Removal of phosphate in secondary effluent from municipal wastewater treatment plant by iron and aluminum electrocoagulation: Efficiency and mechanism

Domestic wastewater quality conditions are complex, which will affect the types of flocs in Fe-electrocoagulation (EC). However, the influence of different types of flocs has not been considered for the removal of phosphate in real wastewater. Herein, the removal of phosphate in real domestic wastewater secondary effluent was investigated during Fe-EC process under low and high dissolved oxygen (DO) concentrations. The removal of phosphate was also investigated during Al-EC process for comparison. The results showed that phosphate (1.3 mg/L) could be efficiently removed with an efficiency of 98% ± 2% both during Fe-EC process with low and high DO concentration, as well as during Al-EC process, with a slight lower removal rate during Al-EC (in 5 min) compared with Fe-EC (in 2 min), at 10 mA/cm2. The flocs composition was mainly green rust during Fe-EC with low DO, amorphous trivalent iron oxide/hydroxide during Fe-EC with high DO, and amorphous alum hydroxide during Al-EC. The removal mechanism was also deduced that the removal of phosphate by Fe-EC with high DO and Al-EC was mainly by coagulation, while by Fe-EC with low DO concentration was mainly by ion-exchange adsorption.