the trend expected for imperfection-free membranes (a better rejection of larger solutes due to a stronger steric exclusion and hindrance), the solute passage through imperfections is demonstrated to be larger for solutes with smaller diffusion coefficients, so in future experiments this can be considered a signature feature of a noticeable impact of imperfections. Quantifying their impact is important for understanding membrane aging, rational selection of membrane cleaning procedures as well as development of better membranes.