Imhoff Replacement Project for the Stickney Water Reclamation Plant

The Stickney Water Reclamation Plant is one of the world’s largest wastewater treatment plants, treating up to 1.44 billion gallons per day for over 2.3 million people. Faced with plant’s existing 108 Imhoff Tanks reaching the end of its useful life, the Metropolitan Water Reclamation District of Greater Chicago implemented a major infrastructure project to replace the aging Imhoff tanks and grit facilities with state-of-the-art technology – improving operation and efficiency; reducing maintenance, energy, and operating costs; and controlling odors, while providing flexibility to meet future capacity needs and regulations and to reduce its carbon footprint and serve as a good neighbor to area communities. 

The initial design for the replacement of the preliminary and primary treatment included the demolition of the Imhoff Batteries A and B and existing Skimming Tanks 1-8 to allow for the planned construction of 18 primary settling tanks (PSTs) and new aerated grit facilities featuring state-of-the-art technology. The new facilities were designed to improve operation and efficiency, reduce maintenance, mitigate odors, and provide flexibility for meeting future capacity needs and regulations, as well as to advance MWRDGC’s initiatives to reduce greenhouse gases, recover resources, achieve energy neutrality, and demonstrate its commitment to serving as a good neighbor to area communities. 

The completed replacement included the construction of nine 160-foot diameter PSTs along with Tunnel Access Pumping Stations located over new operating galleries. Six 132-foot long aerated grit tanks, and other associated support facilities, including grit classifiers for separating and dewatering grit for final disposal, were constructed. The six fine screens for the Westside Plant that remove debris from damaging downstream equipment and piping were also replaced due to age.

A process model was developed for the Stickney WRP to evaluate the size and number of the new PSTs using empirical data from Calumet WRP. Based on the primary sludge settling and removal rates from the Calumet WRP and the process model results, it was determined that only nine PSTs were required to meet the demands of current wastewater flows and characteristics, rather than the 18 tanks originally planned for the upgrade. As a result, the capital cost for this project was reduced by approximately $160 million and the third phase of the project was no longer needed; however, it could be constructed if wastewater flows or regulatory requirements change in the future. 

To provide flexibility for future regulatory and process changes, the system was also designed to accommodate chemically-enhanced primary treatment (CEPT) in the future. The airlift pumps and sludge piping were sized for CEPT. Six of the nine new PSTs were provided with bypass structures. The bypass structures allow degritted influent to fully bypass primary treatment and flow directly to the secondary process to provide a carbon source for biological phosphorus removal. The PST bypasses will minimize the need to provide an additional carbon source and provide MWRDGC with flexibility for the implementation of biological nutrient management at Stickney WRP with minimal capital investment.  

Project Highlights: 

• Recognized with an Honor Engineering Excellence Award by the American Council of Engineering Companies of Illinois 
• State-of-art- technology to upgrade one of the world’s largest wastewater treatment facilities 
• New primary settling tanks and facilities increase biogas production from primary sludge an reduce greenhouse gas emissions 
• Reduced carbon footprint