N₂O Emissions from Danish WWTPs

All nine WWTPs installed Unisense Environment N₂O Wastewater Sensors to provide continuous, real-time measurements of N₂O concentration directly in the wastewater. The data collected using the N₂O sensors were subsequently used for calculating N₂O emissions using N₂O emission models. The emission data were used to calculate an overall average national emission factor: 0.84% N₂O-N/Total-N_inlet. This corresponds to 0.0084 kg N₂O-N/kg Total-N_inlet with a variation of 0.24–1.24% N₂O-N/Total-N_inlet. This is about half of the 1.6% N₂O-N/T-N_inlet emission factor used in the IPCC report from 2019, but the previous reported national emission factor is about 2.5 times higher.

The emission factor calculated from this study will be used as a basis for future inventories, because it is based on the most comprehensive dataset yet. The calculated emission factor represents an estimate and should be adjusted when further data become available.

The study showed a large variation in N₂O emission from plant to plant and in day-to-day emissions from individual plants. The data indicate that increased nitrogen load and a generally highly loaded biological process lead to higher N₂O emissions compared to lower loaded biological processes.

Anammox sidestream processes have high nitrogen loading and nitrogen removal rates. This study found an emission factor of 5–6% N₂O-N/Total-N_inlet, which is significantly higher than the average emission factor found for mainstream processes. Furthermore, this study indicated that there was a relationship between the residual available capacity in the biological treatment and the amount of nitrous oxide emitted, where a larger capacity emits less N₂O.

This study offers indications of mechanisms leading to increased N₂O emissions. However, the data show that ammonium loading, carbon loading, and aeration are important factors for N₂O emissions. Online monitoring should be implemented to understand N₂O emissions and to implement online control strategies. In Denmark, it will be compulsory by 2025 to reduce greenhouse gas emissions. Limiting N₂O emissions from WWTPs will be part of reaching this goal. As most Danish wastewater treatment plants have not taken any steps to reduce emission, the potential for reducing the overall emission is very large. Collecting knowledge on nitrous oxide emission, triggers, and mitigation strategies from national and international projects will contribute to an increased understanding of the subject.