Contaminants in an aqueous stream are oxidized with minimal use of an added oxidant by diffusing the oxidant through a fixed porous element followed by cyclical photolytic treatment of the stream with at least two different wavelengths of light. The porous element is preferably formed in one piece of a durable sintered ceramic material. The photolytic treatment preferably involves the sequential use of ultraviolet light having two different wavelengths, by which complementary oxidizing reactions occur within the stream being decontaminated.