Pilot-scale plant study on iron-rich residual sludge driven multi-path efficient nitrogen removal mediated by niche-specific Fe (III)/Fe (II) cycle for sustainable treatment of municipal wastewater.

Excessive fermentation-derived effluent NH4 constrains the sustainable application of the sludge fermentation-driven denitrification (SFD) process. This study introduced iron-rich residual sludge and an iron "feast-famine" strategy into a 38.00 m3 pilot-scale SFD process. After 120 days, the sludge reduction and denitrification efficiencies reached 77.79 % and ∼ 95.00 %, respectively, with a fourfold increase in NH4+ removal. Further analysis revealed that iron limitation stimulated biofilm sludge (BS) to assimilate Fe(III), increasing Feammox contribution by 5.43-fold. The generated Fe(II) species (e.g., FeO) boosted energy production, improving energy-consuming NH4+ assimilation and NO2- generation. The resulting NO2- and Fe(II) jointly enriched anammox (FC = 1.25), reinforcing multi-path NH4+ removal. Additionally, in suspended sludge, the abundant NOx- induced Fe(II) uptake for NOx- removal via NOx--dependent Fe(II) oxidation and chemical denitrification, with the regenerated Fe(III) further promoting sludge fermentation. This study offers a promising approach to mitigate fermentation-derived NH4+ in the SFD process, advancing its sustainable application.