Evaluation of preformed monochloramine for bromate control in ozonation for potable reuse

Bromate, a regulated disinfection byproduct, forms during the ozonation of bromide through reactions with both ozone and hydroxyl radical. In this study, preformed monochloramine was evaluated for use as a bromate suppression method in pilot testing of wastewater reuse with an average bromide concentration of 422±20 µg/L. A dose of 3 mg/L NH₂Cl-Cl₂ decreased bromate formation by an average of 82% and was sufficient to keep bromate below the MCL at ozone doses up to 8.6 mg/L (1.2 O₃:TOC). Removal of 1,4-dioxane through ozonation decreased with increasing NH₂Cl dose, confirming that monochloramine suppresses bromate formation, at least in part, by acting as a hydroxyl radical scavenger. This may negatively impact oxidation objectives of ozonation in reuse applications. Increasing monochloramine contact time did not improve bromate suppression, indicating that monochloramine probably did not mask bromide as NHBrCl or other haloamines prior to ozonation. However, NHBrCl and NH₂Br may be formed from reactions between HOBr and NH₂Cl and excess free ammonia during ozonation. NDMA was formed by ozonation at concentrations up to 79 ng/L and was not enhanced by NH₂Cl addition.