The Sjölunda wastewater treatment plant in Malmö, Sweden, was upgraded for extended nutrient removal in 1998-1999. The design was based on future effluent standards of 10 mg BOD7/l, 0.3 mg total-P/l and 8 mg total-N/l. The upgrading concept took into consideration existing processes and structures, resulting in a cost-effective and compact upgrading. To introduce nitrification, the existing trickling filters for BOD-removal were converted to a nitrifying mode. A sequencing batch reactor for nitrification of supernatant was necessary to control the ammonia load. Denitrification was accomplished in a moving bed biofilm reactor with addition of external carbon source. The future effluent standards could be met by the upgraded plant. The trickling filters were stable despite varying loading conditions. High rates and low effluent ammonia concentrations were achieved. Essential features for stable post denitrification were control strategies for carbon source dosage and avoiding phosphorus limitation.

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