Mill City Sewage System - Secondary Wastewater Treatment in Mill City, Oregon

Mill City Sewage System is a municipal wastewater treatment facility located at 200 Southeast Whitten Street in Mill City, Oregon. It serves a population of approximately 1,585 residents, classifying it as a small-scale treatment plant within the state's wastewater infrastructure. The plant operates under the regulatory framework of the U.S. Clean Water Act, which requires secondary treatment for municipal discharges to surface waters. The plant provides secondary treatment, a standard biological process that removes organic matter and suspended solids. Its designed capacity is 352.04 thousand cubic meters per year, with an average annual discharge volume of 223.34 thousand cubic meters, indicating operational capacity below design levels. As a small facility, it is subject to National Pollutant Discharge Elimination System (NPDES) permits issued by the Oregon Department of Environmental Quality, which set effluent limits to protect water quality. The treated effluent is discharged into a local water body that ultimately drains into the North Santiam River, a tributary of the Santiam River system. This river network flows into the Willamette River and eventually reaches the Columbia River and the Pacific Ocean. The plant plays a key role in protecting the North Santiam River's water quality, which supports aquatic life and recreational uses in the region.

The plant discharges into a tributary of the North Santiam River, which flows through the Cascade Range foothills into the Santiam River and then the Willamette River. The Willamette River is a major tributary of the Columbia River, which empties into the Pacific Ocean. The North Santiam River supports diverse aquatic species including salmon and trout, and its water quality is critical for downstream ecosystems and human uses such as drinking water and recreation.