WEF's Disinfection 2005 Conference February 6 – 9, 2005 in MESA, AZ “Validation of Computational Fluid Dynamics (CFD) Modeling of Clearwells”
Abstract: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 × t 10 value, where C is the disinfectant concentration and t10 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, tave . To obtain the best results for a fixed volume tank (with the lowest C), t10/tave should be maximized.
Velocity differentials due to the presence of walls, baffles, short-circuiting, and small inlets and outlets generally lead to lower t10/tave 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 smaller grid size provides more accurate results. Inputs to the model consist of a geometrical description of the clearwell and a set of initial and boundary conditions (e.g., inlet/outlet velocities and geometry). The model output produces spatial and temporal solutions for the variables (pressures, velocities, and temperature) in graphical and tabular forms.
This presentation will highlight several clearwell studies in which CFD modeling was successfully validated by field tracer studies. The methods of analysis and results from both the modeling and field studies will be presented and discussed for several clearwells with a range of dimensions, flow rates, and configurations including:
Rectangular clearwell in Salt Lake City, UT: This clearwell is approximately 150 ft long and 78 ft wide with a depth of 5 ft. The average flow rate is 5.5 million gallons per day (MGD) and the average residence time, tave, is less than two hours. The clearwell contains three impervious baffles that are each approximately 130 feet long.
Rectangular clearwell in Oakland, CA: This rectangular clearwell is approximately 160 ft long and 145 ft wide, and has a depth of about 17 ft. The flow rate is 40 MGD and tave is less than two hours. The clearwell contains four solid baffles and four porous cross-baffles.
Trapezoidal clearwell in Tempe, AZ: The dimensions of this clearwell are approximately 450 ft long, 330 ft wide, and 24 ft deep. The average flow rate is 100 MGD and tave is almost five hours. There are two impervious baffles that are each approximately 240 feet long.
Document Type: Research Article
Publication date: January 1, 2005
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