• • The CF-Fe-CS/NAFO system achieved effluent NO3-N below 6.5 mg/L and TN removal of 50-60% at HRT 48 h, versus control effluent >12 mg/L and <20% removal, proving electrochemical Fe(III) reduction sustains NAFO without external carbon.
• • Treating real secondary effluent (influent NO3-N 15.29 mg/L), the system lowered effluent NO3-N to 6.06 mg/L with TN removal >50%, and nitrite/ammonium accumulation <0.6 mg/L, demonstrating robustness to complex matrices.
• • Reducing HRT from 48 h to 24, 12, 6, and 3 h increased effluent NO3-N to 9.5, 11.9, 13.6, and 14.6 mg/L, with TN removal dropping to 32.6%, 19.9%, 10.9%, and 5.8%, respectively, quantifying kinetic limits for design.
• • SEM confirmed chitosan gel forms a uniform coating on carbon felt-iron, preventing iron oxide agglomeration and enhancing microbial attachment; electrochemical analysis showed charge transfer resistance of 3.284 Ω·cm², enabling rapid Fe(III)/Fe(II) cycling; microbial community analysis revealed enrichment of Gallionellaceae, Thiobacillus, Sediminibacterium, and Thauera, forming a synergistic denitrifying consortium.