翻译英文版

Abstract

The removal of COD, suphide and oil–grease from tannery liming drum wastewater was experimentally investigated using direct current (DC) electrocoagulation (EC). In the EC of the wastewater, the effects of initial pH, electrolysis time and current density were examined. The COD, sulphide and oil–grease in the aqueous phase were effectively removed when mild steel electrodes were used as sacrificial anode. The optimum operating range for each operating variable was experimentally determined. The experimental results show that COD, sulphide and oil–grease was removed effectively. The overall COD, sulphide and oil–grease removal efficiencies reached 82%, 90% and 96%, respectively. The optimum current density for removal of COD, sulphide and oil–grease in the tannery liming drum wastewater were 35 mA/cm, 35 mA/cm and 3.5 mA/cm at 10 min electrolysis time and pH 3, respectively. Mean energy

consumptions were 5.768 kWh/m of COD, 0.524 kWh/m of sulphide and 0.00015 kWh/m of oil–grease. Results show that the pseudo-second-order rate equation provides the best correlation for the removal rate of the parameters.

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Keywords: Tannery wastewater; Liming drum wastewater; Electrocoagulation; Mild steel electrodes

Article Outline

1. Introduction 2.

Experimental

2.1. Wastewater samples 2.2. Experimental device 2.3. Experimental procedure 2.3.1. Iron electrodes 2.3.2. Aluminum electrodes 3.

Results and discussion

3.1. Effect of electrode materials 3.2. Effect of initial pH 3.3. Effect of current density 3.4. Effect of electrolysis time 3.5. Electric Energy Consumption

4.

Conclusions References

1. Introduction

The leather industry is well known as a high consumer of water. Variety of chemicals at significant quantities are employed for leather processing. Major chemicals used for leather manufacturing are lime, sodium and ammonium salts, fatliquors, bacterial- and fungicides, tannins, dyes, etc. Wastewater from the leather industry is known to be heavily contaminated with inorganic and organic pollutants. It can create heavy pollution from effluents containing high levels of salinity, organic loading, inorganic matter, dissolved and suspended solids, ammonia, organic nitrogen and specific pollutants (sulphide, chromium and other toxic metal salt residues) [1], [2] and [3]. It was reported that the total global quantity of bovine hides, sheep, goat and pigskins was nearly 8 × 10 t as wet salted weight a year and tanning workshop worldwide used 4 × 10 t of chemicals, produced over 3 × 10 t of wastewater and about 8 × 10 t of solid waste and dewatered sludge [4].

There are many processes for the treatment of tannery wastewater such as chemical coagulation [5], [6] and [7], reverse osmosis membrane [2] and [8], nanofiltration [9], Fenton and H2O2/UV [10] and [11], biodegradation [6], [12], [13], [14] and [15]. Although biodegradation process is cheaper than other methods, it is less effective because of the toxicity of the tannery wastewater that affects the development of the bacteria [16].

Due to the limitations of the primary and biological wastewater treatment processes, alternative processes have been pursued. Amongst them, electrochemical processes

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have been proposed and they have received increasing attention in the last years. Compared to traditional methods, electrochemical processes offer: (a) versatility, since they may be used to treat liquid and solid waste by direct and indirect organic compound oxidation, metal reduction and electrodeposition, not to mention the electrocoagulation (EC) and electroflotation processes; (b) automation, since the current and the potential are parameters that are easily acquired and controlled, facilitating the automation of the treatment process; (c) environmental compatibility, since electrochemical processes are mediated by electron exchange with the electrode surface, dismissing the need for the addition of other chemical agents [17]. Oxide electrodes may be used in the electrochemical treatment of wastewaters containing high concentrations of potentially polluting species, and they can promote the oxidation of their organic constituents [17] and [18].

