Decoupled recovery of phosphorus and iron from sewage sludge via electrochemical anaerobic fermentation

Phosphorus (P) recovery from sewage sludge is essential for sustainable resource management. However, the widespread implementation of Fe-based chemical P removal results in strong Fe-P associations that hinder P release. This study presents a process-level electrochemical Fe-P decoupling anaerobic fermentation (EPD-AF) framework that decouples Fe-P complexes in sludge and directionally partitions Fe and P into two separable product streams. Using a representative applied bias (2.2 V), EPD-AF increased supernatant phosphate (PO₄^3-) by 80 mg/L relative to the 0 V control and recovered 43.63% of total Fe as nanoscale zero-valent iron (nZVI) on the cathode. The Fe-P decoupling process proceeded through an electrochemically driven and microbially facilitated electron transfer pathway. The applied bias provided sufficient thermodynamic driving force for stepwise Fe transformation (Fe(III)→Fe(II)→Fe(0)) and concomitant liberation of Fe-bound PO₄^3-. Meanwhile, microbially derived redox-active mediators maintained the electron transfer network, facilitating electron exchange between the electrode and Fe species, lowering interfacial kinetic barriers, and thereby promoting nZVI formation. EPD-AF defines an operable electrochemical window that enables in situ Fe-P decoupling and product splitting into two recoverable streams, transforming Fe from a background matrix component into a separable cathodic product, providing a new perspective for P recovery from sewage sludge.