Every year, the German Association for Water, Wastewater and Waste1 (DWA) presents the effects of discharge from sewage treatment plants into a body of water in its national ranking of all sewage treatment plants in Germany.
Similarly, the Ruhrverband in its annual Ruhr Quality Report presents the pollution of the River Ruhr, which all in all is “very low”. The analysis covers 70 out of the 72 sewage treatment plants run by the Ruhrverband; two plants were not considered due to their small size and the fact that parts of the municipal sewage were treated in an industrial installation.
For the ranking, the results of the weekly samples of sewage treatment plant effluents are used to establish the annual average of five parameters for every single plant. Oxygen-depleting organic sewage components (measured as sum parameters BOD5 and COD) as well as ammonia nitrogen are classified into five groups depending on oxygen demand (oxygen demand stages, see chart); the eutrophic sewage components of phosphorus and total nitrogen are classified into five groups depending on nutrient load (nutrient load stages, see chart). Stage 1 and 2 represent a "very low" or "low" residual content exceeding legal requirements. In order to sum up individual figures, the results are weighted by number of inhabitants using the design capacity of the sewage treatment plant.
1 Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V. (DWA)