Optimisation of nitrifying activated sludge plants towards nutrient removal (denitrification and enhanced P-removal) leads to a substantial reduction of operating costs and improves effluent and operating conditions. At WWTP Zürich-Werdhöelzli, initially designed for nitrification only, an anoxic zone of 28% of total activated sludge volume was installed and allowed 60% nitrogen elimination besides several other optimisations. In 2001 the operation of WWTP Zürich-Glatt was stopped and the wastewater was connected to WWTP Werdhöelzli. To improve nitrogen removal, WWTP Werdhöelzli co-financed two research projects; one for separate digester supernatant treatment with the anammox process operating two SBRs in series and the other applying NH4 sensors for aeration control in order to decrease energy consumption and raise effluent quality. The results of both projects and the consequences for WWTP Werdhöelzli are discussed in this paper.

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