This study estimates the carbon footprints of 16 municipal wastewater treatment plants (WWTPs), all situated in Scandinavian countries, by using a simple model. The carbon footprint calculations were based on operational data, literature emission factors (efs) and measurements of greenhouse gas emissions at some of the studied WWTPs. No carbon neutral WWTPs were found. The carbon footprints ranged between 7 and 108 kg CO2e P.E.(-1) year(-1). Generally, the major positive contributors to the carbon footprint were direct emissions of nitrous oxide from wastewater treatment. Whether heat pumps for effluents have high coefficient of performance or not is extremely important for the carbon footprint. The choice of efs largely influenced the carbon footprint. Increased biogas production, efficient biogas usage, and decreased addition of external fossil carbon source for denitrification are important activities to decrease the carbon footprint of a WWTP.

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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