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Treatment and operating cost analysis of metalworking wastewaters by a continuous electrocoagulation reactor
Journal Pre-proof Treatment and Operating Cost Analysis of Metalworking Wastewaters by a Continuous Electrocoagulation Reactor Mehmet Kobya, Philip Isaac Omwene, Zubeda Ukundimana
PII:
S2213-3437(19)30649-9
DOI:
https://doi.org/10.1016/j.jece.2019.103526
Reference:
JECE 103526
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
9 September 2019
Revised Date:
30 October 2019
Accepted Date:
5 November 2019
Please cite this article as: Kobya M, Omwene PI, Ukundimana Z, Treatment and Operating Cost Analysis of Metalworking Wastewaters by a Continuous Electrocoagulation Reactor, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103526
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Treatment and Operating Cost Analysis of Metalworking Wastewaters by a Continuous Electrocoagulation Reactor Mehmet Kobya, Philip Isaac Omwene*, Zubeda Ukundimana Department of Environmental Engineering, Gebze Technical University, 41400 Gebze, Turkey. *Corresponding author ([email protected]) Abstract: Environmental pollution is a persistent global challenge of industrialisation. In this study, metalworking wastewater (MWW) containing COD of 17312 mg/L, TOC of 3155 mg/L and
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turbidity of 15350 NTU was treated by continuous electrocoagulation process (CEP). At current densities between 30-90 A/m2, and reactor retention time () of 35 min (Q = 0.10 L/min), removal efficiencies for COD, TOC and turbidity were 68.0-87.0%, 55.2-77.7% and 85.099.6%, respectively for Al electrode. Whereas removals for COD, TOC and turbidity with Fe
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electrode were 75.1-94.8%, 72.2-89.5% and 88.1-99.9%, respectively. Futheremore, varying flow rate from 0.010-0.20 L/min ( of 350-17.5 min) at 90 A/m2, resulted into COD, TOC and
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turbidity removals of 92.6-71.3%, 83.3-64.9% and 99.9-88.9% for Al electrode and 97.878.6%, 94.9-69.9% and 99.9-94.7%, respectively for Fe electrode. Analysis of operating costs
lP
(OC) encompassing consumptions of energy, electrodes, chemicals and landfill disposal of generated sludge for 30-90 A/m2 current density showed a variation from 1.37-4.74 $/m3 and 1.03-3.80 $/m3 for Al and Fe electrodes, respectively. Similarly, the OC for 0.010-0.20 L/min
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flow rates were 4.34-4.88 $/m3 (3.34-1.11 $/kg COD) and 3.58-3.85 $/m3 (7.73-1.41 $/kg COD) for Al and Fe electrodes, respectively. At optimum conditions (EC time = 40 min, 70 min, and 90 A/m2), COD, TOC and turbidity removals from MWW by CEP were 94.3%, 90.1% and
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99.3%, respectively with Fe electrodes. At these optimums, the OC was 3.09 US $/m3 (2.63 US $/kg removed COD). The results showed that CEP was an effective alternative process for
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treatment of the MWW.
Keywords: Continuous electrocoagulation; metalworking wastewater; operating cost.
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1.Introduction Industrial metalworking processes in variety of manufacturing sectors cause significant environmental pollution [1]. The emulsionable fluids used for lubrication and refrigeration in metalworking processes are termed as Metal working fluids (MWF). A MWF concentrate usually contains mineral oils, surfactant mixtures, and various additives. Since MWFs become contaminated with usage, they become ineffective and require new replacements, resulting into generation of wastewaters with high COD, TOC and turbidity. These surface-active organic pollutants are of concern to the environment and must be treated before disposal [1,2]. Various treatment methods including; biological processes (aerobic and anaerobic treatment),
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membrane processes (microfiltration, ultrafiltration and nanofiltration), thermal processes (evaporation and incineration), electrocoagulation and chemical coagulation have been applied to treat MWW [3–8]. Due to the toxic and refractory compounds in MWW, treatment by biological processes is unsuitable as growth of microorganisms is adversely affected. Also, membrane processes are disadvantaged by the generation of membrane concentrate [9]. Thus,
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physio-chemical processes are commonly applied to treat oily industrial wastewaters [10]. Thermal process may not be economically feasible due to high energy requirements [11].
