The growth of the Las Vegas Metropolitan area may eventually lead to increased wastewater discharges into Boulder Basin of Lake Mead (Figure 1). Boulder Basin has experienced several algal blooms over the last few years. As a result, alternate discharge locations and strategies are being investigated. Thus, studying the water quality in Boulder Basin becomes imminent in order to assist various agencies in making decisions on operations within Boulder Basin. Due to its extremely irregular shoreline and large surface area, Lake Mead cannot be simulated adequately by one or two-dimensional models. Therefore, ELCOM (Estuary and Lake COmputer Model), an advanced three-dimensional hydrodynamic model coupled with CAEDYM (Computational Aquatic Ecosystem DYnamics Model) was chosen to simulate threedimensional transport and interactions of flow physics, biology, and chemistry in the reservoir. ELCOM was designed for practical numerical simulation of hydrodynamics and thermodynamics for inland and coastal waters. The code links seamlessly with the CAEDYM model undergoing development at the University of Western Australia Centre for Water Research. The combination of the two codes provides three-dimensional simulation capability for examination of detailed changes in water quality. Figure 2 shows the three-dimensional ELCOM grid used for the Lake Mead simulation. This work involves the setup and application of the models for Boulder Basin. Comparisons between measurements and simulation results show that ELCOM can accurately simulate the temporal and spatial variations of physical (e.g., temperature and conductivity), biological (e.g., chlorophyll-a and total organic carbon), and chemical (e.g., nitrogen and phosphorus) parameters. This study indicates that the hydrodynamic patterns of Boulder Basin are mainly driven by the Colorado River inflow, the Hoover Dam outflow, and meteorological parameters (especially episodes of high wind speed). However, the water quality of Boulder Basin is also affected by the load of nutrients (mainly phosphorus) from the Las Vegas Wash, which carries 3943 WEFTEC®.06
Clearwells are of vital importance in water treatment and distribution systems because they provide disinfection and storage capability to a water supply system. In order to meet drinking water regulations, adequate chlorine contact times in clearwells are necessary. In general, a water system has to meet a minimum C x t 10 value, where C is the disinfectant concentration and t 10 is defined as the time it takes for 10 percent of a tracer introduced at the inlet to exit from the clearwell. Another variable of some importance is the average residence time of the clearwell, t ave . To obtain the best results for a fixed volume tank (with the lowest C), t 10 /t ave should be maximized.Velocity differentials due to the presence of walls, baffles, short-circuiting, and small inlets and outlets generally lead to lower t 10 /t ave values. This ratio can be improved by reducing the size of recirculation zones after turns, carefully designing inlets and outlets, and separating the inflow from the outflow by baffling or other means.Much effort has been devoted to developing, understanding, and improving the predictive capability for the transport and mixing processes in clearwells, and their impacts on water quality. Regulatory requirements, customer expectations, and the desire to minimize water quality deterioration and be able to provide more reliable and safe operation have motivated this effort. A commonly used technique involves simulating the flow in a clearwell using computational fluid dynamics (CFD).CFD provides a rigorous two or three-dimensional hydrodynamic model for simulating chemical mixing and internal patterns of flow within clearwells. A CFD model explicitly considers basic equations governing the approximate motion of water. The resulting model is both accurate and robust, and can be readily applied to all types of clearwell configurations, characteristics, and hydraulic conditions.The basic concepts underlying CFD as applied to a clearwell consist of solving a set of conservation equations (mass, momentum, and energy) using the method of finite differences. The computational domain (i.e., the clearwell) is subdivided into small computational elements over which the conservation equations are solved. In general, a KEYWORDSWater quality, clearwells, computational fluid dynamics (CFD) modeling, shortcircuiting.
Dynamic modeling and probabilistic analysis techniques were used to analyze water quality impacts and to calculate proposed NPDES discharge permit limits for the discharge of secondary treated wastewater from the Sacramento Regional Wastewater Treatment Plant (SRWTP) to the Sacramento River near Freeport, CA. Because of the unique configuration of this discharge, standard models cannot be used describe dilution within the effluent plume downstream of the discharge, and standard methods of calculating effluent limits are inapplicable. Dynamic modeling was used to predict dilution within the plume downstream of the SRWTP diffuser over a 70-year hydrologic period, and a linked probabilistic analysis was used to evaluate water quality constituent concentrations and compliance with relevant water quality criteria. Extensive field studies have been conducted to calibrate and verify models (Fischer Delta Model) and to verify model results (FLOWMOD) used in these analyses. Dynamic modeling and probabilistic analysis techniques, when supported and verified by field data, offer a new and more accurate approach to evaluating water quality impacts and calculating effluent limits.
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