The invention comprises adding bromide to a cooling water system containing corrosion and scale control treatment chemicals at elevated concentrations (40 ppm or greater). A fraction of the recirculating water is drawn off in a side stream and ozonated. Ozone oxidizes bromide to bromine which then serves as a biocide. The bromide levels are chosen such that the ozone/bromide reaction is preferred over the ozone/treatment chemicals reaction. Thus, the addition of bromide at elevated concentrations, while acting as a biocide precursor, serves to protect treatment chemicals necessary for corrosion and scale control.