Two different concepts of the running-in (without inoculation) of activated sludge plants for nitrification and denitrification have been tested on two pilot plants for the extension of a municipal WWTP. During the running-in period, the aeration tank of one pilot plant was fully aerated, whereas the other pilot plant was operated with an anoxic section of 20%. It could be shown that the installation of anoxic zones during the running-in period leads to a higher nitrite peak as compared to complete aeration of the whole volume of the aeration tank. Hence, it is highly recommended to aerate the whole biological reactor, until complete nitrification is achieved. Furthermore, it was found that the activated sludge of the fully aerated pilot plant was not capable of the reduction step of nitrite to gaseous nitrogen. This indicates that for ‘full’ denitrification the heterotrophic biomass has to be adapted to ‘regular’ anoxic conditions at first.

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