Grand Mound WWTP - Secondary Treatment Plant in Centralia, Washington

Grand Mound WWTP is a municipal wastewater treatment plant located near Centralia, Washington, in Thurston County. It serves a population of approximately 2,232 residents, classifying it as a small-scale facility within the state's wastewater infrastructure. The plant is situated inland, about 50 kilometers from the Pacific coast, and its operations are subject to state and federal environmental standards. The plant provides secondary treatment, a standard level required by the U.S. Clean Water Act for most municipal facilities. With a designed capacity of 1,438.45 megaliters per year, it currently treats an annual volume of 378.54 megaliters, indicating significant reserve capacity. As a permitted facility under the National Pollutant Discharge Elimination System (NPDES), it must comply with discharge limits set by the Washington State Department of Ecology to protect water quality. Treated effluent from the plant likely discharges into a local waterway that drains into the Chehalis River basin, eventually reaching the Pacific Ocean via Grays Harbor. This watershed supports diverse aquatic life, including salmon runs, and the plant's secondary treatment helps reduce nutrient and pollutant loads, contributing to the ecological health of the downstream estuary and coastal environment.

The plant's discharge enters a tributary of the Chehalis River, which flows through southwestern Washington to Grays Harbor, an estuary that empties into the Pacific Ocean. This watershed is critical for several salmon species and supports a rich ecosystem of migratory birds and marine life. The secondary treatment process reduces biochemical oxygen demand and suspended solids, helping to maintain water quality in the river and downstream habitats.