SELAYA Wastewater Treatment Plant, Selaya, Cantabria, Spain

The SELAYA wastewater treatment plant is located in the Barrio Arcillal area of Selaya, a municipality in the autonomous community of Cantabria, northern Spain. The plant serves a population of approximately 3,150 residents, classifying it as a small agglomeration under Spanish and EU regulations. As a secondary treatment facility, SELAYA employs biological processes to remove organic matter and suspended solids from wastewater. The plant has a designed capacity of 3,500 m³/day and currently treats an average daily flow of 867.55 m³/day, indicating significant spare capacity for future growth. Under the EU Urban Waste Water Treatment Directive (91/271/EEC), secondary treatment is the minimum required for discharges to freshwater and estuaries from agglomerations of this size. The treated effluent from SELAYA is discharged into a local watercourse that ultimately drains into the Bay of Biscay via the Cantabrian Sea. The plant plays a vital role in protecting the region's rivers and coastal waters from pollution, supporting aquatic life and maintaining water quality for downstream communities and ecosystems.

The SELAYA plant discharges into a small river in the Pas River basin, which flows northward through Cantabria and empties into the Bay of Biscay. This coastal region supports diverse marine and estuarine habitats, including important fisheries and migratory bird routes. The secondary treatment provided by the plant helps reduce nutrient and organic loads, mitigating the risk of eutrophication in the sensitive coastal waters of the Cantabrian Sea.