Batch Electrocoagulation Research Articles

Assessment of electrocoagulation for the treatment of petroleum refinery wastewater

Batch electrocoagulation experiments were carried out to evaluate the removal of sulfate and COD from petroleum refinery wastewater using three types of electrodes: aluminum, stainless steel, and iron. The effects of current density, electrode arrangement, electrolysis time, initial pH, and temperature were investigated for two wastewater samples with different concentrations of COD and sulfate. The experimental results indicated that the utilization of aluminum, as anode and cathode, was by far the most efficient arrangement in the reduction of both the contaminants. The treatment process was found to be largely affected by the current density and the initial composition of the wastewater. Although electrocoagulation was found to be most effective at 25°C and a pH of 8, the influence of these two parameters on the removal rate was not significant. The results demonstrated the technical feasibility of electrocoagulation as a possible and reliable technique for the pretreatment of heavily contaminated petroleum refinery wastewater.

Effect of pH and chloride concentration on the removal of hexavalent chromium in a batch electrocoagulation reactor

In this work, the effect of pH and chloride ions concentration on the removal of Cr(VI) from wastewater by batch electrocoagulation using iron plate electrodes has been investigated. The initial solution pH was adjusted with different concentrations of H 2SO 4. The presence of chloride ions enhances the anode dissolution due to pitting corrosion. Fe 2+ ions formed during the anode dissolution cause the reduction of Cr(VI) to form Cr(III), which are co-precipitated with Fe 3+ ions at relatively low pH. The reduction degree of Cr(VI) to Cr(III) and the solubility of metal hydroxide species (both chromic and iron hydroxides) depend on pH. At higher concentrations of H 2SO 4, the reduction of Cr(VI) to Cr(III) by Fe 2+ ions is preferred, but the coagulation of Fe 3+ and Cr(III) is favoured at the lower H 2SO 4 concentrations.

Treatment of As(V) and As(III) by electrocoagulation using Al and Fe electrode

A batch electrocoagulation (EC) process with bipolar electrode and potentiodynamic polarization tests with monopolar systems were investigated as methods to explore the effects of electrode materials and initial solution pH on the As(V) and As(III) removal. The results displayed that the system with Al electrode has higher reaction rate during the initial period from 0 to 25 minutes than that of Fe electrode for alkaline condition. The pH increased with the EC time because the As(V) and As(III) removal by either co-precipitation or adsorption resulted in that the OH positions in Al-hydroxide or Fe-hydroxide were substituted by As(V) and As(III). The pH in Fe electrode system elevate higher than that in Al electrode because the As(V) removal substitutes more OH position in Fe-hydroxide than that in Al-hydroxide. EC system with Fe electrode can successfully remove the As(III) but system with Al electrode cannot because As(III) can strongly bind to the surface of Fe-hydroxide with forming inner-sphere species but weakly adsorb to the Al-hydroxide surface with forming outer-sphere species. The acidic solution can destroy the deposited hydroxide passive film then allow the metallic ions liberate into the solution, therefore, the acidic initial solution can enhance the As(V) and As(III) removal. The over potential calculation and potentiodynamic polarization tests reveal that the Fe electrode systems possess higher over potential and pitting potential than that of Al electrode system due to the fast hydrolysis of and the occurrence of Fe-hydroxide passive film.

Study of COD and turbidity removal from real oxide-CMP wastewater by iron electrocoagulation and the evaluation of specific energy consumption

This study explores the feasibility of reducing COD and turbidity from real oxide chemical mechanical polishing (oxide-CMP) wastewater. Based on the dynamic characteristics of batch electrocoagulation, three operating stages (lag, reactive, and stabilizing) are proposed to identify the relationships among the zeta potential of the silica particles, solution turbidity, and the corresponding mean particle size. Experiment results show that the silica particles were destabilized and settled at the critical electrolysis time, which was estimated to be about 12 min under an applied voltage of 20 V and a supporting electrolyte of 200 mg/L. The corresponding turbidity removal occurred mostly during the reactive stage. The process variables, including applied voltage and electrolyte concentration, were investigated in terms of COD removal efficiency and turbidity removal. In addition, the effects of applied voltage and supporting electrolyte on COD removal efficiency and specific energy consumption were evaluated. Under the optimum balance, satisfactory removal efficiency and relatively low energy consumption were obtained. The optimum electrolyte concentration and applied voltage were found to be 200 mg/L NaCl and 20 V, respectively. Under the optimum conditions, COD and turbidity decreased by more than 90% and 98% in real oxide-CMP wastewater, respectively.

