WORLD’S LARGEST WATER TREATMENT PLANT OPTIMIZES PROCEDURES BASED ON ALGOR FLUID FLOW RESULTS
|Nearly one billion gallons of water are processed on an average day at James W. Jardine Water Purification Plant, which is located next to Navy Pier in Chicago, Illinois. This plant and the South Water Purification Plant serve nearly 5 million consumers in the City of Chicago and 118 outlying suburbs. Engineers at the plant are using ALGOR’s complete suite of simulation tools to formulate modifications that improve the water purification process.|
Nearly one billion gallons of water are processed on an average day at the James W. Jardine Water Purification Plant in Chicago, Illinois, the largest water treatment plant in the world. This plant and the South Water Purification Plant serve nearly 5 million consumers in the City of Chicago and 118 outlying suburbs. With that volume of water to purify, the chemical treatment process must be as efficient and effective as possible. Recently, the Jardine plant needed to change the location at which activated carbon is added in order to increase its contact time with water. Engineers at the plant studied the water flow patterns in the intake area with ALGOR’s Fluid Flow Analysis software to find the optimal feed point at which to add the carbon so that dispersion time would be minimized.
Approaching this challenge with computer simulation tools diverges from the water industry’s standard practice of building scale models and performing tests in a laboratory. The use of ALGOR’s Fluid Flow Analysis software enabled water plant engineers to determine the best location for the activated carbon feed point in less time and with less expense than would have been possible with a traditional, laboratory testing approach.
The First Step in a Seven-Hour Water Purification Process
There are a number of steps in the 7-hour water purification process, which consists of chemical treatment with activated carbon, polyphosphate, chlorine, fluoride, alum (aluminum sulfate) and polyelectrolyte, followed by sedimentation and filtration. Each chemical additive serves a different purpose, such as killing bacteria, aiding in the removal of micro-organisms or preventing tooth decay. Activated carbon is the first chemical treatment, added to remove objectionable tastes and odors. The tiny carbon particles are tremendously absorbent, like a sponge. In order for it to do its job, the activated carbon must mix thoroughly within the water for as long as possible.
|Adding activated carbon to lake water is the first step of many in the 7-hour water purification process. The diagram above shows a generalized, schematic view of the process used by the City of Chicago Department of Water to purify Lake Michigan water. Activated carbon is used to remove objectionable tastes and odors and must mix thoroughly within the water for as long as possible in order to be effective.|
In the late 1990s, engineers for the City of Chicago Department of Water determined that the activated carbon feed should be moved to the intake basins through which lake water enters the plant. Each spring, plant workers drain and clean each of the two intake basins. Water plant workers have only a short window of time each year to clean the basins and make any needed modifications. The activated carbon feed line was extended and mounted on the inside wall of each intake basin, over a small ledge – a location that was selected for installation convenience.
This modification achieved the goal of increasing contact time that the carbon has with the water. Next, Department of Water Filtration Engineer Anthony Wietrzak, Ph.D., P.E. turned his attention to the challenge of optimizing the dispersion and mixing of carbon within the water. This challenge required Wietrzak to study the hydraulics of the water flow within the intake basin.
“The water industry’s standard practice for approaching such a problem is to build a scale model and perform tests in a laboratory,” said Wietrzak. “Unfortunately, this process is time-consuming and expensive. Chicago’s Bureau of Water Treatment engineering section at the Jardine Water Purification Plant acquired ALGOR’s Professional Multiphysics software package several years ago to provide computer simulation tools for just such a challenge.” In addition to the unsteady laminar fluid flow analysis capabilities Wietrzak would use for this project, the Professional Multiphysics package also offers static stress with linear and nonlinear material models, Mechanical Event Simulation with linear and nonlinear material models, steady and unsteady laminar fluid flow analysis with turbulence, steady-state and transient heat transfer analysis, electrostatic analysis, vibration analysis and the capability to consider the effects of multiple physical phenomena.
Pump Cells Analyzed to Determine Inputs for the Intake Basin Model
To analyze the flow pattern within the intake basin, Wietrzak had to start with the physical characteristic of the system that he knew quantitatively: the capacity of each of the pumps that pulls water from the intake basin into the water treatment plant and the physical dimensions of the intake area. Wietrzak did not want to assume that the flow was uniform coming out of the intake basin. That assumption could result in an unrealistic flow pattern within the intake basin. Thus, he would have to model the pump cells in addition to the intake basin. However, modeling both the pump cells and the intake basin would result in a very large model. Wietrzak was concerned that his computer hardware would not process such a large model and achieve solution convergence in a reasonable amount of time. Therefore, Wietrzak decided to model and analyze one pump cell and the intake basin separately. The pump cell model results at the boundary between the intake basin and the pump cell would determine the input for the intake basin model.
