Coupeville STP - Secondary Wastewater Treatment Plant in Coupeville, Washington

Coupeville STP is a municipal wastewater treatment facility located in Coupeville, Washington, on Whidbey Island in Island County. The plant serves a population of approximately 2,155 residents and operates as a secondary treatment facility, which is standard for small communities in the United States under the Clean Water Act. The plant has a designed capacity of 1,135.62 m³/day and currently discharges about 757 m³/day of treated effluent. As a secondary treatment plant, it uses biological processes to reduce organic matter and suspended solids. The facility is regulated under the U.S. Environmental Protection Agency's National Pollutant Discharge Elimination System (NPDES) permit program, administered by the Washington State Department of Ecology. The treated wastewater is discharged into the waters near Whidbey Island, which are part of the Puget Sound ecosystem. Puget Sound is a large estuarine system that supports diverse marine life, including salmon, orcas, and shellfish. The plant's coastal location requires careful management to protect water quality and marine habitats.

The plant discharges into the nearshore waters of Whidbey Island, which flow into Puget Sound, a major estuary connected to the Salish Sea and ultimately the Pacific Ocean. Puget Sound is ecologically sensitive, supporting critical salmon runs, forage fish, and marine mammals. Secondary treatment helps reduce nutrient and pollutant loads, but coastal discharge requires monitoring to prevent localized impacts on shellfish beds and eelgrass habitats.