As far as the chromium salts is concerned, the tanning process consumes only 60% of the chromium of the tanning bath and the possibility to recover the residual metal represents a main goal in process improvement. A number of articles have been reported regarding removal of Cr especially Cr(VI) from tannery wastewater in the light of its severe hazardous impact upon the environment [19], [20], [21] and [22]. EC/electro-Fenton process can be another alternative process for treating tannery waste effluents [16] and [23]. Compared with other methods, there are a few advantages for the treatment of wastewater by EC [24], [25] and [26]. EC with aluminum and iron electrodes was patented in the US in 1909. The EC of drinking water was first applied on a large scale in the US in 1946 [27] and [28]. In recent years, EC has been successfully tested to treat various industrial wastewaters including decolorization of dye solutions, polishing wastewaters, removal of phenolic

compounds and dairy wastewater treatment [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57] and [58].

The first step in the liming drum is the soaking of the hide. Using water, the raw material is cleaned of any natural hide dirt. The main objective in soaking the hide is to restore it to its natural condition as on the living animal with a water content of

approximately 65%. After this, the liming takes place in the same drum. Here, substances that are not important for the leather-production process, such as natural oils and proteins, are washed out of the hide. By adding lime and sodium sulphide, the pH-value during the dispensing process is raised. By doing so, hair is chemically removed from the surface of the hide. Soaking and liming are carried out in one process, which typically lasts from 24 h to 36 h [59]. The effluent emanating from the beamhouse of tannery contains high concentration of sulphide ions. Since these effluents are toxic to the aquatic environment, it is essential to neutralize them and bring the discharge levels of these species to below the toxic limit [1].

In the present study, the EC effect and mechanisms of chemical oxygen demand (COD), sulphide, oil–grease with different current densities, pH, conductivities and different soluble electrodes (mild steel electrodes and aluminum electrodes) were compared in detail.

2. Experimental

2.1. Wastewater samples

Wastewater was obtained from a tank containing a mixture of the liming drum solutions at the tannery factory in Turkey (Sakarya). The composition of the wastewater is shown in Table 1. Table 1.

Characteristics of wastewater.

pH 12.03 COD (mg L−1) 25,300 BOD5 (mg L−1) 10,850 Suspended matter (mg L−1) 6,130 Sulphide (mg L−1) 3,000 Oil–grease (mg L−1) 185 Conductivity (mS cm−1) 37.2 2.2. Experimental device

The batch experimental setup is schematically shown in Fig. 1. The EC unit consists of an electrochemical reactor, a D.C. power supply and iron electrodes. The electrodes consist of pieces of sheet mild steel or aluminum separated by a space of 2.5 cm and dipped in the wastewater. The electrodes were placed into 400 mL wastewater in a 1150 mL plexiglass electrolytic reactor. There were four electrodes connected in a monopolar mode in the electrochemical reactor, each one with dimensions of 9.3 cm × 7.5 cm × 0.3 cm. The submerged surface area of the electrode plates was 140 cm. The stirrer was used in the electrochemical cell to maintain an unchanged composition and avoid the association of the flocs in the solution. The D.C. source was used to power supply the system with 0–15 V and 0–3 A. A Sunwa Electronics multimeter, model-YX-360TR-EB, was used for measurement the current and the potential between the two electrodes. Electrodes were washed with dilute HCl between the experiments. Experiments were conducted at 20 °C.

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Full-size image (28K)

Fig. 1. Bench-scale EC reactor with bipolar electrodes in parallel connection. (1) Electrocoagulation cell; (2) anode; (3) cathode (iron); (4) bipolar electrodes; (5) magnetic stirrer controller; (6) magnetic stirrer bar; (7) D.C. power supply.

2.3. Experimental procedure

At the beginning of a run the wastewater was fed into the reactor and the pH was adjusted to a desired value using HCl, NaOH solutions. The electrodes were placed into the reactor. The reaction was timed, starting when the D.C. power supply was

switched on. One of the greatest operational issues with EC is electrode passivation. During EC with electrodes, an oxide layer was formed at the anode. Eliminating the oxide formation at the anode could reduce this effect. For this reason, the electrodes were rinsed in the diluted HCl solution after the each experiment. Samples were periodically taken from the reactor. The particulates of colloidal ferric oxyhydroxides gave yellow–brown color into the solution after EC. The sedimentation was filtrated with normal filter paper. Standart Methods for Examination of Water and Wastewater were adopted for quantitative estimation of pH, conductivity, COD, oil–grease and sulphide [60]. All the experiments were repeated twice, and the experimental error was below 4%, the average data were reported.

