How operational strategies influence nitrogen removal and nitrous oxide emissions: Insights from a full-scale...

Nitrous oxide (N₂O) emissions from wastewater treatment plants (WWTPs) pose a significant challenge to the sector's transition toward net-zero greenhouse gas (GHG) emissions. While N₂O is primarily generated during biological nitrogen conversions, its production and emission dynamics remain difficult to predict due to complex interactions between operational parameters, microbial activity, and environmental conditions. In this research, we present a full-scale case study to explore how operational decisions, specifically, feeding point distribution and aeration strategies, can affect the spatial distribution of substrates, microbial processes, and ultimately, N₂O emissions. The results reveal that shifting from step-feed to plug-flow operation significantly altered ammonia (NH₄⁺), nitrite (NO₂^-), and dissolved oxygen (DO) profiles, leading to average increase in N₂O production from 0.38 Kg-N/hr during step-feed to 14.50 Kg-N/hr during plug-flow operation. This increase was not tied to infrastructure changes but rather to dynamic microbial responses. Molecular analyses confirmed shifts in the abundance of key functional genes, further supporting the observed emission trends. Importantly, this study highlights the risks of under- or overestimating emissions when relying on limited sensor data, given the high spatial variability of N₂O within a reactor. Process modeling is suggested as a practical tool to identify emission hotspots and guide sensor placement under uncertainty. Overall, the findings challenge the assumption that N₂O emissions could be dictated by process treatment technology and underscore the need to view emissions as a dynamic outcome shaped by everyday operational decisions. Integrating GHG metrics into routine assessments and adopting a holistic monitoring framework is essential for informed, climate-conscious operation of WWTPs.