Decoupling nitrous oxide emissions from carbon footprint in full-scale completely autotrophic nitrogen removal over nitrite process: key drivers and sustainability assessment.

While the partial nitrification/anammox-based completely autotrophic nitrogen removal over nitrite (CANON) process offers substantial advantages for treating high-ammonium wastewater, significant nitrous oxide (N2O) emissions pose a challenge to its low-carbon sustainability. In this study, long-term field sampling and stable isotope analysis were conducted in a full-scale CANON process to evaluate N2O dynamics and the contributions of different pathways, as well as to perform a scenario-based carbon footprint assessment. The results showed that N2O emissions were low when hydroxylamine oxidation dominated, whereas free nitrous acid (FNA) accumulation was associated with an increased estimated denitrification-associated contribution and higher N2O emissions. Aeration-enhanced gas-liquid transfer likely contributed to dissolved N2O release into the gas phase. During long-term operation of this process, periods with dissolved oxygen (DO) concentrations below 0.20 mg/L were associated with approximately 65 % lower N2O emissions. Under the adopted system boundaries and parameter values, the calculated carbon footprint and operating cost of the CANON scenario were 42 % and 83 % lower than those of the conventional anaerobic-anoxic-oxic process, mainly owing to lower electricity consumption and the absence of external carbon addition. These findings indicate that maintaining stable pH, limiting FNA accumulation, and avoiding transient DO increases may help preserve the low-carbon performance of CANON under site-specific operating conditions.