On the rejection and reversibility of fouling in ultrafiltration as assessed by hydraulic impedance spectroscopy

The manifestation of critical fluxes in membrane filtration has typically served to set an upper value above which detrimental fouling events and a substantial increase of membrane resistance occurs. Despite the usefulness of critical flux concepts for conducting sustainable membrane processes, the specific phenomena causing them cannot be easily identified via conventional membrane filtration modes. Usually arbitrary fouling rates are selected to delimit the boundary between under- and over-critical flux operation. Timescales respective for colloidal matter accumulation are overlooked. Frequency response analysis of transmembrane pressures has been recently presented as a highly-sensitive method to track fouling in ultrafiltration. This hydraulic impedance spectroscopy method allows not only measuring membrane resistances but also corresponding time constants related to colloidal matter accumulation. In this work, hydraulic impedance spectra are gained for the ultrafiltration of different model foulants to assess the origin of critical fluxes for two different membranes. A correlation among characteristic impedance features and the type of fouling could be established: the evolution of phase shift between flux and pressure at increasing fluxes allows identifying the formation of external fouling layers, while registering of hysteresis loops at reversed frequency sequences signals the development of irreversible internal fouling. The hydraulic impedance method emerges as a precise and more sensitive monitoring tool to diagnose the beginning and nature of critical fouling.