Abstract: This invention relates to apparatus and methods for the disinfection of fluids and, in particular, to the disinfection of industrial fluids with ultraviolet radiation. These fluids are typically used in manufacturing as coolants in both long and short assembly lines. They commonly accumulate contaminants from multiple and diverse sources including oil and microorganisms. Fluids can be disinfected by establishing a fluid flow rate sufficient to prevent occlusion of the walls of the ultraviolet transmissible portion by contaminants. Fluids may be so heavily contaminated so as to require removal of at least a minimum percentage of contaminants (MPC) prior to irradiation. Such fluids may be processed to remove the minimum percentage of contaminants according to the equation: MPC=102-(23.45.times.lnV).