Combined Sewers

Combined Sewer Overflow

To avoid flooding, Chicago built a stormwater conveyance system in 1856. Like most cities in this area, Chicago built one underground system that combines both wastewater and stormwater and moves them away from people toward treatment plants. This combined system is large enough to easily handle the city and suburban wastewater that needs to be treated. In fact, the volume of wastewater is so small compared to the stormwater, it is not even considered in designing the size of our sewer system. When there is too much stormwater, the combined sewers overflow and release untreated waste and stormwater into the Chicago River. This practice harms the health and habitat of the river. Homeowners occasionally experience this excess stormwater as flooding in their basements. By investing in green infrastructure, the City will help to reduce the amount of water flowing to the sewer system.

When it rains, some of the stormwater that falls in our neighborhoods soaks into the ground and some flows into the City’s sewer system. With more and more hard surfaces, such as rooftops and roadways, there are fewer and fewer places where rain water can infiltrate the soil, nourish plants and remain part of the natural system.

Without greenspace to absorb it, the sewer system is required to handle more and more water. Stormwater sent to our sewers is no longer available to irrigate our lawns or recharge groundwater. Further, when the sewer system becomes full it discharges into our waterways.

The City of Chicago recognizes the importance of the built infrastructure in terms of managing stormwater. The City’s Department of Water Management spends approximately $50 million per year to clean and upgrade 4,400 miles of sewer lines and 340,000 related structures. Additionally, the City acknowledges the importance of the Tunnel and Reservoir Plan, known as Deep Tunnel, in the long-term management of stormwater.

However, the City believes that the “built” infrastructure alone will not meet all of our needs for managing wastewater and stormwater. Managing stormwater and protecting the quality of our water resources will require a combination of upgrading our “built” infrastructure and creating a “green” infrastructure. Through this green infrastructure, the City will demonstrate forward-thinking ways to reduce the burden on our sewer system and keep stormwater in the environment.

More Background on Chicago’s Combined Sewers

Role of the Metropolitan Water Reclamation District

The city's main sewers convey flow to interceptor sewers. These interceptor sewers are owned and operated by the Metropolitan Water Reclamation District of Greater Chicago

(MWRDGC). The interceptor sewers convey DWF to MRDGC's treatment plants for treatment and release to local waterways. During storm events, flows in excess of the capacity of the interceptor sewers discharge into the MWRDGC's Tunnel and Reservoir Plan (TARP) system for storage. The purpose of TARP is to prevent combined sewer overflows to the city's waterways. The TARP tunnels are generally located 200 feet below the Chicago River system. If the capacity of the tunnels is exceeded during extreme rainfall events, combined sewerage overflows into the Chicago River system. The reservoir phase of TARP will prevent these overflows from occurring. The reservoirs will be completed in stages between 2007-2019.


The Department of Water Management cleans and maintains all city sewers and ancillary structures (manholes, catch basins and inlets) within the public right-of-way. There are about "4,400 miles" of sewer and 340,000 ancillary structures within the sewer system. About 50 million dollars is appropriated each year towards the cleaning and rehabilitation of these facilities. On the average, all sewers and ancillary structures are cleaned once every 4 years.

Water Management

New Construction

The Department upgrades the sewer system with about 30 million dollars worth of its own new construction each year. The department also evaluates the need for new sewers in street improvement projects initiated by the state, county and the Chicago Department of Transportation. These relief sewers or sewer enlargements projects are constructed where existing sewers do not meet design standards.

Design Standards

The city sewer system is designed to accommodate a storm event of a 5-year magnitude. The DWF is minimal compared to storm water flows. Therefore, sanitary loads are usually not considered in the design of city sewers. In terms of rainfall accumulation, a 5-year storm event is equivalent to about 1.2-inches in 20 minutes or about 1.8-inches in 1 hour. Rainfall intensities in excess of these will cause the sewer system to surcharge. The probability that a storm event of a 5-year magnitude or greater will occur is about 20 percent in any one year, or 50 percent over a 3 year period, or 67 percent over a 5 year period.

The size and slope of a sewer determines its 5-year flow capacity. The city sewers are designed with a slope to maintain a minimum flow velocity of 3 feet per second under full flow conditions by gravity. This is the minimum velocity needed to keep the sewers clear of dirt and debris. A maximum slope is also considered in the design to prevent excessive flow velocities.

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