Flexible Nitrite Supply Alternative for Mainstream Anammox: Advances in Enhancing Process Stability

Anaerobic ammonium oxidation (anammox) has attracted extensive attention as a potentially sustainable and economical municipal wastewater treatment process. However, its large-scale application is limited by unstable nitrite (NO₂^--N) production and associated excessive nitrate (NO₃^--N) residue. Thus, our study sought to evaluate an efficient alternative to the current nitritation-based anammox process substituting NO₂^--N supply by partial-denitrification (PD; NO₃^--N → NO₂^--N) under mainstream conditions. Ammonia (NH₄⁺-N) was partly oxidized to NO₃^--N and removed via a PD coupled anammox (PD/A) process by mixing the nitrifying effluents with raw wastewater (NH₄⁺-N of 57.87 mg L^-1, COD of 176.02 mg L^-1). Excellent effluent quality was obtained with< 5 mg L^-1 of total nitrogen (TN) despite frequent temperature fluctuations (25.7-16.3 °C). The genus Thauera (responsible for PD) was the dominant denitrifiers (36.4%-37.4%) and coexisted with Candidatus Brocadia (anammox bacteria; 0.33%-0.46%). The efficient PD/A allowed up to 50% reduction in aeration energy consumption, 80% decrease in organic resource demand, and lower nitrous oxide (N₂O) production compared to conventional nitrification/denitrification process. Our study demonstrates that coupling anammox with flexible NO₂^--N supply has great potential as a stable and efficient mainstream wastewater treatment.