CFD Modeling of Final Settling Tanks: Ramping up from 2D to 3D

New York City Department of Environmental Protection (NYCDEP) and the City College of New York (CCNY) have embarked on 3-D Computational Fluid Dynamics (CFD) modeling of the final settling tanks (FSTs) at their water pollution control plants (WPCPs). NYCDEP is in the process of retrofitting and upgrading its WPCPs from conventional treatment to the biological nitrogen removal (BNR) mode to meet the upcoming State Pollution Elimination Discharge Permit (SPDES) limits. As part of this effort, the City is optimizing the FSTs which often are the weakest link in the process train. With higher solids loadings necessitated by the BNR process and new wet weather operating strategies, the FSTs will be expected to handle the additional solids and hydraulic loadings that it will be subjected to. 3D CFD models are excellent tools for a realistic assessment of the capability of the tanks, a positive step forward from the earlier solids flux analysis method and the 1 and 2-D modeling efforts. CFD models have been extensively used in other fields and only recently have been applied to the wastewater field. This paper will present the approach NYCDEP and CCNY took in developing the aforementioned model and the reasons for its development. A discussion on where 2D is appropriate and the scenarios that demand a 3D investigation will be discussed based on the issues such as geometry interpretation and resulting simplifications, clarifier asymmetry, boundary conditions, turbulence closure models, flocculation modeling, calibration and their effects on the dynamic analyses that predict effluent suspended solids concentrations (ESS).