ZV Obere Iller Sitz Sonthofen Wastewater Treatment Plant, Rettenberg, Bavaria

ZV Obere Iller Sitz Sonthofen is a wastewater treatment plant located in Rettenberg, in the Oberallgäu district of Bavaria, Germany. The plant serves a population equivalent of approximately 110,000, classifying it as a large agglomeration under the EU Urban Waste Water Treatment Directive (91/271/EEC). As a facility in a sensitive area within the Danube basin, it is expected to provide tertiary treatment to protect downstream water quality. The plant operates within Germany's stringent regulatory framework, which implements the EU directive through the Federal Water Act (Wasserhaushaltsgesetz) and state-level regulations. For agglomerations larger than 100,000 population equivalent, the directive requires advanced treatment to remove nutrients such as nitrogen and phosphorus, especially in catchments draining to sensitive waters like the Iller River and ultimately the Danube. Treated effluent from the plant is discharged into the Iller River, a tributary of the Danube. The Iller flows through the Allgäu region before joining the Danube near Ulm. The Danube then travels through Central and Eastern Europe to the Black Sea. The plant plays a critical role in protecting the aquatic ecosystems of the Iller and Danube, supporting biodiversity and water quality for downstream communities.

The plant discharges into the Iller River, which flows into the Danube River and ultimately the Black Sea. The Iller is a fast-flowing alpine river that supports diverse aquatic life, including fish species such as grayling and brown trout. The downstream Danube is a major European waterway with ecologically sensitive floodplains and delta. Nutrient removal at the plant helps prevent eutrophication in the Danube and Black Sea.