The photocatalytic process is considered as one of the advanced oxidation processes (AOPs) for the degradation/mineralization of pollutants, and it is known that ultrasound can significantly enhance these reactions when combined with photocatalytic processes. However little is known about the design parameters of these AOPs for industrial use. In this study, the attenuation of UV light under various conditions including ultrasound irradiation (36 and 935 kHz) and photocatalytic particle concentrations (TiO2: 0 to 100 mg/L) was investigated in a large-scale reactor. In addition, hydroxyl radical-induced oxidation in each process was compared using KI dosimetry. It was found that UV light was attenuated significantly due to the presence of dispersed particles and ultrasonic waves. The effective irradiation radius of UV light using a commercial module was determined to be with less than 10 cm considering the hydroxyl radical-induced activity in the combined processes. The results from this study may be useful to design large-scale UV-light-based AOPs for the removal of organic contaminants from an aqueous environment.