THORNTON STL Wastewater Treatment Plant, Cerro Gordo County, Iowa

THORNTON STL is a municipal wastewater treatment plant located in Cerro Gordo County, Iowa, United States. The facility serves a small population of 422 residents, reflecting its role in a rural community. As a secondary treatment plant, it provides biological treatment to remove organic matter and suspended solids from wastewater. The plant operates under the U.S. Clean Water Act, which requires secondary treatment for municipal wastewater discharges. The National Pollutant Discharge Elimination System (NPDES) permit program, administered by the Iowa Department of Natural Resources, governs the plant's discharge limits and monitoring requirements. The plant's designed capacity of 264.98 megaliters indicates it has room to accommodate future growth. The treated effluent is discharged into local waterways that ultimately drain into the Mississippi River basin. The plant's location in inland Iowa means its discharge contributes to the water quality of streams and rivers that support aquatic life and agricultural uses downstream. Proper treatment helps protect these water bodies from nutrient pollution and pathogens.

The plant discharges into tributaries of the Iowa River, which flows into the Mississippi River and eventually the Gulf of Mexico. The local watershed is primarily agricultural, with row crops and livestock operations contributing to nutrient runoff. The plant's secondary treatment reduces biochemical oxygen demand and total suspended solids, helping to mitigate eutrophication risks in downstream water bodies.