Abstract: The present invention relates to a system for treating contaminated ballast water, comprising water inlets and outlets, piping connected to said inlets and outlets, at least one treatment unit connected to said piping, which treatment unit comprises UV radiation means, and catalysts, capable of, during operation, create photo-catalytic reactions in ballast water flowing through said unit, filter means for said ballast water and valve means for controlling the flow through said piping, treatment unit and filter means, characterised in control unit capable of, controlling and monitoring of said system for ensuring that no untreated water leaves the system in an uncontrolled manner.