The Main Wastewater Treatment Plant of Vienna had to be extended to guarantee sufficient nitrogen removal. After intensive evaluation studies a two-stage activated sludge system was chosen for the plant-extension. Due to the very small specific reactor tank volume of two-stage treatment plants in comparison with low loaded single-stage plants internal cycles had to be applied to ensure sufficient nitrogen removal.

Starting in the year 2000 the plant extension was finished by the beginning of 2005 and the extended plant went into operation in spring. The Austrian effluent standards had to be fulfilled by December 2005.

This paper presents a description of the plant layout and gives an overview of the operating results of the first two years of full scale operation. Mass balances were used to evaluate the pathways of nitrogen removal. The results confirmed the expected flexibility of the chosen concept; nitrogen elimination did not decrease although the effluent recirculation that has been implemented to ensure sufficient nitrogen removal efficiency especially during winter had been reduced markedly due to energy reasons. The treatment efficiency that was observed at pilot plant investigations was clearly exceeded.

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