Direct phosphorus recovery from anaerobic digestion sludge by microbial fuel cell with anolyte circulation: Optimization and key factors identification

Phosphorus (P) recovery from the sludge of wastewater treatment plants is crucial for alleviating global P crisis. Microbial fuel cells (MFCs) represent a promising technology, and the factors influencing P recovery efficiency are critical for practical applications. This study constructed a dual chamber MFC to recover P from anaerobic digestion sludge, and focused on enhancing P recovery rate by circulating anolyte and optimizing the influencing factors. Compared to batch operation, anolyte circulation increased the P recovery rate from 59% to 89% and the average current density from 103 to 179 mA/m³. The optimal conditions were identified as: Fe/P molar ratio 3, initial pH 7.5, and stirring rate 100 rpm for catholyte, dilution ratio 20% and circulating flow rate 2 mL/min for anolyte. Under these optimal parameters, the highest P recovery rate of 97% was achieved. The products were confirmed as newly formed vivianite using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), with a yield of approximately 2.48 mg/g dry sludge (DS) determined by P fractionation. Correlation and Random Forest (RF) analyses identified the Fe/P molar ratio and average current density as key factors for P recovery. Electricity energy output of MFC strongly correlated with the removal of total chemical oxygen demand (TCOD) (R² = 0.8228), indicating that TCOD concentration influences the electricity production, which in turn affects P recovery. Microbial community analysis further confirmed that P recovery and sludge reduction depend on the synergistic activity of anode microorganisms. This study provides a theoretical support for P recovery using MFCs.