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CSO Improvement Project – Cottage Grove Heights

Project Specifications:

Location: Chicago, IL

Application: Storage, Combined Sewer Overflow

Product Used: 5’-8’’ SingleTrap

Number of Basins: 5

Number of Pieces: Basins 1,2,3,4: 58 units each | Basin 5: 48

Total Water Stored: 184,514 cf


Consulting Engineer: Alfred Benesch & Company

Installing Contractor: MQ Construction



Five StormTrap basins were installed in the Cottage Grove Heights neighborhood as part of a combined sewer overflow system for the City of Chicago Department of Water Management.


The existing combined sewer system that runs through the neighborhood was exceeding its capacity during heavy rain events and causing recurring flooding issues, and several residents experienced flooded basements.


“During heavy rains, the runoff overloads the existing 30’’ sewer and mixes with untreated sewage. If the flow rate of the combined sewage exceeds the capacity of the system, it can back up into nearby homes,” explains William L. Schmanski, PE, Project Manager at Alfred Benesch & Company.


The engineering firm contacted StormTrap to assist on this project to add offline storage to upgrade the local combined sewer system. The StormTrap team designed five 5’-8’’ SingleTrap systems that were installed beneath the boulevard of E 97th St between S Ellis Ave & S Avalon Ave. The systems take on storage capacity of combined sewage through a 24’’ diversion pipe during high-intensity rainfall events, minimizing the impact on the downstream sewer system.


The SingleTrap systems were customized for this project. All basins were installed on a sloped concrete slab that incorporates a trench. In the middle of the trench, there is a 4’ precast sump, where the outlet pipe is located, which allows water to drain easily. The systems also have windows on the interior walls of the StormTrap units, which allow for water to flow in both directions. The five systems together are capable of storing 184,514 cf of water and sewage. The tanks are watertight to prevent sewage from contaminating groundwater.


Working closely with Alfred Benesch & Company, MQ Construction, and the City of Chicago Department of Water Management, the project was successfully completed in July 2019, meeting construction timelines and effectively solving the flooding problems. All five basins were installed within two months.


“StormTrap offered a great solution for the project,” stated the project engineer. “The systems allowed for maximizing sewage storage volume while minimizing the project footprint and with customized sloped concrete slab and trench to allow for a complete drain at following the storm.”