Nitrogen and phosphorus removal from kitchen waste biogas slurry by ZnCl₂-modified biogas residue biochar in FCDI

• • ZnCl₂ modification of biogas residue biochar increased specific surface area, adsorption capacity, capacitance, and conductivity, leading to superior electrochemical performance compared to unmodified biochar. • • In simulated digestate, FCDI with 7.5% modified biochar achieved 47.7% NH₄⁺-N and 55.2% RP removal over 12 h, demonstrating effective nutrient recovery potential. • • In actual anaerobic digestion filtrate, maximum removals were 32.2% for NH₄⁺-N and 26.2% for RP, indicating a 15.5% and 29.0% reduction compared to simulated conditions due to membrane fouling. • • Membrane fouling by peptides and amino acids in real digestate increased membrane resistance and blocked ion transport, underscoring the need for anti-fouling strategies in FCDI scale-up.