Electrochemical recovery of H₂ and nutrients (N, P) from synthetic source separate urine water

This study examined an electrochemical method of H₂ production and nutrient recovery from synthetic source separated urine (SSU). The efficacy of H₂ production was examined through hydrogen recovery experiments (HRE) using Ni foam electrodes. Similarly, nutrient (N and P) recovery was also examined in post-nutrient recovery experiments (NRE) with sacrificial Mg electrodes. To achieve higher nutrient recovery in the post-nutrient recovery process, the most important operating parameters (initial solution pH (pH_i) and current density) were optimized. Optimization of NRE revealed that > 90% NH₃-N and PO₄^3--P could be recovered at 8 mA cm^-2 with a pH_i of 6-8. Notable NH₃-N and PO₄^3--P reduction were observed at an equimolar Mg²⁺ dissolution ratio (1:1) of Mg²⁺:NH₄⁺ and a 1.1:1 ratio of Mg²⁺:PO₄^3- respectively. However, poor total Kjeldahl nitrogen (TKN) reduction was observed. Thus, we anticipate that direct electrochemical conversion of urea to N₂ at the anode followed by H₂ generation at the cathode is a more sustainable way to reduce TKN. Batch HRE showed that the initial TKN, 1094 mg L^-1 (934 mg L^-1 from urea-N and 160 mg L^-1 from NH₄Cl), was significantly reduced to 360 mg L^-1 by Ni-Ni electrolysis, whereas around 53.8 g H₂ gas was received from this Ni-Ni electrolysis system with a flow rate of 5-5.8 g mol^-1 day^-1. Overall, this work produced a 68% reduction in TKN due to electrochemical conversion of urea into H₂.