A device for treating ballast water includes a housing having a ballast water inlet and a ballast water outlet, a hydrocyclone connected to the ballast water inlet and having a particle outlet discharging particles from the device and an outlet carrying particle-depleted ballast water, a filter surrounding the hydrocyclone, which filter divides the device into an intermediate chamber disposed between hydrocyclone and filter and an outer chamber disposed between filter and housing. The intermediate chamber is communicatively connected to the outlet of the hydrocyclone carrying particle-depleted ballast water. Ultrasound is generated to act on the outer chamber and/or the filter, UV light is generated to act on the outer chamber and/or the filter, and a gas is introduced into the outer chamber of the device.