Frequently Asked Questions

How Combined Sewers Function

Our first sewers were designed over 100 years ago to carry both stormwater and sewage from homes and businesses. During dry weather, sewage flows safely through our sewers to the Greater Peoria Sanitary District wastewater treatment plant. However, between 20 to 30 times a year, the sewers are overwhelmed by incoming rainwater or melting snow. This causes untreated sewage to overflow into the Illinois River.

During wet weather… Between 20 and 30 times a year, stormwater from rain or melting snow overloads these sewers. They don’t have enough capacity to carry wastewater to the Greater Peoria Sanitary District's (GPSD) treatment plant. So untreated sewage flows over the internal dam into the Illinois River.

During Dry Weather… Peoria’s combined stormwater/sanitary sewers work much like a modern sanitary sewer. All sewage from homes and businesses is sent to the treatment plant by a “regulator,” or small dam, in the sewer.

Peoria built its first sewers in the late 1800s to carry rainwater and melting snow away from homes, businesses, and streets. When indoor plumbing came later, homeowners and business owners hooked their sewage lines to those same sewers, combining stormwater and sewage in one pipe. This was standard practice in many U.S. cities at the time, especially in the Northeast and Midwest. By 1931, the combined sewers were connected to the new Greater Peoria Sanitary District treatment plant through a new riverfront interceptor sewer. However, the old sewers still retained their ability to overflow when sewage levels got too high. If they didn't have this escape valve, raw sewage would back up into people's basements and streets. (In new neighborhoods today, we avoid this problem by building separate sewers for stormwater and sewage.)

Related Documents

Timeline of Watershed Moments (PDF)

Raw sewage in the river is a health hazard, hurts our environment, and harms efforts to revitalize the Peoria Riverfront. Raw sewage carries bacteria, viruses, parasites, and other pathogens. Other pollutants typically found in sewer overflows include oxygen-depleting substances, suspended solids, toxic substances, nutrients, trash, and debris. According to the Illinois Environmental Protection Agency, full-body contact recreation (swimming, jet skiing, water skiing, etc.) is impaired due to bacteria contamination.

Starting in 1987 (through 1994), Peoria proactively undertook about $10 million (in 1980s dollars) in projects to reduce overflows. Projects included: Separating sewers in seven drainage basins by constructing either new sanitary or storm sewers to separate the combined flows; Constructing swirl concentrators at two locations to remove trash from overflows; Using a mile-long, 60-inch and 48-inch diameter sewer to store excess flows until downstream capacity is available in the riverfront interceptor; Installing gates to control the amount of flow discharged to the interceptor sewer and backflow valves to prevent the river from flowing into the interceptor sewers during flood conditions; Constructing treatment plant improvements and installing telemetry to monitor and report on sewer flows.

The benefits included reducing …

• The number of CSO locations is from 20 to 16.
• The average days of overflows from 40 a year to 28 a year.
• The overflow volume from an estimated 840 million gallons averages to 250 million gallons average in a typical year.
• The amount of trash discharging into the Illinois River.

The City has long maintained a National Pollutant Discharge Elimination System (NPDES) program permit that regulates discharges of untreated overflows resulting from combined sewers into the river. Due to evolving regulations, our municipality is required to develop a long-term control plan to reduce the incidence of CSOs. We must work to bring the number down as close to zero as possible. Since about 2007, Peoria has been diligently working to prepare a responsible plan that meets Clean Water Act requirements.

 We have the power to demonstrate our dedication to meeting Clean Water Act requirements while improving public rights of way and beautifying our City. Peoria is proposing a cost-effective approach using 100% green infrastructure. Rather than constructing more capital-intensive "gray" infrastructure (like pipes, tanks, or tunnels), the City seeks to employ proven techniques to prevent stormwater from entering combined sewers in the first place. From a single rainstorm, Peoria needs to be able to capture about 60 Olympic-sized swimming pools of water or about 37 million gallons.

This approach promotes the natural movement of water in a way that complements our City's unique natural topography and soil composition - instead of forcing it to wash down paved streets, into manmade drains, then into massive pipes and tanks. Reducing sewer overflows will reduce the loading of pathogens and other pollutants into the Illinois River. Although it won't solve all the river's problems, like siltation, it will be a start toward a cleaner river and healthier riverfront.

At present, negotiations are continuing in earnest with regulators.

Our proposed green infrastructure approach will be more cost-effective than gray for meeting Peoria's CSO obligation. Nonetheless, making these improvements likely will be in the hundreds of millions of dollars. City leaders and our citizens will be tasked with identifying funding streams for this mandate. There will be costs associated with:

1. Installing green infrastructure (capital). Our current estimate for installing green infrastructure to control CSOs is around $200 million.
2. Ensuring that it continues performing (maintenance, such as street sweeping, keeping plantings healthy, etc.). Maintenance of green infrastructure is estimated between 1 to 3% of capital cost.

Peoria is conducting a global financial analysis to determine what our City and our citizens could reasonably afford to fund. An exact dollar amount is not available at this time because:

• The Peoria City Council, informed by public input, will have policy decisions to make regarding our municipality's approach.
• The location, size, and amount of projects are not yet decided; these also will be informed by public input.
• Our use of green infrastructure will employ adaptive management, so green implementation is consistently monitored and optimized as we go.
• Negotiations continue in earnest with the U.S. EPA, including on an implementation timeline.

This federal mandate is not going away. As a community, we must have serious discussions about how to address it. Continued non-compliance with the Clean Water Act requirements will lead to major fines and penalties. It’s better to keep our dollars here to improve our community than to send fines to Springfield and Washington, D.C.

We all share responsibility for the health of the river and the health of the community. Peoria is here because of the Illinois River. As a community, we need to protect it and stop dumping raw sewage into it. Also, it's important to remember that when the City first built newer neighborhoods, people in older neighborhoods helped pay for some of the new infrastructures. Now it is time to bring the old sewers up to 21st-century standards, and the entire community shares in that responsibility. Also, many of our newer neighborhoods that have separated sewers discharge sewage into the combined sewers as part of the sewage's path to the treatment plant. Therefore, these separate sewers are also contributing to the combined sewer overflow problem.