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The Activated Sludge Model No. 2 (ASM2) (Gujer et al. 1995) and the Activated Sludge Model No. 3 combined with the EAWAG Bio-P Module (EAWAG model) (Rieger et al. 2001) were evaluated under dynamic loading conditions. These models differ mainly in their approach to model decay processes. While the ASM2 model uses the death-regeneration concept, the EAWAG model uses the endogenous respiration approach. In our modeling study, we found that during and after low loading conditions the specific approach of modeling decay significantly affected model predictions. The two models predicted different levels of the effluent PO4 concentrations during and after the 3-day period of low loading. One structural difference between both models found to be important was that ASM2 assumes decay processes to occur both under aerobic and anaerobic conditions, whereas the EAWAG model assumes decay to take place only under aerobic conditions. Both models predicted an increase in effluent PO4 during the low loading period, which does not agree with experimental results and is assumed to be an artifact. This artifact can be linked to a basic assumption in both ASM2 and the EAWAG model where the decay of polyphosphate accumulating organisms (PAOs) and their storage products is assumed to occur at identical rates. Our experimental observations suggest that the rate of decay for active PAOs is larger compared to the rate of decay for poly-P.
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Document Type: Research Article

Publication date: 2003-01-01

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