Model assisted identification of N₂O mitigation strategies for full-scale reject water treatment plants

In a 3-year research project, a new approach to forecast biological N₂O formation and emission at high-strength reject water treatment has been developed (ASM3/1_N2O_ISAH). It was calibrated by extensive batch-tests and finally evaluated by long-term measurement campaigns realized at three wastewater treatment plants (WWTPs) with different process configurations for nitrogen removal of reject water. To enable a model application with common full-scale data, the nitritation-connected supplementary processes that are responsible for N₂O formation are not depicted in the model. Instead, within the new model approach the N₂O formation is linked to the NH₄-N oxidation rate by defining specific formation factors [N₂O-N_form/NH₄-N_ox], depending on the concentrations of NO₂ and O₂ as well as the NH₄ load. A comparison between the measured and the modeled N₂O concentrations in the liquid and gas phase at the full-scale treatment plants prove the ability of the proposed modelling approach to represent the observed trends of N₂O formation, emission and reduction using the standard parameter set of kinetics and formation factors. Thus, enabling a reliable estimation of the N₂O emissions for different operational conditions. The measurements indicate that a formation of N₂O by AOB cannot completely be avoided. However, a considerable reduction of the formed N₂O was observed in an anoxic environment. Applying the model, operational settings and mitigation strategies can now be identified without extensive measurement campaigns. For further enhancement of the model, first results for kinetics of N₂O reduction kinetics by denitrification processes were determined in laboratory-scale batch tests.