An oxidation chamber for use in the treatment of organically contaminated water and waste liquids includes a lamp seal assembly (36) that accomodates the thermal expansion of an ultraviolet lamp (42) while at the same time protecting the lamp (42) from direct contact with the liquid being treated. The lamp seal assembly (36) facilitates replacement of burned out lamps and cleaning of the protective tube (32) that surrounds the lamp (42). In another aspect of the invention, a specially designed family of baffles (28 and 30) and distributors (90, 92, and 94) are provided so that the chamber can accomodate a wide range of flow rates simply by replacing one set of distributors by a different set of distributors.