A decade of nitrous oxide (N 2 O) monitoring in full-scale wastewater treatment processes: A critical review.

Direct nitrous oxide (NO) emissions during the biological nitrogen removal (BNR) processes can significantly increase the carbon footprint of wastewater treatment plant (WWTP) operations. Recent onsite measurement of NO emissions at WWTPs have been used as an alternative to the controversial theoretical methods for the NO calculation. The full-scale NO monitoring campaigns help to expand our knowledge on the NO production pathways and the triggering operational conditions of processes. The accurate NO monitoring could help to find better process control solutions to mitigate NO emissions of wastewater treatment systems. However, quantifying the emissions and understanding the long-term behaviour of NO fluxes in WWTPs remains challenging and costly. A review of the recent full-scale NO monitoring campaigns is conducted. The analysis covers the quantification and mitigation of emissions for different process groups, focusing on techniques that have been applied for the identification of dominant NO pathways and triggering operational conditions, techniques using operational data and NO data to identify mitigation measures and mechanistic modelling. The analysis of various studies showed that there are still difficulties in the comparison of NO emissions and the development of emission factor (EF) databases; the NO fluxes reported in literature vary significantly even among groups of similar processes. The results indicated that the duration of the monitoring campaigns can impact the EF range. Most NO monitoring campaigns lasting less than one month, have reported NO EFs less than 0.3% of the N-load, whereas studies lasting over a year have a median EF equal to 1.7% of the N-load. The findings of the current study indicate that complex feature extraction and multivariate data mining methods can efficiently convert wastewater operational and NO data into information, determine complex relationships within the available datasets and boost the long-term understanding of the NO fluxes behaviour. The acquisition of reliable full-scale NO monitoring data is significant for the calibration and validation of the mechanistic models -describing the NO emission generation in WWTPs. They can be combined with the multivariate tools to further enhance the interpretation of the complicated full-scale NO emission patterns. Finally, a gap between the identification of effective NO mitigation strategies and their actual implementation within the monitoring and control of WWTPs has been identified. This study concludes that there is a further need for i) long-term NO monitoring studies, ii) development of data-driven methodological approaches for the analysis of WWTP operational and NO data, and iii) better understanding of the trade-offs among NO emissions, energy consumption and system performance to support the optimization of the WWTPs operation.