In situ electrical impedance characterization of fouling by calcium agents in reverse osmosis membrane systems using Maxwell Wagner and hydrodynamic models

Abstract

Modeling of electrical impedance spectra acquired from a Filmtec BW30 reverse osmosis membrane system characterized flux-dependent impedances attributed to hydrodynamic flows through and over the surfaces of the membrane as well as conductive, dielectric and geometrical properties of the substrate, support and active layers of the membrane that were consistent with microscopic, spectroscopic, impedance and membrane performance studies in the literature. Those properties of the active layer were consistent with electrochemical and physicochemical modeling of an ion exclusion mechanism involving a bipolar fixed charged structure and a thin and nearly neutral barrier inside the structure also referred to as a depletion layer. The modeling further characterized changes in the electrical, geometrical and hydrodynamic properties arising from exposure of the membrane to a feed seeded with calcium fouling agents over time that were earlier indicators of fouling than rapid flux decline. These indicators included substantive changes in measurements of membrane potential attributed to decreases in the electric potential across the depletion layer, exponential increases in the conductivities of the depletion layer and filtrate consistent with the depletion layer acting as an ionic barrier, immediate manifestation of oscillatory responses of similar periodicity in the membrane potential as well as conductivity and geometrical characterizations of most layers comprising the thin-film-composite structure and detection of a discontinuity in the dependency of hydrodynamic impedance on flux during rapid flux decline. Collectively these indicators suggest that fouling principally occurs via calcium ions binding to carboxylic acid groups fixed in the bipolar polyamide structure.