The invention relates to a device and method for UV irradiation of treated waste water. The device comprises a plurality of UV modules, each UV module being an immersed radiator that comprises a low-pressure mercury radiator surrounded by a round sheathing tube permeable to UV rays. Each UV module is assigned to an irradiation chamber of square cross-section, the midpoint of which is also the midpoint of the UV radiator. The distance between the UV modules is selected in such a way that the cross-sectional area of the irradiation chamber of each UV module is not more than ten times larger than the cross-sectional area of the UV module itself.