Publication date: 21st July 2025
Urine is a rich source of nitrogen and phosphorus, making it a valuable resource for nutrient recovery. However, to harness these nutrients effectively, urine must be collected and stabilized before urease-catalyzed enzymatic degradation occurs. Urease enzymes, present in natural environments and urine treatment systems, rapidly hydrolyze urea—the primary nitrogen compound in urine—into ammonia and carbonic acid. This not only results in nitrogen loss but also causes odor problems, severely limiting the feasibility of decentralized urine management. In this study, we present a bioelectrochemical approach for urea stabilization via pH modulation in microbial electrolysis cells (MECs). By generating hydroxide ions (OH⁻) at the cathode, the system inhibits urease activity by elevating the pH above 11. Experiments using synthetic urine as catholyte revealed that the ratio of cathode surface area to catholyte volume inversely affects the time required to reach the inhibitory pH threshold. Optimized 3D-printed MEC configurations demonstrated that higher current densities (up to 5.4 A m⁻²) could stabilize urea within 1 h, whereas lower currents resulted in prolonged treatment times. The pH-stabilized urine samples remained urease-inactive for approximately 1 month. Additionally, phosphorus recovery efficiencies ranged between 55.6% and 77.6% of the theoretical maximum, constrained by the availability of bivalent cations. Mass transport phenomena and pH gradient-induced inefficiencies were analyzed to assess process limitations. This work underscores bioelectrochemically induced pH elevation as a promising and controllable strategy for on-site urea stabilization and nutrient recovery in decentralized sanitation systems.
L.K. was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. The Authors acknowledge the project no. RRF-2.3.1-21-2022-00009, titled National Laboratory for Renewable Energy has been implemented with the support provided by the Recovery and Resilience Facility of the European Union within the framework of Programme Széchenyi Plan Plus.
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