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). Subject to removal of the MPC, a flow rate can be established to prevent occlusion of ultraviolet-transmissible portions of the flow path and thereby successfully treat the fluid with a disinfecting amount of ultraviolet radiation. Using these methods, microorganism levels can be greatly reduced with a reduced need for biocides or other anti-bacterial or anti-fungal agents. The methods and apparatus of the invention also comprise a flattened-tube mechanism for increased exposure to UV radiation and a turbulence-generating system to increase effectiveness of radiation treatments. Turbulence-generating systems include means for creating pressure differentials or aeration in the fluid stream as well as various types of structures such as ribbons, paddles, cones, beads or vanes that can be placed within the lumen of the tubing system. These methods are highly effective at extending the useful life of fluids such as coolants and reducing or eliminating the risks posed to workers by heavily contaminated or biocide-treated coolants.