During a research program the influence of activated sludge compartment systems with and without anoxic mixing zone on the efficiency of the system and the settleability and dewatering characteristics of the activated sludge was tested. COD-load, number and volume of the compartments and the mixing zones were varied.

Experiments were carried out with two pilot plants, a compartment system and a totally mixed system.

The tests clearly indicate that a four-stage cascade system with a BOD-load of about 1.0 kg/(m3.d) could improve settleability and dewatering characteristics, but bulking sludge could not be avoided.

Using a nonaerated upstream zone with a volume load of 4.0 kg/(m3.d)the settling properties in the compartment system(total BOD-load 1.0 kg/(m3.d)) could be improved to such an extent that the formation of bulking sludge was prevented.

Better settleability leads to the higher solids concentration in the cascade system and to a lower sludge load. Therefore lower BOD-outflow-concentrations and higher efficiency of the activated sludge system were available.

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