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.

The organic acids (OAs) market has been expanding due to their versatility and wide range of uses in several industrial applications, thus prompting a transition from chemical to biological approaches to generate such high-value products. Nevertheless, downstream processes to separate OAs from the fermentation broth represent, to date, the main cost in biological OAs manufacturing. With the goal of limiting this issue, the present study investigated OAs separation from a lactic acid (LA)-rich fermentation broth through physical, i.e., using ethyl acetate (EA), and physico-chemical, i.e., using tributyl phosphate (TBP), solvent extraction. In addition, the salting-out effect of different salts on OAs recovery was investigated. Among the investigated conditions, a combination of pure TBP and 40% ammonium sulfate ensured the highest LA extraction efficiency of 65%. Besides, mixing TBP with EA (50:50) enabled to maintain the same LA extraction efficiency while reducing the process costs, limiting the toxicity of the chemicals involved, and obtaining an extracted OAs mix with lower co-metabolites, i.e. volatile fatty acids, compared to using pure TBP. Overall, solvent extraction assisted by salting-out agents were shown to be a promising method to separate OAs from a fermentation broth in liquid form, especially for fermentation processes operated at low pH.