Abstract: The invention disclosed provides an apparatus and method for disinfection of ship's ballast water, such as sea water, based on hydrodynamic cavitation. The apparatus comprises a cavitating chamber of cross section such as circular or non circular shape housing a single or multiple cavitating element(s) in the form of plates of metallic, ceramic, plastic materials of varying thicknesses placed perpendicular to the direction of flow of liquid and positioned at uniform or non-uniform spacing. The cavitating elements are provided with single or multiple orifices of different cross sections, circular or non-circular, with or without sharp edges and with a fraction of cross-sectional open area of the passage. The disinfected water may be re-circulated through the system for additional disinfection or released from the tank into the surrounding waterways.