Phosphate recovery from digested wastewater using electrochemical reactors: Trade-off between efficiency and stability

Abstract

Phosphate recovery from anaerobic sludge digested (ASD) wastewater via crystallization offers a sustainable solution to phosphate pollution and deficiency. Vivianite, Fe₃(PO₄)₂·8H₂O, a crucial precursor for lithium-ion battery anodes, can be recovered through this process. An iron-air electrochemical reactor has shown successful phosphate recovery as vivianite, but its long-term stability and economic viability are uncertain. To address this, similar reactors were constructed and evaluated for operational stability, anode/cathode/proton exchange membrane (PEM) maintenance frequency, and cost-effectiveness. Results indicated that the double-chamber electrochemical (DCE) reactor degraded after 4 days due to PEM clogging, whereas the single-chamber electrochemical (SCE) reactor degraded after 10 days due to decreased air-cathode catalytic activity. Although the DCE reactor had a higher vivianite yield rate (0.25 mg/L h⁻¹), its cost was significantly greater (¥ -32.32 g⁻¹·P) primarily due to the expensive PEM. The SCE reactor, with a lower yield rate (0.18 mg/L h⁻¹), was more cost-effective (¥ -0.65 g⁻¹·P). Therefore, after improving the conductivity of ASD wastewater to 17 mS/cm, the performance of the SCE reactor surpassed that of the DCE reactor. However, the DCE reactor could treat wastewater with lower conductivity, allowing for a wider scope of application. The conductivity of wastewater was crucial in selecting the appropriate configuration of electrochemical systems. This study offers reference information for aligning electrochemical systems with wastewater treatment, contributing to the objectives of energy saving and emission reduction.