Combined Sewer Overflow

Combined sewer systems are sewers that are designed to collect rainwater runoff and wastewater from homes and businesses in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a Water Resource Recovery Facility (WRRF), where it is treated and then discharged to a water body. However, during periods of heavy rainfall or snowmelt, the wastewater volume in a combined sewer system can exceed the capacity of the pipes or the WRRF. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers or other water bodies. These overflows, called combined sewer overflows (CSO), contain not only storm water but also untreated wastewater and debris.

For many years, the State of Michigan has been requiring municipalities to control their CSO. The City of East Lansing mainly did so by the construction of an in-line retention tunnel and a retention treatment basin (RTB) for the older areas of the City that have combined sewers. Instead of the combined sewers overflowing directly to the river during a wet weather event, they overflow into the retention tunnel that flows to the RTB. The tunnel and the basin provide 7.5 million gallons of storage. With this storage, only a large wet weather event (rain and/or snow melt) will cause the RTB to overflow to the river after treatment. The treatment includes settling, skimming and disinfection with sodium hypochlorite before discharging flow to the river.

There is one other chamber that can overflow to the river, but is designed only to do so if the wet weather event reaches a 25-year/24-hour measurement (approximately four inches of rain in 24 hours). 

Not every rain event will cause a CSO. In a typical year, the City of East Lansing will have only three or four treated discharge events caused by heavy precipitation.

The Michigan Department of Environmental, Great Lakes, and Energy (EGLE) requires that within four hours of a discharge beginning, it shall be reported on the State’s website, MiWaters. The public can access these reports as follows:

  1. Go to the EGLE website:
  2. Find “Online Services” and click on: Combined Sewer Overflow (CSO) and Sanitary Sewer Overflow (SSO) Information.
  3. The CSO/SSO list will appear. You can use the filter to search for the responsible party you are looking for or just scroll down the list.

Additionally, EGLE publishes an annual report of all Retention Treatment Basin (RTB), CSO and SSO discharges. These reports can be accessed as follows:

  1. Go to EGLE website:
  2. In the shaded box titled “Information” you will find the links to the annual reports.