Electrocoagulation technique for refinery wastewater treatment in an internal loop split-plate airlift reactor

Electrocoagulation technique for refinery wastewater treatment in an internal loop split-plate airlift reactor

Journal Pre-proof Electrocoagulation technique for refinery wastewater treatment in an internal loop split-plate airlift reactor Saad H. Ammar, Natheer...

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Journal Pre-proof Electrocoagulation technique for refinery wastewater treatment in an internal loop split-plate airlift reactor Saad H. Ammar, Natheer N. Ismail, Ali D. Ali, Wisam M. Abbas

PII:

S2213-3437(19)30612-8

DOI:

https://doi.org/10.1016/j.jece.2019.103489

Reference:

JECE 103489

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

5 September 2019

Revised Date:

14 October 2019

Accepted Date:

16 October 2019

Please cite this article as: Ammar SH, Ismail NN, Ali AD, Abbas WM, Electrocoagulation technique for refinery wastewater treatment in an internal loop split-plate airlift reactor, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103489

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Electrocoagulation technique for refinery wastewater treatment in an internal loop split-plate airlift reactor Saad H. Ammar*, 1, Natheer N. Ismail1, Ali D. Ali2, Wisam M. Abbas3

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Chemical Engineering Department, Al-Nahrain University, Baghdad, Iraq AL-Nahrain Nanorenewable Energy Research Center (NNERC), Al-Nahrain University, Baghdad, Iraq. 3 College of Pharmacy, Al-Farahidi University, Baghdad, Iraq. Corresponding author E-mail: [email protected] (S.H. Ammar)

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Highlights

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A novel split-plate airlift/electrocoagulation reactor was developed for the treatment of petroleum refinery wastewater. The anode consumption and consumed electrical energy of the split-plate airlift/ electrocoagulation reactor were evaluated. The COD removal efficiencies of oil refinery wastewater were high through the first 30 min of electrocoagulation time. The new reactor arrangement was found to be efficient for treatment of oil refinery wastewater without using mechanical stirring.

Abstract

In the present work, an internal loop split-plate airlift reactor was used as an electrocoagulation cell. The performance of this airlift/electrocoagulation reactor has been assessed by treating petroleum refinery wastewater for COD/TSS removal. Experimental parameters include current density, initial pH, air input (between the two split-plate of airlift reactor), electrocoagulation time and implicitly, and internal liquid circulation velocity were investigated. The results revealed that the COD removal

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increased with increasing current density and electrocoagulation time. The minimum electrocoagulation time required to reach 90% of COD is reduced from 80 to 16 min when the current density increased from 1.8 to 18.5 mA/cm2. Furthermore, the highest COD/TSS removal was accomplished when using initial pH between 7.0 and 9.0. Considerably, the results have shown the technical feasibility of this airlift/electrocoagulation design as a potential method for refinery wastewater treatment by electrocoagulation/flotation without the need of a mechanical stirrer and thereby, low operational costs.

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Key words: refinery wastewater; COD removal; TSS removals; electrocoagulation; split-plate; airlift reactor

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1. INTRODUCTION

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Currently, environmental pollution has become one of the most thoughtful environmental difficulties. Several domestic and industrial activities produce wastewater streams containing different hazard pollutants. These pollutants have very harmful influence on both human beings and the organisms when discharged to the environment [1]. With the continuous growth of manufacturing actions like petroleum refineries, fertilizer, mining, batteries, metal plating and insecticides industries, treatment of wastewater effluents become essential. In oil refining processes, huge quantities of water are used in different units, particularly cooling towers, distillations, hydrotreating units, and crude oil desalting. Therefore, it is clear that there are large amounts of refinery wastewater with different types to be treated. These different types of refinery wastewater are disposed in separated sewer systems depending on the source and characteristics of the wastewater [2, 3].

