Hydrazine promoted nitrite reduction in partial-denitrification by enhancing organic-substrate uptake and electron transport

Partial denitrification coupled with anammox is a promising approach for sustainable nitrogen removal from wastewater. However, this coupling can be influenced by hydrazine (N2H4) released by anammox bacteria. This study aimed to reveal how N2H4 regulates partial denitrification. Short-term batch experiments showed that 0.5 to 10 mg N/L of N2H4 promoted nitrite (NO2−) accumulation, likely by inhibiting the electron transfer from cyt c to nitrite reductase. However, long-term exposure to N2H4 (0.5 and 1 mg N/L) shifted the microbial community and increased NO2− reduction. This exposure enriched the genera OLB8, Thauera, and f_Rhodocyclaceae, and increased the abundance of genes involved in EPS formation, substrate transport and electron transport. The long-term outcome was more NO2− reduction to N2 and more substrate (COD) oxidation. While N2H4 benefits NO2− accumulation in short-term, the mechanism is not sustainable, highlighting the importance of minimizing N2H4 release for successful in such coupled nitrogen removal systems.