Water Online

November 2015

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Measuring Innovation: Chicago is giving its oldest wastewater treatment facility a disinfection facelift and using the world's largest reservoir to curb combined sewer overflows. By Allison Fore W hen the Metropolitan Water Reclamation District (MWRD) of Greater Chicago announced its latest initiative in wastewater treatment this summer, boaters, water skiers, and swimmers rejoiced. This summer's achievements were an added layer in the grand process of water treatment for the billions of gallons of water that pass through MWRD facilities every year and a momentous occa- sion in the long-term quest to protect water along the Chicago Area Waterway System (CAWS), an attraction that is drawing unprec- edented demand. In July, the MWRD introduced disinfection via chlorination and dechlorination to its treatment process to reduce the amount of pathogenic bacteria in the water released from the Calumet Water Reclamation Plant (WRP) into the Calumet River sys- tem. The Calumet WRP serves more than one million people in a 300-square-mile area covering the south side of Chicago and surrounding south suburbs. In opera- tion since 1922, the Calumet WRP is the oldest of the MWRD's seven plants. Calumet WRP treats an average 354 MGD and a maximum 480 MGD, enough to fill every floor of Chicago's Willis Tower. Using an exist- ing chlorine contact chamber retrofitted for more efficient contact, the disinfection technologies neutralize or kill bacteria and other micro-organisms in treated water. The goal is to improve water quality and reduce health risks resulting from contact with the water. "I appreciate the ingenuity that went into this project from staff at the District and the investment made by our area leaders who made this possible," said U.S. Senator Dick Durbin. "Together, we have chosen to make our waterways a priority and take an impor- tant step in making our goal of a cleaner environment a reality." The road to the Calumet WRP ribbon-cutting ceremony began in 2011, when MWRD officials instituted an internal blue ribbon task force to evaluate available disinfection technologies. The task force devoted eight months to research and testing to determine optimal solutions for disinfecting at the lowest cost. On March 1, 2012, the most cost-effective processes were determined to be chlorination/dechlorination for Calumet WRP and UV disinfec- tion with low-pressure high-output lamps for O'Brien WRP in Skokie, IL. Adding The Final Chapter Disinfection occurs after wastewater passes through a series of treatment processes. Primary treatment consists of remov- ing any large material by physical means, including screening debris. Water then flows into chambers where heavy solids, such as sand and grit, sink to the bottom. The water then moves to settling tanks, and a significant portion of the organic solids settles to the bottom, while fats, oils, and grease (FOG) rise to the top. FOG is skimmed from the top, and the untreated sol- ids are scraped from the bottom of the tank and treated further. With the "settle-able solid" material removed, the flow proceeds to secondary treatment. During secondary treatment, a large population of micro-organisms consumes the remaining dissolved organic material. The water flows through a series of large rectangular aeration tanks that are seeded with bacteria and other microbes. Filtered air is pumped through the liquid to enable the microbes to breathe and grow. The microbes flourish and multiply, eating t h e re m a i n i n g organic materials and nutrients in the constantly churning water. This mixture of microbes and water flows into a secondary settling tank. The microbes clump together and settle to the bottom of the tank where they become part of the organic residuals and are removed. Approximately 85 percent of these microbes are recycled to the start of the aeration tanks to continue the biological treatment process. Lastly, the new disinfection process will work to destroy any remaining bacteria in the water. Engineers modified the exist- ing chlorine contact basin by replacing all interior baffle walls and associated walkways; replacing weir gates, discharge gates, drain sluice gates, inlet sluice gates, and a bypass sluice gate; replacing liquid sodium hypochlorite diffuser piping; installing liquid sodium bisulfite diffuser piping; and installing sampling pumps. During this last stage of treatment, the water flows through a labyrinth of tunnel-like passages in the concrete con- tact basin, allowing time for the chlorine to kill off the harmful pathogens. The water is then dechlorinated and empties into the nearby Little Calumet River. The reclaimed water has more than 99 percent of the impuri- ties removed and is deposited into the Little Calumet River and 8 wateronline.com n Water Innovations Greater Chicago: Historic Infrastructure Projects Enhance Windy City Water Quality MWRD was able to do what was needed for less than half the cost of the original estimate.

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