With the continuous increase in environmental requirements and regulations for wastewater treatment a lot of wastewater treatment plants in Austria had to be extended. Extension of the plant requires in general an additional area, which is not always available. This study shows the extension of the WWTP Hard Hofsteig and doubling its capacity by using the Hybrid concept, which is a special two stage activated sludge process for extensive nutrient removal. The plant was extended from 170,000 PE to 270,000 PE without changing the footprint of the plant and the extension costs could be kept very low. Operational data shows that by operating the plant in the two stage Hybrid mode the energy balance of the plant can be improved.

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