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Furthermore, chemical coagulation requires pH control coupled with addition of chemicals (coagulants), which may generate secondary pollutants depending on the applied coagulant
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[12]. Electrocoagulation (EC) process has been previously used to treat various wastewaters including metalworking oil-water emulsions [3,4,6,12]. The EC process is deemed attractive since it is easy to operate, economically viable, simple equipment and reduced amount of
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generated sludge [13,14]. The EC process usually employs sacrificial aluminium (Al) or iron (Fe) electrodes that dissolve to form their respective cations at the anode [15]. Electrolysis of water in the cathode generates hydrogen (H2) gas, which floats the generated flocs on surface
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of EC reactor as separable agglomerates [15,16]. The main anodic and cathodic reactions are as follows:
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For Al electrodes:
𝐴𝑙 → 𝐴𝑙3+ + 3𝑒 − 3𝐻2 𝑂 + 3𝑒 − →
3 2
(1) 𝐻2 + 3𝑂𝐻 −
(2)
For Fe electrodes: 𝐹𝑒(𝑠) → 𝐹𝑒 2+ + 2𝑒 −
(3)
2𝐻2 𝑂 + 2𝑒 − → 𝐻2(𝑔) + 2𝑂𝐻 −
(4)
Fe(OH)n(s) precipitates are formed from hydrolysis of dissolved iron (Fe2+ or Fe3+) 2
4𝐹𝑒 2+ + 10𝐻2 𝑂 + 𝑂2(𝑔) → 4𝐹𝑒(𝑂𝐻)3(𝑠) + 8𝐻 +
(5)
At pH greater than 10, monomeric anionic aluminium and iron species prevail as 𝐴𝑙(𝑂𝐻)− 4 and 3+ 𝐹𝑒(𝑂𝐻)− or Fe3+ cations gradually hydrolyse and transform into 4 [17]. The released Al
𝐴𝑙(𝑂𝐻)3(𝑠) and 𝐹𝑒(𝑂𝐻)3(𝑠) [18,19]. 𝐴𝑙3+ + 3𝐻2 𝑂 → 𝐴𝑙(𝑂𝐻)3(𝑠) + 3𝐻 +
(6)
𝐹𝑒 3+ + 3𝐻2 𝑂 → 𝐹𝑒(𝑂𝐻)3(𝑠) + 3𝐻 +
(7)
4+ − Monomeric and polymeric oxyhydroxides such as 𝐹𝑒𝑂𝐻 2+ , 𝐹𝑒(𝑂𝐻)+ 2 , 𝐹𝑒(𝑂𝐻)2 , 𝐹𝑒(𝑂𝐻)4 , 3+ + 4+ 2+ 𝐹𝑒(𝐻2 𝑂)+ and 2 , 𝐹𝑒(𝐻2 𝑂)6 , 𝐹𝑒(𝐻2 𝑂)5 (𝑂𝐻) , 𝐹𝑒(𝐻2 𝑂)4 (𝑂𝐻)2 , 𝐹𝑒2 (𝐻2 𝑂)8 (𝑂𝐻)2
𝐹𝑒2 (𝐻2 𝑂)6 (𝑂𝐻)2+ 4 are formed from dissolution of iron [18,19] [20]. Similarly, aluminium
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9+ 4+ 4+ − oxyhydroxide like 𝐴𝑙(𝑂𝐻)2+ , 𝐴𝑙(𝑂𝐻)+ 2 , 𝐴𝑙(𝑂𝐻)4 , 𝐴𝑙2 (𝑂𝐻)2 , 𝐴𝑙6 (𝑂𝐻)15 , 𝐴𝑙7 (𝑂𝐻)17 , 7+ 5+ 𝐴𝑙8 (𝑂𝐻)4+ 20 , 𝐴𝑙13 𝑂4 (𝑂𝐻)24 and 𝐴𝑙13 (𝑂𝐻)34 etc are also generated, depending on pH of
solution [19,21]. These monomeric and polymeric oxyhydroxides complexes serve as coagulating agents in the removal of pollutants from wastewater. Several studies on treatment
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of oily wastewaters by EC process exist in the literature [22–26]. However, only few studies have been reported on treatment of MWW by EC process [3–6,27]. The respective operating
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cost, COD and TOC removal efficiencies from real MWW by batch EC reactor for Fe electrode were 1.813 US $/m3, 93% and 82% at pH of 7, 80 A/m2 and 25 min. Whereas for Al electrode,
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these values were 1.190 US $/m3, 93% and 80% at pH of 5, 80 A/m2 and 25 min, respectively [28]. In another study, a continuous EC reactor removed 80% COD at optimum conditions (flow rate 10.5 L/h, oil content of 3000 mg/L, supporting electrolyte of 3000 mg/L NaCl, initial
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pH 8.5, current density of 105 A/m2, and temperature of 25 oC) from oil-water emulsions using Al electrodes [4]. The above-mentioned studies mostly used batch EC processes, and only one study reported EC process in a continuous mode for treatment of synthetic MWW [4]. No
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literature could be found on treatment of real MWW by continuous EC process. Therefore, continuous flow EC process studies for the treatment of MCW are important for application of
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this process.
The present study was aimed at assessing the applicability of continuous EC in treatment of real wastewater from a metal working factory. The influence of process conditions like current density, EC time and flow rate on treatment of MWW by a bench-scale continuous flow EC reactor were examined. Also, operating EC conditions for maximum TOC, COD and turbidity removals were calculated, in addition to the electrode and energy consumptions, operating costs and sludge generation. 3
2. Material and methods 2.1. Characterization of metalworking wastewater The MWW was acquired from automotive engines manufacturing plant in Gebze district of Kocaeli province, Turkey. Collected samples were always utilized immediately, however, as a means of preservation against possible decomposition due to bacterial activity, samples were kept at 2◦C for not more than 5 days. The plant produces approximately two cubic metres of MWW per month. The physio-chemical characteristics of the MWW were; pH = 6.6 0.60, conductivity = 6.1 0.65 mS/cm, total suspended solids of 150 40 mg/L, COD = 17,312
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1200 mg/L, TOC = 3,155 715 mg/L and turbidity = 15,350 2970 NTU.
