Shelton STP Wastewater Treatment Plant, Shelton, Washington

Shelton STP is a municipal wastewater treatment plant serving the city of Shelton, Washington, in Mason County. The plant serves a population of approximately 9,870, placing it in the small to medium agglomeration category under US regulatory frameworks. It is situated near the southern end of Puget Sound, within 50 km of the Pacific coast. As a US facility, Shelton STP operates under the Clean Water Act, with National Pollutant Discharge Elimination System (NPDES) permits issued by the Washington State Department of Ecology. For plants of this scale, secondary treatment is typically required, and permits specify effluent limits to protect receiving waters. The plant's designed capacity is 1.00 million gallons per day, indicating it is sized to handle current and near-term flows. The plant's treated effluent is discharged into the local watershed, which ultimately drains into Puget Sound, a major estuary of the Pacific Ocean. Puget Sound supports diverse marine life, including salmon, orcas, and shellfish, and is ecologically sensitive to nutrient and pollutant inputs. Proper treatment at Shelton STP helps protect this vital ecosystem.

Shelton STP discharges into the Puget Sound basin, a large estuarine system connected to the Pacific Ocean via the Strait of Juan de Fuca. The receiving waters support critical habitats for salmon, steelhead, and other aquatic species. The plant's location within 50 km of the coast means its effluent can influence nearshore marine environments, making nutrient and pathogen control important for maintaining water quality.