Effect of long-term dosage of hydrazine on mainstream anammox process: Biofilm characteristics and microbial community

The impact of the long-term trace hydrazine (N2H4) exogenous supplementation on activity of the anaerobic ammonium oxidation (anammox) biofilm was investigated in a moving bed biofilm reactor (MBBR) for mainstream wastewater treatment. The results of this study demonstrated that the addition of 2–5 mg/L N2H4 enhanced anammox biofilm activity, as evidenced by the augmented nitrogen removal rate (NRR), which increased from 113.4 g/(m3·d) to 126.7 g/(m3·d) with the introduction of 2 mg/L N2H4. However, a higher concentration of N2H4 (10 mg/L) suppressed anammox activity, leading to a reduced NRR of 91.5 g/(m3·d). Bioindicators revealed that the long-term addition of 2 mg/L N2H4 fostered the accumulation of anammox bacteria (AnAOB) biomass, elevating the volatile suspended solids (VSS) content by 12%. Moreover, the structural composition of extracellular polymeric substances (EPS) within the biofilm was altered, resulting in enhanced biofilm strength within the reactor. The protective mechanism of the biofilm was activated, and EPS secretion was stimulated by the continuous N2H4 supplementation. The introduction of an excess dosage of N2H4 led to alterations in the microbial communities, ultimately resulting in a decline in the performance of the reactor. These findings collectively illustrate that N2H4, as an intermediate product, can effectively enhance anammox activity within the MBBR for mainstream wastewater treatment. This study contributes to the understanding of the optimization strategies for anammox processes in wastewater treatment systems. [Display omitted] •Response of anammox biofilm to long-term N2H4 addition was studied.•Trace (2 mg/L) N2H4 supply can promote nitrogen removal of anammox process.•Advanced nitrogen removal rate of 126.7 g/(m3·d) was achieved.•Specific anammox activity increased by 19% at the continuous dosage of 2 mg/L N2H4.•N2H4 of 10 mg/L triggered the protective mechanism of the biofilm and promoted EPS secretion.