Dual-edged effects and mechanisms of hydroxylamine in partial denitrification-anaerobic ammonium oxidation system

The combination of partial denitrification (PD) and anaerobic ammonium oxidation (anammox) is a novel and promising nitrogen removal process. Regulating the synergistic reaction between denitrifiers and anammox bacteria (AnAOB) is the key to achieving stable and efficient PD-anammox performance. In this study, 10 mg/L of hydroxylamine (NH₂OH) was considered to efficiently promote the bacterial activity, microbial energy flow, and the synergy of functional microflora. As a result, the nitrogen removal rate (NRR) significantly increased from 0.05 to 0.30 g N/L/d in parallel with an increase in the nitrogen loading rate (NLR) from 0.10 to 0.40 g N/L/d. However, the dual-edged effect of NH₂OH was also confirmed. The long-term presence of NH₂OH caused overgrowth of complete-denitrifying bacteria and decreased the NRR to 0.11 g N/L/d. Additionally, NH₂OH enhanced nitrous oxide (N₂O) emissions via chemical pathways as well as enhanced denitrification Fortunately, the inhibition caused by NH₂OH was reversible by stopping the dosing, the reactor restored to stable operation with an NRR of 0.27 g N/L/d. Analysis of metabolic intensity and pathways revealed the effecting process and mechanism of NH₂OH on the PD-anammox system. This study verified the dual-edged effects and mechanisms of NH₂OH, therefore proving a theoretical basis and technical reference for the application of PD-anammox.