Under the auspices of the Safe Drinking Water Act (SDWA) the U.S. EPA has promulagated the Surface Water Treatment Rule (SWTR) requiring public water systems using surface water to provide minimum disinfection to control Giardia Lamblia, enteric viruses, and bacteria. The CT concept (concentration of disinfectant in mg/L times time in minutes) is used to establish the appropriate criteria for a surface system to achieve at least 99.9% inactivation of Giardia lamblia and 99.9% inactivation for viruses. In the SWTR, an empirical equation was developed based on water temperature, pH, concentration of chlorine, and inactivation level to predict required disinfection criteria (C times t values). The paper describes the development of an equation based on Chick-Watson kinetics that provides equivalent information but is theoretically more consistent.