Characterization of Internal and External Concentration Polarizations During Membrane Filtration Processes

Concentration polarization (CP) in both osmotically driven and pressure driven membrane processes can significantly increase energy consumption and deteriorate product water quality.  It is therefore of great importance to develop viable methods to predict and to characterize CP.  In this study, three methods were established using reverse osmosis (RO) tests to characterize the effect of CP directly, relying on the measurements of RO water flux, solute (i.e., sodium chloride) rejection and trace contaminant (i.e., boron) rejection, respectively.  The effects of membrane structures and operating conditions on CP were systematically investigated.  Experimental results agreed well with theoretical models developed for each method.  The proposed methods are not only restricted to pressure driven membrane processes, but also can be applied to study other types of filtration processes, e.g., forward osmosis (FO), for which the effect of internal CP (ICP)  is considered critical.  The effective boundary layer thickness contributed by ICP estimated by our methods showed a reasonable and good fit of experimental data.  In addition, current study can be potentially adopted as a feasible approach to characterize the effect of the cake enhanced concentration polarization (CECP) during RO/FO membrane fouling which is an important fouling mechanism for dense membranes.