Owing to the highly variable nature of flows and water quality of CSOs for centralized as well as satellite CSO treatment, specialized methodologies and technologies have been developed to provide effective disinfection; most important has been the introduction of high-rate mixers in combination with dosing equipment to achieve disinfection in much shorter contact times than conventionally accomplished for dry-weather flows. High-rate disinfection or kill is defined as the application of high-rate mixing in combination with a chemical disinfectant to achieve disinfection within five minutes. The effectiveness is typically expressed in terms of log removal. High-rate disinfection utilizes mixing as a substitute for an additional component of time that would otherwise be required. This mixing is expressed as velocity gradient or “G”. These principals were first advanced through field scale applications during U.S. EPA demonstrations in Syracuse and Rochester, New York in the late 1970s (EPA 600/2-79-134, 600/2-79-031B). EPA’s Environmental Technology Verification (ETV) Program has developed generic protocols for the testing of such mixing which is reported. Detailed testing under the ETV Program has further verified the effectiveness of mixing in the disinfection process. Chlorine compounds have traditionally been used to provide disinfection due to their relatively low cost. Since the 1970s, growing awareness of the adverse environmental impacts associated with the byproducts and residuals of chlorination has led to increasingly more restrictive residual chlorine requirements. Efforts to minimize environmental risks, chemical demands and contact times have fostered a strong interest in alternative disinfection technologies. The New York City pilot study addressed, and an ongoing WERF project is addressing, alternative technologies (WERF, 2000). Chlorine dioxide (ClO2) and ultraviolet (UV) disinfection represent a significant advantage over chlorination/dechlorination with respect to the toxicity of the aquatic system.