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 K_w, K_s, and K_c, 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 K_w, K_s, and K_c 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.