Abstract
Nitrous oxides (N2O) emissions contribute to climate change and stratospheric ozone depletion. Wastewater treatment is an important, yet likely underestimated, source of N2O emissions, as recent, long-term monitoring campaigns have demonstrated. However, the available data are insufficient to representatively estimate countrywide emission due to the brevity of most monitoring campaigns. This study showed that the emission estimates can be significantly improved using an advanced approach based on multiple continuous, long-term monitoring campaigns. In monitoring studies on 14 full-scale wastewater treatment plants (WWTPs), we found a strong variability in the yearly emission factors (EFs) (0.1 to 8% of the incoming nitrogen load) which exhibited a good correlation with effluent nitrite. But countrywide data on nitrite effluent concentrations is very limited and unavailable for emission estimation in many countries. Hence, we propose a countrywide emission factor calculated from the weighted EFs of three WWTP categories (carbon removal, EF: 0.1–8%, nitrification only: 1.8%, and full nitrogen removal: 0.9%). However, EF of carbon removal WWTPs are still highly uncertain given the expected variability in performance.The newly developed approach allows representative, country-specific estimations of the N2O emissions from WWTP. Applied to Switzerland, the estimations result in an average EF of 0.9 to 3.6% and total emissions of 410 to 1690 tN2O-N/year, which corresponds to 0.3–1.4% of the total greenhouse gas emissions in Switzerland. Our results demonstrate that better data availability and an improved understanding of long-term monitoring campaigns is crucial to improve current emission estimations. Finally, our results confirm several measures to mitigate N2O emissions from wastewater treatment; year-round denitrification, limiting nitrite accumulation, and stringent control of sludge age in carbon removal plants.
Highlights
Nitrous oxide (N2O) emissions contribute substantially to climate change (IPCC 2014) and stratospheric ozone depletion (Ravishankara et al, 2009)
N2O emission factors (EFs) from wastewater treatment plants (WWTPs) exhibit a strong relation with the effluent NO2− and nitrogen removal efficiency
Applied to Switzerland, the estimations result in an average EF of 0.9–3.6% and total emissions of 410–1690 tN2O-N/year, which corresponds to 0.3–1.4% of total Swiss greenhouse gas (GHG) emissions
Summary
Nitrous oxide (N2O) emissions contribute substantially to climate change (IPCC 2014) and stratospheric ozone depletion (Ravishankara et al, 2009). The atmospheric N2O concentration is expected to rise until the middle of the 21st century (Tian et al, 2020). Wastewater treatment plants (WWTPs) and their N2O production are currently often underestimated emission processes in national greenhouse gas (GHG) inventories. Estimations of countrywide N2O emissions are based on assumed emission factors (EFs). EFs in the standard reporting guidelines are much lower (0.03% to 0.14%; Daelman et al, 2015, IPCC 2006) than reported long-term full-scale measurements. With the refinement of the IPCC methodology in 2019, increased EFs of 0.01% to 2.9% appeared in the reporting guidelines (IPCC 2019)
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