Caffeine removal and mass transfer in a nanofiltration membrane process

The effectiveness of nanofiltration (NF) to remove a wide range of spiked caffeine (0.052–4,500 μg/L) from groundwater at the pilot-scale (60,636 L/h) has been demonstrated. Experiments were conducted using a pilot-scale unit, operating as a two-stage, split-feed, center-exit system that relied on a well supply withdrawn from an average depth of 45 m that contained an average of 11 mg/L of dissolved organic carbon. The average caffeine removal efficiency across the pilot system was 68%, and removal did not vary by solute concentration for constant flux (25.6 L m –2 h –1 ) and temperature (25°C) operating conditions. Mass transfer models evaluated in this work include the homogeneous solution diffusion model (HSDM) with and without film theory (FT), in addition to dimensional analysis, using the Sherwood number, and were shown to predict NF solute mass transfer. Predicted vs. actual caffeine content was linearly compared, revealing correlation coefficients on the order of 0.99, 0.96, and 0.99 for the HSDM without FT, HSDM-FT, and the Sherwood number, respectively. However, the use of the HSDM-FT and the Sherwood number resulted in the over-prediction of caffeine concentrations in permeate streams by 27% and 104%, respectively.