The principle of the activated sludge process for sewage treatment technology was already published in 1914 by Arden and Lockett. The original purification goal of sewage treatment plants was the elimination of the organic load. In the years from 1969 to 1993, for example, the organic residual pollution of public sewage treatment plants in Baden-Württemberg was reduced from approx. 75,000 tonnes BSB5 to below 10,000 tonnes BSB5. Nevertheless, the nutrients nitrogen (N) and phosphorus (P) contribute to the eutrophication of inland waters and marginal seas, and drinking water production is polluted by nitrate.
Since 1 January 1989, the waste water management regulations of the Water Management Act for municipal waste water treatment plants have therefore set stricter limit values for nitrogen and phosphorus as well. Most sewage treatment plants are affected by this, as they were originally designed only to eliminate carbon and the removal of N and P requires further measures.