Emission of nitrous oxide (N 2 O) during biological wastewater treatment is of growing concern since N 2 O is a major stratospheric ozone-depleting substance and an important greenhouse gas. The emission of N 2 O from a lab-scale granular sequencing batch reactor (SBR) for partial nitrification (PN) treating synthetic wastewater without organic carbon was therefore determined in this study, because PN process is known to produce more N 2 O than conventional nitrification processes. The average N 2 O emission rate from the SBR was 0.32 ± 0.17 mg-N L -1 h -1 , corresponding to the average emission of N 2 O of 0.8 ± 0.4% of the incoming nitrogen load (1.5 ± 0.8% of the converted NH 4 + ). Analysis of dynamic concentration profiles during one cycle of the SBR operation demonstrated that N 2 O concentration in off-gas was the highest just after starting aeration whereas N 2 O concentration in effluent was gradually increased in the initial 40 min of the aeration period and was decreased thereafter. Isotopomer analysis was conducted to identify the main N 2 O production pathway in the reactor during one cycle. The hydroxylamine (NH 2 OH) oxidation pathway accounted for 65% of the total N 2 O production in the initial phase during one cycle, whereas contribution of the NO 2 -reduction pathway to N 2 O production was comparable with that of the NH 2 OH oxidation pathway in the latter phase. In addition, 3 spatial distributions of bacteria and their activities in single microbial granules taken from the reactor were determined with microsensors and by in situ hybridization. Partial nitrification occurred mainly in the oxic surface layer of the granules and ammonia-oxidizing bacteria were abundant in this layer. N 2 O production was also found mainly in the oxic surface layer. Based on these results, although N 2 O was produced mainly via NH 2 OH oxidation pathway in the autotrophic partial nitrification reactor, N 2 O production mechanisms were complex and could involve multiple N 2 O production pathways.

The identification of the key nitrous oxide (N2O) production pathways is important to establish a strategy to mitigate N2O emission. In this study, we combined real-time gas-monitoring analysis, (15)N stable isotope analysis, denitrification functional gene transcriptome analysis and microscale N2O concentration measurements to identify the main N2O producers in a partial nitrification (PN) aerobic granule reactor, which was fed with ammonium and acetate. Our results suggest that heterotrophic denitrification was the main contributor to N2O production in our PN aerobic granule reactor. The heterotrophic denitrifiers were probably related to Rhodocyclales bacteria, although different types of bacteria were active in the initial and latter stages of the PN reaction cycles, most likely in response to the presence of acetate. Hydroxylamine oxidation and nitrifier denitrification occurred, but their contribution to N2O emission was relatively small (20-30%) compared with heterotrophic denitrification. Our approach can be useful to quantitatively examine the relative contributions of the three pathways (hydroxylamine oxidation, nitrifier denitrification and heterotrophic denitrification) to N2O emission in mixed microbial populations.

We measured the atmospheric emission rates of methane (CH4) and nitrous oxide (N2O) in two wastewater treatment plants in Southwest Germany, which apply different treatment technologies. Dissolved gas concentrations and fluxes were measured during all processing steps as well as in the discharge receiving streams. N2O isotopocule analysis revealed that NH2OH oxidation during nitrification contributed 86–96% of the N2O production in the nitrification tank, whereas microbial denitrification was the main production pathway in the denitrification tank in a conventional activated sludge (CAS) system. During wastewater treatment using a modified Ludzack-Ettinger system (MLE) with energy recovery, N2O was predominantly produced by the NO2- reduction by nitrifier-denitrification process. For both systems, N2O emissions were low, with emission factors of 0.008% and 0.001% for the MLE and the CAS system, respectively. In the effluent-receiving streams, bacterial denitrification and nitrification contributed nearly equally to N2O production. The CH4 emission from the MLE system was estimated as 118.1 g-C d-1, which corresponds to an emission factor of 0.004%, and was three times lower than the emission from the CAS system with 0.01%.

Under the highest aeration condition, NH2OH oxidation (nitrification) was the main contributor to N2O production at about 90% and 57%, respectively, at the aeration tank middle and end. At other sampling points, 55-63% of the N2O was produced by bacterial NO2(-) reduction (nitrifier-denitrification) with a lower nitrification contribution. For all sampling points in the lower aeration experiments, NO2(-) reduction was a major N2O production pathway.