A method and system for ballast water treatment are provided. A vacuum is applied directly to an existing vent of a vessel's ballast water tank during the vessel's voyage. The applied vacuum is in a range of approximately -2 to -4 pounds per square inch. To further assure success, living organisms can be added into the ballast water tank to consume (i) oxygen dissolved in the ballast water, and (ii) food sources for selected organisms contained in the ballast water. No modifications to the vessel's ballast water tank are required as all system components are coupled to the existing vent.