If iron or aluminum electrodes are used, the generated Fe(aq) or Al(aq) ions will immediately undergo further spontaneous reactions to produce corresponding hydroxides and/or polyhydroxides. The Fe(II) ions are the common ions generated the dissolution of iron. In contrast, OH ions are produced at the cathode. By mixing the solution, hydroxide species are produced which cause the removal of matrices (dyes and cations) by adsorption and coprecipitation.

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2.3.1. Iron electrodes

In the study of iron anodes, two mechanisms for the production of the metal hydroxides have been proposed [24], [25] and [26]: Mechanism 1

pH < 4

(1)

anode: 4Fe(s) → 4Fe(aq) + 8e

2+−

(2)

bulk of solution: 4Fe(aq) + 10H2O(l) + O2(aq) → 4Fe(OH)3(s) + 8H(aq)

2++

(3)

cathode: 8H(aq) + 8e → 4H2(g)

+−

(4)

overall: 4Fe(s) + 10H2O(l) + O2(aq) → 4Fe(OH)3(s) + 4H2(g)

4 < pH < 7

(5)

anode: 4Fe(s) + 24H2O(l) → 4Fe(H2O)4(OH)2(aq) + 8H(aq) + 8e

+−

(6)

bulk of solution: 4Fe(H2O)4(OH)2(aq) + O2(aq) → 4Fe(H2O)3(OH)3(s) + 2H2O(l)

(7)

bulk of solution: 4Fe(H2O)3(OH)3(s) → 2Fe2O3(H2O)6(s) + 6H2O

(8)

cathode: 8H(aq) + 8e → 4H2(g)

+−

(9)

overall: 4 Fe(s) + 16H2O(l) + O2(aq) → 2Fe2O3(H2O)6(s) + 4H2(g)

6 < pH < 9

Precipitation of Fe(III) hydroxide (7) continues, and Fe(II) hydroxide precipitation also occurs presenting a dark green floc.

(10)

bulk of solution: 4Fe(H2O)4(OH)2(aq) → 4Fe(H2O)4(OH)2(s)

Mechanism 2

pH < 4

(11)

anode: Fe(s) → Fe(aq) + 2e

2+−

(12)

cathode: 2H(aq) + 2e → H2(g)

+−

(13)

overall: Fe(s) + 2H → Fe(aq) + H2(g)

+2+

4 < pH < 9

(14)

anode: Fe(s) + 6H2O(l) → Fe(H2O)4(OH)2(aq) + 2H(aq) + 2e

+−

(15)

bulk of solution: Fe(H2O)4(OH)2(aq) → Fe(H2O)4(OH)2(s)

(16)

cathode: 2H(aq) + 2e → H2(g)

+−

(17)

overall: Fe(s) + 6H2O(l) → Fe(H2O)4(OH)2(s) + H2(g)

Mechanism 3

4 < pH < 9

(18)

anode: 2Fe(s) + 12H2O(l) → 2Fe(H2O)3(OH)3(aq) + 6H(aq) + 6e

+−

(19)

bulk of solution: 2Fe(H2O)3(OH)3(aq) → 2Fe(H2O)3(OH)3(s)

(20)

bulk of solution: 2Fe(H2O)3(OH)3(s) → Fe2O3(H2O)6(s) + 3H2O(l)

(21)

cathode: 6H(aq) + 6e → 3H2(g)

+−

(22)

overall: 2Fe(s) + 12H2O(l) → Fe2O3(H2O)6(s) + 3H2(g)

In the oxygenated water and at lower pH, Fe is easily converted to Fe. The Fe(OH)n(s) formed remains in the aqueous stream as a gelatinous suspension, which can remove the waste matter from wastewater either by complexation or by

electrostatic attraction followed by coagulation. Ferric ions electrogenerated may form monomeric ions, ferric hydroxo complexes with hydroxide ions and polymeric species, namely, Fe(H2O)6 Fe(H2O)5OH, Fe(H2O)4(OH)2, Fe2(H2O)8(OH)2,

Fe2(H2O)6(OH)4 and Fe(OH) depending on the pH range [26]. The complexes (i.e. hydrolysis products) have a pronounced tendency to polymerize at pH 3.5–7.0 [25] and [26].

