Our Combined Sewer Overflow Operations

GLWA operates and maintains our Combined Sewer Overflows and Combined Sewer Overflows control Facilities in accordance with our Michigan Department of Environmental Quality (MDEQ) National Pollution Discharge Elimination System (NPDES) permit.  This permit is issued once every five years to GLWA by the MDEQ.

Here you will find information pertaining to the GLWA sewage collection system and its relationship to our treated and untreated Combined Sewer Overflows outfalls. This will include, but is not limited to, current and past discharge information, a brief history lesson of how we got here, and FAQs to help educate and inform the public or Combined Sewer Overflows knowledge seekers! GLWA is committed to being a good steward of our environment and this information will help you understand the steps we’ve taken and the steps we will take in the future (as they are known now) to continue our good stewardship of the environment. Click here to learn more about our Combined Sewer Overflows facilities.

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GLWA Combined Sewer Overflow Discharge Information

For information on GLWA CSO Discharges, please visit the Michigan Department of Environment Great Lakes and Energy (EGLE) MI-Waters website by clicking below.

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CSO Notification Plan

The Great Lakes Water Authority (GLWA) has developed a public notification plan (PNP) for combined sewer overflows (CSO) that may occur from outfalls located along Detroit and Rouge Rivers, in accordance with NPDES Permit #MI0022802 and 40 CFR Part 122.38(c).

CSO Annual Reports

Here are the annual CSO Discharge reports for the State of Michigan.

Combined Sewer Overflow Facilities
Billion in Combined Sewer Overflow Control Facility Infrastructure Investment
Reduction in untreated discharge since 2012

Combined Sewer Overflow FAQs

As is typical in many established communities, the Great Lakes Water Authority (GLWA) operates a combined sewer system, which means our sewers transport rain and sewer water in the same pipe. A combined sewer overflow will at times occur during a significant rain event. A combined sewer overflow may occur during a significant rain event because the collection system rapidly reaches capacity, which increases the risk of flooding and basement backups. To address this risk, the combined overflow is discharged through various points along the Detroit and Rouge Rivers.

Approximately 95 percent of combined sewer overflow passes through a Combined Sewer Overflow (CSO) Control Facility, where it is treated and disinfected to meet the Michigan Department of Environmental Quality’s (MDEQ) strict treatment requirements before discharge.

Typically, all sewage is treated at the GLWA Water Resource Recovery Facility (WRRF). However, during those significant rain events when combined sewer overflows occur, approximately 95 percent of the flow that is not treated at the Water Resource Recovery Facility (WRRF) is routed through CSO Control Facilities, which treat and disinfect the water to meet the MDEQ’s requirements for treatment.

The remaining five percent of water that is not sent to the WRRF, or to CSO Control Facilities is discharged at untreated points.  Contact with the water during and shortly after overflow events is not recommended and signage is placed at these points to remind the public to avoid such contact.

CSO Control Facilities are mostly found in older sewer systems and together with interceptor sewers provide extra protection during significant rain events. Initially, combined sewer systems were constructed with very large pipes (up to 20 feet wide by 20 feet tall) to prevent flooding and remove waste near homes and neighborhoods by sending flow to nearby water bodies (the Rouge and Detroit rivers). For GLWA, these are referred to as trunk sewers.

Over time, however, it was recognized that the flow in these trunk sewers needed to be treated at a wastewater facility to limit water quality impacts to the nearby water bodies. Therefore, interceptor sewers were built to intercept trunk sewer flow previously going to the river and take it to a wastewater treatment plant, or in our case, a Water Resource Recovery Facility (WRRF). These facilities treat the water to ensure that it meets all MDEQ and EPA water quality requirements before being released back into the environment.

Interestingly, most interceptor sewers are built along rivers because the previous trunk sewers all went straight to the river. This is also how it got its name “interceptor,” because it intercepts the flow from going to the river and takes it to the wastewater treatment facility.

