A contactor apparatus is provided to treat ballast water that has exited a ballast tank before being expelled from a water-borne ship. Within the contactor apparatus, means are provided for supplying a combination of acoustic energy and dissolved ozone. The acoustic energy can be provided at two or more frequencies (e.g., at 16 and 20 kHz). The acoustic energy and dissolved ozone, together, offer a more-effective and more-efficient mode of disinfecting the ballast water before release.