Electrochemical Phosphorus Removal and Recovery from Cheese Wastewater: Function of Polarity Reversal

Electrochemically mediated calcium phosphate precipitation (ECaPP) on the cathode offers a chemical-free process for phosphorus removal and recovery from wastewaters. However, this process is limited by the requirement of a large cathode surface area for precipitation. Here, we present a concept of overcoming this limiting factor by periodically swapping the polarity of the electrodes. In batch studies, the surface of the cathode was cleared from precipitates after deposition by polarity reversal under all studied current densities ranging from 62.5 to 187.5 A/m(2). The characterization of morphology and elemental distribution of the cathode surface with scanning electron microscopy-energy-dispersive X-ray spectroscopy over the polarity reversal process further confirms the feasibility of polarity reversal in removing deposits from the cathode. The phosphorus removal efficiency was enhanced at a low current density (62.5 A/m2), 95.0% compared to 80.0% without reversing the polarity. The recovered solids were also found to contain lower amounts of byproducts [i.e., CaCO3 and Mg(OH)(2)], as evidenced by the higher P content (13.5% vs 10.0%), lower Mg content (2.5 g/kg vs 12.5 g/kg), and lower Ca/P atomic ratio (1.7 vs 2.4) with and without polarity reversal. In the long-term continuous flow operation mode, without polarity reversal, phosphorus's removal efficiency decreased significantly due to the coverage of the cathode by deposits. However, with polarity reversal, the ECaPP system realized a relatively stable phosphorus removal in the steady-state stage, about 20.5% higher than without polarity reversal. Accordingly, a lower power consumption (39 kW h/kg P) was achieved, around 31.6% lower than without polarity reversal. This study proves that the drop in ECaPP systems' efficiency under long-term operation can be solved with the proven concept of refreshing electrode surfaces by reversing their polarity, making the necessity for a sufficiently big cathode (precipitation area) superfluous. This can pave the way for the ECaPP system to move toward automation and implementation.