What is a Combined Sewer Overflow (CSO)?

A CSO is the release of untreated waste and storm water from a combined sewer system directly into a waterway.  The release of untreated water occurs a a CSO structure.  The Village's combined sewer system contains six (6) overflow structures.  Four (4) structures can discharge to Weller Creek and two (2) can discharge to Feehanville Creek.  Most of the time, the structure receives wastewater from the Village, and allows if to fall into a large interceptor sewer owned and operated by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC).  The MWRDGC interceptor then transports the wastewater for treatment at the North Side Wastewater treatment plant in Skokie or the Kirie Wastewater treatment plant in Des Plaines.

During wet weather, the interceptor system can become inundated with storm water.  At such times, the interceptor can be relieved by discharging flow to the Tunnel and Reservoir Plan (TARP or "Deep Tunnel").

During severe events, both the interceptor and Deep Tunnel can be filled to capacity.  When this occurs, the CSO structures can discharge flow into adjacent waterways.  Arguably, most of the water discharged into streams in this manner is storm water that does not need to be treated.  However, it is also undeniable that at least a portion of the discharge contains sanitary wastewater.

Why are CSOs important to me?

Although most water discharged into streams is storm water that does not need to be treated a small percentage of discharge contains sanitary wastewater.  Therefore, swimming, canoeing , or other water activities should be avoided during and immediately following a significant rainfall.

CSO Overflow Structure locations

There are a total of  six (6) CSO locations in the Village.  Four (4) structures can discharge to Weller Creek and two (2) can discharge to Feehanville Creek.  The six (6) CSO locations are visually monitored by Public Works staff on a monthly basis or following rain events.  The Village has posted caution signs next to all CSO locations.

Where can I find out about CSO events?

The MWRDGC posts records of all known CSO events on their website MWRDGC CSO Event Notice.

In the interest of avoiding a redundancy of efforts, the MWRD has invited TARP municipalities to use the MWRD Public Notification Plan for their public notification compliance. The MWRD has developed an opt-in public notification system. Members of the public are able to sign up to receive e-mail and/or text notification of CSO events by accessing the MWRD CSO Notification Registration.

Where can I learn more about CSOs?

You can refer to the Village's CSO Operation and Maintenance Plan and the Pollution Prevention Plan, which are available under the documents section found at the top of the page.