MARISSA STP Wastewater Treatment Plant, Marissa, Illinois

MARISSA STP is a municipal wastewater treatment plant located in Marissa, Illinois, a small community in Saint Clair County. The facility serves a population of about 2,527 people, reflecting its role in managing domestic wastewater for this rural area. As a U.S. plant of this scale, it operates under the Clean Water Act and is subject to National Pollutant Discharge Elimination System (NPDES) permits issued by the Illinois Environmental Protection Agency. These permits set effluent limits to protect water quality, and secondary treatment is typically required for municipal plants serving populations of this size. The plant's treated effluent discharges into a local waterway that ultimately drains into the Mississippi River basin. This downstream connection underscores the importance of proper treatment to prevent nutrient and pathogen loading into the larger river system, which supports diverse aquatic life and provides drinking water for downstream communities.

The plant discharges into a tributary of the Kaskaskia River, which flows into the Mississippi River. The Mississippi River is a major ecological corridor supporting diverse fish, bird, and plant species. Proper wastewater treatment is critical to prevent nutrient pollution that could contribute to hypoxia in the Gulf of Mexico, where the Mississippi River terminates.