Combined Sewer Overflows

The Chicago region’s combined sewer system conveys both sewage and rainwater in the same pipes to our wastewater treatment facilities. This system functioned sufficiently before climate change increased precipitation in the Midwest, but bigger storms overwhelm the treatment facilities, which then dump untreated sewage into the Calumet and Chicago River systems and—in cases of extremely heavy rainfall—back into Lake Michigan. Friends calls for a zero tolerance policy for these combined sewer overflows (CSOs).

On a Dry Day

All of our wastewater 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. Learn more about MWRD’s water treatment process.

When it Rains, it Overflows

Heavy rains disrupt the water treatment process. Research shows that as little as 0.3 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 litter contaminate rainwater. This polluted rainwater then combines with domestic, commercial, and industrial wastewater into the combined sewer pipes, which frequently become overwhelmed.  An increase in water use at home or the office can trigger a CSO, and sewage ends up in the river.

To address sewer overflows, MWRD has built over 17 billion gallons of stormwater capacity across its Tunnel and Reservoir 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.

What can you do to keep our river clean?