2.2. Description of Continuous EC process and experimental procedure
Treatment of MWW was carried out in continuous mode EC process (Fig. 1). The set-up comprised of a wastewater storage tank, a feed pump, a continuous flow EC reactor, a DC
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power supply, and sedimentation tank. A Plexiglas EC reactor with dimensions; 19 cm length, 8 cm width, and 25 cm depth was used. The actual volume of the reactor was 4800 cm3. The
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electrodes (two anodes and two cathodes of dimensions 22 cm × 5 cm × 0.4 cm) consisted of four pieces of iron (99.50% purity) or aluminium (99.53% purity) spaced approximately 4.5 cm
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apart and submerged in the EC reactor solution. The electrodes were arranged in a monopolar parallel mode, yielding a total effective electrode surface area of 660 cm2 and connected to a
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digital dc power supply (0-18 A, 0-120 V, Agilent 6675A model, galvanostatic mode).
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Fig. 1 Schematic set up of a continuous flow electrocoagulation process
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Wastewater in the feed tank was continuously stirred by a mechanical stirrer (Heidolph RZR 2021 model), and the influent was continuously fed to the reactor by a peristaltic pump (Cole-
lP
Parmer 7553-75 model) at a desired flow rate. Effluent from the EC reactor was collected in a sedimentation tank, whereas sediments settled at the bottom of sedimentation reactor after EC. Samples for analysis were collected from effluent point of the continuous flow EC reactor at
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designated time intervals. Samples collected were filtered through 0.45 m micropore membrane filter before analysis.
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2.3. Analytical details
The COD, TOC, and turbidity analysis were done in accordance with the Standard Methods
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[29]. The COD analyses were performed spectrophotometrically (Perkin Elmer Lambda 35 UV/VIS spectrophotometer, USA) using closed reflux colorimetric method. TOC was obtained by sample combustion using a non-dispersive IR source (Tekmar Dohrmann Apollo 9000) at 680 oC. The turbidity of wastewater (in NTU units) was determined by Mettler Toledo 8300 model turbidimeter, whereas total suspended solids (TSS) were determined through vacuum filtration on a Whatman model GF-C filter (1mm) and subsequently drying at 105 oC to constant weight. The pH and conductivity were measured using Mettler Toledo SevenGo duo portable
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meter. Deionised water was used in preparation of all the reagents and all chemical used were of analytical grade. The experimental runs were performed in duplicate, and the analysis of each parameter was done in triplicate for each run.
3. Results and Discussion 3.1. Effect of current density The amount of electrochemically generated coagulant (Al3+ or Fe2+) in EC process is widely influenced by applied current density. In this work, current density was varied from 30-90 A/m2 at a flow rate Q = 0.10 L/min. The influence of these variations on COD, TOC and turbidity
min operation at different current densities. 100
100
90
70 60 50 40
2
CD (A/m ) 30 50 70 90
30 20 10
80 70 60
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TOC Removal Efficiency (%)
80
50 40
2
CD (A/m ) 30 50 70 90
30 20
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COD Removal Efficiency (%)
Al electrode
Al electrode
90
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removal from the MWW by EC was investigated. Figs. 2 and 3 show the data obtained for 50
10
0
0 0
5
10
15
20
25
30
35
40
45
50
55
0
5
10
15
20
25
30
35
40
45
50
55
Operating Time (min)
lP
Operating Time (min)
Fig 2 Effect of current density on COD and TOC removals from MWW by CEP process using
100 Fe electrode
80 70
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60 50 40 30 20
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COD Removal Efficiency (%)
90
2
CD (A/m ) 30 50 70 90
10
5
10
15
Fe electrode
90 80 70 60 50 40
2
CD (A/m ) 30 50 70 90
30 20 10
0
0
100
TOC Removal Efficiency (%)
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Al electrode (Q = 0.10 L/min)
0 20
25
30
35
Operating Time (min)
40
45
50
55
0
5
10
15
20
25
30
35
40
45
50
55
Operating Time (min)
Fig. 3 Effect of current density on COD and TOC removals from MWW by CEP process using Fe electrode (Q = 0.10 L/min)
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At applied current densities of 30, 50, 70, and 90 A/m2, maximum COD removal efficiencies were obtained as 75.1%, 82.3%, 88.5% and 89.5%, respectively for Fe electrode. Also, maximum TOC removal efficiencies at these current densities were obtained as 72.2%, 79.2%, 82.73% and 89.5% respectively (Fig. 2). Similarly, at 30, 50, 70, and 90 A/m2, the respective maximum COD removals with Al electrode were 68.1%, 79.0%, 83.1% and 87.0%. On the other hand, TOC removals were 55.2%, 66.9%, 74,2% and 77.7%, respectively (Fig. 3). As can be seen in Figs. 2 and 3, COD and TOC removal efficiency for both electrodes increased with current density. This may be explained by high dissolution rate of anodes at high current densities, which in turn increases coagulant production rates (Eqs. 1-7), and the amount of metal
