Electrochemically mediated phosphorus and energy recovery from digested effluent

The growing global concern over the high phosphorus concentration in discharged wastewaters has driven the demand for exploring the means to recover it from wastewater. We previously demonstrated the possibility of phosphorus recovery by iron-air fuel cells from digested effluent. The present study focused on further optimizing the performance of the fuel cell by adjusting the wastewater properties (initial pH) and device parameters (anode/cathode area ratio, electrode spacing). Under neutral or slightly alkaline conditions, the HCO₃^- ions accelerated the formation of iron anode passivation layer, resulting in a decreased phosphate removal efficiency and vivianite yield. Additionally, the occurrence of oxygen crossover with small electrode spacing and anode/cathode area ratio significantly influenced the efficiency of fuel cells in terms of phosphate removal, vivianite production, and electricity generation. The results showed that an acidic pH (5.78), an adequate anode/cathode area ratio (1.3), and an appropriate electrode spacing (5 cm) were prone to increase vivianite yield. Furthermore, the fuel cell achieved the highest electric energy output with an initial pH of 5.78, an anode/cathode area ratio of 0.4, and an electrode spacing of 7.5 cm. As far as operational cost was concerned, the iron-air fuel cell system exhibited a potential cost-saving advantage of about 65.6% compared to the traditional electrochemical crystallization system.