External electron donor and transporter of iron triggers the paths to enhancing collaboration of anammox and denitrification in granular sludge: Bacterial resilience and synergetic promotion mechanism.

[Display omitted] • Role of iron in collaboration of anammox and denitrification was revealed. • Iron as electron donor and transporter promoted NH 4 +-N removal. • PD/A with iron exhibited resilience in shocking effect of COD, pH and salinity. • Thauera hold versatile metabolism pathway for NO 2 −-N production utilizing Fe(II). • Candidatus Brocadia participated in iron metabolism with Fe(II) as electron donor. Autotrophic nitrogen removal via anaerobic ammonium oxidation (anammox) technology currently faces significant challenges in treating low-strength wastewater due to inefficient retention of bacteria and unstable activity under fluctuating conditions. This study demonstrates enhanced resilience and stability of anammox coupled with partial denitrification (PD) in granular sludge, assisted by ferric iron (Fe(III)) and nano zero-valent iron (nZVI). The systems treated wastewater with low concentrations of ammonia (NH 4 +-N of 50 ∼ 80 mg/L) and nitrate (NO 3 −-N of 60 ∼ 100 mg/L). Fe(III) and nZVI were intermittently added to two PD coupled with anammox (PD/A) systems, which were subjected to increasing nitrogen loading rates (NLRs) and decreasing temperature. Remarkably, both Fe(III)- and nZVI-assisted systems achieved satisfactory total nitrogen (TN) removal efficiencies of 98.1 % and 96.6 %, respectively, even as the temperature dropped from 28.5℃ to 13.4℃ and the NLR increased from 0.22 to 0.72 kgN/m3/d. Significantly, these two systems exhibited pronounced resilience under the shocking effect of excessive organic load, extremely low pH, and high salinity. The presence of iron, acting as an accelerated electron transporter, enhanced anammox activity and microbial NH 4 +-N oxidation. Additionally, iron reduced the requirement for an organic carbon source, further stimulating the competitiveness of anammox for nitrite (NO 2 −-N) against denitrification. Interestingly, metagenomic analysis revealed that the functional bacteria Thauera, responsible for PD, employed versatile metabolic pathways for NO 3 −-N reduction to NO 2 −-N using Fe(II) as an electron donor. Overall, this study provides new insights into microbial interactions and versatile metabolism in iron-assisted anammox systems, contributing to more energy-efficient wastewater treatment. [ABSTRACT FROM AUTHOR]