Three series of data obtained in a pilot-scale alternating-type activated sludge nutrient removal process are examined in light of the recently introduced Activated Sludge Model No. 2 (ASM2). The first data series was collected while adding acetate to the inlet to improve phosphate removal. ASM2 can explain why phosphate levels first rise before an improvement occurs based on a slow build-up of internally stored polyhydroxyalkanoates (PHA). The second and third data series represent batch experiments performed in the pilot plant, and show the effect of nitrate on phosphate release. ASM2 could describe the second data series well after including one additional reaction to account for denitrification by phosphate accumulating organisms. The third data series examines the effect of the rate of acetate addition, and shows that this affects the rate of anoxic phosphate release more strongly than it does the denitrification rate.

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