Chemical and electrochemical coagulation processes as a tertiary treatment for residual phosphate removal from domestic wastewater: effect of operating parameters and calculation of operating cost

In this study, we aimed to compare the treatment efficiency and techno-economic analysis of chemical coagulation (CC) and electrocoagulation (EC) processes for residual phosphate (PO₄³⁻–P) removal from a secondary domestic wastewater treatment plant. The effect of two different coagulants (FeCl₃ and alum) and different pH values was evaluated in the CC process. On the other hand, the effect of different metal scrap anodes (Fe, Al, and hybrid Fe + Al), applied current (i = 0.1–0.4 A), and charge loading (q = C L⁻¹ or F m⁻³) were evaluated in the EC process. In the CC process, the highest removal efficiency of 99.7% PO₄³⁻–P was obtained with 0.5 kg FeCl₃ m⁻³ and pH = 7.5. However, in the EC process, the highest removal efficiency of 99.9% PO₄³⁻–P was obtained with Fe scrap anodes under optimum conditions (EC time = 15 min, i = 0.4 A, and q = 6 C L⁻¹ or 3.73 F m⁻³). The removal capacity and metal/PO₄³⁻–P molar ratios in the CC process were calculated as 85.7 mg PO₄³⁻–P g⁻¹ Fe and 19.8 mol mol⁻¹ for FeCl₃, and 218.9 mg PO₄³⁻–P g⁻¹ Al and 7.7 mol mol⁻¹ for alum, respectively. On the other hand, the EC process was calculated as 85.0 mg PO₄³⁻–P g⁻¹ Fe and 20.0 mol mol⁻¹ for Fe scrap, 132.5 mg PO₄³⁻–P g⁻¹ Al and 26.6 mol mol⁻¹ for Al scrap, and 84.1 mg PO₄³⁻–P g⁻¹ Fe + Al and 13.6 mol mol⁻¹ for Fe + Al scrap, respectively. At the end of the study, total operating costs ($ per m³ and $ per kg PO₄³⁻–P) were calculated for both processes. The results of the study showed that using Fe scrap anodes with the EC process for PO₄³⁻–P removal results in higher removal efficiencies and lower operating costs.