Enhancement of hydrazine accumulation in anammox bioreactors

The role of hydrazine (N₂H₄) in anammox metabolism has been widely studied; however, studies on N₂H₄ biosynthesis by anammox bacteria are limited in the literature. In this context, the current research aims to investigate the enhancement of biological N₂H₄ production in the anammox process in a long-term manner. The experimental studies started with the optimization of the operating conditions to achieve maximum N₂H₄ accumulation. Under favorable conditions (pH = 8.97 ± 0.08; T = 35.5 ± 0.5 °C; initial hydroxylamine dose = 1.46 ± 0.01 mM), 17.16 ± 0.64 mg L^-1 of N₂H₄ accumulated in the batch systems. The continuity of N₂H₄ bioproduction was then evaluated by long-term observations. A continuous flow bioreactor was operated in four consecutive manipulated periods under optimized conditions. In the long-term operated bioreactor, 55.10 ± 0.30 mg L^-1 N₂H₄ was accumulated at optimal conditions, which was 2.5 times higher than reported in the literature. Although manipulation of the bioreactor operating conditions initially resulted in a significant increase in N₂H₄ bioaccumulation, it subsequently caused a severe deterioration in anammox activity. However, this could be mitigated by increasing the biomass concentration in the anammox systems. In addition, the relative abundance of Candidatus Kuenenia decreased by 1.88% throughout the long-term operation.