Magnetic biochar as dual-functional mediator for low-field magnetic separation of vivianite and membrane fouling mitigation in anaerobic membrane bioreactors.

Anaerobic membrane bioreactors (AnMBRs) are considered a promising technology for sustainable wastewater treatment. However, membrane fouling remains a major obstacle to their practical application. Meanwhile, efficient phosphorus recovery through vivianite crystallization is essential for improving the resource recovery potential of AnMBRs. This study investigated the use of magnetic biochar (MBC) as a dual-functional mediator to simultaneously address these two challenges. The results showed that MBC efficiently mitigated membrane fouling, reducing the average fouling rate from 1.11-1.74  kPa/d to 1.07-1.28  kPa/d. This improvement was mainly attributed to the regulation of sludge extracellular polymeric substances (EPS). Specifically, the tightly bound EPS content in the MBC-AnMBR was 22 % lower than that in the control, resulting in the formation of a loose and less adhesive fouling layer. More importantly, MBC acted as a magnetic mediator that promoted vivianite crystallization. During the initial 5  days of operation, the ion activity product and saturation index in the MBC-AnMBR were significantly higher than those in the control system, accelerating vivianite nucleation. Solid-phase elemental mass balance analysis demonstrated a high theoretical magnetic separation potential for phosphorus and iron, with phosphorus recovery reaching 98 % under a low magnetic field intensity, compared with only 55 % in the control system. A synergistic effect between membrane fouling mitigation and vivianite recovery was observed, highlighting MBC as a novel and practical strategy for the simultaneously optimizing AnMBR performance and phosphorus resource recovery, thereby advancing the sustainable operation of AnMBRs.