Ellsworth Development Authority Wastewater Treatment Plant, New Underwood, South Dakota

Ellsworth Development Authority is a projected wastewater treatment plant located on Highway 1416 in New Underwood, Pennington County, South Dakota, United States. The facility is designed to serve a population of approximately 28,084, placing it in the medium-agglomeration category for US inland plants. As a projected facility, specific treatment processes and capacity details are not yet finalized. However, US plants of this scale typically operate under EPA National Pollutant Discharge Elimination System (NPDES) permits issued by the South Dakota Department of Agriculture and Natural Resources. The Clean Water Act requires secondary treatment as a baseline for municipal wastewater facilities, with additional nutrient removal standards applicable in sensitive watersheds. The plant's discharge will likely enter a local watercourse within the Missouri River basin, which ultimately drains to the Mississippi River and the Gulf of Mexico. The region's semi-arid climate and reliance on groundwater and surface water resources make proper wastewater treatment critical for protecting downstream aquatic ecosystems and drinking water supplies.

The plant is located inland in western South Dakota, within the Missouri River watershed. Treated effluent would likely discharge into a tributary of the Cheyenne or Bad River, which flows into the Missouri River and eventually the Mississippi River and Gulf of Mexico. The region supports mixed-grass prairie ecosystems and is important for migratory birds and native fish species. Proper nutrient management is essential to prevent downstream eutrophication in the Missouri River system.