Greenhouse gas emissions from wastewater treatment processes: identifying triggering factors at laboratory and full-scale systems

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This thesis focuses on the study of the global green house gas (GHG) emissions from wastewater systems. Nitrous oxide (N2O) and methane (CH4) are the main GHG directly emitted from wastewater treatment plants (WWTP). In this thesis, different studies were performed in order to determine the N2O production when using different combination of electron acceptors during denitrification. Firstly, three different external carbon sources where used in a mixed denitrifying culture. Secondly, a denitrifying polyphosphate accumulating organism (dPAO) and a denitrifying glycogen accumulating organism (dGAO) enriched cultures were used to assess the effect of using an internal carbon source (polyhydroxyalkanoates, PHA) for denitrification on the N2O production. Results indicated that electron competition during the reduction of different nitrogen oxides is a significant factor in ordinary heterotrophic denitrification processes using external carbon sources as the electron donor, but not in PHA-driven denitrification processes conducted by dPAO or dGAO. Results also showed that generally, higher N2O accumulation was detected in the tests conducted with dGAO than those conducted with dPAO, especially when nitrite was used as electron acceptor. Later, the effect of DO at a constant pH level and the effect of pH at a constant DO level on N2O and NO production in a partial nitrification sequencing batch reactor (SBR) were explored. Also, the relationship between NO production and the ammonia oxidation rate (AOR) as well as the N2O production rate and the AOR were studied. Results showed that these relationships were linear and exponential, respectively. This investigation highlighted the importance of also monitoring NO emissions since they may lead to N2O emissions. The last investigation of this thesis was a long-term full-scale study in the WWTP of Girona in order to assess the N2O and CH4 emission dynamics of the plug-flow reactors. Results showed seasonal and spatial variations on N2O emissions but only spatial variations on CH4 emissions. Finally, the overall carbon footprint of the plug-flow reactor was assessed. ​
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