Ammonia oxidizing bacteria (AOB) denitrification and bacterial denitrification as the main culprit of high N2O emission in SBR with low C/N ratio wastewater.

A large amount of greenhouse gas nitrous oxide (N 2 O) will be produced during the biological nitrogen removal process for organic wastewater of low C/N ratio. One of the effective methods to solve this problem is to incorporate inexpensive carbon source. In this study, raw wastewater (RW) from pig farm, that was not anaerobically digested, was utilized as exogenous carbon in both A/O and SBR aerobic reactor to treat liquid digestate with high ammonia nitrogen and low C/N ratio. The results showed that N 2 O emission in SBR was higher than that of A/O process under the same nitrogen load. The N 2 O conversion in the biological nitrogen removal process was investigated by the strategy of integrating stable isotope method and metagenomics. The δO18-N 2 O, δN15-N 2 O, and SP values of the SBR were closer to the denitrification values of Ammonia-Oxidizing Bacteria (AOB) than those of A/O. The abundance of AOB in the SBR reactor was higher than that in the A/O reactor, while the abundance of denitrifying bacteria was lower. The amoA/B/C gene abundance in the SBR was greater than that in the A/O, and the NOS gene abundance was the opposite. The results indicated that both AOB denitrification and bacterial denitrification led to the increase of N 2 O emissions of the SBR. [Display omitted] • RW as source of exogenous carbon source improved nitrogen removal while reducing N 2 O emissions. • Insufficient carbon source would result in more N 2 O emission. • A/O had more N 2 O emission reduction potential than SBR. • The denitrification of ammonia oxidizing bacteria (AOB) resulted in more N 2 O emission from the SBR. [ABSTRACT FROM AUTHOR]