Recent progress in highly effective electrocoagulation-coupled systems for advanced wastewater treatment

Abstract

Electrocoagulation (EC) has been a well-known technology for wastewater treatment over the past centuries, owing to its straightforward equipment requirements and highly effective contaminant removal efficiency. This literature review emphasizes the influence of several input variables in the EC system such as electrode materials, applied current, pH, supporting electrolyte, and inner-electrode distance on effluent removal efficiency and energy consumption. Besides that, depending on the intrinsic properties of effluents, EC is recommended to hybridize with other methods such as physical-, biological-, chemical-, and electrochemical methods in order to enhance removal performance and reduce energy consumption. Subsequently, a comprehensive analysis of EC performance is presented, including power consumption, and evaluation of the synergistic effect of multiple input variables using statistical methods. Finally, this review discusses future perspectives such as the environmentally friendly utilization of post-EC treated sludges, the development of renewable energy-driven EC systems, and the challenges of EC management by artificial intelligence.

Keywords: Chemical Engineering; Environmental engineering; Water geochemistry; Water resources engineering.

Graphical abstract

Figure 1

Timeline of EC research in wastewater treatment

Figure 2

Annual publications of electrocoagulation in various fields The literature search was conducted using Elsevier’s Scopus database with the keyword “electrocoagulation”.

Figure 3

Schematic illustration of the electrocoagulation cell

Figure 4

The effect of electrode materials to EC performance (A and B) SEM images of (A) pre-treated and (B) post-treated Al electrode of the anode Al surface (passivation film) when operating at pH 11 in electrolyte with different dyes. (C) Proposed mechanism of fouling layer formation on the anode during EC reaction and polarity reversal for surface cleaning: (1) pristine anode, (2) porous iron oxide layer formation at the early stage, (3) thick and dense black rust layer (containing goethite, lepidocrocite, and magnetite with passivating effects) formation during long-term performance, (4) film-free area after polarity reversal. Insets are photos of Fe electrode (i.e., carbon steel, 3.0 cm × 5.5 cm) after extended EC operation. The removal efficiency of (D) COD and (E) energy consumptions versus pH for the electrodes (5 mA/cm² and 90 min).

Figure 5

The effect of input varieties to EC performance (A) Percentage of nitrogen forms in influent and treated drainage water by EC system in different current densities. (B) The distribution of Fe²⁺ species in the aqueous solution at different pH values. (C) The influence of interelectrode gap on micro polystyrene (μPS) removal performance. (D) Effect of NaCl and CaCl₂ electrolytes on removal efficiency.

Figure 6

The integrated biological-EC system (A) The anaerobic batch reactor. (B and C) Overall performance of the hybrid systems on ΝΗ⁴⁺-Ν, color, and d-COD removal efficiencies: (B) AD-EC-BIO and (C) AD-BIO-EC (AD: after adsorption with zeolite; EC: after electrocoagulation; BIO: after biological treatment with plastic tubes).

Figure 7

The integrated physical-EC system (A) Schematic illustrator of the hybrid EC-ultrafiltration treatment system. (B) Energy consumption (in kWh/m³) and % color removal for the two positions of the electromagnetic field (EMF) compared to the experiment without EMF. (C) Schematic diagram of sono-EC.

Figure 8

The integrated chemical-EC system (A and B) (A) Experimental setup of ozone-assisted EC and (B) comparison of ozonation, electrocoagulation, and ozone-assisted EC (current density: 3 A/dm², effluent COD concentration: 3,000 ppm, pH = 7, inter-electrode distance: 1.8 cm, electrolysis time: 5 h, and ozone flow rate: 15 L/min and concentration: 2 g/h). (C and D) (C) Comparison of DR89 dye removal efficiency by different processes and (D) energy consumption for different processes. Experimental condition: electric current = 0.1 A, (PS) = 0.4 mM, (H₂O₂) = 0.4 mM, (Na₂SO₄) = 2 mM, pH = 7, (DR89) = 100 mg/L, reaction time = 15 min, electrode: Al//Fe.

Figure 9

The integrated electrochemical-EC systems (A) Schematic diagram of the EC coupled with electrooxidation for the simultaneous treatment of multiple pollutants in contaminated sediments, and (B) time course of phenanthrene concentrations in the sediment as a result of different treatments. For single mixed metal oxide (MMO) and single Fe anodes, the current density was 20 mA/cm². For dual MMO/Fe anodes, the current density applied to MMO and Fe was 10 mA/cm² and 10 mA/cm², respectively. The initial contaminant concentration was 1.00 ± 0.1 mg/kg). (C) Schematic diagram of the synchronous degradation and removal mechanism in the Fe-carbon fiber brush (CFB) system. (D) SEM images of CFB cathode before and after use.