Abstract

The aim of this study was to expand the IWA Activated Sludge Model No. 2d (ASM2d) to account for a newly defined readily biodegradable substrate that can be consumed by polyphosphate accumulating organisms (PAOs) under anoxic and aerobic conditions, but not under anaerobic conditions. The model change was to add a new substrate component and process terms for its use by PAOs and other heterotrophic bacteria under anoxic and aerobic conditions. The Gdansk (Poland) wastewater treatment plant (WWTP), which has a modified University of Cape Town (MUCT) process for nutrient removal, provided field data and mixed liquor for batch tests for model evaluation. The original ASM2d was first calibrated under dynamic conditions with the results of batch tests with settled wastewater and mixed liquor, in which nitrate uptake rates, phosphorus release rates and anoxic phosphorus uptake rates were followed. Model validation was done with data from a 96-hour measurement campaign in the full-scale WWTP. 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. Both models were compared based on their predictions of the effect of adding supplemental carbon to the anoxic zone of an MUCT process. In comparison with the ASM2d, the new model better predicted the anoxic behaviors of COD, NO3-N and PO4-P in batch experiments with ethanol and fusel oil. However, when simulating ethanol addition to the anoxic zone of a full-scale BNR facility, both models predicted similar effluent NO3-N concentrations (6.6-6.9 g N/m3). For the particular application, effective enhanced biological P removal was predicted by both models with external carbon addition but for the new model, the effluent PO4-P concentration was about half of that found from ASM2d. On a PO4-P removal percentage basis, the difference was very small, i.e. 94.1% vs. 97.1%, respectively, for the ASM2d and expanded ASM2d.

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Authors (5)