O Fallon City Wastewater Treatment Plant - Saint Clair County, Illinois

O Fallon City wastewater treatment plant is located in Saint Clair County, Illinois, serving a population of around 27,190. The facility is part of the municipal infrastructure for the O'Fallon area, situated near Interstate 64. As a medium-sized agglomeration, the plant plays a key role in managing wastewater for the community. Under the US Clean Water Act, the plant operates under an NPDES permit issued by the Illinois Environmental Protection Agency. For a population of this scale, secondary treatment is typically required, with additional nutrient removal if the receiving water body is sensitive. The plant's design capacity and treatment processes are consistent with standard practices for similar facilities in the region. The treated effluent is discharged into local streams that flow into the Kaskaskia River basin, ultimately reaching the Mississippi River. This watershed supports diverse aquatic life and provides water for downstream communities. The plant's operations help protect water quality in the Mississippi River system, which is ecologically and economically significant.

The plant discharges into tributaries of the Kaskaskia River, which flows into the Mississippi River. The Mississippi River is a major migratory corridor for fish and birds, and its floodplains support diverse habitats. Protecting water quality in this basin is critical for downstream ecosystems, including the Gulf of Mexico, where nutrient pollution contributes to hypoxic zones.