Chlorite-Mediated Regulation of Nitrite Accumulation and Nitrogen Removal in Integrated Partial Denitrification/Anammox Biofilter.

Achieving stable and efficient nitrite accumulation in integrated partial denitrification/anammox (PD/A) processes is typically constrained by substrate competition among microorganisms, posing a significant challenge for large-scale engineering applications. This study proposes a novel strategy for regulating nitrogen transformation pathways using chlorite (ClO2-) as a bioregulator, by inducing denitrification to remain at the nitrite stage and thereby providing a stable substrate supply for the anammox process. In the continuous-flow PD/A biofilter, the increase of ClO2- concentration from 0 to 1.0 mg/L was associated with an increase in total nitrogen removal efficiency (Re.TN) from 62.74% to a peak value of 85.27%, accompanied by an increase in effluent nitrite accumulation rate (NAR) from 62.24% to 81.77%. 15N stable isotope tracing further confirmed that the anammox pathway dominated the system, contributing up to 98.9% of the total N2 production and serving as the primary functional support for deep nitrogen removal. The system demonstrated exceptional process robustness across a wide hydraulic retention time (HRT) range of 5-12 h, with Re.TN consistently maintained above 80%. In addition, the system exhibited good conversion capacity for ClO2-, with the residual concentration in the effluent maintained within the safety limits, thereby reducing the potential risk of secondary pollution. Microbial community analysis indicated that ClO2- regulation was associated with changes in the functional microbial community structure, accompanied by increased relative abundances of the core autotrophic anammox bacteria (Candidatus Brocadia and Candidatus Jettenia) and the succession of heterotrophic Thauera populations toward nitrite-producing subgroups. Metagenomic functional profiling revealed that the functional potential of core nitrogen metabolism changed under ClO2- regulation, with increased relative abundances of nitrate reduction-related genes (narG/H/I) and decreased relative abundance of the nitrite reduction-related gene (nirS), while the relative abundance of the Anammox-related marker gene hdh increased by 27.5%. This study provides new insights into optimizing the stability of the PD/A system and offers new perspectives for the further engineering application of efficient nitrogen removal processes.