Advancing sustainable nutrient recycling and carbon neutrality: urea recovery and hydrolysis inhibition via a novel urea electro-forward osmosis system (UEFOS) from source-separated urine

Urine, commonly known as "liquid gold" in wastewater, constitutes merely 1 % of domestic sewage by volume yet contributes approximately 80 % of the nitrogen content. Urea accounts for 75-90 % of the total nitrogen in urine, presenting considerable recovery potential. This study developed an innovative urea electro-forward osmosis system (UEFOS) for urea recovery from source-separated urine. The results demonstrated that UEFOS markedly enhanced urea recovery efficiency, achieving a 90.36 % improvement compared to an open circuit system. Kinetically, the UEFOS improved the urea recovery rate and the simulated equilibrium capacity. Furthermore, UEFOS effectively inhibited urease activity by producing alkali in situ at the cathode. The contribution distribution indicated that the primary contributor to urea recovery was the electric field. Driving factors identification revealed that the electric field indirectly promoted water permeation by directly enhancing the osmotic pressure difference, thereby achieving efficient urea transport and recovery. Mechanistic exploration disclosed that ion migration driven by electric field increased the osmotic pressure difference between feed solution (FS) and draw solution, enhancing urea recovery. Simultaneously, OH⁻ generated in the cathode migrated to the FS by electric field, inhibiting urease activity and consequently reducing urea hydrolysis losses. These multiple synergistic mechanisms collectively ensured efficient urea recovery. The UEFOS demonstrated considerable economic and carbon reduction benefits through urea recovery, with an estimated annual net profit of $395 million and a corresponding annual carbon reduction of 13.8 million tons. This study provides an effective strategy to recover urea from urine and provide guidance for practical application of UEFOS.