Development of a Computational Fluid Dynamics (CFD) Model of a Full-Scale Oxidation Ditch Incorporating Activated Sludge Model (ASM)-1

Abstract

Wastewater treatment facilities combine biological, physical, and chemical unit processes to remove pollutants and restore wastewater to a quality that is harmless. This work develops a water-sludge multiphase computational fluid dynamics (CFD) model of a full-scale oxidation ditch, a biological unit process that couples hydrodynamics with a bio-kinetics model, namely the activated sludge model (ASM)-1, to analyze the spatial and temporal distribution of the ASM-1 components. This model represents one of the oxidation ditches at the Valrico Advanced Wastewater Treatment Facility in Dover, Florida. The ditch is oval-shaped and equipped with two vertical axis surface mechanical aerators, one in either curved end of the ditch. The motion of the aerators was simulated with the multiple reference frame approach. Turbulence was accounted for via the standard k-epsilon model with standard wall functions. ASM-1 consists of 12 components and 8 kinetic processes that describe carbon oxidation, nitrification, and denitrification. Each component of ASM-1 was incorporated into the CFD model via Reynolds-averaged scalar advection-diffusion-reaction transport equations. The model was ran in single and multiphase modes, where in single phase mode the hydrodynamics account for water only and in the multiphase mode the hydrodynamics account for both water and sludge phases. It is found that the vertical stratification induced by the sludge phase can impact the dissolved oxygen distribution in the ditch and thus ASM components such as active heterotrophic biomass and particulate products from biomass decay.