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Oil refineries produce huge quantities of contaminated wastewater, containing chemical oxygen demand (COD), biochemical oxygen demand (BOD) content of about 300–600 ppm and 150–250 ppm, respectively; phenol content of 10–200 ppm; oil and grease content of 100–400 ppm in desalting wastewater; benzene content of 1–100 mg/l; heavy metals up to 10 ppm for lead and up to 150 ppm for chromium; and additional contaminants [4]. Oil refining processes also dispose sludge and solids (1 to 6 kg/ton crude), 80% of which may be considered hazardous due to the existence of toxic organic materials and heavy metals [5, 6]. Several treatment techniques are provided toward removal of these contaminants from refinery wastewater. Electrochemical methods, membrane separation, flotation and chemical coagulation, biological treatment and adsorption are examples. The electrocoagulation technique is one of the most effective and promising methods applied for treatment of different pollutants (such as emulsified oils, heavy metals, dyes, and other organic pollutants) in domestic and industrial wastewater effluents [7, 8]. Electrocoagulation technique with the simple design, low cost, low

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sludge production and good removal efficiency, consequently, has extensive uses and substitutes the conventional and expensive chemical coagulation methods which consume chemical coagulants [9, 10].

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Depending on the complication of the phenomena involved and pollutant types, the electrocoagulation theory has been explained in previous reports. It can be plotted in three sequential stages [11, 12]. Initially, two simultaneous reactions occur; the first reaction is oxidation on the anode electrode forming metal ions, concurrently, the second reaction occurs at the cathode electrode where micro-sized bubbles and hydroxide ions are produced. Furthermore, O2 bubbles may also produce at the anode electrode [13]. The generated metal ions (M3+) from the anode electrode reacts with hydroxide ions generated at the cathode electrode giving the coagulating agents (e.g. M(OH)3). Then, the generated coagulating agents neutralize the surface charges and destabilize the colloidal pollutants, adsorbing on them, and producing and growing flakes. H2 bubbles at the cathode and O2 at the anode (if present) will float to the surface, colliding with and capturing the flakes, carrying the particles to the surface, and helping the elimination of the contaminated media. Other dissolved ionic metal hydroxides, such as M(OH)2+, M2(OH)24+, M(OH)2+, M(OH)4−, M6(OH)153+, and so on may be similarly generated. These ionic coagulating agents have very strong attraction strength to destabilize contaminants causing elimination of these matters from the liquid media by electrostatic attraction and hence, electrocoagulation [14]. Continuous production of hydrogen and oxygen bubbles makes the contaminants- coagulants complex to float. The interaction between electrocoagulation and electroflotation assist extraordinary exclusive performance for the elimination of numerous contaminants from wastewater effluents prior to discarding or recycle [15].

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On the other hand, airlift contacter including two or three phases has been extensively used for treatments of wastewater from different sources to perform several applications [16]. Airlift contactors are distinctive state of bubble columns [17]. It contains two characteristic regions; riser region and the downcomer region. Gas or air may be fed in one region (riser) while the other region became downcomer. The gas bubbling in the riser region may cause decreases in the density of the mixture in this region, resulting in the differences in gas phase holdup between the two regions. Consequently, the static pressure between the two regions will be different, leading to the circulation of liquid between the riser and the downcomer [18]. Generally, according to liquid circulation, there are two designs for airlift contactors: internal loop (including draft-tube airlift and split-plat airlift) and external loop airlift reactors [19]. These unique properties of airlift contactors such as good internal mixing attracted us to take advantage of these properties to address the problem of improper distribution of coagulation coefficients inside the conventional electrocoagulation cells. Therefore, the electrocoagulation reactor may be designed as split-plates airlift unit. The two electrodes of electrocoagulation process may be fixed inside the riser region, hydrogen bubbles generated from the anode between the two

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split-plates causes the fluid to be circulated between the two regions without the need for air pumping, thus increasing the mixing and good distribution between the pollutants and the coagulations. Another good addition to the use of this reactor design in the process of electrocoagulation/flotation is to dispense with the use of mechanical mixer and thus rationalize energy consumption as expected. Therefore, the aim of this work is to introduce a new design of split-plates airlift internal-loop contactor as an electrocoagulation/flotation reactor for oil refinery wastewater treatment. Several experimental variables were investigated on the COD and TSS removals including electrical current density, electrocoagulation time, value of solution initial pH and air flow.

