A Newly Isolated Rhodococcus sp. S2 from Landfill Leachate Capable of Heterotrophic Nitrification and Aerobic Denitrification

Abstract

Nitrogen removal through heterotrophic nitrification–aerobic denitrification (HN–AD) has been acknowledged as one of the most efficient and cost-effective nitrogen removal processes. This study involved the isolation of a novel HN–AD bacterium (Rhodococcus sp. S2) from landfill leachate. Rhodococcus sp. S2 exhibited high nitrogen removal performance under aerobic conditions without the accumulation of nitrite as an intermediate. The maximum removal efficiencies for NH4+-N, NO2−-N, and NO3−-N were found to be 99.97 ± 0.3%, 99.79 ± 0.14%, and 83.53 ± 0.31%, respectively. Additionally, Rhodococcus sp. S2 demonstrated simultaneous nitrifying and denitrifying capabilities and showed a preference for utilizing NH4+-N in mixed nitrogen sources. The optimal nitrogen removal conditions for Rhodococcus sp. S2 were as follows: sodium acetate as a carbon source, a C/N ratio of 16, a shaking speed of 200 rpm, a pH of 9, and a temperature of 35 °C. Genome sequencing results revealed the presence of nitrate reductases (NarG), nitrate oxidoreductase (NxrA), and nitrite reductase (NirBD) in Rhodococcus sp. S2, providing further evidence of its HN–AD capability. In treating raw wastewater under the aforementioned experimental conditions, S2 achieved a maximum TN removal efficiency of 57.16 ± 0.52% with the addition of sodium acetate as a carbon source. These results suggest that Rhodococcus sp. S2 might be a promising candidate for wastewater nitrogen removal.