Electrocoagulation is an electrochemical method that has been used in the treatment of polluted wastewaters. Aluminum or iron are usually used as electrodes and their cations are generated by dissolution of sacrificial anodes to release active coagulant precursors in the solution. EC occurs in formation of metal hydroxides by electro-dissolution of sacrificial anode under DC electrical field. The formed metal monomeric and polymeric hydroxide complex species serve as coagulants to trap and destabilization of colloidal pollutants from solution.
Electrocoagulation process has advantages when compared to conventional methods such as simple equipment, easy to operate, reduction of sludge, produces effluent with less total dissolved solids (TDS), removal of the small colloidal particles and requires no addition of chemicals that might create secondary pollution.
Electrocoagulation (EC) is also a technique that can be used as experimentally as a pre-treatment process during the membrane process for the treatment of wastewaters that contain concentrated oil-water (O/W) emulsions successfully to treat oil containing wastewater, organic pollutants from bilge water, dairy wastewater, Textile wastewater, Fluoride containing wastewater, Silica containing wastewaters Arsenic containing wastewater solutions , Boron containing wastewaters, Phosphate containing solutions, restaurant wastewater, slaughterhouse wastewater , organic matter from leachates and so on.
Theory of Electrocoagulation
Electrocoagulation was proposed before the turn of 20th century. Several wastewater treatment plants were built in London in 1889 for the treatment of sewage by mixing it with sea water and electrolyzing, the electrolytic sewage treatment of plants were also built in the United States with similar plants to treat the municipal wastewaters in the following decades. However, due to high operating costs and ready availability of other alternatives such as dosing chemical coagulant resulted all such plants to be abandoned in 1930s.
Electrocoagulation is an electrochemical technique used to remove pollutants from wastewater whereby sacrificial anodes dissolve to produce active coagulant precursors usually can be iron or aluminum and the electrolyte is either water or wastewater. The cathode produces hydroxyl ions and hydrogen gas. The released gas carries the pollutants (especially oil and grease) to the top of the water where it can be collected and removed. The metallic ions (Fen+ and Al3+) and the hydroxide ions react to produce insoluble hydroxides during hydrogen gas evolution that will adsorb the oil or other pollutants out of the solution and will contribute the coagulation-flocculation process. Figure 1 shows the schematic view of electrocoagulation mechanisms.
Factors Affecting Electrocoagulation Treatment
Electrocoagulation process for the treatment of wastewaters may be affected by several operating parameters such as current density, initial pH, operating time, conductivity, electrode arrangement, temperature and wastewater concentration.
Effect of Current Density
Current density has been reported as one of the most important operating parameters that can affect the efficiency of electrocoagulation treatment process. Aouni et al. (2009), reported the effect of current density on the treatment of textile wastewater by a hybrid electrocoagulation. The applied current density was between 3.33 to 100 mAcm-2. The experimental data indicated that the COD removal efficiency increased faster when current density increased. The increase of the current density increased the removal efficiency of the pollutants. When the intial phosphate concentration, pH, temperature and sodium chloride was kept constant the removal efficiency of phosphate increased from 85 to 95% by increasing the current density from 1.13 to 4.54 mA/cm2 .
Experiment of electrochemical removal of phenol from oil refinery wastewater was reported. The experiment was conducted for 2 hours period for various current densities. The maximum percentage of phenol removal efficiency was 84, 88, 95 and 97% for current densities of 6.4, 12.9, 19.3 and 23.6 mAcm-2, respectively after 120 min. A lot of studies reported that increase of current density resulted the increase of COD removal efficiency in the wastewater. This increase is due to the fact that at high current density the amount of Al3+ ions from the anode dissolution increases according to Faraday’s law.
Effect of Initial pH
It has been established that pH as a very important parameter that affect the performance of electrochemical reactions [Bensadok et al., 2011]. The effects of pH of wastewater on electrocoagulation are reflected by current density as well as the solubility of metal hydroxides. The pH of the treated wastewater increase as operating time of electrocoagulation process increase. This increase is due to the OH– ion accumulation in an aqueous solution during the electrocoagulation process.
Wang et al. (2009), investigated the effect of COD removal from laundry wastewater by electrocoagulation process. The initial pH of the solution was adjusted from 2.5 to 9.5 by adding NaOH(aq) and H2SO4(aq). As the pH increased from 2.5 to 5.1 the COD removal efficiency also increased from 30% to 66% and then started gradual decrease until pH 9.5 where removal efficiency became 54%. The optimal initial pH of this was found to be 5.1.
Effect of the Time
In all treatment processes, the duration of the treatment is also an important parameter in electrocoagulation process. Treatment of oily wastewater from bilge by electrochemical process were performed. The experiment carried out between 5-120 min at 6.8 pH and a temperature of 20 oC and a current density of 7.5 mA/cm2. They obtained the major part of their removal efficiency the first 10 min of the test. The first 10 min of the test, the COD and oil-grease removal efficiency was 64% and 53% respectively, while these values were 69% and 68% after 60 min.
Electrocoagulation was used the treatment of textile wastewater and the effect of the reaction time was investigated (S.M. Palácio et al., 2009). The ranging between the time of electrolysis was 15-90 min. After 15 min of electrocoagulation treatment, the removal percentage of the turbidity, COD and TOC value was 92%, 62% and 41% respectively while these values reached 97%, 77% and 55% after 60 min of electrocoagulation treatment. Shalaby et al. , reported that the effect of time on treatment of phosphate removal from wastewater by electrocoagulation.
