Simultaneous nitrogen and phosphorus removal from anaerobic digested wastewater with struvite recovery using magnesium-air fuel cell

Abstract

In this study, a magnesium-air fuel cell (Mg-O₂-FC) was adopted to treat anaerobic digested wastewater, and long-term operation was conducted to evaluate its applicability in terms of ammonium and phosphorus removal efficiencies. The results showed that the phosphorus and ammonium removal efficiencies were 96.76 % and 48.08 %, respectively, when the Mg-O₂-FC was operated in sequential batch mode for six cycles. Meanwhile, a two-stage approach named MaFuce-T further enhanced the ammonium and phosphorus removal efficiencies of the Mg-O₂-FC and maximized the utilization of magnesium resource, achieving optimal recovery of ammonium and phosphorus resources in the form of struvite (MgNH₄PO₄·6H₂O). Under this approach, the volumetric removal rates of 3.26 kg/(m³·d) for phosphate and 8.79 kg/(m³·d) for ammonium, with magnesium ion utilization efficiency of 64.04 %. Under the maximum lifespan of the magnesium plate (24 h), this MaFuce-T approach increased the amount of phosphate removed by approximately 3.47 times, while ammonium removal by roughly 1.02 times. By SEM-EDS, FTIR, and XRD qualitative analysis, the product was determined as struvite with a purity of 86.81 %. Furthermore, the maximum output voltage generated by the fuel cell was recorded at 0.42 V, with a power output of 10.50 mW·h. This approach demonstrated significant economic potential and contributes to the sustainable development of anaerobic digested wastewater treatment systems.