Evaluation of the nitrogen removal mechanism by analysing the kinetic characteristics, microbial compositions and functional genes in an aerobic fluidized bed biofilm reactor at different carbon to nitrogen (C:N) ratios

The nitrogen removal mechanism in an aerobic fluidized bed biofilm reactor (AFBBR) was explored by analysing the nitrogen removal efficiencies, pollutant degradation kinetics, microbial characteristics, and functional gene abundances. The carbon to nitrogen (C:N) ratios significantly changed the treatment efficiencies for the chemical oxygen demand (CODcr), total nitrogen (TN), and ammonia nitrogen (NH4+-N). Moreover, the average removal rates of NH4+-N (73.2%) and TN (72.7%) under the C:N = 13.94 condition were higher than those under the other conditions. High C:N ratios improved the simultaneous nitrification and denitrification (SND) rate (98.8%) in the AFBBR system. The pollutant degradation kinetics conformed to the Haldane fictitious kinetic model. Actinobacteria and Protrobacteria were the dominant bacteria in the AFBBR system. Additionally, low C:N ratios increased the relative Protrobacteria (43.5%) abundance. The real-time quantitative polymerase chain reaction (qPCR) results indicated that high C:N ratios increased the SND through enhancing the nirS, nirK, and amx-16S gene abundances. These findings showed that high C:N ratios increased the nitrogen removal rates by improving the functional gene abundances of denitrification and anaerobic ammonia oxidation (ANAMMOX).