Organisms carried with ballast water can find a way that enables them to spread into a new habitat, becoming invasive species. This can generate large impacts threatening the ecosystem and human activities. The effectiveness of microbiological disinfection by UV/H2O2 treatment on Enterococcus faecalis has been evaluated in this study at laboratory scale, in both buffered distilled water (DW) and saltwater (SW). A Collimated Beam Reactor was used to determine optimal H2O2 concentration with DW and a Continuous Flow Reactor was tested with DW and SW. The optimal concentration of hydrogen peroxide found was 5 mg/L. The improvement of adding H2O2 increased efficacy by 28.9% in SW compared with UV alone; while results indicated that water salinity did not induce strong interference in treatment. In addition, re-growth of surviving bacteria was prevented 24 h after the treatment; even an additional one-log inactivation was obtained. The results suggest that the addition of small concentrations of H2O2 leads to an improvement in UV treatment. Finally, the operational costs were estimated for typical cargo vessels; UV/H2O2 treatment was considered to be competitive for ballast water treatment, since it could improve the effectiveness of the process with similar costs per 1000 m(3) of treated water: 14 for UV treatment and 16 for UV/H2O2 treatment. (C) 2015 Elsevier BM. All rights reserved.