Low-dose chlorite drives stable nitrite accumulation for partial denitrification

Achieving stable and efficient nitrite accumulation in partial denitrification (PD) requires precise control techniques, which poses a challenge to its practical application. This study introduces a new strategy using chlorite (ClO₂^-) as a regulatory factor to halt denitrification at the nitrite stage, ensuring stable and efficient PD. The effectiveness of this approach was assessed in a continuously operated PD biofilter. At different ClO₂^- concentrations, the nitrite accumulation rate (NTR) reached its peak at 97.6 % at 1.00 mg/L ClO₂^-, with effluent nitrate-nitrogen (NO₃^--N) as low as 0.61 mg/L. The chlorite-regulated PD filter exhibited a rapid start-up period, reaching optimal performance within 7 days and maintaining stable operation for over 30 days. Moreover, this PD biofilter sustained a nitrite accumulation rate above 67.5 % across a wide range of carbon/nitrogen ratios (C/N) (2-4) and remained efficient at an extremely short hydraulic retention time of 30 min. Additionally, ClO₂^- easily degrade into non-toxic chloride ion (Cl^-) and oxygen (O₂) in the environment, thereby preventing secondary pollution. Metagenomic analysis identified Thauera (16.4 %), Simplicispira (14.7 %), and Flavobacterium (14.0 %) as key contributors to nitrite accumulation. Furthermore, according to the real-time quantitative polymerase chain reaction (qPCR) results, chlorite exposure upregulated the nitrate reductase gene (nar) and downregulated the nitrite reductase gene (nir). Finally, the final effluent Total Inorganic Nitrogen (TIN) concentration of 10.26 mg/L was achieved through the Partial denitrification/Anammox (PD/A) coupling process. This study provides new insights into nitrite accumulation in PD/A and presents valuable guidance for optimizing the PD/A process in engineering applications.