GILMAN STL Wastewater Treatment Plant, Marshall County, Iowa

GILMAN STL is a municipal wastewater treatment plant located in Greencastle Township, Marshall County, Iowa, United States. The facility serves a small community of approximately 509 residents, reflecting its role in managing local domestic wastewater within a rural setting. The plant provides secondary treatment, a standard biological process that significantly reduces organic matter and suspended solids. With a designed capacity of 264.98 volume units and an average daily discharge of 227.13 volume units, the facility operates below its maximum capacity. As a US plant serving fewer than 10,000 people, it falls under the Clean Water Act's National Pollutant Discharge Elimination System (NPDES) permitting framework, which sets effluent limits to protect water quality. The treated effluent is discharged into a local water body within the Mississippi River basin. Iowa's landscape is dominated by agriculture, and the plant's discharge contributes to the overall water quality management in the region. The downstream environment supports diverse aquatic life and is part of the larger Mississippi River watershed, which ultimately drains into the Gulf of Mexico.

The plant discharges into a local watercourse within the Mississippi River basin, which flows southward to the Gulf of Mexico. The surrounding region is predominantly agricultural, and the plant's effluent helps maintain water quality in streams that support fish and other aquatic species. The Mississippi River system is a critical migratory corridor for birds and fish, and nutrient management is a key concern due to downstream hypoxia zones.