Fate of calcium, magnesium and inorganic carbon in electrochemical phosphorus recovery from domestic wastewater

Abstract

Calcium (Ca), magnesium (Mg), phosphate and (bi)carbonate are removed simultaneously in electrochemical recovery of phosphorus (P) from sewage. However, the fate of these ions is not completely understood yet. In this paper, through wastewater acidification and current density altering, we clarified the precipitation process and electrochemical interaction of phosphate and coexisting ions. The removal of P is attributed to amorphous calcium phosphate (ACP) formation, whereas the removal of bicarbonate is mainly due to calcite (CaCO3) formation and acid-base neutralization. While both ACP and calcite results in Ca removal, Ca predominantly ends up in calcite. For Mg, it is exclusively removed as brucite (Mg(OH)2). Regardless of the acidification, 53 ± 2% P and 32 ± 1% Mg were removed in 24 h at 8.3 A/m2. By contrast, in response to the acidification, the removal of Ca dropped from 42% to 19%. The removal of Mg depends on the current density, with less than 5% removed at 1.4 A/m2 but 70% at 27.8 A/m2 in 24 h. Based on the precipitation mechanisms, the formation of calcite and brucite can be reduced by acidification and operating at a relatively low current density, respectively. Accordingly, we achieved the lowest Ca/P molar ratio (1.8) and the highest relative abundance of ACP in the precipitates (75%) at bulk pH 3.8 with a current density of 1.4 A/m2.