Optical technologies applied alongside on-site and remote approaches for climate gas emission quantification at a...

Plant-integrated and on-site gas emissions were quantified from a Swedish wastewater treatment plant by applying several optical analytical techniques and measurement methods. Plant-integrated CH₄ emission rates, measured using mobile ground-based remote sensing methods, varied between 28.5 and 33.5 kg CH₄ h^-1, corresponding to an average emission factor of 5.9% as kg CH₄ (kg CH_4production) ^-1, whereas N₂O emissions varied between 4.0 and 6.4 kg h^-1, corresponding to an average emission factor of 1.5% as kg N₂O-N (kg TN _influent) ^-1. Plant-integrated NH₃ emissions were around 0.4 kg h^-1, corresponding to an average emission factor of 0.11% as kg NH₃-N (kg TN _removed) ^-1. On-site emission measurements showed that the largest proportions of CH₄ (70%) and NH₃ (66%) were emitted from the sludge treatment line (mainly biosolid stockpiles and the thickening and dewatering units), while most of the N₂O (82%) was emitted from nitrifying trickling filters. In addition to being the most important CH₄ source, stockpiles of biosolids exhibited different emissions when the sludge digesters were operated in series compared to in parallel, thus slightly increasing substrate retention time in the digesters. Lower CH₄ emissions and generally higher N₂O and NH₃ emissions were observed when the digesters were operated in series. Loading biosolids onto trucks for off-site treatment generally resulted in higher CH₄, N₂O, and NH₃ emissions from the biosolid stockpiles. On-site CH₄ and N₂O emission quantifications were approximately two-thirds of the plant-integrated emission quantifications, which may be explained by the different timeframes of the approaches and that not all emission sources were identified during on-site investigation. Off-site gas emission quantifications, using ground-based remote sensing methods, thus seem to provide more comprehensive total plant emissions rates, whereas on-site measurements provide insights into emissions from individual sources.