Nutrient recovery from hydrolysed urine by high-rate electrodialysis: A proof-of-concept study

Human urine represents a valuable resource for sustainably meeting the growing demand for nitrogen (N), phosphorus (P) and potassium (K) fertilisers as it is rich in these NPK nutrients. This study demonstrates the suitability of electrodialysis (ED) for simultaneous N, P and K recovery from hydrolysed urine at a high current density of 30 mA/cm 2 , facilitating rapid ion transport. The concentrate product obtained from a synthetic urine at 30 mA/cm 2 contained 20.0 ± 0.3 g NH 4 + -N/L, 9.9 ± 0.2 g K + /L, and 1.6 g PO 4 -P/L. The corresponding concentration factors of N, P, and K were 3.5, 4.1 and 3.4, respectively. At 30 mA/cm 2 , the process approached the limiting current density (LCD) of the system when the total ammonia nitrogen (TAN) concentration in the diluate decreased to around 2 g/L. At this point, the TAN removal efficiency remained high at ~64 % with a stable specific energy consumption of 9.0 ± 0.2 kWh/kgN and an average nitrogen migration rate of 104 gN/m 2 /h. Experiments with real hydrolysed urine confirmed stable operation in high-rate ED, achieving NH 4 + -N concentrations of up to 30 gN/L, among the highest reported for ED-based nutrient recovery from urine. These findings validate the feasibility and efficiency of high-rate ED for nutrient recovery from source-separated urine. The study provides a practical reference for designing continuous ED systems, where membrane area and diluate/concentrate concentrations can be adjusted to maintain efficient nutrient recovery while operating below the LCD to minimise energy loss and reduce capital costs. • High-rate ED at 30 mA/cm 2 achieved efficient N, P, and K recovery. • LCD reached when TAN in synthetic urine dropped to ~2 g N/L (~64 % removal). • Specific energy consumption stable at ~9–10 kWh/kg N at 30 mA/cm 2 . • A nitrogen migration rate of 104 gN/m 2 /h guides design of continuous ED systems. • Real urine trials confirmed stable operation, reaching NH 4 + -N up to 30 g/L.