Three-Dimensional Simulation of Water Quality in a Wastewater Stabilization Pond
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Wastewater stabilization ponds (WSP) are effective secondary or tertiary treatment systems, however simple water quality models that assume the ponds behave as either completely stirred tank reactors (CSTR) or plug flow systems are commonly used in design. These models consider a WPS to be a homogeneous or a one-dimensional system, respectively, and critically neglect spatial variability in hydrodynamics that leads to poor characterization of variability in residence time, biogeochemistry and removal efficiency. Despite these shortcomings, CSTR and plug-flow models are still used for pond design because of their simplicity. This thesis explores the application of fully three-dimensional (3D) computational models capable of being applied to develop better design guidelines that will assist municipalities in meeting effluent regulations. Some 3D models that have been applied to WSPs, neglect complex biochemical reactions and are limited to use first order decay removal. To date, no calibrated water quality model that can simulate complex biochemical processes and is coupled with a calibrated 3D hydrodynamic model has been applied to a WSP. The present study focuses on development of a 3D water quality model of a secondary facultative pond coupled with a 3D hydrodynamic model. The calibrated water quality model was also coupled with CSTR (0D) and plug flow (1D) hydrodynamic models to show the disadvantages of these simplified and widespread assumptions in WSP modelling. The 3D numerical model reproduced the observed WSP effluent nutrient, dissolved oxygen, pH, and phytoplankton seasonal dynamics and quantitatively reproduced nutrient removal (5-7% error). The model was extended to investigate the effect of physical changes in pond design (inlet location, baffle configuration, wind sheltering, depth) on performance, which revealed that increasing the depth of the pond and constructing wind sheltering improved pond performance. Removing baffles had no impact on nutrient removal efficiency and relocating the inlet to the surface reduced the pond efficiency. The 0D and 1D models over-predicted effluent concentration of all substances, with the exception of pH and dissolved oxygen since the nutrients were typically sequestered near the bottom of the pond during stratified conditions. Mixing events from wind and buoyant inflows caused the model predictions to converge with consistent vertical distributions. The numerical results from this thesis provide a detailed view of biogeochemical processes in WSPs and suggest that future designs should consider the influence of 3D hydrodynamics on nutrient removal and pond performance.