Enhanced electrochemically mediated calcium phosphate precipitation for phosphorus recovery simply by carbon felt barrier cathode

Phosphorus removal and recovery from wastewater is crucial for environmental pollution control and sustainable phosphorus fertilizer production. Electrochemical technologies are promising, but disadvantages like high energy consumption remain. Here, we improved the electrochemical system's performance by placing carbon felt in front of the cathode at a distance of 3 mm with a current density as low as 1 A/m2, which separated the reactor into two parts: near the cathode area and bulk solution. The phosphorus removal near the cathode can reach 75 % in 1 h, while it takes 8 h for the bulk solution to achieve 75 % removal efficiency. The main reason for the enhancement was the retention of OH- ions between the cathode and carbon felt with enriched calcium ions, providing an appropriate environment for calcium phosphate precipitation. The functional groups on the carbon felt surface were another reason for the retention of OH- ions. Furthermore, our results show that the carbon felt system has considerable stability under different mass transfer efficiencies. By adjusting the stirring rate, we could control the proportion of precipitation on the carbon felt or cathode, thereby reducing the scaling on the cathode and avoiding significantly reduced removal efficiency. A long-term continuous flow experiment also illustrated this. Interestingly, we observed that the phosphorus removal had a specific correlation with pH and fit an empirical formula. Therefore, we could estimate the removal efficiency by simply measuring the effluent pH of the solution after treatment. In treating real phosphorus-rich wastewater, the carbon felt system removed 63 %-87 % of 1500 mg/L P after 8 h to 48 h treatment at 20 A/m2, respectively, resulting in lower energy consumption (31 kWh/kg P, 8 h) compared to the system without carbon felt (48 kWh/kg P, 8 h). This study provides a simple yet efficient method for enhancing electrochemical phosphorus recovery.