Comparative study on the nutrient recovery from source-separated urine using magnesium-air fuel cell in different operating modes

To address the paucity of phosphorus reserves, on -site resource recovery from urine was a viable approach in line with sustainable development imperatives. In this context, an electrochemically induced precipitation system based on the principle of Mg -air fuel cells (MAFC), capable of precipitating NH 4 + -N and PO 4 3- -P as struvite from urine, offered an alluring prospect. Given the dilemma of the recovered struvite quality and insufficient Mg utilization in batch mode MAFC (B-MAFC), a continuous -flow mode MAFC (CF-MAFC) was proposed to facilitate struvite crystallization via regulating liquid flow rates. The hydraulic retention time (HRT), electrode distance (D), and external resistance (R) of reactor were optimized to enhance CF-MAFC performance. Orthogonal experimental results showed that under optimal operating conditions (HRT = 108 min, D = 1 cm, and R = 10 Omega ), more than 98% of PO 4 3- -P could be recovered in CF-MAFC. The PO 4 3- -P treatment load of CF-MAFC outperformed B-MAFC by 1.32 times, with a 22.57% increase in Mg utilization. The mass percentage of struvite in the recovered precipitate exceeded 85%, and the total nutrient content surpassed 35.70%, fulfilling China ' s compound fertilizer quality criteria. The study findings demonstrated that a continuous -flow MAFC held significant potential for the on -site resource -oriented treatment of urine.