Ballast water transfers aquatic species among ports and poses biological threat to the world's oceans. This work investigated the use of combined ultraviolet (UV)/Ag–TiO 2 +ozone (O 3 ) processes for ballast water treatment using Escherichia coli and Vibrio alginolyticus as indicator microorganisms. In all tests, the V. alginolyticus cells are more resistant to UV/O 3 inactivation than are the E. coli cells. Bacterial cells under stressing stimulated activities of antioxidants for their protection. The combined UV+O 3 and UV/Ag–TiO 2 +O 3 tests yield synergistic removal of both strains compared with individual tests. The combined UV/Ag–TiO 2 +O 3 process is potentially applicable for onboard treatments for ballast waters.