The invention relates to hydrocyclones for ballast water treatment. The invention further relates to ballast water treatment systems comprising such hydrocyclones and methods of operating such hydrocyclones and ballast water treatment systems. The configuration of the hydrocyclone is adapted to remove particles of at least µm particularity well. The hydrocyclone thus reduces the need for poweful or prolonged UV irradiation in a downstream decontamination apparatus of a preferred ballast water treatment system.