Continuous, Self-Driven Liquid Fertilizer Extraction from Human Urine Using Microbial Fuel Cells for Circular Economy: Catholyte Characterization

This work presents the development of urine filtration microbial fuel cells, converting urine into a purified catholyte in which key macronutrients such as nitrogen, phosphorus, and potassium (NPK) are recovered, and chemical oxygen demand (COD) is removed, with energy being continuously produced in the electrofiltration process. In this study, electrofiltrating-MFC (EF-MFC) bioreactors using ceramic-based membranes were assembled to oxidize urine and generate an electric current, which facilitated the movement of cationic species and water toward the cathode. As a result, a flux of purified cathodic filtrate was achieved at a 9.5 higher rate than the control, while the organic content was removed. The reactor design enabled continuous electropumping of the newly formed catholyte throughout the experiment. In addition, NPK were also recovered in the cathodic filtrate, thus demonstrating the electrofiltration capacity of the ceramic bioreactors. This work exploited pH and ion splitting mechanisms that were directly related to the electric field generated by the EF-MFC system and to study electro-osmotic drag as a continuous electropumping mechanism responsible for sustained extraction of water/nutrients from urine. Furthermore, the free chlorine detected in the catholyte produced may contribute to the quality and disinfection (safety) of the purified biofertilizer. The proposed technology allows for the additional oxidation of the organic fraction in urine on the cathode side of the half-cell while retaining a high proportion of all other key macronutrients for potential reuse as a liquid fertilizer as a byproduct of stable electrical current production in the process.