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 ...