About 15,000 Population Equivalents are connected to the Lüdinghausen Waste Water Treatment Plant of the Lippeverband. The activated sludge stages, designed for 30,000 PE and a BOD5 sludge load of 0.3 kg/(kg · d), are, thus, at present operated with a sludge load of 0.15 kg/(kg · d), i.e. in the nitrification range. In order to reduce energy costs for aeration and prevent possible sludge losses resulting from the denitrification processes, the aeration tanks are equipped with denitrification zones.

Because of the loading conditions, the only possible denitrification method was that of primary denitrification. However, contrary to usual practice, a multi-stage cascade, comprising two denitrification and two nitrification tanks, was installed.

Apart from a very high degree of nitrification, nitrate is eliminated to a farreaching degree. Better reduction of BOD5 and COD is also attained. Moreover, the settlability of the activated sludge is, compared to earlier conditions, substantially more favourable.

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