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

Progress in developing a means for control and prevention of fouling has be en impeded by the lack of a suitable noninvasive fouling-measurement techni que. In addition, fouling remediation strategies have traditionally relied upon the end-of-the-cycle recovery of productivity for estimating the degre e of fouling removal. This paper describes the application of ultrasonic ti me-domain reflectometry (TDR) for real-time measurement of membrane fouling layer growth and its removal. The experimental results obtained using an a utomated reverse osmosis system under controlled pressure, temperature and CaSO4 feed-concentration conditions show that ultrasonic signal amplitude m easurements provide a sensitivity to the dynamics of fouling-layer growth t hat is comparable to that obtained from the flux-decline behavior. For expe riments 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 obser ved; the layer growth occurred as randomly-oriented rosettes initially, fol lowed by the growth of laterally-oriented flat crystals during the later st ages. Ultrasonic TDR measurements were capable of distinguishing these two modes of growth. Overall, the implications of the ultrasonic TDR behavior w ith respect to fouling were confirmed by independent measurement techniques . The ultrasonic technique was also successfully employed for monitoring me mbrane cleaning at ambient conditions. The end-of-the-cleaning-cycle membra ne characterization showed that the ultrasonic measurements correspond well with the permeability recovery and surface analysis.