Cake-layer deposition, growth, and compressibility during microfiltration measured and modeled using a noninvasive ultrasonic technique
A noninvasive measurement method was developed to monitor particle deposition and cake formation in situ and to determine the thickness of the cake layer formed during cross-flow microfiltration with nylon membranes. An ultrasonic signal reflection technique was used. The study was carried out with paper mill effluent (average particle size of 0.947 μm) from a wastewater treatment plant. Ultrasonic response signals were obtained as particles initiated and a cake layer formed on the membrane surface. The ultrasonic testing technique can measure the cake thickness as a function of time. Results also showed that the ultrasonic unit is a suitably programmed microprocessor and can be used to compare reference and test signals. It produced a separated signal (from reference and test signals), indicating the state and progress of the cake layer on the membrane surface under operating conditions. A predictive modeling program, ultrasonic reflection, was developed to better understand the processes related to the deposition of cake layers on the membrane surface. The modeling program can predict the shape of the curve at the beginning of the signals, indicating the cake compressibility. In microfiltration, the flux decline is related not only to the layer thickness but also to the cake density (compressibility). The changes in the densities of a cake layer as well as the thickness can be substantiated by the modeling. The predicted results were in good agreement with the actual measurements.