2. Materials and experimental procedure

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2.1 Petroleum refinery wastewater sample

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The wastewater sample used in the current study is real oil refinery wastewater sample collected in June 2018 from Al-Dura oil refinery, southern Baghdad, Iraq. The refinery wastewater sample was used without any treatment and analyzed at the wastewater treatment unit laboratory according to standard methods [20] for different parameters as shown in Table 1. Sodium chloride (NaCl, 99.8%, Merck) was used if required to raise the solution conductivity. The value of initial pH was modified by using 1 M of HCl or NaOH if required.

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Table 1 Characteristics of Al-Dura oil refining wastewater sample. Property pH (-) Oil and grease (ppm) TSS (ppm) TDS (ppm) COD (ppm) BOD (ppm) Density (kg/m3) Conductivity (ms/cm) Turbidity (NTU) Phenols (ppm) Chloride (ppm) Sulfate (ppm) Phosphate (ppm)

2.2 Airlift/electrocoagulation reactor and procedure

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Value 6.8 326 2163 1175 562 213 1015 16.8 416 2.44 995 625 0.73

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The proposed split-plates airlift/electrocoagulation reactor is composed of two rectangular split-plates (80 mm width and 200 mm length). Two aluminum plates (50 mm* 120 mm and thickness of 1.0 mm giving an electrode surface area of 120 cm2) act as electrodes for electrocoagulation were inserted into the riser region (between the two split-plates of airlift reactor). The space between the electrodes is 3 cm. The reactor tank is cylindrical, made from Perspex (100 mm diameter and 35 cm long) with 2 L working volume. Therefore, electrocoagulation tank was designed as an internal loop airlift contactor. The two electrodes were linked to an ammeter to deliver the required current, by means of a 30V, 5A DC-power supply (Hyelec, HY3002, India). Fig.1 illustrated the schematic representation of the split-plate airlift/electrocoagulation reactor.

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The general performance of oil refinery wastewater treatment was investigated in terms of COD and TSS removals. The effect of different experimental parameters were studied, including electrical current density (1.8 to 18.5 mA/cm2), electrocoagulation time (5-80 min), air flowrate (0.2 to 2 LPM), and initial pH (4.0 to 12). . At time, 3 ml samples were taken, filtered and analyzed for COD content.

Figure 1 Schematic diagram of the split-plate airlift/electrocoagulation reactor.

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2.3 Methods of analysis COD analysis was carried out using photometric detection by Lovibond COD VARIO photometer (COD Setup, MD 200, UK). TSS and turbidity of wastewater samples were measured in this study by Gravimetric determination (Hach LXV322.99.00002) [21]. The COD/TSS removals (R%) were determined using the following formula: (COD/TSS removals %) =

𝑋𝑖 −𝑋𝑡 𝑋𝑖

∗ 100

(1)

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Where Xi is the initial COD or TSS content while Xt is the residual COD or TSS content (mg/l) of effluent electrocoagulation time of (t). The oil content was analyzed by TD-500D Handheld oil-inwater analyzer (US) using CCl4 as solvent. The TDS was measured by TDS-meter (161002 PurePro Inc.) and the pH was determined through pH-meter (HANNA Instruments Co.). Electrical conductivity was measured using (HANNA HI-99301). The BOD was measured by DO-meter (HI2040-01 edge). The chloride and sulfate ions were analyzed by ionic chromatography system (Dionex ICS-2000).

