Spatially resolved nitrous oxide emissions in wastewater treatment processes

Nitrous oxide (N₂O) is a potent greenhouse gas emitted from wastewater treatment plants (WWTPs). However, the origins of emitted N₂O remain insufficiently understood, and emissions from primary treatment processes are often overlooked due to methodological limitations. In this study, N₂O emissions were monitored at seven WWTPs across China using zonal odor control systems to obtain spatially resolved measurements. The average N₂O emission factors ranged from 0.014% to 0.122%, with either primary or biological treatment process serving as the dominant contributor depending on plant conditions. To further characterize emissions from different emission zones, intensive monitoring covering both diurnal and seasonal variations was conducted at two representative WWTPs. In the biological-dominant plant, influent NH₄⁺-N loads exhibited a strong positive correlation with N₂O emission rates in aerobic treatment (ρ=0.866, p < 0.01). In the primary-dominant plant, N₂O transported from the sewer system accounted for 104-168% of total full-scale emissions, which was likely due to N₂O consumption via denitrification in the anaerobic and anoxic treatment. Meanwhile, on-site batch experiments revealed that primary treatment also generated N₂O, with microbial production contributing 43.6% of total gaseous emissions from the primary treatment. Elevated emissions were observed during winter months at this plant. Overall, this study highlights the substantial and previously underestimated contribution of primary treatment to N₂O emissions and provides new insight into identifying emission hotspots and understanding N₂O formation mechanisms in WWTPs.