Discussion
4.1. Process performance: regular operation and process perturbations
During the majority of the experimental campaign carried out at
Granollers WWTP, the treatment system performed correctly with
high COD and N removal rates, as described before. Process parameters
(Table 1) given under this study could be considered normal for a municipalWWTP
of these characteristics and configuration. The variations
in aeration flow and the reject wastewater influxes were part of the
regular operation of the plant. The period of nitrification instability occurred
accidentally. These events altered or disrupted the functioning
of the bioreactor and were recorded in order to study their impact on
CH4 and N2O emissions.
4.2. CH4 emissions
During the experimental campaign of this study, a complete screening
of dissolved CH4 concentrations and emission dynamicswas performed at
theWWTP of Granollers. This allowed a better understanding of the spatial
allocation and transportation of CH4 along theWWTP, with a special
focus on the activated sludge bioreactor. A fraction of the emitted CH4
was arriving dissolved in the wastewater and most likely originated
during the transport of this wastewater through the anaerobic sewer
pipes as has been previously reported (Guisasola et al., 2008). Another
fraction was coming from the reject wastewater stream being produced
in situ during the anaerobic digestion of the waste sludge from the