Reduction of N2O emission in wastewater treatment by modified basalt fiber bio-nest technology

This study applied umbrella-shaped modified basalt fiber (MBF) bio-carrier to an integrated fixed-film activated sludge system to investigate the performance of MBF bio-nest in wastewater treatment and N2O emission reduction. The effect of dissolved oxygen (DO) on N2O reduction in the bio-nest was investigated by changing the DO concentration. The results showed that under the same operating conditions, TN removal efficiency was increased by 63.87% and N2O emission was reduced by 71.32% in the bio-nest system compared with the sequencing batch activated sludge bioreactor. According to the 16sRNA sequencing results, a variety of functional microregions existed within MBF bio-nests, with a high diversity of microbial populations. Saccharibacteria genera incertae sedis were the main carbon-removing bacteria in the reactor, and heterotrophic nitrification-aerobic denitrification (HN-AD) genera were the main nitrifying bacteria, which did not emit N2O during the nitrification process. Denitrification genera were dominated by conventional heterotrophic denitrification bacteria (HDN) in the inner and middle layers of the bio-nest (17.42%, 23.02%), and HN-AD bacteria in the outer layer of the bio-nest and suspended sludge (29.70%, 27.53%). Aerobic/anoxic/anaerobic genera were distributed in all layers of the bio-nest, and denitrification genera had higher relative abundance in the MBF bioreactor than in the SBR, which facilitated denitrification and mitigated the accumulation of intermediate products, reducing N2O emissions. The MBF bio-nest reactor had the highest TN removal rate of 86.64%±1.14% and the lowest N2O emission of (0.4±1.0) mg N2O/g TN when the DO concentration was 2.5mg/L (M2). The bio-nest microbial genus categories were basically the same in each DO gradient, but differed in their relative abundance. In M2, HN-AD bacteria were the main nitrifying functional bacteria in the reactor (the relative abundance of the layers from inside to outside was 44.24%, 61.34%, and 36.16%), which was conducive to N2O reduction; HDN were the main functional bacteria in the M2reactor, with moderate relative abundance of 20.17%, 12.00%, and 21.20% from inside to outside layers; the concentrations of NO2--N and NO3--N in the effluent were (0.011±0.002) and (1.65±0.46) mg/L; denitrification was carried out completely, which was conducive to the reduction of N2O emissions.