Abstract: The purification apparatus includes UV-tubes and secures a continuous and uniform disinfection or purification, respectively, of a flowing medium such as water, independent which portion of the space containing the UV-tubes is flowed through by such medium. The purification apparatus comprises a cylindrical pressure vessel with two chambers arranged on top of each other and separated from each other by a partition wall. The lower chamber comprises an inlet opening for the medium which is to be purified. The cylindrical upper chamber communicates with the lower chamber by inlet parts for the medium arranged on the partition wall and arranged groupwise along various concentric circles, each having a different radius.