A lab-scale study on heterotrophic nitrification-aerobic denitrification for nitrogen control in aquatic ecosystem

Nitrogen (N) loss is generally caused by denitrification under anaerobic conditions and the N loss in the heterotrophic nitrification^_aerobic denitrification (HN^_AD) system is of recent research interest. However, previous studies are generally focused on pure cultures-based system and the information on HN^_AD in the complex aquatic ecosystem is limited. In this study, HN-AD system was established in the mixed cultures of the sediments and the performances of HN-AD were evaluated under different conditions. Further, the N loss mechanism in HN^_AD system was explored. The study found that the N was lost in the sediment cultures with ammonium-N (NH₄^+_N) (or) and nitrate-N (NO₃^-_N) as N source under aerobic conditions. The highest N loss rate was achieved under the TOC/TN mass ratio of 10 with citrate as the carbon source. Under this condition, the N loss percentages of NH₄^+_N (201.91 mg/L) and NO₃^-_N (130.00 mg/L) reached 99.61% and 100.00%, respectively, which were higher than those in the pure HN^_AD strains reported in the literature. High NH₄^+_N removal efficiencies were also achieved at low C/N mass ratio and high NH₄^+_N concentration (493.12 mg L^-1). The N loss pathway in the system was investigated by adding Na₂WO₄ as the nitrate reductase inhibitor. The study found that the N was not lost via partial nitrification/denitrification pathway, i.e., NH₄⁺ → NH₂OH → NO₂^- → N₂O (N₂), instead via full nitrification/denitrification pathway, i.e., NH₄⁺ → NH₂OH → NO₂^- → NO₃^- → NO₂^- → N₂O (N₂), since nitrate was a key intermediate. The variation in NH₄^+_N, NO₃^-_N, and NO₂^-_N concentrations in the HN^_AD processes further confirmed the N transformation pathway. Therefore, HN^_AD may occur and cause N loss in natural aquatic ecosystems. The results of this study demonstrate that N was lost through HN-AD and that the well-cultured HN-AD sediments could be useful biological tool to remediate eutrophic water bodies.