Various investigations into batch reactors as well as into continous flow activated sludge plants in laboratory and pilot scale had the aim to determine the difference of the nitrification and denitrification capacity of fixed and suspended biomass in combined systems. By adding porous support material a certain amount of biomass is fixed on this material in a conventional aeration basin of an activated sludge plant. The dimensioning of activated sludge plants for nitrification considers the sludge age of the activated sludge. With fixing the biomass on the support material in the aeration basin, the sludge age can be raised without increasing the MLSS and therefore without increasing the loading of the secondary clarifier.

Measurements of the oxygen consumption of the suspended sludge and the fixed sludge in situ, e.g. without removing the biomass from the support material, as well as the nitrogen balances showed that the nitrification capacity of the fixed biomass is about 20-30 % less than that of the suspended biomass, related to the organic dry matter (MLSS). Also the fixed sludge had a lower organic dry matter than the suspended sludge. This led to the conclusion that there is no linear dependency between increasing the biomass in the aeration basin by adding support material and the nitrification capacity of the whole sludge mass. Regarding the denitrification the measurements showed that a simultaneous denitrification takes place inside the support material although the oxygen concentration in the liquid phase is between 2-4 mg/l.

The denitrification is caused by a lack of oxygen in the deeper regions of the biofilm. This can also be a reason for the observed lower nitrification capacity of the fixed sludge. However, the combination of fixed biomass on porous support material and suspended biomass has the advantage of increasing the total biomass in the system. Furthermore under certain circumstances a simultaneous denitrification takes place in the totally aerated basin. The measurement itself and the findings of the investigations will be presented and discussed in detail in the paper.

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