In situ measurement of particle deposition and its removal in microfiltration by ultrasonic time-domain reflectometry

An ultrasonic time-domain reflectometry (UTDR) technique was used for the non-invasive, in situ visualization of particle deposition and its removal during crossflow microfiltration with a nylon membrane. The study was carried out with 0.5 g/l kaolin (average particle size 2 µm). Ultrasonic response signals were obtained upon particle deposition and subsequent cake layer formation on the membrane surface. A cake layer echo appeared in the time domain. The UTDR technique was also used to quantify the thickness of a fouling layer on the membrane surface. Moreover, UTDR was applied to monitor membrane cleaning and evaluate the effectiveness of various cleaning methods. Flux measurements and SEM analyses corroborated the UTDR results. Overall, the results confirmed the effectiveness of the UTDR technique for visualizing particle deposition on and removal from membrane surfaces.

This study describes the application of the UTDR technique to the non-invasive, in-situ, continuous visualization of particle deposits and growth on and removal from MF nylon membranes. Both the UTDR signal amplitude and arrival time of a fouling peak as a function of operating time provide useful information on the fouling process. Results of the present study show that UTDR can monitor particle initiation, and the formation and growth of a cake layer on a MF membrane surface in real time, according to changes in UTDR response signals obtained. A dense cake layer formed in a short time, with a corresponding flux decline. Although there is an obvious change (with time) in the thickness of the kaolin layer after the formation of a dense layer, there is little change in flux. This implies that the pore size of the top kaolin layer is bigger than that of the bottom layer. UTDR can also be used to monitor membrane cleaning. Ultrasonic cleaning is an effective cleaning method. Traditional flux measurements and SEM analyses are in good agreement with the UTDR results. Acknowledgments The authors thank the Water Research Commission (WRC) of South Africa for financial support (WRC project No. 930). The authors gratefully acknowledge the help from Professors Guoyong Chai of the Central Research Institute of China Chemical Science and Technology, P.R. China. We also wish to express our appreciation to Professor A.G. Fane of the University of New South Wales, Australia, for constructive suggestions and comments.