Hydrazine-Based Strategies to Control Denitrification Bacteria Overgrowth in Anammox-Denitrification Systems

Abstract

In simultaneous anammox-denitrification (SAD) systems, the overproliferation of denitrification bacteria severely impairs nitrogen removal efficiency by competing with anammox bacteria for nitrite under organic-rich conditions. This study proposes a hydrazine (N2H4)-based strategy to selectively inhibit denitrification bacteria while stimulating the anammox activity. Batch experiments revealed a concentration-dependent dual role of N2H4: at 10 mg N/L, the anammox activity increased to 123% of baseline performance, whereas denitrification activity declined to 59%. Higher concentrations (≥ 50 mg N/L) progressively suppressed both anammox and denitrification processes. A dual inhibitory mechanism for N2H4 was proposed: (1) disrupting electron carriers (e.g., cytochrome c) and (2) inactivating key metabolic enzymes like HDH or Nar. In SAD systems, short-term (5 h) exposure to 10 mg N/L N2H4 effectively curtailed denitrifier overgrowth, elevating the ammonium removal rate from 96.2 ± 0.3 to 116.4 ± 2.5 mg N/g VSS·d, and reducing COD degradation rates from 345.5 ± 13.0 to 268.0 ± 9.7 mg N/g VSS·d. However, the TN removal efficiency and the proportional contribution of anammox to TN removal remained suboptimal compared to the control, suggesting that future studies should explore either higher concentrations or extended exposure durations of N2H4 shock to achieve optimal performance. These findings highlight the dual role of N2H4 in modulating microbial competition and propose its application as a control agent to optimize the SAD performance under the organic stress.

Keywords: Anammox, Denitrification, Hydrazine, Cytochrome c, Enzyme activity