[Nitrification, Denitrification, and N₂O Production Under Saline and Alkaline Conditions]

A sequencing biofilm batch reactor (SBBR) running continuously in an anaerobic/aerobic/anoxic (A_n/O/A) mode was adopted to study the characteristics of nitrification and denitrification process and nitrous oxide (N₂O) production under high saline and alkaline conditions. Different carbon and nitrogen ratios (C/N) were also investigated. An influent C/N ratio of 5, 2, and 0 (control), achieved the following results:TN removal efficiency was (98.17±0.42)%, (65.78±2.47)%, and (44.08±0.27)%, respectively; total N₂O production was (32.07±2.03) mg·L^-1, (21.81±0.85) mg·L^-1, and (17.32±0.95) mg·L^-1, respectively; and the N₂O conversion rate (i. e., the ratio of total N₂O production to total nitrogen removal) was (29.75±0.93)%, (30.04±2.17)%, and (41.69±0.80)%, respectively. The nitrification process proceeded normally during the nitrite stage, and nitrite-oxidizing bacteria (NOB) were strongly inhibited under the high saline and alkaline conditions. Due to the inhibition of N₂O reductase under these conditions, N₂O production was higher during the heterotrophic denitrification process than during the ammonia oxidation process. With an increase in the carbon to nitrogen ratio, more carbon sources were available for denitrification meaning that the total nitrogen removal rate and N₂O production were both increased. As the ratio of carbon to nitrogen was increased, the N₂O conversion rate decreased, which may have been caused by electron competition among the nitrogen oxide reductases during the denitrification process; the higher the ratio of carbon to nitrogen, the weaker the electron competition. High-throughput sequencing indicated that ammonium-oxidizing bacteria (AOB) were enriched and NOB were almost entirely absent in the SBBR. Thauera, Azoarcus, and Gemmobacter were the dominant heterotrophic denitrifying bacteria identified in the system.