Long-term nitrous oxide monitoring data on 3 full-scale wastewater treatment plants

The currently applied approaches to estimate direct greenhouse gas (GHG) emissions of nitrous oxide (N2O) and methane (CH4) from wastewater treatment (sector 5D) in the national greenhouse gas (GHG) inventories of Switzerland are based on the IPCC guidelines. (IPCC 2006). A review of the Swiss application of guidelines showed that the guidelines are only partly representative for Switzerland. (Eawag 2018). Hence, an extended study was performed including extensive monitoring campaigns on Swiss wastewater treatment plants (WWTPs), analysis of existing data sets and literature reviews. The present document summarizes the study and suggests an improved methodology for an assessment of the GHG emissions of sector 5D. N2O emission are drastically underestimated with the current methodology. Additionally, source allocation of emissions is not representative. Hence, we suggest a new methodology based on 17 long-term monitoring campaigns on full-scale WWTPs in Switzerland and an analysis of nitrogen flows in Swiss WWTPs. The decreasing N2O emissions since 1990 link to improvements of nitrogen removal rates on WWTPs. Further reductions are expected in the coming decades because of further planned improvements of nitrogen removal on Swiss WWTPs. Major uncertainties within the new methodology can be linked to the emission factor of WWTPs with carbon removal only. The occurrence of such plants decreased substantially between 1990 and 2020. CH4 emissions are overestimated with the current methodology. Additionally, source allocation of emissions is not representative. Hence, we propose a new methodology based on monitoring campaigns and a literature review. Emissions remained relatively stableincreased moderately since 1990 despite along with an increase in population and accompanied by due to technological improvements at the WWTPs (covering open sludge storage tanks). This data package contains long-term online monitoring data for three Swiss WWTPs over at least one year used for the methodology described above. The operators of one of the WWTP decided to stay anonymous (Carbon removal WWTP). Other monitoring campaigns are described unter the following doi: https://doi.org/10.25678/0003XE. The method to monitor the emissions is described under the following doi:https://doi.org/10.25678/0003WD.