Overcoming temperature and substrate limitations of anammox via partial denitrification: Stable performance,...

The widespread application of the anaerobic ammonium oxidation (anammox) process remains constrained by its narrow substrate range (nitrite-to-ammonia ratio of 1.32), sensitivity to low temperatures, and difficulty in controlling nitrous oxide (N₂O). This study demonstrated that under different substrate types and proportions (nitrate-nitrite, NO_x^--N), the partial denitrification coupled with anammox (PD/A) process with high nitrogen load (1.35 kg N/m³/d) still maintained stable nitrogen removal efficiencies (83.5 %∼98.5 %), even at long-term continuous cooling (29.4∼11.6°C) and persistent low temperature (15.1∼9.6°C). In the presence of coexisting NO₃^--N and NO₂^--N (NO₃^--N: NO₂^--N of 1:1), carbon sources were preferentially utilized for the reduction of NO₃^--N to NO₂^--N, effectively circumventing the strict substrate dependence inherent to anammox. The stable nitrogen removal of the PD/A process at low temperatures was attributed to the low activation energy (35.1 kJ/mol) of partial denitrification and the cold-resistant granular sludge, ensuring a highly efficient supply of the essential NO₂^--N for anammox. Cold-resistant granule sludge provided a spatial environment for the synergistic symbiosis of partial denitrification, anammox, and other heterotrophic bacteria, facilitating anammox adaptation to persistent low-temperature conditions. Metagenomic sequencing revealed a high relative abundance of Candidatus Brocadia (14.9 %) within the granular sludge, while Thauera (18.0 %) dominated the flocculent sludge at low temperatures. It further revealed the cross-feeding relationship of the bacterial community between granular and flocculent sludge. Notably, N₂O emissions from the PD/A process can be effectively regulated by controlling the COD/NO₃^--N ratio, achieving a balance between improving nitrogen removal efficiency and mitigating N₂O emissions. This research provides a theoretical foundation for the stable operation and N₂O control of non-specifically dependent PD/A process under long-term low-temperature conditions.