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Modeling External Carbon Addition in Combined N-P Activated Sludge Systems with an Extension of the IWA Activated Sludge Models

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The aim of this study was to expand the IWA Activated Sludge Model No. 2d (ASM2d) to include the effect of adding a readily biodegradable exogenous substrate to anoxic zones of biological N/P removal systems, where it can be used as growth substrate by the Polyphosphate Accumulating Organisms (PAOs) but can not be used by PAOs in the anaerobic zone. The model change was to add a new biodegradable substrate component and process terms for its use by PAOs and other heterotrophic bacteria. The new model and the original ASM2d were first calibrated/validated under dynamic conditions with the results of both batch tests to observe nitrate uptake rates (NURs), phosphorus release rates (PRRs) and anoxic phosphorus uptake rates (PURs) with the settled wastewater and a 96–hour measurement campaign in the full-scale MUCT bioreactor. The results of similar batch tests with ethanol and fusel oil as the external carbon sources were used to adjust the kinetic and stoichiometric coefficients in the expanded ASM2d. The results of these two tests were used to compare predictions of the new model and the original ASM2d. For this purpose, it was assumed that the added external carbon sources were treated as the new component SA,1 (in the expanded ASM2d) or a fraction of SA (in the ASM2d). In the latter case, much higher COD utilization rates were predicted under anoxic conditions, whereas the anoxic PURs in the two-phase experiments were underestimated. In spite of the mechanistically inappropriate approach to treat some external sources as the ASM2d components, the original (calibrated) ASM2d predicted NO3-N and PO4-P concentrations in the full-scale bioreactor only slightly different from the new model.
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Keywords: ASM; Activated sludge; denitrification; external carbon; mathematical modeling; nutrient removal; simulation

Document Type: Research Article

Publication date: 2011-01-01

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