Hydrazine-enhanced Anammox and microbial compartmentalization in a single-stage partial nitritation/anammox reactor...

Partial nitritation/anammox (PN/A) systems treating low chemical oxygen demand (COD)/NH₄⁺-N ratio (C/N) municipal wastewater face challenges due to limited substrates and microbial imbalance. This study assessed the regulatory effect of low-dose hydrazine (2 mg/L) in a single-stage PN/A reactor fed with post-carbon-capture influent (C/N ≈ 1.5-2) over 118 days. Hydrazine selectively suppressed nitrite-oxidizing bacteria (NOB), raising the ammonia-oxidizing bacteria/NOB ratio from 1.41 to 10.64 and increasing autotrophic nitrogen removal from 38 % to 51 %. Upon withdrawal, NOB recovered, confirming reversible inhibition. Suspended sludge showed reduced diversity and protein-rich extracellular polymeric substances, while biofilm communities exhibited increased diversity and enriched anammox-compatible populations (Candidatus Kuenenia and Brocadia). Functional prediction indicated a shift from heterotrophic denitrification to anammox-dominant pathways under low C/N conditions. These shifts enhanced process stability and microbial resilience. The findings demonstrate that low-dose hydrazine promotes autotrophic nitrogen removal by restructuring the microbial community, offering a practical strategy for optimizing PN/A performance in low-strength wastewater treatment.