DESIGN AND OPERATION OF THE O'HARE CUP RESERVOIR: CSO CONTROL BECOMES A REALITY

Under the Metropolitan Water Reclamation District of Greater Chicago's (District) Tunnel and Reservoir Plan (TARP), the O'Hare Chicagoland Underflow Plan (CUP) Reservoir was the first of the District's CUP Reservoirs to be completed and was dedicated in July 1998. As part of the Upper Des Plaines TARP system, the reservoir provides relief from sewer backup flooding for more than 21,000 homes and businesses in combined sewer areas of Arlington Heights, Mount Prospect, and Des Plaines. The reservoir provides 343 million gallons of floodwater storage capacity. It will prevent more than 2.3 million in average annual flood damage in a 7.5 square mile combined sewer area containing a population of 61,000.In 1972, the District's Board of Commissioners designated TARP as the Chicago area's plan for cost effectively bringing Chicago area waterways up to Federal and State water quality standards. TARP is designed to prevent combined sewer overflows and waterway backflows to Lake Michigan by intercepting the excess combined sewer flow and storing it until it can be pumped to water reclamation plants for treatment.TARP consists of four tunnel systems and three reservoirs: the Mainstream and Des Plaines Systems, which will be served by the McCook Reservoir; the Calumet System, which will be served by the Thornton Composite Reservoir; and the now completed Upper Des Plaines System, which is served by the O'Hare CUP Reservoir. Under Phase I of TARP, tunnels were to be constructed, providing capture of the “first flush” during storms and the corresponding pollution control benefits. Under Phase II, large reservoirs were to be constructed, augmenting the storage volume of the tunnels and providing flood control benefits.The total cost of the O'Hare CUP Reservoir was 44 million. The U.S. Army Corps of Engineers (Corps) designed and built the reservoir and financed 75 percent of the total project cost. As the local sponsor with the Corps, the District provided 25 percent of the total cost, which included land acquisition and utility relocations.All inflow and outflow from the O'Hare CUP Reservoir is by gravity. The rate of outflow is remotely controlled by remote operation at the District's Kirie Water Reclamation Plant (WRP) where it is treated prior to discharge into Higgins Creek. The Kirie WRP has a design average flow of 72 MGD, design maximum flow capacity of 110 million gallons per day (MGD), and maximum hydraulic capacity of 145 MGD. Annual average daily flow is currently approximately 38 MGD. The amount of time required to empty the reservoir may be a function of several variables including 1) the excess capacity available at the Kirie WRP; and 2) inflow to the tunnel/reservoir system subsequent to fill events. Upon draining, the sediments remaining from the combined sewage are sluiced from the reservoir into the tunnel by contracted cleaning services.An aeration system prevents offensive odors associated with storing combined sewage in the reservoir. The aeration system consists of nine 100-horsepower floating mechanical aerators and is designed to maintain aerobic conditions in the top 15 to 20 feet of water.Since its inception and through November 2001, the O'Hare CUP Reservoir received inflow from the adjoining TARP tunnel system 12 times, capturing a total volume of approximately 1.261 billion gallons that would have otherwise backflowed within the combined sewer area.This paper will review the design concepts for the O'Hare CUP Reservoir and actual operating experience gained to date. Issues such as volume captured, odor generation, impact on Kirie WRP operation, economic flood control benefits, and post-fill cleanup will be addressed.