Investigation of the Electrocoagulation Treatment of Cotton Blue Dye Solution using Aluminium Electrodes

AbstractThis study was performed to investigate the variables that influence the removal efficiency of an acid dye, i. e., cotton blue (CB) (chemical name: aniline blue WS) dye, from aqueous solution by an electrocoagulation (EC) technique. Batch EC studies were performed using aluminum electrodes to evaluate the influences of various experimental parameters, i. e., initial pH (pH0): 3–11, electrolysis time (t): 0–30 min, initial concentration (C0): 100–1000 mg/L, electrode gap (g): 0.5–1.3 cm, number of electrodes (N): 4–10, and applied voltage (Vap): 7–11 V, on the removal of CB dye. The optimum values of pH0, Vap and t for CB removal were found to be 6.0, 11 V and 15 min, respectively. The removal efficiency increased with decreasing values of C0 and g. For a CB solution having C0 = 100 mg/L, 97% removal efficiency was obtained at the optimized conditions. It was found that the EC sludge can be dried and thermally degraded. The bottom ash obtained after its combustion can be blended with cementitious mixtures. This approach for EC sludge disposal ensures energy recovery along with safe disposal of the EC sludge.

Electrocoagulation Studies on Treatment of Biodigester Effluent using Aluminum Electrodes

In the present paper, electrocoagulation (EC) has been employed for the reduction of chemical oxygen demand (COD) reduction of a distillery wastewater (biodigester effluent) in a batch EC reactor using aluminum electrode. The individual and interactive effects of the four main independent parameters have been studied on the COD removal efficiency using a central composite experimental design. The parameters studied are current density (j): 44.65–223.25 A/m2; initial pH (pH 0): 2–8; inter-electrode distance (g): 1–3 cm; and electrolysis time (t): 30–150 min. Pareto analysis of variance of the results showed a high coefficient of determination value (R 2 = 0.9693) and satisfactory prediction for second-order regression model. Maximum COD removal efficiency of 52.23% was obtained with j = 120 A/m2, pH 0 = 6.0, g = 1 cm, and t = 150 min.

Treatment of bio-digester effluent by electrocoagulation using iron electrodes

The present paper deals with chemical oxygen demand (COD) reduction of a bio-digester effluent (BDE) in a batch electrocoagulation (EC) reactor using iron electrode. A central composite (CC) experimental design has been employed to evaluate the individual and interactive effects of four independent parameters on the COD removal efficiency. The parameters studied are current density ( j): 44.65–223.25 A/m 2; initial pH (pH 0): 2–8; inter-electrode distance ( g): 1–3 cm and electrolysis time ( t): 30–150 min. The results have been analyzed using Pareto analysis of variance (ANOVA). Analysis showed a high coefficient of determination value ( R 2 = 0.8547) and satisfactory prediction for second-order regression model. Graphical response surface and contour plots have been used to locate the optimum values of studied parameters. Maximum COD and color reduction of 50.5% and 95.2%, respectively, was observed at optimum conditions. Present study shows that EC technique can be employed in distilleries to reduce the pollution load before treatment in aerobic treatment plants to meet the discharge standards.

Electrochemical treatment of a distillery wastewater: Parametric and residue disposal study

Most of the distilleries in India and the world over employ bio-methanation of molasses spent wash in a bio-digester plant followed by two-stage aerobic treatment as a treatment strategy. The bio-digester effluent (BDE) generated after this treatment strategy still has a high chemical oxygen demand (COD) and dark brown color, and cannot be directly discharged into a water body. In the present study, electrocoagulation (EC) has been employed for the COD and color removal of a BDE in a batch EC reactor using stainless steel (SS) electrode. A central composite (CC) experimental design has been employed to evaluate the individual and interactive effects of four independent parameters namely initial pH (pH0: 3.5–9.5), current density (j: 39.06–195.31 A m−2), inter-electrode distance (g: 1–2 cm) and electrolysis time (t: 30–150 min) on the COD and color removal efficiency. Pareto analysis of variance (ANOVA) showed a high coefficient of determination value for COD (R2 = 0.9144) and color (R2 = 0.7650) between the experimental values and the predicted values by a second-order regression model. Maximum COD and color removal of 61.6 and 98.4%, respectively, were observed at optimum conditions. Detailed physico-chemical analysis of electrode and residues (scum and sludge) of the EC process has also been carried out. A strategy for the disposal of EC residues has been proposed.

