The wastewater treatment plant (WWTP) of Wörgl-Kirchbichl is located on the bank of the river Inn in the municipality of Kirchbichl in Tirol, Austria. It is a medium-size two stages WWTP that encounters strong seasonal variation due to tourism, with high nitrogen returns in sludge dewatering effluent (SDE). This case study offers a great opportunity to integrate and evaluate at full-scale an optimized concept for the treatment of the sludge dewatering effluent to support the development of an energy positive WWTP. Emphasis will be placed on the integration of the side stream treatment in the whole wastewater treatment concept.


  • Full-scale demonstration of integrated SDE management via nitritation at a two-stage WWTP:
    • Operational and long-term experience
    • Investigations on specific factors as e.g. C/N ratio or seasonal impacts
    • Evaluation of the expected reduced aeration demand and increased biogas yield
  • Development of a simulation model for the whole biological wastewater two-stage treatment process, including nitritation of SDE and electricity input/output
    • Process optimization of nitration in SDE and identification of technical/biological limits
  • Verification of the model’s outcomes for two-stage and even one-stage activated sludge plants.
  • Modelling based comparison of nitritation in SDE with state-of-the-art processes in regard to efficiency and costs.
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Sludge dewatering effluent (SDE) from dewatering of digested sludge contains a significant load of nitrogen (mainly NH4), which is usually recycled to the inlet of the WWTP where it contributes by 15-20% to the total nitrogen load of the plant and leads to a higher aeration demand in the activated sludge tank. The conventional biological side-stream treatment options are based on well-established wastewater treatment processes like nitrification/denitrification (over nitrate) or nitritation/denitritation (over nitrite). In these processes organic carbon (COD) needs to be added as electron donor for the reduction of nitrite/nitrate to N2 gas.

In alternative, SDE can be pre-treated in side stream by nitritation only, before being recycled to the main stream of the WWTP. Nitrite contains chemically bound oxygen which can be used for COD removal by denitritation, thus reducing the energy demand for aeration. Additionally, in two-stage WWTPs the reduced carbon demand in the first stage will increase the biogas yield in the anaerobic digestion.

The case study 5 will integrate this innovative approach in a two-stage WWTP and develop an optimised treatment and management concept considering plant layout and wastewater composition. The implementation of nitritation in SDE will provide an optimised energy balance of the WWTP by significantly decreasing the aeration demand and enhancing the biogas yield.

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Local Associated Partners

At Kirchbichl, POWERSTEP project partners rely on the support of: