INDUSTRY

BY J. WILLIAM BELL - Hydrosystems research on NCSA's Titan cluster aids the U.S. Army Corps of Engineers as it chooses aeration systems for a pair of massive reservoirs. About 25 million people visited Chicago's World Columbian Exposition in 1893. They streamed in from around the world, arriving by ship, locomotive, and "El" train. Because of the city's dubious drinking water and fear of a dysentery outbreak, their water was piped in from a Wisconsin spring. Chicago still faces its share of water woes more than a century later. The Metropolitan Water Reclamation District of Greater Chicago serves more than three million people with a combined sewer system. Rain water and the sanitary sewer share a single set of pipes, tunnels, reservoirs, and treatment plants. Though the facilities can treat about two billion gallons of water a day, a single large rainstorm can produce more than five billion gallons before a single sink is turned on or toilet is flushed, according to data from the water reclamation district. On the wettest days, the treatment plants can't keep up. Sewage is diverted into the area's waterways, and the Environmental Protection Agency levies fines. People go home to flooded basements. To avoid these problems, the U.S. Army Corps of Engineers and the water reclamation district are building a pair of reservoirs that will bring the district's total storage capacity to 15.6 billion gallons. (A third reservoir, with a capacity of more than 325 million gallons, is already complete.) The plan certainly holds water. But it's not without drawbacks, especially in a metropolis with an average of 12,000 people per square mile. "The reservoirs are in residential areas," says Heather Henneman, a Corps of Engineers hydrologist working on the project. "Can't be helped. Everything's populated in Chicago." The longer the water sits in open reservoirs, the more it begins to smell. Aeration systems, not unlike the one in your aquarium, can be used to oxygenate standing water and reduce odor. But an 87-acre reservoir is no mere fish tank. "Aeration at this scale is not common," says Fabián Bombardelli. As part of his PhD work at the University of Illinois at Urbana-Champaign, Bombardelli used NCSA's Titan cluster to model the plumes of air bubbles produced by aeration systems. The models are the first of their kind to consider not only the physics of aeration but also the amount of oxygen consumed by organic particles in the wastewater. It is used in conjunction with an existing Corps of Engineers model to help the Corps make decisions about the final design of the reservoirs. Bombardelli worked with Marcelo García, the Chester and Helen Siess Professor of Civil and Environmental Engineering at the University of Illinois, and Professor Gustavo Buscaglia of the Instituto Balseiro and Centro Atómico Bariloche in Argentina. An early description of the model can be found in the November 2002 edition of the International Journal of Multiphase Flow. An Aerobic Exercise Given the opportunity, organic matter in a reservoir will begin to break down via aerobic reactions, which generate carbon dioxide, carbonates, and other innocuous chemical species. In the absence of oxygen, anaerobic reactions take place. Products such as methane and sulfide hydrogen--which, in sufficient amounts, are sure to annoy the neighbors--result. Snapshots of velocity magnitude (left) and vorticity (right) in the aeration simulations. Vorticity values are multiplied by 1,000. Aeration is a simple way to prevent anaerobic reactions. Just blow air bubbles into the reservoir. The bubbles carry a high concentration of oxygen. The water that surrounds them carries a lower concentration. The process of mass transfer forces oxygen out of the bubbles and into the water. Provide enough oxygen, and you get nothing but aerobic reactions. The Corps of Engineers is comparing two possible means of aeration, one submerged and one that sits on the surface. Bombardelli's models help the Corps hash out the fine points of a submerged system. Under their current scheme, the 7.3-billion-gallon McCook reservoir would include a total of 2,140 submerged diffusers each blasting between 16 and 23 cubic feet of air per minute into the water. The Corps of Engineers has to position the diffusers, decide when to turn them on and off, and reckon how the resulting plumes of bubbles will interact. They also want to know how efficiently the system will operate when the reservoir is filled to different levels. This figuring currently relies on relatively small-scale experiments, historical data, and educated guesswork. "Most mass transfer experiments are limited to the depth of storage tanks at sewage treatment plants," which are typically less than half the depth of the Chicago reservoirs, according to Henneman. With the computer models, however, the Corps of Engineers can analyze various design configurations in greater detail. "They are asking very ambitious questions," says Bombardelli, who is now an assistant professor of civil and environmental engineering at the University of California at Davis. By the end of the year, the Corps of Engineers intends to use answers to those questions to pick which type of aeration system they will use in the reservoirs. Losing Some Bulk Early bubble models in environmental engineering were not very sophisticated affairs. They showed only the bulk parameters of the plumes and only on a vertical axis. "You saw how the plume spread and how fast but comparatively little about the bubbles themselves," Bombardelli says. His new simulations, however, rely on a set of submodels to create a much more complicated picture. After months of experimenting with different turbulence models and comparing the quality and efficiency of their results to experimental data, Bombardelli can now create some of the most-detailed, highest-resolution views of bubble plumes available. It takes about seven hours of computing time on 20 processors of NCSA's Titan cluster to simulate nine seconds of action. The models track the buoyancy, drag, and turbulent dispersion of bubbles in three dimensions as the bubbles collide, break apart, and coalesce. The mass transfer of oxygen and nitrogen out of the bubbles is also replicated. Finally, a biochemical oxygen demand model simulates how much oxygen is consumed by organic material in the water, how this demand varies in different parts of the plume, and how it changes over time. All of these features are critical to understanding the rate and degree of oxygenation caused by the aeration process, and all give the Corps of Engineers information that it can't easily derive from experiment. "And [computational models] are odorless, which is an accomplishment in this field," says the University of Illinois' Marcelo García. Joking aside, he is pleased with the way the team has been able to build a new, powerful tool by combining knowledge from a variety of existing fields. "We've borrowed from chemical engineering, nuclear engineering, computer science, and mechanical engineering and brought them all into environmental engineering." This research is supported by the U.S. Army Corps of Engineers. Team members: Fabián Bombardelli Gustavo Buscaglia Marcelo García