𝑘𝑊ℎ

𝑘𝑔 𝐶𝑂𝐷

)=

𝟏𝟎𝟎𝟎 𝑼𝑰𝒕 𝑽 𝑪𝑶𝑫𝒊 𝑹%

(2)

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In this reactor type, the specific electrical power consumption of oil refinery wastewater treatment by electrocoagulation/electroflotation method was determined by using the following formula [22]:

Where E = electrical power consumption (

𝑘𝑊ℎ

𝑘𝑔 𝐶𝑂𝐷

), I = applied current (A), U = voltage (V), V =

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volume (L) of oil refinery sample in the reactor, t = electrocoagulation time (h), CODi = initial COD content (562 mg/l) and R% = removal efficiency. At each run, liquid circulation velocity was calculated in the two regions by using the neutral-buoyancy flow follower method as described in previous work [10].

3. Results and Discussion

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3.1 Influence of current density and electrocoagulation time Figures 2 and 3 revealed the percentage reduction of COD and TSS respectively in the split-plate airlift/electrocoagulation reactor with time at different current densities (from 2.67 to 21.4 A/cm2), using initial pH of 6.8, temperature of 30 oC and without air pumping. The results demonstrated that the COD and TSS percentage removal increased linearly practically through the first 20 min. the COD reduction efficiency improves with the electrical current density increase. Relatively, increasing current density leads to generate more H2 microbubbles and cause further upwards fluxes

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and therefore, rapid elimination of contaminants by gas floatation [11, 23]. Mainly, increasing the applied current caused a higher dissolution rate of anode electrode which increased the aluminum ions released (more coagulation agents were available) in the oil wastewater, thereby improves the pollutants reduction rate [9, 24]. During the electrocoagulation process, the amount of sacrificial electrode (anode) consumed depends mainly on the value of current applied as defined by Faraday’s law [25, 26]. 𝐼𝑡𝑀𝑤

(3)

𝑍𝐹

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𝑊=

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Where W is the aluminum consumed (g), I is the electrical current (A), t is the electrocoagulation time (s), Mw is the aluminum molecular weight, Z is the number of electrons involved in the redox reaction (Z=3 for aluminum) and F is the Faraday’s constant (96,500 C/mol electrons).

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Figure 4 displays the calculated quantity of aluminum consumed (g) with time at different current density. For this configuration of split-plate airlift/ electrocoagulation reactor, it was observed when the current applied increased, more H2 bubbles were generated, leading to the increase of the liquid circulation velocity among the two regions even though no air flow was used in the riser region. It was detected also, that there was a measurable liquid circulation velocity between the two regions, even at low H2 generation rate with a low applied current. The circulation velocity of liquid in two regions versus current density is revealed in Figure 5.

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Increasing the generation of micro-sized hydrogen bubbles from cathode cause to raise gas-holdup in riser region and hence, liquid circulation velocity increases clearly due to differences in density among the two regions of the split-plate airlift contactor. The dual positive outcome of current density on the COD reduction efficiency is attributed to the high formation rate of coagulating agents through anode oxidation reactions and also, to the efficient mixing in the split-plate airlift reactor caused by liquid circulation between the two regions. This leads to increased contacting probability/frequency between coagulating agents and pollutants leading to enhance the COD reduction without the need of mechanical stirring.

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100 90 80

60 50

1.8 mA/cm^2

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11.3 mA/cm^2

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4.6 mA/cm^2

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COD emoval %

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Figure 2 Influence of current density and electrocoagulation time on the COD removal (initial pH: 6.8, no air flow and temperature of 30 oC). 100 90

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TSS removal %

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1.8 mA/cm^2 4.6 mA/cm^2 11.3 mA/cm^2 18.5 mA/cm^2 60

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Electrocoagulation time (min)

Figure 3 Influence of current density and electrocoagulation time on the TSS removal (initial pH: 6.8, no air flow and temperature of 30 oC).

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1.8 mA/cm^2

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4.6 mA/cm^2

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Amount of aluminum dissolved (g)

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Figure 4 Consumed aluminum electrode (g) as a function of electrocoagulation time at different current densities (initial pH: 6.8, no air flow, and temperature: 30 oC).

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Downcomer

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Liquid circulation velocity cm/s

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Current density (mA/cm^2)

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Figure 5 liquid circulation velocities in the riser and downcomer of split-plate airlift reactor at different current density (initial pH: 6.8, electrocoagulation time: 30 min, without air flow and temperature: 30 oC).

