Heterotrophic Nitrification-Aerobic Denitrification by Bacillus sp. L2: Mechanism of Denitrification and Strain Immobilization

The biological denitrification of low-C/N wastewater is a great challenge in treatment plants due to the lack of microorganisms with heterotrophic nitrification-aerobic denitrification (HN-AD) abilities. In this study, Bacillus sp. L2 was isolated from aeration tank water samples using a nitrification medium and screened for its ability to perform HN-AD in low-C/N wastewater. The strain showed a maximum NH4+-N removal rate of 98.37% under low-C/N conditions. In the presence of a mixed N source, strain L2 was capable of completely removing NH4+-N within 24 h. Furthermore, optimal nitrogen removal conditions for strain L2 were found to be C/N = 9, pH = 9, and sodium acetate as the C source. Under optimal conditions, the strain was able to maintain a high NH4+-N removal rate under 0-3% salinity and an NH4+-N concentration of 200 mg/L or less. The denitrification pathways of strain L2 were NH4+-> NH2OH -> NO2-(<-> NO3-)-> NO -> N2O -> N-2 and NH4+-> NH2OH -> NO -> N2O -> N-2. Furthermore, semi-continuous wastewater treatment was conducted using immobilized technology, which resulted in more than 82% NH4+-N removal after three cycles of reuse. This study demonstrates the great potential of Bacillus sp. L2 in wastewater treatment applications.