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4+

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2.3.2. Aluminum electrodes

The generated Al and OH react with each other to form Al(OH)3 [25] and [26]:

(23)

3+

−

anode: Al(s) → Al(aq) + 3e

3+−

(24)

cathode: 3H2O(l) + 3e → 3/2H2(g) + 3OH(aq)

−−

(25)

overall: Al(aq) + 3H2O(l) → Al(OH)3(s)

3+

The electrolytic dissolution of the aluminum anode produces the cationic monomeric species such as Al and Al(OH) at low pH, which at appropriate pH-values are transformed initially into Al(OH)3 and finally polymerized to Aln(OH)3n.

(26)

3+

2+

nAl(OH)3 → Aln(OH)3n

3. Results and discussion

3.1. Effect of electrode materials

First of all, treatment of liming drum wastewater, by using mild steel and aluminum electrode materials was investigated. As shown from Fig. 2, there were no significant differences between mild steel electrodes and aluminum electrodes for the elimination of oil–grease under the same condition. Under the same condition, the elimination rate of COD and sulphide using mild steel electrodes was higher than that of aluminum electrodes. As a result, mild steel electrodes are superior with respect to aluminum as sacrificial electrode material for treatment liming drum wastewater.

Full-size image (17K)

Fig. 2. Effect of electrode materials on treatment of liming drum wastewater (pH 3; C0,COD = 22,500 mg L; C0,sulphide = 2100 mg L; C0,oil–grease = 1 mg L, i = 6.42 mA/cm; T = 298 K; d = 2.5 cm; agiation speed = 200 rpm; conductivity = 35 mS cm).

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3.2. Effect of initial pH

It has been established that pH is an important parameter influencing the performance of the EC process [26]. The effect of initial pH on the COD removal efficiency is presented in Fig. 3. High COD removal percent may be attained in acidic mediums, the efficiency with increasing pH at pH 3–5, maximum COD removal attainable is 62% with iron electrode.

Full-size image (13K)

Fig. 3. Effect of initial pH on the removal efficiency of liming drum wastewater (electrodes: mild steel; C0,COD = 23,800 mg L; C0,sulphide = 2480 mg L; C0,oil–grease = 210 mg L; i = 7.85 mA/cm; T = 298 K; d = 2.5 cm; agiation speed = 200 rpm; conductivity = 37.2 mS cm).

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The major portion of COD in wastewater may arise from sulphide since sulphide can be oxidized by Cr in COD test. Therefore, there are no significant differences between COD and sulphide removal efficiency graphics as shown in Fig. 3.

Depending on the pH, sulphide exists as H2S, HS and S. Interaction between Fe ions and H2S, HS and S species leads to the formation of FeS precipitate that is insoluble. Both, bulk and surface reactions between S and Fe/Fe are expected under experimental conditions. Reactions that occur in solution phase are:

(27)

2−

2+

3+

−

2−

−

2−

2+

6+

Fe + H2S → FeS + 2H

2++

(28)

Fe + HS → FeS + H

2+−+

(29)

Fe + S → FeS

3+

2+2−

Elemental sulphur is formed due to oxidation at anodic sites and also by Fe reduction:

(30)

HS + OH → S + H2O + 2e

−−0−

In addition to monosulphides, FeS2 formation by polysulphide-pathways is also suggested according to the following equation [1]:

(31)

FeS + S → FeS2

2+

3+

0

The kinetics of Fe conversion to Fe are strongly affected by the pH; the surface charge of the coagulating particle also varies with pH. In general, at lower and higher pH, Fe is increasingly soluble [61].

As shown in Fig. 3, the removal efficiency of oil–grease was not affected by the pH.

3.3. Effect of current density

From Fig. 4, it is apparent that the removal efficiency was not improved continuously with increasing current density. Fig. 4 illustrates that the COD removal efficiency increased slowly from 60.8% to 82.1% by increasing the current density from 3.5 mA/cm to 35 mA/cm after 10 min reaction time at pH 3. As the applied current density was increased from 35 mA/cm to 70.7 mA/cm, the COD removal efficiency did not rise significantly. The graphics of the removal efficiency of COD and sulphide was analogous because the substantially reason of COD in wastewater was sulphide. After 10 min of electrolysis oil–grease efficiency reached a maximum at 3.5 mA/cm current density; a 94–96% oil–grease removal was achieved under this condition.

