Enhanced bromate control during ozonation: The chlorine-ammonia process
Potentially carcinogenic bromate forms during the ozonation of bromide-containing waters. Some water treatment facilities have had to use ammonia addition and pH depression to minimize bromate formation, but these processes may prove to be insufficient to comply with upcoming regulations. The chlorine-ammonia process (Cl-2-NH3), consisting of prechlorination followed by ammonia addition prior to ozonation is shown to cause a 4-fold decrease in bromate formed when compared to the ammonia-only process. Experiments revealed three key mechanisms: (i) oxidation by HOCl of Br- to HOBr and its subsequent masking by NH3 as NH2Br; (ii) decrease of HO. exposure through halogenation of Dissolved Natural Organic Matter (DNOM) by HOCl and scavenging of HO. by NH2Cl; and (iii) DNOM acting as a bromine sink after oxidation of Br- to HOBr. At an ozone exposure of 6 mg/L(.)min and pH 8, conventional ozonation of Lake Zurich water spiked with 560 mug/L Br- formed 35 mug/L BrO3-, whereas the application of the Cl-2-NH3 process resulted in 5 mug/L BrO3-. Additional pH depression to pH 6 further decreased bromate formation by a factor of 4. Trihalomethanes (THM) and cyanogen chloride (CNCl), that may form during prechlorination and monochloramination, respectively, were well below regulatory limits. The chlorine-ammonia process holds strong promise for water treatment facilities struggling with a bromate formation problem during ozonation.