Addition of H2O2 has been employed to repress bromate formation during ozonation of bromide-containing source water.However,the addition of H2O2 will change the oxidation pathways of organic compounds due to the generation of abundant hydroxyl radicals,which could afect the removal efcacy of trihalomethane precursors via the combination of ozone and biological activated carbon(O3-BAC).In this study,we evaluated the efects of H2O2 addition on bromate formation and trihalomethane formation potential(THMFP)reduction during treatment of bromide-containing(97.6–129.1μg/L)source water by the O3-BAC process.At an ozone dose of 4.2 mg/L,an H2O2/O3(g/g)ratio of over 1.0 was required to maintain the bromate concentration below 10.0μg/L,while a much lower H2O2/O3ratio was sufcient for a lower ozone dose.An H2O2/O3(g/g)ratio below 0.3 should be avoided since the bromate concentration will increase with increasing H2O2 dose below this ratio.However,the addition of H2O2 at an ozone dose of 3.2 mg/L and an H2O2/O3 ratio of 1.0 resulted in a 43% decrease in THMFP removal when compared with the O3-BAC process.The optimum H2O2/O3(g/g) ratio for balancing bromate and trihalomethane control was about 0.7–1.0.Fractionation of organic materials showed that the addition of H2O2 decreased the removal efcacy of the hydrophilic matter fraction of DOC by ozonation and increased the reactivity of the hydrophobic fractions during formation of trihalomethane,which may be the two main reasons responsible for the decrease in THMFP reduction efcacy.Overall,this study clearly demonstrated that it is necessary to balance bromate reduction and THMFP control when adopting an H2O2 addition strategy. Addition of H2O2 has been employed to repress bromate formation during ozonation of bromide-containing source water. Even though the addition of H2O2 will change the oxidation pathways of organic compounds due to the generation of abundant hydroxyl radicals, which could afect the removal efcacy of trihalomethane precursors via the combination of ozone and biological activated carbon (O3-BAC) .In this study, we evaluated the efects of H2O2 addition on bromate formation and trihalomethane formation potential (THMFP) reduction during treatment of bromide-containing (97.6-129.1 μg / L) source water by the O3-BAC process. At an ozone dose of 4.2 mg / L, an H2O2 / O3 (g / g) ratio of over 1.0 was required to maintain the bromate concentration below 10.0 μg / L, while a much lower H2O2 / O3ratio was sufcient for a lower ozone dose. An H2O2 / O3 (g / g) ratio below 0.3 should be avoided since the bromate concentration will increase with increasing H2O2 dose below this ratio. dose of 3.2 m g / L and an H2O2 / O3 ratio of 1.0 resulted in a 43% decrease in THMFP removal when compared with the O3-BAC process. optimum H2O2 / O3 (g / g) ratio for balancing bromate and trihalomethane control was about 0.7- 1.0. Fractionation of organic materials showed that the addition of H2O2 decreased the removal efcacy of the hydrophilic matter fraction of DOC by ozonation and increased the reactivity of the hydrophobic fractions during formation of trihalomethane, which may be the two main reasons responsible for the decrease in THMFP reduction efcacy. Overall, this study shows that it is necessary to balance bromate reduction and THMFP control when adopting an H2O2 addition strategy.