Use of capacitive microsensors and ultrasonic time-domain reflectometry for in-situ quantification of concentration polarization and membrane fouling in pressure-driven membrane filtration

In membrane separation processes, the presence of the concentration polarization boundary layer (CPBL) often leads to a decline in the permeate flux and an increased risk of membrane fouling. Although the study of membrane fouling has benefited from the recent development of real-time, non-invasive acoustic measurements of the fouling process under realistic conditions, measurement of concentration polarization is still difficult and rather limited. This paper describes the development of interdigitated capacitive microsensors for real-time in-situ monitoring of CPBL growth in a cross-flow nanofiltration membrane module using calcium sulfate (CaSO4) solution as the feed. The developed capacitive sensors respond to changes in solute concentration as a function of distance from the membrane surface. Detection capability was determined in a series of comprehensive experiments with systematic changes in feed concentration and module operating parameters including upstream pressure, flow rate, and feed composition. The specially designed microscale capacitor assembly provides a unique capability for real-time characterization of the transient and steady-state concentration boundary layer polarization behavior. The capacitive microsensor concentration polarization measurements were corroborated via independent membrane morphological analysis and standard performance metrics. The experimental data were in excellent qualitative agreement with theoretical predictions from a mathematical model of concentration polarization behavior. In addition, the use of microcapacitor sensors in combination with ultrasonic time-domain reflectometry, enabled real-time observation of the development of scaling from the initiation of concentration polarization through the onset of membrane fouling. These results clearly demonstrate the advantages of a multisensor approach to the measurement of membrane phenomena, and provide a basis for the development of "smart" membranes. (c) 2005 Elsevier B.V. All rights reserved.