In 1999 the existing activated sludge unit of the Southpest Wastewater Treatment Plant was supplemented by a two-stage biofilter system aiming for nitrification and post-denitrification. In this arrangement excess biomass of the filters is wasted through the activated sludge unit, facilitating backseeding, and recirculation of the nitrate-rich effluent of the N-filter serves for decreasing the methanol demand of the DN-filter and for saving aeration energy at the same time. The paper reports on the development of an ASM1-based mathematical model that proved to be adequate for describing the interactions in the combined system and was used to compare the efficiency of different treatment options. Full-scale results verified that backseeding may considerably improve performance. However, nitrification ability of the activated sludge unit depends on the treatment temperature and, if unexpected, can be limited by insufficient oxygen supply. The upgrading possibilities outlined may serve as a new perspective for implementation of combined activated sludge-biofilter systems.

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