Stormwater and Sewer Overflows

Although the Chicago River is cleaner than ever before, a number of significant water quality threats still remain. Chicago and our older suburbs have combined sewer systems that convey rainwater and sewage in the same pipes. Heavy rain events can overwhelm this system, flushing a mix of stormwater and sewage directly into the Chicago River, and—in extreme cases—back into Lake Michigan.

All of this effluent is managed by the Metropolitan Water Reclamation District of Greater Chicago (MWRD). On a dry day, MWRD has the capacity to treat 1.4 billion gallons of wastewater across its three main Chicagoland water reclamation plants—Calumet, O’Brien and Stickney. Water enters the plants where it is strained, separated, centrifuged and, at the Calumet and O’Brien facilities, disinfected.

Click here to learn more about MWRD’s water treatment process.

Heavy rains complicate the water treatment process. Research by the City of Chicago shows that as little as .67 inches of rain, depending on location and severity, can trigger a sewer overflow at any number of the more than 300 outfalls that flow directly into the Chicago and Little Calumet Rivers. Of course, stormwater is clean before it hits the pavement, but pollutants like oil and garbage contaminate the rainwater. This harmful first flush enters the sewer system for treatment along with household and industrial wastewater, where it is often diverted into the Chicago River.

To address sewer overflows, MWRD has built nearly 10 billion gallons of stormwater capacity across its Tunnel and Reservoir System Plan (TARP). TARP functions simply by holding stormwater in reservoirs until MWRD has capacity to treat it, effectively preventing the release of raw sewage into the Chicago River. MWRD estimates that TARP has eliminated at least half of all possible sewer overflows since 1980, and the system will expand to 18 billion gallons by 2029.

When it rains, the combined sewer pipes, which carry domestic, commercial, and industrial wastewater along with stormwater, frequently become overwhelmed.  An increase in sewer volume can trigger a sewer overflow and sewage ends up in the river.