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 (NH 4 +_ N) (or) and nitrate-N (NO 3 −_ 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 NH 4 +_ N (201.91 mg/L) and NO 3 −_ 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 NH 4 +_ N removal efficiencies were also achieved at low C/N mass ratio and high NH 4 +_ N concentration (493.12 mg L −1 ). The N loss pathway in the system was investigated by adding Na 2 WO 4 as the nitrate reductase inhibitor. The study found that the N was not lost via partial nitrification/denitrification pathway, i.e., NH 4 + → NH 2 OH → NO 2 − → N 2 O (N 2 ), instead via full nitrification/denitrification pathway, i.e., NH 4 + → NH 2 OH → NO 2 − → NO 3 − → NO 2 − → N 2 O (N 2 ), since nitrate was a key intermediate. The variation in NH 4 +_ N, NO 3 −_ N, and NO 2 −_ 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.