Abstract: The present invention provides an apparatus and method for treating fluid media by exposure to ultraviolet radiation. A fluid flow path from a fluid inlet to a fluid outlet is provided. The fluid flow path is proximate to a source of ultraviolet radiation. Reagents may be added to the system so that the reagent is present in the circulating fluid while the fluid is being exposed to the ultraviolet radiation. Samples of fluid may be withdrawn from the system so that process parameters may be adjusted. In a system in which the fluid flow path is defined by a plurality of interconnected pipes, a connecting means between the pipes includes a port through which reagent may be added or samples withdrawn from the system.