LEON Wastewater Treatment Plant, Onzonilla, Castilla y León, Spain

The LEON wastewater treatment plant is located in Onzonilla, a municipality in the province of León, within the autonomous community of Castilla y León, Spain. It serves a population equivalent of approximately 330,000, covering the city of León and surrounding urban areas. The plant is situated inland, about 50 km from the coast, and operates under Spain's national water regulations which transpose the EU Urban Waste Water Treatment Directive (91/271/EEC). As a large agglomeration serving over 150,000 people, the plant is required by EU law to provide at least secondary treatment, and likely includes nutrient removal given the sensitivity of the region's water bodies. The facility is part of the municipal wastewater infrastructure managed by the relevant local authority, ensuring compliance with discharge standards set by the Confederación Hidrográfica del Duero, the river basin authority. Treated effluent from the plant is discharged into the Bernesga River, a tributary of the Esla River, which flows into the Duero River and eventually reaches the Atlantic Ocean near Porto, Portugal. The Duero basin supports diverse aquatic life and is an important agricultural and ecological corridor in the Iberian Peninsula. The plant plays a critical role in protecting water quality in this transboundary river system.

The LEON plant discharges into the Bernesga River, a tributary of the Esla River, which flows into the Duero River. The Duero basin is one of the largest in the Iberian Peninsula, supporting irrigated agriculture, wetlands, and diverse fish species. Downstream, the river reaches the Atlantic Ocean via the Douro estuary in Portugal, an ecologically sensitive area for migratory birds and marine life. The plant's treatment helps prevent eutrophication and maintains water quality for downstream communities and ecosystems.