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hydroxides flocs; Fe(OH)3(s) or Al(OH)3(s) available to form precipitate ions and complexes in solution. These generated metallic hydroxides in the EC process are reported to be good adsorbents for emulsified and colloidal dispersed oils [24]. Morever, the rate of bubble generation increased with increase in current density, leading to faster upward flow of colloidal molecules and Al or Fe hydroxide flocs with the sorbed organic compounds. In this case, oily
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and colloidal pollutants in the MWW were flocculated by the generated H2 gas bubbles at the cathode, hence separating pollutant molecules from aqueous phase. As a result, at about 30-40
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minutes of treatment (as seen in Figs. 2 and 3), when the repulsive potential energy between charged particles from anodic dissolution of Al or Fe electrodes (Al3+ or Fe2+) is neutralised in
lP
the EC reactor, destabilisation of organic compounds in the MWW occurs. At high current densities and longer EC time (especially, 90 A/m2 and 30-40 min) for both electrodes, addition of the electrochemically dissolved Al3+ or Fe2+ ions to the solution becomes ineffective as the
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removals of COD and TOC (>%85) approach their maximum level. Figs. 2 and 3 also indicates that the kinetic regime for COD removal with EC process shifted from mass transfer to kinetically-limited at different current density and EC time. Moreover, at the beginning of the
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EC process, the concentration of pollutants was more abundant; hence the rate of pollutant removal was high followed by a gradual decrease. On the other hand, the reduced concentration
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of TOC and COD at the end of the process resulted in a slower reaction rate (above 35-40 min). Similar findings have also been reported in different wastewater treatment by EC procces [3,26,30]. If the treated water is to be discharged to receiving environments such as rivers and lakes, effluent turbidity should also be taken into consideration. High levels of turbidity may obstruct light from reaching lower water depths, hence affecting aquatic life. The turbidity removal efficiencies for Al electrode at 30, 50, 70, and 90 A/m2 were 85.1% (2302 NTU), 99.3% (107 NTU), 99.4% (92 NTU) and 99.6% (61 NTU), respectively. Similarly, the turbidity removal efficiencies at these current density conditions for Fe electrode were determined as 7
88.0% (1842 NTU), 99.6% (61 NTU), 99.4% (92 NTU) and 99.9% (15 NTU), respectively. As exhibited by the COD and TOC removal trends, the turbidity removal efficiencies also
2,5 15 2,0 3
Wsludge (kg/m ) 3
1,5
ELC (kg/m ) 3 ENC (kWh/m )
10
1,0 5 0,5 0,0 20
30
40
50
60
70
80
90
0 100
2,5
8
Al electrode
2,0 6 1,5 4 3
Wsludge (kg/m )
1,0
3
ELC (kg/m ) 3 ENC (kWh/m )
0,5
2
0,0 20
30
40
50
60
70
80
90
0 100
Energy Consumption (ENC, kWh/kg COD)
3
Energy Consumption (ENC, kWh/m )
20
Al electrode
3
Sludge and Electrode Consumptions (kg/m )
3,0
Sludge and Electrode Consumptions (kg/kg COD)
increased with increased in applies current density.
2
2
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Current Density (CD, A/m )
Current Density (CD, A/m )
Fig. 4 Generated sludge amount, electrode and energy consumptions at different current densities in the CEP process for Al electrode
10 3
2,0
Wsludge (kg/m )
8
3
ELC (kg/m ) 3 ENC (kWh/m )
1,5 1,0
6 4
0,5 0,0 20
30
40
50
60
70
80 2
Current Density (CD, A/m )
90
18 16
3
14 12
2 100
2
1
10 8
Wsludge (kg/kg COD)
6
ELC (kg/kg COD) ENC (kWh/kg COD)
4 2
0 20
30
40
50
60
70
80
90
Energy consumption (kWh/kg COD)
12
2,5
20
Fe electrode
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3,0
4
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14
3
3,5
Energy Consumption (kWh/m )
16
Sludge and electrode consumptions (kg/kg COD)
18 Fe electrode
4,0
lP
3
Sludge and Electrode Consumptions (kg/m )
4,5
0 100
2
Current Density (CD, A/m )
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Fig. 5 Generated sludge amount, electrode and energy consumptions at different current densities in the CEP process for Fe electrode
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The energy consumption costs encompassing pumping, continuous EC reactor, (ENC, kWh/m3 or kWh/removed kg COD) and electrode consumptions (ELC, kg/m3 or kg/removed kg COD)
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for both electrodes were calculated based on COD removed using Eqs. 8-12. Most national and international environmental regulatory authorities consider COD as the most important parameter for discharge of treated industrial wastewater. In this regards, we have also expressed costs in terms of kg COD removed per m3 treated wastewater. Figs. 4 and 5 show the generated sludge amounts, electrode and energy consumptions for the range of current densities investigated in this study.
