Nitrogen recovery through fermentative dissimilatory nitrate reduction to ammonium (DNRA): Carbon source comparison and metabolic pathway

The dissimilatory nitrate reduction to ammonium (DNRA) process was known as a short circuit in nitrogen cycle, a key step in recovering nitrogen from nitrate wastewater, and has attracted people's attention for a long time. Here, three organic substances supported DNRA systems, sodium succinate, glucose, and ethanol, were constructed in membrane bioreactor at three different COD/N ratios (6.5, 7.7, and 10) for nitrogen recovering from nitrate wastewater. The experiment results indicated that the maximum ammonium nitrogen transformation efficiency of 71.49%, 66.92%, and 92.05% were achieved at a COD/N ratio of 7.7 in sodium succinate, glucose, and ethanol supported DNRA systems. The high-throughput sequencing, which targeted nrfA gene, showed a greater diversity of DNRA microbiota at a COD/N ratio of 7.7. The quantitative PCR and 15N isotope tracing experiment showed that the ethanol-supported DNRA system had the highest nrfA gene abundance and DNRA potential rate. Meanwhile, ethanol-supported DNRA systems were found to contain a large number of fermenting bacteria (such as Acetoanerobium, Lentimicrobium, and Fusibacter), suggesting that anaerobic digestion may accompany the DNRA system. The potential metabolic pathways of three carbon sources were further predicted. Glucose and succinate may enter pyruvate metabolism via glycolysis and the citrate cycle. The high abundance of genes encoding phosphate acetyltransferase in ethanol system suggested that ethanol was most likely to be degraded via acetyl-CoA pathway and ensured electron supply to nitrate-reducing bacteria. This study is expected to provide new ideas for efficient recovery of nitrogen resources.