Removal turbidity and separation of heavy metals using electrocoagulation–electroflotation technique: A case study

The electrocoagulation (EC) process was developed to overcome the drawbacks of conventional wastewater treatment technologies. This process is very effective in removing organic pollutants including dyestuff wastewater and allows for the reduction of sludge generation. The purposes of this study were to investigate the effects of the operating parameters, such as pH, initial concentration ( C 0), duration of treatment ( t), current density ( j), interelectrode distance ( d) and conductivity ( κ) on a synthetic wastewater in the batch electrocoagulation–electroflotation (EF) process. The optimal operating conditions were determined and applied to a textile wastewater and separation of some heavy metals. Initially a batch-type EC–EF reactor was operated at various current densities (11.55, 18.6, 35.94, 56.64, 74.07 and 91.5 mA/cm 2) and various interelectrode distance (1, 2 and 3 cm). For solutions with 300 mg/L of silica gel, high turbidity removal (89.54%) was obtained without any coagulants when the current density was 11.55 mA/cm 2, initial pH was 7.6, conductivity wa s 2.1 mS/cm, duration of treatment was 10 min and interelectrode distance was 1 cm. The application of the optimal operating parameters on a textile wastewater showed a high removal efficiency for various items: suspended solid (SS) 86.5%, turbidity 81.56%, biological oxygen demand (BOD 5) 83%, chemical oxygen demand (COD) 68%, and color over 92.5%. During the EC process under these conditions, we have studied the separation of some heavy metal ions such as iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), lead (Pb) and cadmium (Cd) with different initial concentrations in the range of 50–600 mg/L and initial pH between 7.5 and 7.8. This allowed us to show that the kinetics of electrocoagulation–electroflotation is very quick (<15 min), and the removal rate reaches 95%.

Silica particles settling characteristics and removal performances of oxide chemical mechanical polishing wastewater treated by electrocoagulation technology

The purpose of this study was to explore the feasibility of removing silica particles and reducing turbidity from oxide chemical mechanical polishing (oxide-CMP) wastewater. Based on the dynamic characteristics of batch electrocoagulation, three operating stages (lag, reactive, and stabilizing) are proposed to identify the relationships among the zeta potential of the silica particles, solution turbidity, and the corresponding mean particle size of the silica. Experimental results show that the silica particles were destabilized and settled at the critical mean particle size, which was estimated to be above 520 nm after 10 min, and the corresponding turbidity removal mostly occurred during the reactive stage. Furthermore, the corresponding mean particle size varied from 520 to 1900 nm as the treatment time progressed from 10 to 20 min, which also occurred during the reactive stage. Several parameters, including different electrode pairs, electrolyte concentration, applied voltage, and the optimum condition of power input were investigated. Experimental results indicate that a Fe/Al electrode pair is the most efficient choice of the four electrode pair combinations in terms of energy consumption. The optimum electrolyte concentration and applied voltage were found to be 200 ppm NaCl and 30 V, respectively.

Electrocoagulation study for the removal of arsenic and chromium from aqueous solution

This study was undertaken to investigate the removal of arsenic (As) and chromium (Cr) from aqueous solution using electrocoagulation (EC) technique. Batch EC studies were performed using iron electrodes to evaluate the influence of various experimental parameters on the removal of metal ions. The parameters were initial pH (pH 0), electrolysis time (t), initial concentration (C 0), electrode gap (g), stirring rate (r) and current density (j). Effect of pH 0 was studied in the range 2.0 to 8.0 while C 0 was varied from 10 to 100 mg/L. As and Cr removal by EC was governed by the chemical dissolution of iron, and the formation of metal-hydrous ferric oxide complexes, which in turn was strongly, influenced by pH 0 and j. Optimum value of pH 0 and j for As and Cr removal were found to be 4.0 and 2.0; and 75 and 50 A/m2, respectively. Removal efficiency increased with decrease in the value of C 0 and g. The r value of 100 rpm produced sufficient agitation for the proper agglomeration of flocs and optimum removal of ions.