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𝐸𝑁𝐶 (𝑘𝑊ℎ⁄𝑚3 ) =
𝑈×𝑖×𝑡𝐸𝐶 𝑣
𝐶𝑝𝑢𝑚𝑝 (𝑘𝑊ℎ⁄𝑚3 ) =
(8)
𝜌×𝑔×𝑄×ℎ×𝑡𝐸𝐶
(9)
1000×𝑣
𝐸𝑁𝐶 (𝑘𝑊ℎ⁄𝑘𝑔 𝐶𝑂𝐷) = 𝐸𝐿𝐶 (𝑘𝑔⁄𝑚3 ) =
+ 𝐶𝑝𝑢𝑚𝑝
𝑈×𝑖×𝑡𝐸𝐶 𝑘𝑔 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝐶𝑂𝐷
+ 𝐶𝑝𝑢𝑚𝑝
(10)
𝑖×𝑡𝐸𝐶 ×𝑀𝑤
(11)
𝑧×𝐹×𝑣
𝐸𝐿𝐶 (𝑘𝑔⁄𝑘𝑔 𝐶𝑂𝐷) =
𝑖×𝑡𝐸𝐶 ×𝑀𝑤
(12)
𝑧×𝐹×𝑘𝑔 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝐶𝑂𝐷
where U represents cell voltage (V), i represents current (A), tEC is the EC time (sec for ELC or
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hour for ENC) and v represents volume (m3) of the solution, Mw represents molecular mass (Fe: 55.85 g/mol and Al: 26.98 g/mol), z represents the number of electron transferred (z for Fe = 2 and z for Al = 3) and the Faraday’s constant (F) is (96487 C/mol e-).
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The faradic yield or current efficiency (Al or Fe = mexp/mth) for an electrode (Al or Fe) was computed as the ratio of the experimental weight loss of the anodes (mexp) to the theretically
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consumed amount of the anode (mth) [6,31]. The current efficiency at 30, 50, 70 and 90 A/m2 was calculated as 120%, 132%, 124% and 115% for Al electrodes and 109%, 110%, 106% and
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112% , respectively for Fe electrodes. The experimental electrode consumptions (ELC) for 30, 50, 70 and 90 A/m2 at 50 min was 0.189 kg/m3 (0.0342 kg/kg COD), 0.348 kg/m3 (0.0838 kg/kg COD), 0.458 kg/m3 (0.1469 kg/kg COD) and 0.546 kg/m3 (0.243 kg/kg COD), respectively for
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Al electrode. Meanwhile, for Fe electrode the respective ELC was 0.357 kg/m3 (0.0828 kg/kg COD), 0.601 kg/m3 (0.1959 kg/kg COD), 0.810 kg/m3 (0.4068 kg/kg COD) and 1.100 kg/m3 (1.222 kg/kg COD). Energy consumption (ENC) at 30-90 A/m2 for Al electrode ranged from
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1.98-17.11 kWh/m3 (0.35-7.60 kWh//kg COD). For Fe electrode, ENC was calculated as 2.3117.67 kWh/m3 (0.53-19.63 kWh/kg COD) for the same range of current density. Moreover, the
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amount of sludge produced during the CEP process at 30-90 A/m2 ranged from 2.30 to 2.93 kg/m3 and 2.74 to 4.12 kg/m3 for Al and Fe electrodes respectively. Sludge production in EC process is affected by wastewater characteristics such as number of settable solids and presences of destabilized matter due to coagulation and flocculation [18]. We noted that EC time and applied current density were the major factors influencing the quantity of electrogenerated sludge, for both electrodes. This can be explained by the increase of dissolved metal (Fe or Al) anodes with time and applied current (Faraday’s law).
