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Direct and indirect greenhouse gases emissions of two model municipal WWTPs were estimated using carbon footprint analyses. One WWTP was designed with anaerobic digestion, the other one with simultaneous aerobic stabilization of sewage sludge. Emission factors were derived from literature values. For direct nitrous oxide emissions, a new estimation model based on measurements at eight Austrian WWTPs was applied. Results show that the direct nitrous oxide emission from the activated sludge tanks dominates the carbon footprint of WWTPs with a moderate nitrogen removal. Anaerobic digesters and anaerobic sludge storage tanks can also become a relevant source of direct methane emissions.
Parravicini et al. (Tue,) studied this question.