BACKGROUND: Ballast water discharge from ships is regarded as one of the four major risk factors that threaten global marine environmental safety, and ballast water treatment is vital to prevent the introduction of potentially invasive species. The UV/Ag-TiO2/O-3 process has been investigated for its potential use for ballast water treatment using Escherichia coli (E. coli) as an indicator bacterium. Inactivation curves were obtained, and the occurrence of oxidants was studied. RESULTS: Compared with individual unit processes with ozone or UV/Ag-TiO2, the inactivation of E. coli by the combined UV/Ag-TiO2/O-3 process was enhanced, and the inactivation efficiency was improved with increasing ultraviolet intensity and ozone dose. The initial total residual oxidant (TRO) concentration was positively correlated with ozone dose, and resulted in faster decay rate for lower initial concentration. Persistence of TRO resulted in a cumulative bacteria mortality in the effluent. CONCLUSION: The UV/Ag-TiO2/O-3 process was found to be efficient for E. coli inactivation in simulated ballast water. (C) 2011 Society of Chemical Industry