When the electrolysis time increased, the generation of the flocs increased resulting an increase of the pollutant removal efficiency. The decolonization of C.I. Acid Yellow 23 solution by process was investigated. The increase of electrolysis time resulted an increase of ion concentrations and hydroxide flocs. As the time increased from 2-6 removal efficiency of color reached from 15.53 to 98.98% . Xu and Zhu (2004), studied the effect of reactive time on the treatment of refectory oily wastewater by electrocoagulation process. More than 99% of oil and 77% of CODCr removal efficiencies were achieved within 60 min, and the main removal occurred in the first 20-30 min.
Effect of Conductivity
Increasing the conductivity in the water or wastewater have a great importance in the reducing of power consumption. Sodium chloride (NaCl) salt is usually used to increase the conductivity of the water or wastewater to be treated. The increase of conductivity from 443 to 2850 μS/cm resulted decrease of the voltage between the electrodes from 7.25 to 1.8 V. It also resulted a decrease of the power consumption from 1.29 to 0.32 kWh/m3. An increase the amount of sodium chloride salt resulted the increase of removal efficiency.
This also resulted an increase the amount of aluminum generation as sodium chloride dose varies from 0.5 to 5 g/L . Conductivity decreases due to the electrochemical reactions taking place in the reactor during water and wastewater treatment. Conductivity was shown to depend on the time of electrolytic reactions and current density. A current density 348 A/m2 resulted the decrease of conductivity from 19.6 to 18.6 mS with aluminum electrode and 19.6 to 18.6 mS with iron electrode. A further decrease of conductivity was observed when the current density of 631 A/m2 was used. The conductivity decreased from 19.6 to 18.2 mS with aluminum electrode and 19.6 to 18.3 mS with iron electrode.
Effect of Electrode Material
Electrode material is a very important parameter and the heart of electrocoagulation process. Therefore, the electrode materials and the distance between electrode materials are concerned greatly. The selection of electrode materials usually depends their availability. Aluminum and iron electrodes are the two most commonly used electrode materials in electrocoagulation applications due to cheap, readily available and proven effective. It was suggested to use iron electrode for wastewater treatment and aluminum electrode for water treatment because iron is relatively cheaper.
The electrode connection and the number of electrodes can affect the removal efficiency. Monopolar and bipolar electrode materials were studied to find the optimal arrangement for the removal of COD from laundry wastewater by electrocoagulation process. The monopolar electrode found to be superior over bipolar electrode. The removal efficiency was about 62 and 7% for monopolar and bipolar respectively.
This study also investigated the effect of electrode numbers by using monopolar electrode composed of 2, 4 and 6 number of plates and the COD removal efficiency became 51.3, 54, and 62% respectively. This result indicated that the use of more electrode connections in parallel does not affect too much the COD removal efficiency. Kobya et al. , found that the iron electrode to be superior to aluminum electrode as sacrificial electrode material from COD removal efficiency and the consumption of energy.
Effect of Temperature
The effect of temperature on the electrocoagulation technology got a little attention from the researchers.
Ulucan and Kurt (2015), reported the effect of temperature increase on comparative study of electrochemical wastewater treatment process for bilge water as oily wastewater. The removal efficiency of COD and oil-grease were investigated at range of bilge water temperature values from 4 to 60 oC. The study were carried out at the pH of 6.86 which was the wastewater’s original pH at current density of 7 mA/cm2 for about 10 min.
Using aluminum electrode, the removal efficiency of COD and oil-grease became 29 and 23% at 4 oC respectively; these values increased to 64 and at the temperature of 40 o When iron electrode was used, the removal efficiency of COD and oil-grease became 34 and 9% at 4 oC respectively, while these values reached to 39 and 16% at the temperature of 40 oC.
The temperature increases due to the reactions occurring inside the reactor. This increase depending on the period of the electrolytic reactions, the type of the electrode material and current density. A current density of 348 A/m2 resulted an increase of temperature from 23.8 to 25.5 oC with aluminum electrode and from 23.8 to 25.3 oC with iron electrode.
Effect of Wastewater Concentration
The initial wastewater concentration is a key parameter in electrocoagulation process and it is important for the selection of suitable concentration range. Mollah et al (2004), reported the effect of initial concentration on the treatment of orange II azo-dye by electrocoagulation process in continuous flow of cell using sacrificial iron electrodes. They investigated the effect of initial contaminant concentration by taking 10, 25, 30, 40 and 50 ppm of dye solution. The experiment was carried out by making constant voltage of 40V and a constant current density of 159.5 A/m2. The initial conductivity of the solution was 3.76 mS/cm and finally decreased to 3.60 mS/cm after the experiment.
They saw the decrease of removal efficiency from 90.4 to 55% almost linearly with increase in concentration of the dye. This decrease of removal efficiency was attributed the formation of insufficient number of iron hydroxide complexes generated by the electrode that cannot adsorb all the pollutants in the solution. The effect of initial concentration between 10 to 150 mg/L on phosphate removal using electrocoagulation was examined. It was found that increase in concentration at constant electrocoagulation time and current density terminated in the increase of decrease of removal efficiency.