Investigation of membrane fouling and cleaning using ultrasonic time-domain reflectometry

Abstract

Progress in developing a means for control and prevention of fouling has been impeded by the lack of a suitable noninvasive fouling-measurement technique. In addition, fouling remediation strategies have traditionally relied upon the end-of-the-cycle recovery of productivity for estimating the degree of fouling removal. This paper describes the application of ultrasonic time-domain reflectometry (TDR) for real-time measurement of membrane fouling layer growth and its removal. The experimental results obtained using an automated reverse osmosis system under controlled pressure, temperature and CaSO₄ feed-concentration conditions show that ultrasonic signal amplitude measurements provide a sensitivity to the dynamics of fouling-layer growth that is comparable to that obtained from the flux-decline behavior. For experiments conducted at axial velocities of 4.6 and 9.8 cm/s (Re= 178 and 379, respectively), a sequential two-mode CaSO₄ fouling layer growth was observed; the layer growth occurred as randomly-oriented rosettes initially, followed by the growth of laterally-oriented flat crystals during the later stages. Ultrasonic TDR measurements were capable of distinguishing these two modes of growth. Overall, the implications of the ultrasonic TDR behavior with respect to fouling were confirmed by independent measurement techniques. The ultrasonic technique was also successfully employed for monitoring membrane cleaning at ambient conditions. The end-of-the-cleaning-cycle membrane characterization showed that the ultrasonic measurements correspond well with the permeability recovery and surface analysis.