As part of a consent decree recently entered into with the U.S. government and the state of Illinois, the city of Peoria, Illinois, and the Greater Peoria Sanitary District have pledged to dramatically reduce combined sewer overflows into the Illinois River in the coming years. For its part, the city has developed an ambitious blueprint to comply with the consent decree’s requirements, primarily by means of implementing green infrastructure such as permeable pavement, bioswales, infiltration trenches, and tree filtration systems.
With plans to monitor its ongoing performance closely, Peoria could serve as a model for cities looking to use green infrastructure as a cost-effective means of improving environmental performance while optimizing existing sewer infrastructure.
The product of many years of discussion between the city, the GPSD, and state and federal regulatory authorities, the proposed consent decree was lodged with the U.S. District Court for the Central District of Illinois on Dec. 23. To comply with the requirements of the consent decree, the city expects to spend $109 million in capital costs between 2022 and 2039. Meanwhile, the GPSD, which provides wastewater treatment for the Peoria region, estimates that it will spend $30 million by 2032, mainly on improvements to its collection system.
A second go-round at CSO reduction
Serving an area of approximately 66 sq mi, the GPSD treats wastewater collected by 717 mi of sewer, according to Tim Leach, P.E., M.ASCE, the director of planning and construction at the GPSD. Within this area, the GPSD owns and operates 523 mi of separate sanitary sewer, while the city of Peoria owns and operates 130 mi of combined sewer and 64 mi of separate sanitary sewer, he says. Located in the city’s older urban core, the combined sewer system includes multiple outfalls that discharge to the Illinois River.
Depending on rainfall conditions, Peoria experiences from 20 to 60 CSOs per year, resulting in average discharges of about 180 million gal. annually, says Greg Myroth, P.E., a project manager for Symbiont Science, Engineering, and Construction Inc. The engineering and construction firm is serving as program manager for the implementation of the city’s CSO control projects. Symbiont, together with Wood Environment and Infrastructure Solutions, helped the city develop its long-term control plan and negotiate the consent decree.
The current consent decree marks the second time that the city is taking steps to reduce overflows. “This isn’t Peoria’s first bite at the apple in terms of upping our game at CSO improvements,” says Jane Gerdes, P.E., a project manager for Peoria’s Public Works Department. In the 1980s, the city conducted sewer separation within some small sewersheds close to the Illinois River, installed two swirl concentrators to remove floatable debris, and added a large sewer line to increase system storage capacity. Before making the improvements, Peoria had discharged an annual average of about 640 million gal. of CSOs to the environment, according to Myroth.
A cost-effective solution
By the end of 2039, Peoria is required to meet four interim milestones as well as a final deadline.
Each milestone includes a goal for a certain volume of CSO reduction, eventually leading up to 100 percent reduction of overflows during six-month design storms that have a total rainfall depth of 1.53 in. “Our model indicates that the design storm right now results in about 23 million gallons of CSOs under the existing conditions,” Myroth says.
Peoria opted to pursue green infrastructure as its primary means of CSO reduction only after evaluating multiple alternatives. “We probably looked at over 30 different solutions,” Gerdes says. Such options included sewer separation, storage shafts, storage tanks, large-diameter tunnels, and end-of-pipe primary treatment and disinfection. Ultimately, the city selected green infrastructure because it is expected to cost approximately two-thirds of what traditional gray infrastructure would cost. “We’re using green infrastructure as our first … solution to CSOs because of its cost-effectiveness,” Gerdes says.
Because green infrastructure “can be employed anywhere within the CSO area,” the various technologies offer the city an enormous amount of flexibility, Gerdes says. For example, green infrastructure can be added as part of transportation projects or other elements of the city’s capital plan. The practices also confer certain “co-benefits,” including urban beautification, Gerdes notes.
Monitoring progress and performance
All told, Peoria plans to install approximately 19.4 acres of green infrastructure by the end of 2034, mainly in the form of permeable pavement, bioswales, infiltration trenches, and tree filtration systems. However, the city will “keep an open mind and look for new technologies as they come about,” Myroth says. “That’s what’s cool about the plan. The city has the flexibility to utilize adaptive management and modify that balance of projects.”
To ensure that its selected green infrastructure practices are working as intended, Peoria will conduct extensive testing and modeling throughout the consent decree period. In this way, the city will keep tabs on its overall progress toward its CSO reduction goal. To this end, Peoria will conduct flow monitoring of a project site before and after the installation of green infrastructure.
“We’ll be able to see how effective that particular project is,” Myroth says. However, “one project is not going to show you a lot,” he notes. “But when you start piecing together a whole year’s worth of projects, then that’s going to tell a better tale.” Just as intriguing, Peoria will be able to use its monitoring to assess the performance of individual green infrastructure treatment technologies. “This is a unique way of utilizing green infrastructure to an extent that really hasn’t been done elsewhere,” Myroth says.
More storage, higher flows
Although most of the work to be conducted as part of each stage of the compliance period involves green infrastructure, the city also will make certain improvements early on to boost the storage capacity of its existing sewer system. For example, weirs will be added at strategic locations within a relatively flat section of large trunk sewer.
In the event that green infrastructure does not reduce CSO volumes sufficiently, Peoria will add more storage capacity to its collection system in the final years of the consent decree. “It’s kind of our fail-safe or catch-up at the end of the 18-year program,” Gerdes says. If needed, this additional capacity likely will comprise offline storage in the form of storage tanks or piping.
As for the GPSD, its compliance efforts mainly entail modifications to allow more flow to enter its approximately 7.5 mi long riverfront interceptor. Constructed in 1929, the riverfront interceptor collects wastewater from an approximately 4 sq mi area served by separate sanitary sewers and an 8.3 sq mi area served by combined sewers. Ranging in diameter from 12 to 84 in., the interceptor conveys flows to the GPSD’s 37 mgd wastewater treatment plant.
The GPSD “will be modifying the regulator structures that control the amount of flow that is diverted from the city’s combined sewer system to the riverfront interceptor,” says Leach, of the GPSD. In addition, the GPSD will clean sediment from the riverfront interceptor and construct additional sewers to direct flows to the interceptor, Leach says.
The GPSD also must add storage capacity at its two remote facilities that treat excess wet weather flows by means of sedimentation and ultraviolet disinfection, according to Leach. The environmental engineering firm Greeley and Hansen LLC, has conducted modeling, preliminary design, and final design of the GPSD’s projects pertaining to the consent decree.
