Optimization of nitrifying activated sludge plants towards nutrient removal (denitrification and enhanced P-removal) will lead to substantial reduction of operating costs and improves effluent and operating conditions. At the WWTP Zürich-Werdhölzli, initially designed for nitrification, an anoxic zone of 28% of total activated sludge volume was installed in the initially fully aerated activated sludge tanks and allowed with optimization of the sludge blanket in the secondary clarifiers a nitrogen elimination of 60–70%. Parallel, different small process alterations at the grit removal tank, the primary clarifiers and return sludge pumps reduced odour problems and oxygen input and improved denitrification. Separate treatment of the digester supernatant, that contains about 20% of the inlet nitrogen load, will increase nitrogen elimination to about 75% and allows partial enhanced biological O-removal in the activated sludge system which will reduce precipitant feed and excess sludge production.

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