The upgrading and nitrification was a requirement in 1986 for the conventional sewage treatment plant Geiselbullach, west of Munich, Germany, designed for 250 000 inhabitants equivalents. The possibility was tested to use submerged bio-film reactors in the aeration tanks to increase the MLSS concentration. Half-scale experiments were undertaken with three different reactor types. A rope type material, called Ring-laceR was selected for the large-scale application, because it did not produce anaerobic sludge deposits, as the other tested reactor types did. The design criteria had to be developed. The process operation started in January 1988; a few months later the phosphorus removal by chemical precipitation was also put into operation. After stable conditions were assured the concentration of the MLSS could be increased to about 10 g/l, due to sludge volume indices of about 50, formerly 180 to 300. A nearly complete nitrification was achieved, which could even be continued in winter times at water temperatures of 8 to 10 °C.

Many highly developed microorganisms in the sessile sludge occurred (nematodes, tubifex…), which grew excessively under certain conditions and reduced the normal bacteria to unacceptable low quantities. A worm cure could reduce the worms to acceptable counts. Problems with the longtime stability of the material arose and were investigated.

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