Since the beginning of the 20th Century, the Emscher and its tributaries have been operated as open waste water courses (Emscher System). Between 1966 and 1977, the Emscher Mouth Treatment Plant was constructed for the biological treatment of domestic and industrial discharge from the 774 square kilometre Emscher catchment area. The plant was designed for 5 million population equivalents. Since the beginning of the Nineties, the Waste Water Management Regulations require, apart from the elimination of carbon, also the elimination of nitrogen and phosphorus, in order to reduce, as far as possible, the load on natural, flowing bodies of water and to protect the North Sea. While the COD values are observed with a narrow margin, the nitrogen values make it necessary to expand the aeration volume of the Emscher Mouth Treatment Plant by 50% to 216,000 m3. For this purpose, comprehensive studies, as well as pilot studies on the subject of filtration, have been carried out. Construction of the new aeration tanks was begun in spring 1993. The expansion will be completed at the end of 1996. Investments totalling 102 million DM are required.

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