A protocol is demonstrated for the design and evaluation of ultraviolet (UV) disinfection systems based on a mathematical model. The disinfection model incorporates the system's physical dimensions, the residence time distribution of the reactor and dispersion characteristics, the UV intensity within the reactor, and the quality of the wastewater being treated. The data requirements for calibration of the model are discussed. An example is then given, using information from a pilot study at the Port Richmond Water Pollution Control Plant, New York, New York. Reactor hydraulics are characterized in terms of the dispersion coefficient and the dispersion number. Intensity is computed by the point source summation method; this is adjusted by attenuation factors to account for surface fouling of the UV lamp enclosures and the aging of the lamps. Water quality is related to the UV absorbance coefficient, the initial bacterial density, and the suspended solids (SS). The bacteria inactivation rate is related to the intensity of the reactor, while bacteria occluded by particulate matter, and unaffected by UV, are described as a function of the SS concentration. (Copyright (c) J. Water Pollution Control Federation, 59(1):25-31, January 1987.)