Combined Sewer Overflows
The Chicago region’s combined sewer system conveys both sewage and stormwater through the same pipes to wastewater treatment facilities, where the water is typically treated before being released into local waterways. During larger storms, however, the system can get overwhelmed, causing untreated sewage and stormwater to discharge directly into the Chicago-Calumet River system. These events are known as combined sewer overflows (CSOs).
In the 1970s, CSOs were estimated to occur once every three days, and the river knew no relief from untreated sewage discharges. Thanks to Metropolitan Water Reclamation District of Greater Chicago (MWRD)'s construction of the Tunnel and Reservoir Plan (TARP or Deep Tunnel), a system of large tunnels and vast reservoirs to reduce flooding and capture water, CSOs are much less frequent. However, overflows still occur during the heaviest rain events, and with climate change, storms are becoming more intense than even TARP is designed to handle.
Friends of the Chicago River calls for a zero tolerance policy for combined sewer overflows through:
- The completion of TARP
- Large-scale adoption of green stormwater infrastructure
- More stringent NPDES permits to require stronger controls and monitoring
- Taking individual action with Overflow Action Day Alerts™
On a Dry Day
For most of the Chicago-Calumet River watershed, wastewater is managed by MWRD. On a dry day, MWRD has the capacity to treat 1.4 billion gallons of wastewater across its water reclamation plants. Water enters the plants where it is cleaned before being released into the river system.
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-Calumet River system. Stormwater may be clean before it hits the pavement, but pollutants like oil, road salt, and litter contaminate rainwater. This polluted water then combines with domestic, commercial, and industrial wastewater into the combined sewer pipes. All of this pollution goes straight into the river when sewer overflows happen.
Overflows harm people, water, and wildlife by:
- Releasing untreated sewage, bacteria, and pathogens into the river. During CSOs, stormwater mixed with raw sewage bypasses treatment and enters waterways, introducing pathogens that can cause diseases in people who kayak, fish, or contact the water.
- Depleting oxygen needed by fish and aquatic life. Organic waste in sewage fuels microbial decomposition that consumes dissolved oxygen, creating low-oxygen conditions that stress or kill fish and invertebrates.
- Introducing toxic pollutants that harm wildlife. CSOs can contain metals, oil and grease, pharmaceuticals, and household and industrial chemicals that impair fish health, reproduction, and development.
- Smothering habitat and aquatic food webs with litter. Litter from roads and other solids in overflow discharges can settle on riverbeds, degrading spawning habitat and benthic communities that support fish and wildlife.
What can you do to keep our river clean?
Sign up for our Overflow Action Day Alerts™, which email reminders to conserve water and decrease the amount going into the system and frequency of CSOs.
