N2O Emissions from Danish WWTPs - a two year monitoring project
N2O emission varies in time and between plants
Based on limited data, the WWTPs have used a factor to estimate the N2O emission. However, the available data show that the N2O emission varies in time and between plants. Therefore, more data is needed to achieve a reliable estimate of the N2O emission from Danish WWTPs.
To quantify the N2O emission from Danish WWTPs, the Danish Environmental Protection Agency (EPA) launched a funding scheme aimed at Danish utilities to collect data on N2O emissions. In the period 2018-2020, nine different plants monitored the N2O emission. The nine WWTPs cover a range in terms of plant size, nitrogen load, aeration technology, sludge treatment, and reject water handling.
Continuous, real-time N2O measurements
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-Ninlet. This corresponds to 0.0084 kg N2O-N/kg Total-Ninlet with a variation of 0.24–1.24% N2O-N/Total-Ninlet. This is about half of the 1.6% N2O-N/T-Ninlet 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.
Variations in individual measurements
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-Ninlet, 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.
Large potential in the reduction of N2O emissions
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.