Abstract Ballast water is used to supply stability on ships that have no load. It is loaded on before departure and often carries stowaways in the form of tiny organisms. To avoid that, the ballast water can be treated on board. Treating ballast water to free it from potentially harmful invasive species is a challenging problem. Alfa Laval in cooperation with Wallenius Water has been developed one system that cleans ballast water during ballasting and de ballasting operation. The paper presents how this technology actions and what are the costs for implementing the system on board.