What you need to know about the sewer systems
Every day, the MWRD treats an average of 1.3 billion gallons of used water after it goes down drains, leaves households and runs off streets and sidewalks.
Understanding Your Sewer Brochure - English, Español, Polish
Most sewer systems in the Chicago area – and older cities around the world – were built over 100 years ago. This was well before wastewater treatment ever existed. In fact, the local sewer systems were originally designed to drain sanitary flow and a limited amount of stormwater directly to the waterways. Today, most of the local sewers are required to carry much more water than they did when they were first put into service; consequently, they can exceed their capacity, resulting in backups and overflows due to insufficient flow capacity at some point in the sewer system.
A combined sewer system can lead to backups and overflows. This can occur when the intercepting sewers and water reclamation plants reach capacity during heavy rain. The local sewers in the surrounding communities continue to drain, or overflow, to a waterway.
When a lot of rain falls in a concentrated area over a short period of time, stormwater may enter sewers faster than it can flow through them, exceeding their flow capacity. When this happens, some of the combined sewage cannot reach the MWRD’s tunnels and reservoirs or the waterway quickly enough. Thus, it can back up into streets and unprotected basements.
- The Chicago area sewers are a web of combined sewer systems. Unlike a separate sewer system where the sanitary and stormwater drain to separate pipes, in a combined sewer system, both sanitary sewage and stormwater drain into the same pipes. These local sewers contain water from homes, businesses and street drains. After flowing into local drains the water makes its way to the MWRD’s intercepting sewers, which then divert the water to the MWRD water reclamation plants (seven total) for treatment before it is discharged into the waterways.
During a heavy rain, pay attention to the amount of water you’re using. In fact, The MWRD encourages the public to minimize its use of water before and during storm events to reduce the amount of water flowing into the sewer system. See our Overflow Prevention page for more information.
The MWRD owns and operates 560 miles of intercepting sewers, which are larger sewers that receive flow from approximately 10,000 local sewer system connections, and force mains, which are pipes that move water under pressure by using pumps or compressors. The water reclamation plants mimic the purification process that occurs naturally in rivers, condensing what would take one or two weeks to less than 12 hours. Once considered waste, the discarded water that flows to our plants for treatment is now considered a collection of resources to be recovered and reused beneficially.
Visit EPA.gov to learn more about combined sewer overflow and sewer separation.
Working hard to prevent sewer backups
The MWRD and your municipality are committed to working together to prevent problems with the sewer systems. The MWRD and your municipality regularly inspect, maintain, and clean sewers to ensure they are operating properly.
The MWRD is completing the last remaining TARP reservoir -- McCook Reservoir Stage II. When completed, this reservoir will provide an additional 6.5 billion gallons of storage, in addition to 3.5 billion gallons of capacity already made available through Stage I, which was launched in 2017. Click here to see a video of the McCook Reservoir filling up during a storm.
The MWRD is also working with municipalities and nongovernmental organizations to adopt green infrastructure. Technologies such as porous pavement, rain gardens, native plant landscaping, bio-swales, green roofs, and greenways allow stormwater to be absorbed into the ground instead of flowing into the sewer system. Reducing the burden on the sewer system will help reduce sewer overflows and the likelihood of backups.
Tips to help prevent sewer backups
You can help prevent or reduce the chance of sewer backups in your home by connecting your downspouts to rain barrels. You can then save the rainwater for reuse. Click here for a demonstration on installing a rain barrel.
Also, consider replacing your driveway and other paved areas on your property with pervious surfaces that can absorb stormwater. For more tips, see Frequently Asked Questions below or download our “Understanding Your Sewer” (Español) (Polish) brochure.
Frequently Asked Questions
Does my municipality have a combined or separate sewer system?
If you are in our Cook County service areas, click here to see if your community has a combined (C), separate (S) or combined and separate (C&S) sewer system.
How much wastewater and stormwater does the MWRD treat?
Although we treat a total capacity of 1.3 billion gallons of wastewater per day, the MWRD has a total treatment capacity of more than 2 billion gallons. We collect, clean and return that water to the environment in a matter of hours.
