The Formation and Control of Bromate During Ozonation: Impact of Water Quality Parameters and Treatment Conditions
Creators: name: Hickey, Tara L.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 1997

description: Bromate is formed by the ozonation of bromide-containing water. Bromate is categorized as a probable human carcinogen. The proposed Disinfectants/Disinfection By-Products (D/DBP) Rule contains an MCL (maximum contaminant level) of 10 μg/L for bromate. The World Health Organization (WHO) has a recommended limit for bromate of 25 μg/L. In this research, bromate formation was examined on a dynamic basis, in a continuous-flow, completely-mixed ozonation reactor. Bromate formation was investigated in model waters as a function of ozone dose, pH, bromide concentration, total organic carbon concentration, and inorganic carbon concentration. Chemical additions of ammonia, tertiary butyl alcohol (a hydroxyl radical scavenger), and hydrogen peroxide were also examined to assess their impact on bromate formation. The source of organic material for these studies was hydrophobic organic carbon extracted from the Kralingen Plant of Water Supply Company Europoort in Rotterdam in The Netherlands. The extract was provided by KIWA, the sponsors of this research, because of their concern regarding bromate formation in Dutch waterworks employing ozonation. Synthetic waters with the desired water quality characteristics were ozonated in a 500-mL completely-mixed flow reactor with a hydraulic residence time of ten minutes. Various ozone doses were applied to achieve effluent dissolved ozone residuals of 0.05 to 1.0 mg/L. The effluent was analyzed to determine bromate concentrations corresponding to these ozone residuals. The influent baseline conditions for the experiments were pH 7.0, 2.5 mg/L TOC, 300 μg/L Br-, and 1 mM carbonate. The range of variations for the water quality parameters included: influent pH from 6.0 to 8.3, total organic carbon concentrations from 0.0 to 5.0 mg/L as C, bromide ion concentrations from 50 to 500 μg/L, and carbonate concentrations from 0.0 to 2.0 mM. The hydraulic residence time was varied from 2.0 to 20 minutes and the gas flow rate was investigated at 100 mL/min and 39 5mL/min. The results of this study indicate that bromate formation increased with increasing pH. For any given ozone dose, the formation of bromate was found to be enhanced with increasing influent Br- concentration and carbonate concentration. As TOC concentration increased, the organic matter created an ozone demand that resulted in a decreased formation of bromate. The addition of t-butanol provided for both an increase in dissolved ozone residual and an appreciable decrease in bromate production for all ozone doses tested. The impact of ammonia was found to be pH-specific, where ammonia additions decreased bromate formation at a pH of 8.3 while addition of ammonia did not affect bromate formation at pH 7.0. Additionally, as the liquid flow rate increased, dissolved ozone residuals increased, but with lower bromate production. A decrease in the gas flow rate also provided increased dissolved ozone residuals and partial pressures and decreased bromate production. Effluent ozone residuals and bromate concentrations appeared to follow an exponential increase with increasing ozone dose. Exponential models were fitted to the experimental data for illustrative purposes and were used to discuss the implications of the experimental results to ozonation in practice. The results show that under the best conditions, i.e., warm water temperatures and 0.5 and 2.0-log inactivation of Giardia and viruses, respectively, the USEPA's MCL of 10 μg/L for bromate could be met for waters with a wide range of water quality characteristics. When 0.5-log inactivation of Cryptosporidium at cold water temperatures are considered, bromate formation significantly exceeds the USEPA's MCL and the WHO's suggested levels for bromate under a number of water quality conditions.