This paper outlines the design, assessment, and installation of a prototype ballast water treatment system for a double-hull tank ship. The IMO has established a timetable by which vessels must be equipped with ballast water treatment systems that meet a prescribed performance standard. Tank ship installations are particularly challenging because ballast piping is typically located in spaces defined by classification society rules as hazardous. These same rules do not readily allow the treatment equipment and piping to communicate with these spaces. The treatment system described in this paper has been demonstrated, through a risk assessment, to meet applicable classification society rules. Planned full-scale testing intends to show the operational safety, efficiency, and biological efficacy of this promising technology