An apparatus for purifying contaminated water by photochemical oxidation has at least a subflow of water that is directed through a flow channel in which the water is irradiated with UV electromagnetic radiation from at least one UV lamp assembly. The UV lamp assembly includes a high-pressure UV halogen lamp, which is mounted generally parallel with the flow direction in the channel. A method of irradiating the water flow with UV radiation includes using the at least one UV halogen high-pressure lamp assembly, which has energy intensive wave lengths in the range of 150 nm to 260 nm, preferably in the range of 160 nm to 220 nm, and most preferably in the range of 192 nm to 205 nm.