Evaluation of Wet Weather Strategies and Clarifier Optimization Using State-of-the-Art Tools

The development of more stringent effluent nutrient standards requires improved understanding and reliability of unit process performance during typical and stressed conditions. State of the art tools are needed to ensure optimum performance during these conditions. This paper present the uses of a calibrated BioWinTM model in conjunction with a secondary clarifier Computational Fluid Dynamics (CFD) model for the evaluation of wet weather strategies and for the assessment, design and retrofitting of secondary clarifiers. Two case histories are presented to illustrate the use of CFD model for the evaluation of wet weather strategies and clarifier optimization. In the first case, the calibrated CFD model was applied to asses the capacity of existing 130-ft diameter clarifiers and to the design of a new 160- ft clarifier. In conjunction with BioWinTM, the CFD model was used to evaluate the impact of using step-feed, non-step feed and polymers during a storm event. The results show that the combined used of step-feed and polymers during a wet-weather event can improve the performance of the secondary clarifiers by 80%, reducing the effluent suspended solids from around 160 mg/L to below 30 mg/L. In the second case, the CFD model was applied to identify geometry and operation limitations of existing clarifiers that could not be identified by traditional evaluation methods such as the state point analysis.