Source identification of nitrous oxide on autotrophic partial nitrification in a granular sludge reactor

摘要

Emission of nitrous oxide (N_2O) during biological wastewater treatment is of growing concern since N_2O is a major stratospheric ozone-depleting substance and an important greenhouse gas. The emission of N_2O 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_2O than conventional nitrification processes. The average N_2O emission rate from the SBR was 0.32 ± 0.17 mg-N L ~1h~(-1), corresponding to the average emission of N_2O 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_2O concentration in off-gas was the highest just after starting aeration whereas N_2O 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_2O production pathway in the reactor during one cycle. The hydroxylamine (NH_2OH) oxidation pathway accounted for 65% of the total N_2O production in the initial phase during one cycle, whereas contribution of the NO_2 reduction pathway to N_2O production was comparable with that of the NH_2OH oxidation pathway in the latter phase. In addition, spatial distributions of bacteria and their activities in single mi-crobial 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_2O production was also found mainly in the oxic surface layer. Based on these results, although N_2O was produced mainly via NH_2OH oxidation pathway in the autotrophic partial nitrification reactor, N_2O production mechanisms were complex and could involve multiple N_2O production pathways.