Annually the National Association of Clean Water Agencies (NACWA) honors MWRD water reclamation plants for meeting decades of compliance in National Pollutant Discharge Elimination System (NPDES) permit requirements and federal Clean Water Act standards.
How does TARP work to manage CSOs?
Our TARP or “Deep tunnel” system is designed to reduce flooding and pollution caused by combined sewer overflows. TARP essentially is a larger intercepting sewer. TARP captures the excess flow from the combined sewers before the water can reach the waterway. This water is stored in the tunnels but will also be sent to large storage reservoirs.
What is a combined sewer?
In a combined sewer system, sanitary sewage and stormwater drain into the same pipes. Homes, businesses and street drains are connected to the local sewers, which are owned and maintained by municipalities. Local sewers flow by gravity into the MWRD intercepting sewers, which then convey the flow to MWRD water reclamation plants for treatment.
What is a separate sewer?
In a separate sewer system, the sanitary sewage and stormwater drain to separate pipes; thus, the stormwater is conveyed to a stormwater outfall for discharge directly into the receiving water body.
What is a combined sewer overflow?
If the intercepting sewers and water reclamation plants reach capacity during heavy rain, the local sewer continues to drain, or “overflow,” to a waterway.
My sewer line is clogged, should I call the MWRD to inspect?
No, contact a qualified plumber to investigate the blockage. You should also contact the water management department of your local municipality. Your municipality manages the local sewer lines. The MWRD manages the intercepting sewer lines that run beneath them.
Can the MWRD recommend a plumber to evaluate my drains or sewer lines?
No, we recommend you contact the Better Business Bureau, check your local phone directory, or seek out references from trusted family, friends or neighbors.
Why do our sewers overflow into waterways?
They were designed that way. Most local sewers in the Chicago area (and in other older cities around the world) were built over 100 years ago, before wastewater treatment existed. They were designed to drain sanitary flow and a limited amount of stormwater directly into the river.
Most of these local sewers are required to carry much more water today than they did when they were first put into service, and as a consequence, they can exceed their flow capacity, causing backups.
Where can I purchase a rain barrel?
The MWRD offers rain barrels at cost. Click here to learn more.
What causes sewer backups?
Backups are usually caused by insufficient flow capacity at some point in a sewer system. The system cannot drain as quickly as it is being filled.
What you can do to prevent sewer backups
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Have a qualified, experienced plumber who guarantees their work inspect your basement. To protect your home from sewer backups, have a check valve or overhead sewer installed. These are valuable home improvements that will prevent sewer backups if properly installed.
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Disconnect your home’s downspouts so they drain onto pervious (absorbent) surfaces instead of into the sewer system. It is important to make sure the disconnected downspout does not direct water toward your foundation or a neighbor’s foundation. Consider directing flow to a rain barrel or rain garden area. Your neighbors are likely served by the same local sewer, so encourage them to do the same.
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You can connect downspouts to rain barrels to save the rainwater for reuse.
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Consider replacing your driveway and other paved areas on your property with pervious surfaces that can absorb stormwater.
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Don’t flood yourself. Have your lateral connection inspected for blockages or damage that could cause basement backups.
Does the MWRD process and deliver drinking water?
No. Here is what happens between the lake and your faucet:
- Water from Lake Michigan enters the intake crib at depths of 20 to 30 feet.
- Water enters the purification plant's intake basin through a tunnel beneath the lake bed.
- Water is filtered through eight traveling screens to catch debris.
- Water is pumped by low lift pumps up to 25 feet for the first chemical treatment.
- Water flows from the chemical application channels.
- Water flows through mixing basins to begin the flocculation process.
- Flocculated water passes into settling basins to sit for hours allowing floc to settle.
- Water is filtered through precisely graded sand and gravel performing a "natural polishing".
- Filtered water flows into clearwells for its final chemical application.
- From finished water reservoirs water flows to the distribution system.
Read more about it here: https://www.chicago.gov/.../education/water_treatment.html
