McHenry South WWTP | Crystal Lake, Illinois Wastewater Treatment Plant

McHenry South WWTP is a municipal wastewater treatment facility located in Crystal Lake, Illinois, within McHenry County. The plant serves a population of approximately 15,122 people, making it a medium-sized agglomeration under state and federal classifications. Its location in the Fox River watershed places it within a region of mixed urban and suburban development. As a U.S. facility, the plant operates under the Clean Water Act, administered through the National Pollutant Discharge Elimination System (NPDES) permit program. The Illinois Environmental Protection Agency issues permits that set effluent limits for parameters such as biochemical oxygen demand, total suspended solids, and nutrients. For a plant serving this population, secondary treatment is the minimum standard, with potential requirements for nutrient removal if the receiving water body is sensitive. The plant discharges treated effluent into a local waterway that ultimately flows into the Fox River, a tributary of the Illinois River, which drains into the Mississippi River and eventually the Gulf of Mexico. The Fox River supports diverse aquatic life and is used for recreation, making effluent quality important for downstream ecological health.

The plant's treated effluent enters a tributary of the Fox River, which flows through northeastern Illinois before joining the Illinois River. The Fox River watershed supports a variety of fish species and provides habitat for migratory birds. Downstream, the Illinois River connects to the Mississippi River, contributing to the vast Mississippi River Basin ecosystem. Nutrient loading from wastewater can impact algal growth and oxygen levels in these waters, making treatment standards critical for maintaining water quality.