Hydroxylamine enhances nitrogen removal by reshaping microbial community and nitrogen fate in activated sludge.

Abstract

Hydroxylamine (NH2OH, HA), a highly reactive intermediate of ammonia oxidation, shows great potential for modulating multiple nitrogen removal pathways in activated sludge systems. However, the effects of repeated low-dose HA addition on nitrogen fate and the potential coupling of nitrogen removal pathways remain unclear. In this study, a step-feed sequencing batch reactor (SBR) was operated under progressively carbon-limited conditions to investigate HA-mediated nitrogen removal. At an HA dose of 5 mg/L and an overall C/N ratio of 3.61 ± 0.06, the HA-amended reactor achieved an effluent total nitrogen concentration of 3.57 ± 0.70 mg/L and a TN removal efficiency of 91.96 ± 1.57%, compared with 11.74 ± 0.84 mg/L and 71.42 ± 2.05%, respectively, in the control reactor. Cycle profiles and batch tests showed enhanced nitrite retention, inhibited nitrite-oxidizing bacterial activity, and delayed initial COD decline. Notably, the second feeding step promoted transient NH4+-NO2- coexistence, creating conditions that could favor nitrite-based denitrification and potential anammox activity. Microbial community and functional gene analyses provided complementary evidence of shifts in nitrogen transformation potential. Specifically, the relative abundance of Nitrospira decreased, whereas Alcaligenes, Candidatus Competibacter, and functional genes associated with nitrogen removal were enriched under HA exposure. Overall, low-dose HA enhanced nitrogen removal by reshaping nitrogen fate and microbial community functions in activated sludge. These findings offer new insights into achieving efficient biological nitrogen removal in wastewater treatment under low C/N conditions.