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-TiO2 + 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. call cells. Bacterial cells under stressing stimulated activities of antioxidants for their protection. The combined UV + O-3 and UV/Ag-TiO2 + O-3 tests yield synergistic removal of both strains compared with individual tests. The combined UV/Ag-TiO2 + O-3 process is potentially applicable for onboard treatments for ballast waters. (C) 2011 Elsevier By. All rights reserved.