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 NH4+-N and PO43--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 = 108min, D = 1cm, and R = 10 Ω), more than 98% of PO43--P could be recovered in CF-MAFC. The PO43--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.