Formation of Bromate and Chlorate during Ozonation and Electrolysis in Seawater for Ballast Water Treatment

Abstract

Many effective technologies have been developed and approved for ballast water treatment, and the effectiveness of a process should be evaluated considering its environmental friendliness. The aim of this study was to evaluate the formation and formation mechanisms of bromate (BrO3−) and chlorate (ClO3−) in seawater during ozonation, electrolysis, and a combined (ozonation/electrolysis) process. In seawater ozonation, BrO3− was generated exceeding a 5 mg/L ozone dose despite the high Br− (65 mg/L). The formation of BrO3− and ClO3− by electrolysis depended on the electrode materials where two types of electrodes (IrO2/Ti and Pt/Ti) were used. The combined (ozonation/electrolysis) process generated much higher levels of BrO3− and ClO3− than either individual process. In ozonation or electrolysis, mixed oxidant including HOCl/OCl− and HOBr/OBr− is the main parameter for inactivation, which is called total residual oxidant, TRO. In this study, a predictive model for BrO3− and ClO3− was suggested in terms of TRO formation. This predictive model can recommend allowable TRO (generated by ozonation or electrolysis) for practical applications in seawater treatment.