Estimation of Hydroxyl Radical Exposure in a three-stage Countercurrent Ozone Contactor – Reduction in Musty Odor Compounds and Formation of Bromate

ABSTRACT The Hanshin Water Supply Authority is the first bulk water supplier in Japan that currently operates two drinking water treatment plants. Both plants employ an ozonation process for the reduction of musty odor compounds, 2-methylisoborneol and geosmin, and trihalomethanes. In this study, we aim to estimate hydroxyl radical (HO∙) exposure in a three-stage countercurrent ozone contactor applied at the Inagawa drinking water treatment plant. We estimated the HO∙ exposure as the product of the ozone exposure multiplied by the RCt value, that is, the ratio of the HO∙ exposure to the ozone exposure. The ozone exposure was estimated by a methodology we developed earlier, and the RCt values were experimentally evaluated in this study. The estimated HO∙ exposure ranged from 0.4 × 10−10 to 6.7 × 10−10 M·s across the contactor during our research period. The highest HO∙ exposure of 6.7 × 10−10 M·s indicated that the reduction ratio of musty odor compounds exceeded 95%. At these exposures, the detected concentration of bromate was 6 µg/L, which is equal to the 60% concentration level of Japan’s water quality standard for drinking water. Further, we attempt to balance the reduction in musty odor compounds with control of bromate formation.