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3.2. Effect of flow rate The treatment efficiency of a CEP process is also affected by flow rate (Q) to the reactor. Lowering the flow rate results into more hydraulic retention time in the EC reactor, hence restabilising the generated metal hydroxides flocs and enhancing their removal mechanisms (oxidation, precipitation and adsorption). Mixing is also required during the EC process to promote growth and precipitation of flocs. The influence of flow rate on removals of COD and TOC for metalworking wastewater was analysed for variations of flow rate from 0.01-0.20 min/L ( τ of 350-17.5 min) at 90 A/m2. 100
Al electrode
90
80
60 50 40 Q (L/min) 0.01 0.05 0.10 0.20
20 10
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70
30
80
TOC Removal Efficiency (%)
COD Removal Efficiency (%)
100
Al electrode
90
70 60 50 40
Q (L/min) 0.01 0.05 0.10 0.20
30 20 10
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0
0
0
5
10
15
20
25
30
35
40
45
50
55
0
Operating Time (min)
5
10
15
20
25
30
35
40
45
50
55
Operating Time (min)
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Fig 6 Effect of flow rate on COD and TOC removals from MWW by Al electrodes (CD = 90
100 Fe electrode
80 70
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60 50 40 30 20 10 0 0
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COD Removal Efficiency (%)
90
5
10
15
20
25
30
35
Fe electrode 90 80 70 60 50 40 Q (L/min) 0.01 0.05 0.10 0.20
30 20 10 0
40
45
50
55
0
5
10
15
20
25
30
35
40
45
50
55
Operating Time (min)
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Operating Time (min)
Q (L/min) 0.01 0.05 0.10 0.20
100
TOC Removal Efficiency (%)
lP
A/m2)
Fig. 7 Effect of flow rate on COD and TOC removals from MWW by Fe electrodes (CD = 90 A/m2)
Figs. 6 and 7 show the effect of flow rate on COD and TOC removal from MWW by CEP. For Al electrode, increase in flow rate from 0.010 to 0.20 L/min, resulted into decrease in COD and TOC removal efficiencies from 92.6% (1281 mg/L) to 71.3% (4968 mg/L) and 85.8% (448 mg/L) to 64.9% (1107 mg/L), respectively. Similarly, COD and TOC removal efficiencies 10
decreased from 97.8% (380 mg/L) to 78.6% (3704 mg/L) and 94.9% (160 mg/L) to 69.9 % (949 mg/L), respectively, for the same variations in flow rate with Fe electrode. Under the prescribed flow rates, both electrodes obtained > 77% COD and TOC removals except at 0.20 L/min. Low flow rates favoured mixing of the electrochemically generated coagulants and the pollutants in the wastewater, hence improving the rate of pollutant removal. The turbidity removal efficiencies at 50 min of CEP process with Al electrode were 99.9 % (15 NTU), 99.9 % (15 NTU), 99.6 % (61 NTU) and 88.9% (1703 NTU) at flow rates of 0.010, 0.050, 0.10 and 0.20 min/L, respectively. Whereas with Fe electrode, turbidity removals were recorded as
Wsludge (kg/m )
1,5
ELC (kg/m ) 3 ENC (kWh/m )
16
3
1,0
15
0,5 0,0 0,00
0,05
0,10
0,15
0,20
14 0,25
Flow Rate (Q, L/min)
11
2,0
10 9
1,5
1,0
8 7
Wsludge (kg/kg COD)
6
ELC (kg/kg COD) ENC (kWh/kg COD)
5
0,5
0,0 0,00
0,05
0,10
4 0,15
0,20
Energy Consumption (kWh/kg COD)
3
2,0
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17
2,5
12
Al electrode
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3
Energy Consumption (kWh/m )
3,0
2,5
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18 Al electrode
3
Sludge and Electrode Consumptions (kg/m )
3,5
Sludge and Electrode Consumptions (kg/kg COD)
99.9 % (15 NTU), except at 0.20 min/L (94.7%).
3 0,25
Flow Rate (Q, L/min)
Al electrodes
3,0
3
Wsludge (kg/m )
1,5 1,0
3
ELC (kg/m ) 3 ENC (kWh/m )
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0,5
ur
2,5
0,0 0,00
0,05
0,10
0,15
Flow Rate (Q, L/min)
17,5 17,0 16,5 16,0 15,5 15,0
0,20
14,5 0,25
Sludge and Electrode Consumptions (kg/kg COD)
3,5
18,0
3
4,0
18,5
Energy Consumption (kWh/m )
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4,5
2,0
19,0
Fe electrode
3
Sludge and Electrode Consumptions (kg/m )
5,0
3,5
45 Fe electrode 40
3,0
35 2,5 2,0
Wsludge (kg/kg COD)
30
ELC (kg/kg COD) ENC (kWh/kg COD)
25 20
1,5
15 1,0 10 0,5 0,0 0,00
5 0,05
0,10
0,15
0,20
Energy Consumption (kWh/kg COD)
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Fig. 8 Generated sludge amount, electrode and energy consumptions at different flow rates for
0 0,25
Flow Rate (Q, L/min)
Fig. 9 Generated sludge amount, electrode and energy consumptions at different flow rates for Fe electrodes The calculated current efficiencies ( Φ Al or Fe) at flow rates of 0.010, 0.050, 0.10 and 0.20 L/min for Al electrode were 108%, 111%, 115% and 125%, respectively. Under the same flow rates, 11
the respective current efficiencies for Fe electrode were 120%, 113%, 112% and 122%. The generated sludge, energy and electrode consumptions for Al and Fe electrodes at various flow rates are presented in Figs. 8 and 9. At 0.010-0.20 L/ min, electrode and energy consumptions ranged from 0.512-0.593 kg/m3 (or 0.400-0.119 kg/kg COD) and 14.57-17.32 kWh/m3 (or 11.37-3.49 kWh/ kg COD) for Al electrode, and 1.18-1.20 kg/m3 (or 3.09-0.32 kg/kg COD) and 14.57-18.81 kWh/m3 (or 38.25-5.08 kWh/kg COD), for Fe electrode (Fig. 9). Electrogenerated sludge in the CEP process ranged from 3.22-2.45 kg/m3 for Al electrode and 4.68-2.98 kg/m3 for Fe electrode for change in flow rates from 0.010-0.20 L/min (Figs. 8 and 9). Generally, increasing flow rate resulted into a decrease in amount of produced sludge,
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energy and electrode consumptions.