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Full-size image (18K)

Fig. 4. Effect of current density on the removal efficiency of liming drum wastewater (electrodes: mild steel; pH 3; C0,COD = 25,300 mg L; C0,sulphide = 3000 mg L; C0,oil–grease = 185 mg L; T = 298 K; d = 2.5 cm; agiation speed = 200 rpm; duration of electrolysis = 10 min; conductivity = 37.2 mS cm).

An oil–water emulsion is a colloidal dispersion in which oil constitutes the dispersed phase and water forms the continuous phase. Emulsions are normally stabilized by the presence of an emulsifying agent, such as a surfactant. The anionic head groups on the surfactant molecules prevent aggregation and coagulation of the oil droplets via electrostatic repulsion. During electrolysis in an EC procedure, the sacrificial iron anode is oxidized to polymeric ionic species. With progressive electrolysis the ionic strength of the medium increases. Ionic polymeric iron species can neutralize the surface charge of surfactant molecules. They can either generate bridges between

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surfactant molecules. Simultaneously, hydrogen as well as hydroxides is generated at the cathode (Eqs. (3) and (12)). The pH of the medium rises as a result of this electrochemical process. The net result of the reactions is that the emulsion is destabilized, and the colloidal oil particles begin to coalesce [54].

It is well known that the amount of current density determines the coagulant production rate, and adjusts the rate and size of the bubble production, and hence affects the growth of flocs [25] and [26].

However, it is advisable to limit the current density in order to eliminate other adverse effect, like heat generation [62]. With increasing current density the amount of oxidized iron and the bubble generation is increased and consequently the amount of the hydroxyl polymers available for the attraction of the colloidal particulates, oils, or other contaminants are also increased.

The COD removal after 10 min of EC can be calculated according to:

(32)

where ΔCOD is the concentration (mg L) removed

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from the wastewater for 10 min electrolysis time, mFe is the mass of dissolved iron during the electrolysis (g/L), Cres,COD is the residual mass concentration of COD (mg L) for 10 min electrolysis time, the coefficient k denotes the COD removal capacity (mg COD per g iron) and n is an other coefficient (L/g iron), mFe was calculated from Faraday's law. Eq. (32) corresponds simply to the mass balance of the COD (concentration removed = initial concentration − residual concentration). The values of k and n were calculated from the slope and intercept of the linear plot, ΔCCOD/mFe vs. Cres,COD and are represented in Table 2.~~~~~~~~~~~~~~~~~~~~ Table 2.

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k and n constants.

k −101,449 n 28.52 R2 0.998 A relationship between Eq. (32) and Faraday's equation can be derived:

(33)

where C0 is initial COD concentration

in the wastewater, i is current density.

The theoretical ΔCCOD values for various i values were calculated using Eq. (33). Fig. 4 shows a comparison between the theoretical values and the experimental data. As can be seen in Fig. 4, the calculated values were found to generate a satisfactory fit to the experimental data.

3.4. Effect of electrolysis time

As shown in Fig. 5, as the time of electrolysis increases comparable changes in the removal efficiency of COD, sulphide and oil–grease are observed.

Full-size image (22K)

Fig. 5. Effect of electrolysis time on the removal efficiency of liming drum wastewater (electrodes: mild steel; pH 3; C0,COD = 22,100 mg L;

C0,sulphide = 2560 mg L; C0,oil–grease = 165 mg L; T = 298 K; d = 2.5 cm; agiation speed = 200 rpm; i = 7.85 mA/cm; conductivity = 34 mS cm).

The removal efficiency of the parameters depends directly on the concentration of hydroxyl and metal ions produced on the electrodes. After 10 min of electrolysis, COD, sulphide and oil–grease removal efficiency at pH 3 and 7.85 mA/cm current density were 65.7%, 62.5% and 91.4%, respectively.

