Electrochemically-mediated calcium phosphate precipitation (ECaPP) on the cathode offers a chemical-free process towards phosphorus removal and recovery from wastewaters. However, this process is limited by the requirement of a large cathode surface area for precipitation. Here, we show 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 by polarity reversal under all studied currents (200 to 600 mA). The phosphorus removal efficiency was enhanced at low current (200 mA), 95.0% compared to 80.0% without reversing polarity. The recovered solids were also found to contain lower amounts of by-products (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 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, which is about 20.5% higher than without polarity reversal. The ECaPP systems’ efficiency decrease with time can be solved with the proven concept of cleaning 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 towards automation and implementation.

Refreshing electrodes via reversing polarity in electrochemical recovery of phosphorus from wastewater

ACS Spring 2022

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