In all waste water treatment processes, the effluent parameters must be evaluated in accordance with the accepted environmental discharge regulation. The industrial effluent discharge standards at Gebze industrial zone are regulated by the Turkish Water Pollution Control
-p
Regulation. The sewage discharge standard for metal working effluents is < 1000 mg/L COD. In the present study, over 94.2% COD removal was obtained by Fe electrode (i.e., from 17312
re
mg/L to < 1000 mg/L), at flow rate < 0.050 L/min ( τ = 70 min), applied current density of 90 A/m2 and EC time of 40 min. The final effluent concentrations for COD, TOC and turbidity
lP
with Fe electrode at these optimum conditions were 987 mg/L, 312 mg/L and 107 (NTU), respectively.
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3.3. Evaluation of operating cost
The costs entailed in electrochemical treatment process are related to electrical energy, peristatic pump, sacrificial electrodes, sludge handling costs, maintenance and labour. The
ur
computation of operating cost (in terms of $/m3 of wastewater treated and $/kg COD removed)
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in this work considered energy consumptions and electrode consumptions as the main costs.
𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝐶𝑜𝑠𝑡 (𝑂𝐶) = 𝑎 × 𝐸𝑁𝐶 + 𝑏 × 𝐸𝐿𝐶 + 𝑐 × 𝑊𝑠𝑙𝑢𝑑𝑔𝑒 + 𝑑 × 𝐶𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙
(13)
Where; ENC (energy consumption; kWh/m3 and kWh/kg COD) and ELC (electrode consumption; kg Fe/m3 and kg Fe/kg COD) are experimentally obtained values from the treatment process. The parameter “a” represents the electrical energy cost and “b” represents cost of electrode material. As of May 2018, in Turkey, the electrical energy cost was 0.12 US $/kWh, and cost of electrode materials were 0.952 US $/kg Fe and 4.10 US $/kg Al. “c” landfill 12
disposal cost of sludge 0.152 US $/kg (sludge moisture of ~20%), “d” for chemical consumptions (CC) such as NaOH and H2SO4 (for adjustment of desired pH) with assigned prices of respectively, 1.01 US $/kg and 0.40 US $/kg.
Operating costs were calculated according to energy consumptions as indicated in Eqs.8-10, experimental electrode consumptions as indicated in Eqs. 11-12, sludge handling operating and use of chemical. 4,9
2,25
1,50
3,0
1,25
2,5
1,00
2,0
0,75 3
1,5
20
30
40
50
60
70
80
90
4,5
2,0 1,5
4,4
1,0
4,3 0,00
0,25 100
2,5
3
$/m $/kg COD
4,6
0,50
$/m $/kg COD
1,0
3,0 4,7
Operating Cost ($/kg COD)
3,5
3
1,75
Operating Cost ($/m )
4,0
3,5
4,8
Operating Cost ($/kg COD)
2,00
3
Operating Cost ($/m )
4,5
4,0 Al electrode
Al electrode
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5,0
0,05
2
0,10
0,15
0,5 0,25
0,20
Flow Rate (Q, L/min)
-p
Current Density (CD, A/m )
4,0
3,90
Fe electrode
7 6
3,80
2,5
2,0
2,0
1,5
1,5
1,0
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3
1,0
$/m $/kg COD
0,5 20
30
40
50
60
70
80
2
0,0 100
5
3,75 4
3,70 3
3,65
2 3
3,60 3,55 0,00
$/m $/kg COD 0,05
0,10
1
0,15
0,20
0 0,25
Flow Rate (Q, L/min)
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Current Density (CD, A/m )
90
0,5
3
2,5
Operating Cost ($/m )
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Operating Cost ($/kg COD)
3,85
3,0
3,0
3
Operating Cost ($/m )
8
Fe electrode
3,5
3,5
Operating Cost ($/kg COD)
4,0
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Fig.10 Effect of current density and flow rates on the operating cost for Al electrode
Fig.11 Effect of current density and flow rates on the operating cost for Fe electrode
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The operating cost with Al and Fe elecrodes at different current densities and flow rates are shown in Fig. 10 and Fig. 11. At 30-90 A/m2 with Al electrode, operating costs were calculated as 1.37-4.74 $/m3 or 0.399-2.140 $/kg removed COD. In addition, for 0.010-0.20 min/L ( τ of 350-17.5 min) flow rate, operating costs with Al electrode were 4.34-4.88 $/m3 or 3.34-1.11 $/kg removed COD (Fig. 10). While for Fe electodes at 30-90 A/m2, the operating costs were 1.03-3.80 $/m3 or 0.43-3.97 $/kg removed COD. For 0.010-0.20 min/L flow rate, operating costs with Fe electrode were 3.58-3.85 $/m3 or 7.73-1.41 $/kg COD removed (Fig. 11). Both 13
electrodes showed a tremedously increase in operating cost for applied current density increase from 30 to 90 A/m2. In general, operating costs for Fe electrode were lower than those for Al electrode. Moreover, Fe electrode presented better COD, TOC and turbidity removal efficiencies, making it more suitable for treatment of metal working wastewaters.
