In 2008, the Ruhrverband has kept the efficiency of its sewage treatment plants at a constantly high level. The overall design capacity of all 72 sewage treatment plants covers a total of 3,282,000 population equivalents. All the sewage treatment plants of the Ruhrverband comply with the legal requirements concerning the purification of municipal sewage.
Compared to 2006, population figures in the Ruhr catchment area have changed only marginally with around 2.134 million inhabitants on June 30th, 2007. The share of inhabitants connected to sewage treatment plants has also remained more or less unchanged at around 98 per cent.
The following diagram illustrates the development of mean design capacity and the number of sewage treatment plants from 1991 to 2008. While the total number of sewage treatment plants in the Ruhr catchment area is decreasing, the figure shows that the specific design capacity - in other words, the performance - of plants is clearly on the rise.
Requirements regarding the efficiency of sewage treatment plants have also become stricter over the past few decades, as shown by the stepped diagram illustrating the case of nitrogen elimination.
Current inspections of compliance with legal requirements regarding the discharge of municipal sewage have shown that the four most important parameters - chemical oxygen demand (COD), phosphorus (Ptot), ammonia nitrogen (NH4N), and inorganic nitrogen (Ninorg) - stayed well below the maximum allowance in the reporting period of the Ruhr Quality Report 2008.
The chart shows that the concentration of all parameters has increased slightly but remained below the levels of 2006. The lower values achieved in 2007 are partly due to dilution caused by above-average waste water volumes in 2007, a year with heavy precipitation resulting in a 17 per cent increase of waste water in the sewage treatment plants.