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

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 CH4production) −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.