Modelling the full-scale N2O emissions from wastewater treatment plants for identifying mitigation strategies

Sammanfattning: Nitrous oxide (N2O) emissions, which primarily originate from the biological nitrogen removal process, dominate the release of greenhouse gases (GHGs) in wastewater treatment plants (WWTPs). N2O production occurs through dynamic microbial pathways that have a significant impact on the environment compared with other GHG emissions. Reducing these substantial emissions aligns with the objectives of minimizing the carbon footprint of WWTPs and achieving sustainability. This study aims to investigate the pathways responsible for N2O production through a comprehensive model-based approach on a full-scale basis to identify effective mitigation strategies. To model N2O emissions, this study employs the activated sludge model (ASM), specifically ASM2dISS, which incorporates two N2O production pathways: nitrifier denitrification and heterotrophic denitrification pathways. The collected measurements are pre-processed in MATLAB by filling in the missing values and removing the outlier values using flow balances and the data is analyzed for obtaining clean data for modelling the treatment plant and for choosing the periods for model simulations. With the available data, the model is constructed in the WEST and firstly, steady-state simulations are calibrated, and then dynamic calibrations are performed using dynamic data of 24-hour daily measurements. Calibrations are executed for solids, nitrogen species and N2O emissions of the treatment process and the calibrated model is validated for different periods. Kinetic parameters are adjusted in the model and calibration and validation results are compared with the plant measurements. Model simulations generally predicted well with the measured values in most of the simulated days except nitrate content in the activated sludge units which is slightly underestimated than the measurements. It is identified that nitrate concentration is strongly correlated with N2O emissions and as calibration is focused on N2O emissions, nitrate calibration is limited considering its effect on N2O emissions. Model results revealed that N2O emissions are contributed by both nitrifier denitrification and heterotrophic denitrification pathways with heterotrophic denitrification as a dominant pathway. The calibrated model is used for implementing the mitigation strategies in the activated sludge process. Two approaches have been proposed as mitigation strategies, one is to vary the SRT of the system and another approach is to introduce internal recirculation in the secondary treatment process. The second approach is evaluated with four cases, by varying recirculation flow rates and the number of anoxic zones in the process. Results are compared with calibrated values as a reference, in which results from both approaches showed a good reduction of N2O emissions. When results from all the strategies are compared, 3 times the internal recirculation with 1 anoxic zone showed good possibilities for lowering N2O emissions from the treatment process. Each strategy comes with certain positives and negatives in terms of process modifications and energy demands, therefore every aspect needs to be considered for implementing the proposed mitigation strategies.