3.3 Influence of wastewater initial pH The pH has a distinguishing impact on the electrocoagulation/electroflotation efficiency. Therefore, oil refinery initial pH is an important parameter in the COD removal efficiency. The effect of the initial pH on the COD removal from oil refinery wastewater sample was assessed by changing the value of initial wastewater pH from 4 to 12 by adding 0.1 M of HCl or NaOH if required. The results displayed in Figure 6 reveals that the optimal pH value for the COD and TSS reduction was found to be between 7.0 and 9.0. It is suggested that the pH affects the COD removal from the 9

solution in term of Al(OH)3 solubility. The reduction in the COD removal at a pH value below 6.0 and higher than 9.0 may be credited to the amphoteric actions of aluminum hydroxide (e.g. Al(OH)3) coagulants in the solution. The solubility of Al(OH)3 has been described to be lowest at the solution pH ranging from 6 to 8 [27]. Al(OH)3 coagulants precipitate at pH from 6 to 7, nonetheless when the solution becomes either more acidic or alkaline, the Al(OH)3 solubility increases. Therefore, initial pH of the solution below 6.0 and higher than 9.0 does not facilitate the Al(OH)3 precipitation, and therefore, the optimum condition would be a pH value between 7 and 9. 100

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COD

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TSS

10 0 3

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7 8 initial pH

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COD/TSS Removal (%)

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Figure 6 Influence of initial pH on the COD removal. (current density: 11.3 mA/cm2, electrocoagulation time: 30 min, no air flow and temperature of 30 oC)

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The changes in pH value through the electrocoagulation process were monitored. Figure 7 displays the pH profile during electrocoagulation time (the inserted figure illustrated the initial and final pH relationship). It is noted that the pH gradient depends on the values of initial pH, however, because of the equilibrium OH− ions production/consumption, pH values does not varied considerably [10].

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The initial/final pH relationship reveals an incease in the value of final pH for low values of initial pH, this can be explained by the additional OH- ions produced at low pH media besides the releas of OH- due to the incidence of an incomplete exchange of Cl- with OH- ions in aluminum hydroxide [28]. When the initial pH value excess 8.0, the creation of Al(OH)4 coagulants beside with transfer of extra hydroxide ions on the cathode leading to an inconsiderable decrease in the value of final pH.

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Figure 7 pH gradient through electrocoagulation process as effected by initial pH values (current density of 11.3 mA/cm2, no air flow and temperature: 30 oC).

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The effect of current on the electrical power consumption has been also investigated. Power consumption was calculated in kWh per kg of COD removed using Eq.2 at electrocoagulation time of 30 min, initial COD content of 562 mg/l and initial pH of 6.8. Figure 8 displays the power consumption in the split-plate airlift reactor as affected by current density (from 1.8 to 18.5 mA/cm2). It was observed normally, that increasing current density increases the electrical power consumption. Comparable effects of power consumption in the electrocoagulation/electroflotation method were described through pervious reports [27, 28].

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Of course, the minimum electrocoagulation time needed for achieving COD removal more than 90% affects the electrical power consumption. However, decreasing this electrocoagulation time does not recompense the influence of electrical current density increasing on the power consumption. In traditional electrocoagulation cells, the electrical power cost represents from about 30% of the overall operational costs, whereas the supplementary costs comprises of electrode materials consumed and mechanical stirring [29]. In the present split-plate airlift electrocoagulation reactor, no mechanical mixer is used. Therefore, from the power consumption point of view, in addition to avoid discharging additional aluminum hydroxides in the solution, the optimum operating conditions resemble to a reduced applied electrical current (4.6 mA/cm2) with enough electrocoagulation time (say 45 min).