Wietrzak began by modeling the volume within a pump cell in Superdraw III, ALGOR’s precision finite element model building tool. The pump cell model resulted in 6,492 solid “brick” elements. Wietrzak applied zero velocity constraints to the surfaces on the walls of the pump cell. To the free surface at the top of the pump cell, he applied a zero shear constraint. Next, Wietrzak converted the pump capacity of 300 million gallons per day to a volumetric flow rate in feet per second. The flow rate was then applied to the pump cell outlet as a velocity boundary constraint. No constraints were placed on the inlet to the pump cell.
An unsteady fluid flow analysis was performed on the pump cell model with the applied velocities ramping up over 50 time steps. Performing an unsteady fluid flow analysis and ramping up the velocities over time facilitated convergence.
Wietrzak first analyzed one of the pump cells that pulls water out of the intake basin and into the water treatment plant. The known pump capacity was used as the input to the analysis in order to find the velocity profile at the inlet to the pump cell. The sketch shows where the water enters and exits the pump cell (upper left) while the velocity contour resulting from the ALGOR fluid flow analysis shows the fluid dynamics within the volume of the pump cell.
“The largest assumption I made for the pump cell model is that the flow is uniform coming out of the pump,” said Wietrzak. “I ran several variations of the pump cell model in which I varied the outlet velocity constraints at the pump discharge and none made a significant difference in the velocity profile at the inlet to the pump cell. All models yielded higher velocities at the top of the entrance and lower velocities near the bottom. Since I was concerned only with the velocity profile boundary condition at the outlet of the basin (which is also the inlet to the pump cell), I am satisfied that this is a sensible assumption.”
Running several variations on the pump cell model gave Wietrzak the opportunity to experiment with different model constraint techniques and gauge the effectiveness of those techniques in terms of model convergence. “I know the theory and how to handle problems with a textbook approach. However, there were several constraint techniques that I needed to learn in order to run this model and get realistic results,” said Wietrzak, who holds a Ph.D. in fluid dynamics from Northwestern University. “For example, the free surface boundary condition had to be properly applied in order to get the solution to converge. ALGOR’s technical support team was very helpful in answering any questions that arose during the solution process.”
Intake Basin Model Reveals the Need to Change Activated Carbon Feed Point
Armed with the results of the pump cell fluid flow analysis, Wietrzak was ready to tackle the intake basin model. He began by modeling the volume within the basin in Superdraw III, a model which resulted in 8,138 solid “brick” elements. As with the pump cell model, Wietrzak applied zero velocity constraints to the surfaces on the walls of the basin and zero shear constraints to the free surface. The velocity results of the pump cell analysis were then applied to the area where the basin connects to the pump cells. An unsteady fluid flow analysis was performed on the intake basin model with the applied velocities ramping up over 50 time steps.
|The simulated velocity profile at the inlet to the pump cell was applied to the intake basin model. The sketch shows where the water enters and exits the intake basin (upper left); the location of the original activated carbon feed point (Add Point 1); and the location of the new feed point based on analysis results (Add Point 2). The velocity contour resulting from the ALGOR fluid flow analysis shows the fluid dynamics within the volume of the intake basin.|
Wietrzak then studied the simulated flow patterns by slicing through the model layer by layer and viewing the flow patterns throughout the model. He also used animated analysis replays to see how the solution progressed over time. He discovered a recirculation pattern next to the wall, very near to the current feed point. “The feed point is located above a narrow ledge,” describes Wietrzak. “The flow pattern predicted that the carbon would tend to be dragged along the ledge rather than mixing quickly and thoroughly with water throughout the volume of the basin. Our observations of the carbon deposits qualitatively confirmed the fluid flow analysis results.”
Wietrzak therefore determined that the activated carbon feed pipe should be moved out between 5 and 10 feet from the wall in order to maximize mixing. This modification was implemented in one of the two intake basins in the Spring of 2000.
Wietrzak has also used the ALGOR Multiphysics package to model other areas of the plant that may benefit from modifications. For example, since completing the activated carbon feed point project, Wietrzak has studied the effectiveness of the air scrubber system on exchanging the air in the chlorine battery room, from where chlorine is supplied for the treatment process. The scrubber releases caustic gas to neutralize airborne chlorine. Wietrzak used fluid flow analysis to find the “dead spots” in the air flow.
“We do not redesign everything, but ALGOR’s complete suite of simulation tools is enabling us to formulate modifications that improve the water purification process,” said Wietrzak. “The variety of simulation tools offered by the Professional Multiphysics package will enable us to study many phenomena on the computer and make many enhancements without the need for laboratory testing.”