Growth carriers from a technical deammonifying moving-bed WWTP were used in batch tests to determine possible N-conversion reactions under varying oxygen and substrate conditions. Deammonification, denitrification, and nitrification reactions could be proved using conventional analysis, combined with gas chromatography/mass spectrometry analysis to get additional information about 15N-isotope labelled gaseous end products of the different reactions. In this orientating study N2O could be observed in some cases up to 12% of the total gas production. N2O production came from incomplete denitrification processes under anoxic or oxygen-limiting conditions and in the absence of organic substrate. It seems as if structural components of deammonifying biofilms play a crucial role for the portion of side-reactions, leading to undesirable gaseous end products.

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