Plant-integrated methane (CH) and nitrous oxide (NO) emission quantifications were performed at five Scandinavian wastewater treatment plants, using a ground-based remote sensing approach that combines a controlled release of tracer gas from the plant with downwind concentration measurements. CH emission factors were between 1 and 21% of CH production, and between 0.2 and 3.2% of COD influent. The main CH emitting sources at the five plants were sludge treatment and energy production units. The lowest CH emission factors were obtained at plants with enclosed sludge treatment and storage units. NO emission factors ranged from <0.1 to 5.2% of TN influent, and from <0.1 to 5.9% of TN removed. In general, measurement-based, site-specific CH and NO emission factors for the five studied plants were in the upper range of the literature values and default emission factors applied in international guidelines. This study showed that measured CH and NO emission rates from wastewater treatment plants were plant-specific and that emission rates estimated using models in current guidelines, mainly meant for reporting emissions on the country scale, were unsuitable for Scandinavian plant-specific emission reporting.

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 CH emission rates, measured using mobile ground-based remote sensing methods, varied between 28.5 and 33.5 kg CH h, corresponding to an average emission factor of 5.9% as kg CH (kg CH) , whereas NO emissions varied between 4.0 and 6.4 kg h, corresponding to an average emission factor of 1.5% as kg NO-N (kg TN ). Plant-integrated NH emissions were around 0.4 kg h, corresponding to an average emission factor of 0.11% as kg NH-N (kg TN ). On-site emission measurements showed that the largest proportions of CH (70%) and NH (66%) were emitted from the sludge treatment line (mainly biosolid stockpiles and the thickening and dewatering units), while most of the NO (82%) was emitted from nitrifying trickling filters. In addition to being the most important CH 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 CH emissions and generally higher NO and NH emissions were observed when the digesters were operated in series. Loading biosolids onto trucks for off-site treatment generally resulted in higher CH, NO, and NH emissions from the biosolid stockpiles. On-site CH and NO 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.

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