For the rate of removal the parameters can be represented by the following pseudo-second-order kinetics:

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−1

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−1

(34)

where C represents the residual parameter

concentration at the t time in the solution, Ce concentration coefficient, k2 the reaction rate coefficient and t the time.

According to the above equation, a plot of t/C against t will yield a straight line with a slope of 1/Ce. Fig. 5 reveals reasonably good fit of pseudo-second-order kinetic model to the observed data. The coefficients for the pseudo-second-order removal rate of COD, sulphide and oil–grease were shown in Table 3. Table 3.

The coefficients of pseudo-second-order kinetic model.

Parameter Cmax (mg L−1) k (L mg−1 min−1) R2 COD 5000 Sulphide 238 Oil and grease 8.56 −2.50 × 10−5 −4.25 × 10−5 −2.97 × 10−2 0.991 0.987 0.996 3.5. Electric Energy Consumption

Electrical energy consumption and current efficiency are very important economical parameters in EC process. Electrical energy consumption was calculated using the Eq. (35):

(35)

E=UItECwhere E is the electrical energy in Wh, U the cell voltage in volt (V), I the

current in ampere (A) and tEC is the time of EC process per hour.

In Fig. 6 and Fig. 7, current density has been plotted against power consumed. As shown from the figures, COD removal has highest energy consumption. Both the plots are from results after 10 min of EC carried out at different current density values.

Full-size image (20K)

Fig. 6. Effect of current density on the energy consumption and the removal efficiency (electrodes: mild steel; pH 3; C0,COD = 25,300 mg L;

C0,sulphide = 3000 mg L; C0,oil–grease = 185 mg L; T = 298 K; d = 2.5 cm; agiation speed = 200 rpm; duration of electrolysis = 10 min).

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Full-size image (53K)

Fig. 7. Effect of current density on the energy consumption and the removal efficiency: (a) COD; (b) sulphide; (c) oil–grease (electrodes: mild steel; pH 3; C0,COD = 25,300 mg L; C0,sulphide = 3000 mg L; C0,oil–grease = 185 mg L;

T = 298 K; d = 2.5 cm; agiation speed = 200 rpm; duration of electrolysis = 10 min).

As shown in Fig. 7(a), the minimum energy consumption for COD removal was 5.768 kWh/m at 35 mA/cm current density for 10 min electrolysis time. The energy consumption in the high current density was increased because of polarization. The effect of the current density on the energy consumption and the removal efficiency of sulphide are shown in Fig. 7(b). Apparently, minimum energy consumption is 0.524 kWh/m. The effect of current density on energy consumption on the removal of oil–grease is shown in Fig. 7(c). From Fig. 7(c), it is apparent that energy consumption

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for the removal efficiency of oil–grease has a lower value then others as 0.00015 kWh/m.

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4. Conclusions

This investigation has demonstrated that EC with mild steel electrodes is an effective method to clarify tannery wastewater by reducing the COD, sulphide and oil–grease content of the wastewaters and will lead to reduce waste disposal costs for tannery companies. EC is a feasible process for treating the tannery wastewater, characterized by high COD, sulphide and oil–grease concentrations. The effect of various operational parameters on EC operation was investigated and optimized. The mild steel electrodes were more effective for the removal of COD and sulphide compared with the aluminum electrodes. The results showed that COD, sulphide and oil–grease was effectively removed at initial pH 3 when the initial concentration of COD, sulphide and oil–grease was 25,300 mg L and 3000 mg L, and 185 mg L, respectively. The results also indicated that the removal efficiency of the COD, sulphide and oil–grease was raised to 82%, 90% and 96%, respectively. The optimal current density for COD, sulphide and oil–grease removal was 35 mA/cm, 35 mA/cm and 3.5 mA/cm for an operating time of 10 min, respectively. At above optimal conditions, the power requirements were 5.768 kWh/m of COD, 0.524 kWh/m of sulphide and 0.00015 kWh/m of oil–grease. The results showed that a pseudo-second-order kinetic model was found to be in good agreement with the experimental results. Finally, a mathematical equation showing the relation between the amount of the COD removal with the current density at pH 3 was presented. The predictions of this mathematical model are in very good agreement to the experimental data.

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