A Multi-Faceted CFD Evaluation of an Existing Secondary Clarifier

As part of the Great Lakes Water Authority’s Water Resource Recovery Facility Comprehensive Regional Wastewater Master Plan, computational fluid dynamics (CFD) modeling was used to evaluate the performance and capacity of an existing secondary clarifier. The hydraulic performance was evaluated using three-dimensional (3D) CFD models, and process performance was evaluated using a two-dimensional (2D) CFD model. The clarifier inlet configuration is very complex and unique, where flow is conveyed into the circular clarifiers from a series of diffuser pipes around the periphery. To accurately represent flow into the clarifier from the diffusers, it was essential that the flow through the diffuser be modeled. Modeling flow through each of the 73 inlet diffusers and the clarifier simultaneously proved to be impractical considering the computational resource requirements. Therefore, two separate 3D CFD models were created for a single inlet diffuser and the entire secondary clarifier. The high-resolution single inlet diffuser model was used to both determine and define the inlet boundary conditions of the clarifier model. The velocity field and shear flocculation potential predicted by the 3D CFD models were considered as well as the 2D CFD simulations, which included sludge characteristics and density currents to predict clarifier process performance and capacity. In this paper, the multi-faceted CFD evaluation used to determine the future capacity potential for the GLWA WRRF secondary clarifiers will be discussed. Overall, the model results indicated the performance of the existing secondary clarifiers would not be compromised with an increased surface overflow rate. The paper will detail the approach and results that led to that conclusion.