The fouling phenomenon in spiral wound membrane modules was investigated using ultrasonic time domain reflectometry (UTDR) and computational fluid dynamic (CFD) simulations. This study utilized a high-frequency 50-MHz ultrasound system to measure the fouling distribution in the spiral wound ultrafiltration and reserve osmosis membrane modules. The results show that for a porous membrane, the voltage decreases as the fouling increases while the opposite is true for a dense membrane. The effects of gravity and module orientation on fouling were also investigated. It is clear from the results that gravity and membrane curvature play a role in the deposition of foulants on the membrane surface. This finding is supported by CFD simulations revealing that the inner membrane experiences higher axial velocities as the fluid flows through the spacers, thus generating a higher wall shear stress in the interior, which can aid in reducing membrane fouling. This study demonstrated that UTDR with a high-frequency transducer can be used to analyze the fouling phenomenon in spiral wound membrane modules. (C) 2015 Elsevier B.V. All rights reserved.