Is There a Precipitation Sequence in Municipal Wastewater Induced by Electrolysis?

Electrochemical wastewater treatment can induce calcium phosphate precipitation on the cathode surface. This provides a simple yet efficient way for extracting phosphorus from municipal wastewater without dosing chemicals. However, the precipitation of amorphous calcium phosphate (ACP) is accompanied by the precipitation of calcite (CaCO₃) and brucite (Mg(OH)₂). To increase the content of ACP in the products, it is essential to understand the precipitation sequence of ACP, calcite, and brucite in electrochemical wastewater treatment. Given the fact that calcium phosphate (i.e., hydroxyapatite) has the lowest thermodynamic solubility product and highest saturation index in the wastewater, it has the potential to precipitate first. However, this is not observed in electrochemical phosphate recovery from raw wastewater, which is probably because of the very high Ca/P molar ratio (7.5) and high bicarbonate concentration in the wastewater resulting in formation of calcite. In the case of decreased Ca/P molar ratio (1.77) by spiking external phosphate, most of the removed Ca in the wastewater was used for ACP formation instead of calcite. The formation of of brucite, however, was only affected when the current density was decreased or the size of cathode was changed. Overall, the removal of Ca and Mg is much more affected by current density than the surface area of cathode, whereas for P removal, the reverse is true. Because of these dependencies, though there is no definite precipitation sequence among ACP, calcite, and brucite, it is still possible to influence the precipitation degree of these species by relatively low current density and high surface area or by targeting phosphorus-rich wastewaters.