Abstract: A hydrodynamic cavitation unit for a hydrodynamic cavitation device is provided. The hydrodynamic cavitation device is configured for fluid communication with an inlet conduit and an outlet conduit and may include an expansion chamber downstream of the inlet conduit. The hydrodynamic cavitation unit includes a body; and a plurality of lumens through the body. Each of the lumens is configured for fluid communication from the inlet conduit, through the body, and to the outlet conduit. A ratio of a total cross-sectional area of the lumens to a cross-sectional area of the inlet conduit is about 1:1 in at least a first point in the body, less than 1:1 in at least a second point in the body, and greater than 1:1 in at least a third point in the body. The hydrodynamic cavitation unit may include a cap or a vortex unit including the cap. The vortex unit may extend into the expansion chamber. The at least one lumen may include a constriction relative to the inlet conduit.