A target biokill of species for water to be discharged from a ballast tank of a sea faring vessel is determined. A diverted portion of sea water is regulated prior to charging the water into the ballast tank. Regulation of the diverted portion of water and a rate of injection of the ozone into the portion are adjusted to attain the target biokill. Then ozone at the determined rate into the regulated diverted portion to attain the target biokill when the portion is recombined into the water for charging to the ballast tank.