The development of a non-destructive, real-time observation technique to detect and monitor fouling during liquid separation processes is of great importance to the development of strategies to improve operating conditions. In this study ultrasonic time-domain reflectometry (UTDR) was used to measure organic fouling, in real time, during ultrafiltration (UF) with polysulfone (PS) membranes. The feed solution was a paper-mill effluent, which contains breakdown products of lignin or lignosulphonate, from a wastewater treatment plant. An asymmetric, composite PS membrane can be detected by UTDR. Experimental results showed that the ultrasonic signal response can be used to monitor fouling-layer formation and growth on the membrane in real-time. Traditional flux measurements and analysis of the membrane surface by microscopy corroborated the UTDR results. Further, the differential signal developed indicate the state and progress of the fouling layer and gives warning of advanced fouling during operation. Moreover, a predictive modelling program - ultrasonic reflection (USRF) was developed to model the fouling deposits. This contributed to a better understanding of the fouling layer formation and its recognition during ultrasonic testing. This modelling was applied to model the density and thickness of a fouling layer on a membrane surface so as to predict the fouling behavior. The modelling results were in a good agreement with the actual observations.