Optimal secondary clarifier performance is crucial to meet secondary treatment requirements, especially when treating peak wet weather flows (PWWFs), to prevent high effluent suspended solids (ESS) concentrations and elevated sludge blankets. A state-of-the-art computational fluid dynamic (CFD) model was successfully used as a design and diagnostic tool to (1) predict secondary clarifier performance and (2) optimize performance for municipal wastewater treatment plants subject to significant PWWFs. Three case studies are presented. For Case Study 1, the model was used to determine the capacity of an existing plant to treat PWWF and to identify modifications that could be made to increase secondary clarifier capacity. For Case Study 2, the model was used to determine the number of secondary clarifiers that will be necessary to treat future PWWF conditions for a plant under design. For Case Study 3, the model was used to identify modifications that are currently being made to increase the clarifier capacity for handling PWWF