Combined Sewer Overflows

Draft General NPDES Permit No. ILM580012 for Discharges from Combined Sewer Overflows (CSOs)
Combined Sewer Overflow Operational and Maintenance Plan Summary
Pollution Prevention Plan

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. A portion of the Village of Wilmette’s combined sewer system east of Ridge Road is diverted to the North Shore Channel by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) after their “Deep Tunnel” is full and can no longer accept any more sewage. The Village has no control over the diversion. A combined sewer system is designed to collect a mixture of rainfall runoff, domestic and industrial wastewater in the same pipe for conveyance to a waste water treatment plant. A CSO may occur during heavy rainfalls when the inflow of combined wastewater exceeds the capacity of the MWRDGC deep tunnel and the wastewater treatment plant. The CSO outfalls to the waterway act as relief points for the excess flow in the sewers, thereby reducing the frequency and severity of sewer backups and flooding.

Why are Combined Sewer Overflows (CSO) important to me?

Although a CSO may contain highly diluted sewage that could include bacteria which may cause illness, they may also cause temporary water quality degradation in the waterways. Regardless of a CSO activity, the Chicago Area Waterways may contain disease-containing bacteria at any time. Therefore, swimming, canoeing, or other activities where immersion in water is possible should be avoided and is not recommended, particularly during and immediately following rainfall.

Potential Combined Sewer Overflow (CSO) event areas

There are a total of six potential CSO locations along the North Shore Channel (NSC) in the Village of Wilmette. The MWRDGC has one CSO location numbered #101 near Sheridan Road. The other five potential CSO locations are visually monitored by the Public Works Department on a monthly basis. The Village has posted caution signs at each of the potential CSO locations listed below:

  1. West side of the NSC at Sheridan Road – 96” Pipe
  2. East side of the NSC at Sheridan Road – 12” Pipe
  3. Between Girard and Maple on the East side of NSC – 12” Pipe
  4. East side of the NSC at Laurel Avenue – 15” Pipe
  5. East side of the NSC at Laurel Avenue – 12” Pipe

What is being done to reduce the occurrence of Combined Sewer Overflows (CSO)?

The MWRDGC’s ongoing Tunnel and Reservoir Plan (TARP) Project was implemented to alleviate the polluting effects of CSOs and to provide relief from local flooding by providing holding capacity for 18 billion gallons of combined sewage in its tunnels and reservoirs until it can to be pumped to the water reclamation plant for full treatment. Although the reservoir portion of TARP is scheduled for completion in stages between 2013 and 2023, significant benefits have already been realized. It is estimated that since the first of the tunnels went on line in 1985 until 2003, more than 741 billion gallons of CSOs have been captured and conveyed to the water reclamation plants for full treatment. Since TARP went on line, the waterways have seen an increase in both the fish population and number of species present; basement and street flooding has been reduced; and there are fewer floodwater discharges to Lake Michigan. To date, more than $2 billion have been spent on the project. In addition to TARP, the District maintains and operates its collection system to maximize storage and optimize transportation of combined sewage to the treatment plants. This is accomplished by conducting a regularly scheduled prograon lineaintenance which includes sewer inspection, cleaning, videotaping activities, and inspection of diversion and bypass structures.

Who treats the sewage?

The MWRDGC receives and treats the sewage from the Village of Wilmette. If you observe a dry weather discharge through a CSO outfall or witness someone dumping wastes into our waterway or sewer system, contact the District’s hotline at (800) 332-DUMP.

Where are the Combined Sewer Overflow (CSO) events posted?

The District has developed an electronic “Address Book” containing a list of email addresses of interested parties. The Address Book will be updated on an as-needed basis as other members of the affected public are identified and members of the public are able to sign up to receive e-mail notification of CSO events by accessing the District’s website ( These parties will be sent an email alert in the event of a known CSO or diversion to a waterway.

Why do floodwater discharges to Lake Michigan occur?

During extremely heavy rainfall in the Chicagoland Area, storm runoff empties into the waterways system causing the water level to rise. The water level may rise to a level sufficient to submerge the CSO outfalls, thereby reducing the rate of discharge from the outfall. This can result in basement backups and local flooding. The discharge of floodwaters to Lake Michigan occurs when the waterways reach high levels and threaten flooding of structures along the waterway and submergence of CSO outfalls. Since the initial operation of TARP in 1985, the number of times that floodwaters are discharged to Lake Michigan has been reduced. When TARP is fully complete in 2023, the number will decrease further.

How can the public reduce CSOs?

During periods of high flow, every gallon of wastewater and stormwater kept out of the sewer system is a gallon that will not add to a CSO discharge. Examples of ways to reduce the wastewater load include avoiding unnecessary water usage, toilet flushing, dishwashing, clothes washing, and showering. Additionally, individuals and businesses could install rain barrels/cisterns to collect rainwater runoff from their roofs. This water would be used for garden/lawn watering and similar uses, thereby reducing both the impact of heavy rain events and the use for potable water for non-potable uses.

Where can I get more information regarding CSO?