When these interceptors and wastewater facilities were built, however, they didn’t account for the major storms that we see today. What this functionally means is that during significant wet-weather events, the interceptor sewers do not have sufficient capacity to carry all the wet-weather generated flow to the wastewater treatment facility, and therefore still need an outlet to the river to keep homes and basements from flooding with combined sewer water.  During significant rain events CSO Control Facilities are used to treat and disinfect excess flow in a manner that meets the MDEQ’s requirements for treatment

Newer sewer systems are constructed with separate storm and sanitary sewers, so they do not have the same capacity challenges that combined systems do when it comes to treating the water in the pipe during wet-weather events.

What we constantly strive for is to keep people’s basements dry during wet-weather while minimizing the impact of any combined sewer overflow events on our waterways.

As with many regulations, we as a service sector always strive to do better. We are constantly working to improve our operations and ensure that the public is fully informed and aware of our system and effects of the system during storms.


No. The water treated at our WRRF continues to meet and/or surpass our water quality permit requirements for effluent water quality, and our CSO Control Facilities continue to meet or surpass water quality requirements.

Working with the United States Environmental Protection Administration (EPA) and the MDEQ as a part of our National Pollutant Discharge Elimination System (NPDES), GLWA is working towards minimizing untreated discharges.

GLWA operates and maintains nine CSO Control Facilities, as well as 62 untreated discharge points. Of those 62 untreated discharge points, six of them discharge only in the event of an emergency that jeopardizes property (i.e. wide-spread basement flooding). The remainder of the points discharge at varying frequencies. It should be noted that these 62 sites are responsible for approximately 5 percent of total combined sewer overflow discharge volume.

We have so many discharge points because of the size of the system, which serves approximately 3 million people, including the city of Detroit and 77 additional communities. Some of our pipes are 20 feet wide by 20 feet tall (big enough to drive two semi-trucks through at the same time side-by-side). Each discharge site serves as a relief point for the sewer system to diminish flooding when there is not sufficient capacity in the interceptor (mentioned above) to send flow to the WRRF.

While combined sewer overflow discharge must continue in some form to protect residents’ property and prevent flooding, we are also working on plans to treat more of the untreated discharge through screening and disinfection (at a minimum). Screening means that we remove things like pop bottles, trash, or other items that make it into the sewers and prevent them from being sent to the river. Disinfection means that we kill harmful bacteria in the water.

The only way to completely remove combined sewer overflow discharge is to create separate sewers, dividing storm water and sanitary water. For old systems, the cost of doing this has proven to be incredibly expensive and would put an undue burden on resident’s water bills.

Discharge water from the CSO Control Facilities are screened and disinfected water that meets the MDEQ’s requirements for treatment of Combined wastewater.

Discharge water from the untreated combined sewer overflow discharge points are considered diluted wastewater. There are still elements of wastewater in a discharge that may include debris (leaves, garbage, cans, etc.), bacteria, as well as rain water. Combined sewer overflow discharges also have a great deal of inorganics, such as dirt coming off roadways, during wet-weather events.

Take note of the pipes, and observe them during dry and wet-weather. If you see them discharging and it is not raining, please contact the number on the signs. If you see them discharging during rain, we recommend that you stay away from these points, as the flow can be potentially dangerous and could pull a person into the water, causing drowning.


You can help minimize debris in the system by throwing trash away rather than littering. If you see leaves blocking a catch basin (drain in the road), clean them up and throw them away. The public can also help to improve overall water quality by minimizing runoff of fertilizer from their property during rainfall (fertilizer has phosphorus, and phosphorus contributes to algae blooms, like those in Lake Erie).  When buying fertilizer there will be three series of numbers like 10-15-5. The number 10 in this instance represents the available phosphorus in the fertilizer. The closer to zero this number is, the better for the environment in terms of phosphorus containing water runoff into receiving waters (i.e. the river or lake).

GLWA is working to add more informational pieces to its website to explain the history of combined sewer overflows and how our system was designed. In the meantime, if you’re looking for more information, please contact Chris Nastally, Manager of GLWA’s CSO Control Program at christopher.nastally@glwater.org.

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Contact Us

For general information about GLWA's Combined Sewer Overflow Operations please contact Chris Nastally via email.