Ammonia nitrogen (NH4-N) can enter the body of water from diffuse sources (e.g. agriculture) and in sewage. In the 1970s, the Ruhr in Essen-Rellinghausen would sometimes reach peak concentrations of more than 2 mg/l in winter; the mean annual value would be up to 1 mg/l. Today, the mean annual concentration of ammonia nitrogen in the Ruhr is at a very low level of less than 0.05 mg/l. This is the result of stricter legal requirements on sewage treatment which were established as a response to seal deaths and massive algae growth in the North Sea. As a consequence of those stricter thresholds, the Ruhrverband has been refitting its sewage treatment plants until the end of the year 2005 so that ammonia nitrogen can now be converted into nitrate nitrogen which, in turn, is converted into atmospheric nitrogen.
The development of Ruhr water quality
Phosphorus is a nutrient limiting the eutrophication of a body of water. It mainly enters the water body through municipal sewage and agricultural inputs. It was mainly tertiary treatment (phosphate precipitation) in sewage treatment plants that helped to achieve a considerable reduction of phosphorus loads in the body of water. This can clearly be seen from data at Essen-Rellinghausen where the mean annual level of phosphorus in the Ruhr has remained below 0.2 mg/l since the early 1990s and has even been recorded at less than 0.1 mg/l since 2006.
Chemical oxygen demand (COD) specifies the amount of oxygen necessary for oxidising organic content in the water. Hence, it is a yardstick for the organic load of a body of water. Over time, mean annual COD has decreased continuously. While levels of around 20 mg/l would still be common in the middle of the 1970s, COD is down to about 10 mg/l today.
Heavy metal is present in every body of water. Its concentration is partly of natural origin and partly man-made with regional variations. The fact that the quality of Ruhr water has improved sustainably can best be seen from the case of nickel content. Thanks mainly to waste water engineering measures taken by metal processing companies in the Ruhr catchment area, the average nickel load which used to see peaks of up to 65 µg/l could be reduced continuously. In the early 1990s, nickel content was already down to less than 10 µg/l; the current annual average is at around 3 µg/l.