Electrocoagulation/electroflotation in an external-loop airlift reactor—Application to the decolorization of textile dye wastewater: A case study

A 20 L external-loop airlift reactor was used as an electrochemical cell in order to carry out water depollution using batch electrocoagulation (EC) without mechanical agitation, pumping requirements or air injection. Mixing and complete flotation of the pollutants were achieved using only the overall liquid recirculation induced by H 2 microbubbles generated by water electrolysis. A red dye from the Moroccan textile industry was used in a case study to validate this innovative application of airlift reactors. Experimental results showed that the axial position of the Al electrodes and the residence time in the separator section were the key parameters to achieve good mixing conditions, to avoid bubbles/particles recirculation in the downcomer and to prevent floc break-up/erosion by hydrodynamic shear forces. Such optimum conditions corresponded to an optimum liquid overall recirculation velocity that was correlated to current, electrode position and dispersion height. Operation time required to achieve 80% COD and 80% color removal efficiencies was modeled as a function of current density. Similarly, specific energy and electrode consumptions were correlated to current, electrode gap and conductivity, which provided the necessary tools for scale-up and process optimization. Operation time and removal efficiencies were similar to those reported in conventional EC cells, but specific energy and electrode consumptions were even smaller without the need for mechanical agitation, pumping requirements and air injection, which could not be achieved in other kinds of conventional gas–liquid contacting devices.

Sludge conditioning characteristics of copper chemical mechanical polishing wastewaters treated by electrocoagulation

Treatment of copper chemical mechanical polishing (Cu-CMP) wastewaters by batch electrocoagulation was found effective in simultaneously removing the very fine metal oxide particles, copper ion and organic pollutants in the previous investigations. In the present work, the continuous electrocoagulation tests were performed to explore their treatment efficiencies and to identify the optimum operating conditions. Inherently, this electrocoagulation process, in either batch or continuous operating mode, generates a significant amount of sludge that needs to be properly disposed. In this study, the freeze/thaw conditioning of sludge obtained from this process was investigated in an aim to greatly reducing the sludge volume. Experimental tests were conducted to identify proper freeze/thaw operating conditions. Several fundamental aspects, such as the moisture bonding energy estimated using DSC test data, were examined to elucidate the conditioning results.

Mathematical analysis of a batch electrocoagulation reactor

Electrocoagulation treats water by delivering coagulant from a sacrificial anode (aluminium) in an electrochemical cell. Hydrogen is evolved from the inert cathode. In the batch electrocoagulation reactor numerous interactions occur with settling and flotation identified as the dominant removal paths. Current determines both coagulant dosage and bubble production rate. The bubbles influence the mixing, and hence mass diffusion within the reactor. Rate of flotation and settling were experimentally determined for currents 0.25-2.0 A and pollutant loading 0.1-1.7 g/L. The performance of the electrocoagulation reactor was quantified by analysis of experimental results. First-order ordinary differential equations were developed to describe the pollutant's settling and flotation behaviour. Kinetic rate constants were calculated considering this pair of irreversible reactions. At low current (0.25A), sedimentation dominates with slow release of coagulant and gentle agitation provided by low bubble production. Removal is slow and hence the low rate constants calculated were appropriate. At high currents (1.0 and 2.0 A) faster removal occurs due to greater bubble density. This resulted in greater mass floated to the surface and higher rate constants were observed. Thus the developed rate equations successfully quantified the reactor's performance over a variety of conditions.

A quantitative comparison between chemical dosing and electrocoagulation

Abstract A renewed interest in electrocoagulation has been spurred by the search for reliable, cost-effective water treatment processes. This technology delivers the coagulant in situ as the sacrifcial anode corrodes, due to an applied potential, while the simultaneous evolution of hydrogen at the cathode allows for pollutant removal by flotation. By comparison, conventional chemical dosing typically adds a salt of the coagulant, with settling providing the primary pollutant removal path. This paper provides a quantitative comparison of these two approaches based on turbidity removal associated with a clay pollutant. Chemical coagulation was evaluated via jar tests using aluminium sulphate (alum). This proved more effective than electrocoagulation under acidic conditions (pH ∼4) and low coagulant levels (4 mg-Al l −1 being the minimum able to effectively destabilise the colloidal clay particles). Highly effective coagulation was observed at intermediate alum dosage levels (4–20 mg-Al l −1 ), where the isoelectric point occurred at pH ∼7.8. Three operating stages (lag, reactive and stable) were identified in a batch electrocoagulation reactor with the operating current determining the pollutant removal rate. At the isoelectric point, which occurs during the reactive stage, the greatest turbidity reduction occurs, indicating aggregation by a sorption mechanism (compared to the charge neutralisation as in the case of chemical coagulation). During the stable stage, continued precipitation of aluminium hydroxide and a decrease in turbidity indicated a sweep coagulation mechanism. The highest current (2 A) reduced the pollutant level in the shortest time, 1% residual turbidity after 30 min, though the highest efficiency (in terms of pollutant removed per unit of aluminium added) was achieved at the lowest current (0.25 A).