Nitrogen and phosphorus removal from kitchen waste biogas slurry by ZnCl₂-modified biogas residue biochar in FCDI
The digestate from anaerobic digestion of food waste is separated into solid residue and liquid filtrate. The filtrate retains high nutrient and carbon content, making it a viable resource for recovery. This study prepared biochar from food waste digestate residue and employed it as an electrode active material in a flow-electrode capacitive deionization (FCDI) system, with activated carbon as a control, to assess nitrogen and phosphorus removal from kitchen waste biogas slurry. ZnCl₂ modification significantly enhanced the biochar's specific surface area, adsorption capacity, capacitance, and conductivity. The optimal mass fraction of modified biochar in the electrode liquid was 7.5%. In simulated digestate, the FCDI system achieved removal efficiencies of 47.7% for NH₄⁺-N and 55.2% for reactive phosphorus (RP) over 12 hours. Performance ranking of electrode materials was activated carbon > ZnCl₂-modified biochar > unmodified biochar. In continuous operation with actual anaerobic digestion filtrate, maximum removal efficiencies were 32.2% for NH₄⁺-N and 26.2% for RP. The reduced performance in real digestate is attributed to organic foulants such as peptides and amino acids, which block ion-exchange membrane channels, increase membrane resistance, and impede ion transfer and charge transport, thereby diminishing deionization efficiency.