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Water purification is a crucial process in the operation of a municipality. Ensuring that water treatment plants are meeting regulatory requirements is a vital, but complicated and costly process. Because water treatment plant influent and effluent rates are demand driven, and vary both diurnally and seasonally, controlling flow rates for the disinfection stage can be challenging both operationally and economically. Thus, performing large-scale field experiments to verify water quality regulatory criteria such as modal transport time of conservative tracers in a chlorine disinfection contact tank under extreme operating conditions can range from difficult and costly to impossible. In this paper, a computational fluid dynamics (CFD) approach is used to verify the compliance of a water reclamation plant disinfection stage with respect to modal time. CFD allows a large parameter space to be tested without the need to build large physical models or taking functioning systems in a treatment plant offline. This can save facilities large amounts of time and money when designing, optimizing, and developing plants or checking compliance. This paper introduces a hybrid approach of computational analysis of a water reclamation plant's chlorine contact tank in Southern California. The method uses a hybrid approach which combines three-dimensional CFD with hydraulic grade line analysis of the open water surface. Verification cases were compared to experimental measurements at a functioning, full-scale plant with modal contact time differences below 15%. The method was then used to predict residence time distributions (RTDs) for cases which could not be artificially induced at the plant, but represented peak flow conditions that could be expected. PRACTITIONER POINTS: The hybrid 3-dimensional CFD method allows low cost simulation of the disinfection stage. By using head loss calculations to properly define the water level at each section, a steady-state single phase flow simulation can be run in 3D without the need to scale down geometries. This allows for more accurate transport results and parameter studies.