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1.8 1.6

E (kwh/kg COD

1.4 1.2 1 0.8 0.6 0.4 0.2

4.6 11.3 18.5 Current density (mA/cm^2)

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Fig.8. Influence of electrical current density on power consumption (E) (electrocoagulation time: 30 min, no air flow and temperature: 30 oC, initial pH: 6.8)

3.4 Influence of air flowrate

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Figure 8 displays the influence of air flow on the COD removal from the oil refinery wastewater sample using the proposed design of split-plate airlift reactor. 0.2 to 2 l/min electrocoagulation time of 30 min) were fixed.

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can be efficiently utilized as electrocoagulation/electroflotation unit even if no air flows in the riser region. Increasing COD removal when using air may be attributed to the efficient mixing achieved in the reactor and therefore, increasing the probability of particle-bubble contact and increasing the flotation rate of pollutants–coagulates complex. However, using air in this reactor design may be unnecessary and therefore, the additional cost could be avoided. In addition, there is no significant increase in the COD removal when the air flowrate is increased. As revealed in Figure 8, the COD removal improved only from 89% to 93.4% (by 4%) when the air flowrate increased from 0 to 2 l/min respectively. Furthermore, using high air flowrate can lead to excessive mixing of the solution, and this may break the flocs layers of pollutants.

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COD/TSS removal (%)

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COD TSS 0

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Fig.8. Effect of air flowrate on the COD removal (electrocoagulation time: 30 min, initial pH: 6.8, current density of 11.3 mA/cm2 and temperature: 30 oC)

3.6 Overall performance of airlift/electrocoagulation reactor

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The overall performance of airlift/electrocoagulation reactor for oil refinery wastewater treatment was investigated in terms of COD, BOD, TSS, TDS, turbidity, oil, chloride and sulfate removals under the optimal experimental conditions (current density of 11.3 mA/cm, initial pH: 6.8, temperature: 30 oC, electrocoagulation time: 30 min and air flowrate: 0.8 LPM). Fig.9 presents the overall performance results. The overall electrocoagulation process produces COD and TSS removal efficiencies of 93.1% and 90% respectively. Furthermore, the BOD and Oil contents in the treated samples had values of 6.5 and 1.9 mg/L, respectively.

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The suggested electrocoagulation reactor configuration in this study allows for high treatment efficiency and, simultaneously, low electrical power consumption, when compared to other proposed electrocoagulation reactor designs. El-Ashtoukhy et al. studied the phenolic compounds removal from oil refinery wastewater using fixed-bed anode (aluminum ranching rings) electrochemical reactor. They reported 53% COD removal efficiency (for initial phenol content of 40 mg/L) through 120 min electrocoagulation time, 8.6 mA/cm2 current density and initial solution pH of 7.0. With these conditions, the energy consumption was more than 2 kWh/g phenol removed. While in the present study, the energy consumption is 1 kWh/g COD removed at a current density of 11.3 mA/cm2 and up to 93.1% of COD was removed within 30 min [30].

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Before treatment

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After treatment

1500 1175 995

1000 625

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500 216 39

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BOD

Turbidity

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chloride sulfate

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Fig.9. The overall performance results of the split-plates airlift/electrocoagulation reactor in terms of various parameters of oil refinery wastewater treatment under optimum conditions (current density of 11.3 mA/cm, initial pH: 6.8, temperature: 30 oC, electrocoagulation time: 30 min and air flowrate: 0.8 LPM).

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4. Conclusions

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Electrocoagulation/flotation was evaluated as a considerable system, for the COD removal from real oil refinery wastewater using new and simple electrocoagulation reactor configuration composed of two aluminum electrodes inserted between the split-plates of the airlift reactor. The operating variables include current density, electrocoagulation time, liquid circulation velocity, and air flowrate (if used) were investigated on the COD removal efficiency. The results have revealed the technical feasibility of split-plate airlift/electrocoagulation contactor as a potential and dependable method for treatment of severely contaminated oil refinery wastewater without the use of mechanical stirrer and thus, reduced power consumption. The performance of the airlift/electrocoagulation reactor was found to be highly affected by the current density and liquid circulation velocity. The experimental results revealed that electrocoagulation in this design can achieve COD percentage removal of up to 94%.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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