An apparatus for and method of treating a volume of fluid. The apparatus includes a fluid passage (35) through which a fluid flows, at least one source of irradiation (32), external to the passageway, and at least two elongated elliptical reflecting troughs (22 and 24) for reflecting irradiation from the at least one source of irradiation onto the fluid passageway. Open ends of the troughs face each other to define a space between the closed ends of the troughs. The fluid passageway and the at least one source of irradiation are spaced from the local axes so as to produce a substantially uniform irradiation distribution within the fluid passageway.