The City of St. Charles owns and operates a sanitary sewer collection system and two wastewater treatment facilities: The Main Wastewater Treatment Facility and the West Side Water Reclamation Facility. The collection system tributary to the Main Wastewater Treatment Facility (Main WWTF) consists of approximately 152 miles of sanitary sewers, 5 miles of force main and 13 lift stations. The Main WWTF is located at the Public Works Facility, 1405 S. 7th Avenue on the eastern shore of the Fox River. The treatment facility has a design average treatment capacity of 9 million gallons per day (MGD). The facility generally serves the community’s wastewater needs east of Randall Road and discharges to the Fox River. The facility provides preliminary, primary, and secondary treatment for flows received from the tributary communities to meet strict effluent water quality requirements before discharging to the Fox River.

The St. Charles Main WWTF was required to comply with a 1 mg/L phosphorus limit. It was likely that the treatment facility will need to achieve lower phosphorus effluent limits prior to 2030. Trotter and Associates completed the Facility Plan and Phosphorus Removal Feasibility Study in 2015. The analysis utilized BioWin™ to evaluate various treatment methods and technologies. The selected alternative was an A²O process to address the phosphorus limit and future Total Nitrogen requirements. The recommendations also included construction of a chemical feed system for polishing, a chemical buffering system for struvite prevention, and a primary sludge fermenter for enhanced BPR. 

The 2015 Facility Plan also identified the need for a Digester Improvements project. The existing anaerobic digesters were constructed in 1989. The previous digested sludge storage tank was constructed in 1951 and repurposed multiple times. The Facility Plan recommended rehabilitation of the anaerobic digesters and replacement of the digested sludge storage tank. It was further recommended that the project be completed jointly with the Phosphorus Removal Project to expedite the completion schedule and minimize disruption to treatment facility operations.