Combining Stress Testing and Dynamic Linking of Whole Plant Simulators and CFD for the Evaluation of WWTP Wet Weather Capacity

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 for the evaluation of these units in order to ensure optimum performance during these conditions. Two of the most power tools available today are whole plant simulators such as BioWin™, GPS-X; and Computational Fluid Dynamics (CFD) models. Through the application of two case studies, this paper illustrates the combined use of a whole plant simulator and a secondary clarifier CFD model for the evaluation of wet weather strategies and for the assessment, design and retrofitting of secondary clarifiers.In the first case study, after calibration and validation with stress testing data, a BioWin model was linked to a 2Dc CFD clarifier model for the determination of the wet weather capacity of a Kentucky wastewater treatment plant. The results indicate that clarifier improvements and step feed are needed in order to sustain the extreme wet weather flows for this plant. In the second case study, the calibrated 2Dc model was applied to assess the capacity of existing 39.6 m (130 ft) diameter clarifiers and to the design of a new 48.8 m (160 ft) clarifier. In conjunction with BioWin, the 2Dc 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 approximately 160 mg/L to below 30 mg/L.