Bromide measurements and mass balances in the catchments of major Swiss rivers revealed that chemical industry and municipal waste incinerators are the most important bromide sources and account for similar to 50% and similar to 20%, respectively, of the similar to 2000 tons of bromide discharged in the Rhine river in 2014 in Switzerland. About 100 wastewater treatment plants (WWTPs) will upgrade their treatment for micropollutant abatement in the future to comply with Swiss regulations. An upgrade with ozonation may lead to unintended bromate formation in bromide containing wastewaters. Measured bromide concentrations were <0.05 mg L-1 in similar to 75% of 69 WWTPs, while they ranged from 0.4 to similar to 50 mg L-1 in WWTPs with specific bromide sources (e.g., municipal waste incinerators, landfill leachate, and chemical industry). Wastewater ozonation formed little bromate at specific ozone doses of <= 0.4 mg O-3/mg DOC, while the bromate yields were almost linearly correlated to the specific ozone dose for higher ozone doses. Molar bromate yields for typical specific ozone doses in wastewater treatment (0.4-0.6 mg O-3/mg DOC) are <= 3%. In a modeled extreme scenario (in which all upgraded WWTPs release 10 mu g L-1 of bromate), bromate concentrations increased by <0.4 mu g L-1. in major Swiss rivers and by several micrograms per liter in receiving water bodies with a high fraction of municipal wastewater.