A nanofiltration retention model for trace contaminants in drinking water sources

A diffusion-convection controlled mass transfer model was developed for predicting the retention behaviour of full-scale nanofiltration (NF) plants. The model is based upon a previous diffusion-only-controlled transport model and is described by an analytical equation in which the retention can be calculated as a function of the recovery. The three main parameters in the model are Kw, Ks, and Kc, respectively, the water transfer coefficient, solute transfer coefficient and the convective hindrance factor. These parameters can be determined experimentally for a given membrane and solute or can be taken from the literature for a given membrane and solute. Both laboratory-scale and bench-scale NF experiments were carried out with several trace organic contaminants using TriSep TS80 NF membranes. High retentions were found for trace organics with a molecular weight higher than the MWCO of the membrane. From these experiments Kw, Ks, and Kc were determined for MgSO4 and atrazine in order to predict the retention behaviour of a full-scale NF process. Especially at higher recoveries, e.g., 0.6-0.95, the differences in predicted retentions are large compared to models using diffusion-only-controlled transport.