4. Conclusions Continuous EC process was used to treat wastewater from a metal working factory. Operating parameters like applied current density, flow rate and EC operating time were analysed to determine their influence on treatment of MWW by a bench-scale continuous flow EC reactor.
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Optimum conditions to obtain maximum TOC, COD and turbidity removals in the EC process were determined. In addition, evaluation of economic aspects of the treatment process was also done. For Al electrode at optimums of EC time of 50 min, flow rate of 0.010 L/min ( τ of 350 min), and current density of 90 A/m2, the obtained removals of COD, TOC and turbidity were
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92.6% (1281 mg/L), 85.8% (448 mg/L) and 99.9% (16 NTU), respectively, and the calculated operating costs were 4.34 $/m3 (3.33 $/ kg COD). On other hand, optimum operating conditions
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for Fe electrode were determined as EC time of 40 min, flow rate of 0.05 L/min ( τ of 70 min) and applied current density of 90 A/m2. Removal efficiencies of COD, TOC and turbidity at
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these optimums were 94.3% (987 mg/L), 90.1% (312 mg/L) and 99.3% (107 NTU), respectively, with operating costs of 3.09 $/m3 (2.63 $/kg COD). Consequently, the continuous EC process results with Fe electrodes were most suitable with respect to the legal sewage COD
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discharge limit of 1000 mg/L for metal working wastewater. Also, the costs entailed in EC treatment in terms of $/m3 treated wastewater or $/kg removed COD were lesser for Fe electrode than for Al electrode. Increase in current density and flow rate resulted into increase in cost of
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EC treatment.
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Acknowledgements
This study was funded by the Scientific and Technological Research Council of Turkey (TUBITAK) through project number104Y267. The authors thank to TUBITAK for their financial support of this work. References [1]
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17
PII:
S2213-3437(19)30649-9
DOI:
https://doi.org/10.1016/j.jece.2019.103526
Reference:
JECE 103526
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
9 September 2019
Revised Date:
30 October 2019
Accepted Date:
5 November 2019
Please cite this article as: Kobya M, Omwene PI, Ukundimana Z, Treatment and Operating Cost Analysis of Metalworking Wastewaters by a Continuous Electrocoagulation Reactor, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103526
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Treatment and Operating Cost Analysis of Metalworking Wastewaters by a Continuous Electrocoagulation Reactor Mehmet Kobya, Philip Isaac Omwene*, Zubeda Ukundimana Department of Environmental Engineering, Gebze Technical University, 41400 Gebze, Turkey. *Corresponding author ([email protected]) Abstract: Environmental pollution is a persistent global challenge of industrialisation. In this study, metalworking wastewater (MWW) containing COD of 17312 mg/L, TOC of 3155 mg/L and
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turbidity of 15350 NTU was treated by continuous electrocoagulation process (CEP). At current densities between 30-90 A/m2, and reactor retention time () of 35 min (Q = 0.10 L/min), removal efficiencies for COD, TOC and turbidity were 68.0-87.0%, 55.2-77.7% and 85.099.6%, respectively for Al electrode. Whereas removals for COD, TOC and turbidity with Fe
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electrode were 75.1-94.8%, 72.2-89.5% and 88.1-99.9%, respectively. Futheremore, varying flow rate from 0.010-0.20 L/min ( of 350-17.5 min) at 90 A/m2, resulted into COD, TOC and
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turbidity removals of 92.6-71.3%, 83.3-64.9% and 99.9-88.9% for Al electrode and 97.878.6%, 94.9-69.9% and 99.9-94.7%, respectively for Fe electrode. Analysis of operating costs
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(OC) encompassing consumptions of energy, electrodes, chemicals and landfill disposal of generated sludge for 30-90 A/m2 current density showed a variation from 1.37-4.74 $/m3 and 1.03-3.80 $/m3 for Al and Fe electrodes, respectively. Similarly, the OC for 0.010-0.20 L/min
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flow rates were 4.34-4.88 $/m3 (3.34-1.11 $/kg COD) and 3.58-3.85 $/m3 (7.73-1.41 $/kg COD) for Al and Fe electrodes, respectively. At optimum conditions (EC time = 40 min, 70 min, and 90 A/m2), COD, TOC and turbidity removals from MWW by CEP were 94.3%, 90.1% and
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99.3%, respectively with Fe electrodes. At these optimums, the OC was 3.09 US $/m3 (2.63 US $/kg removed COD). The results showed that CEP was an effective alternative process for
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treatment of the MWW.
Keywords: Continuous electrocoagulation; metalworking wastewater; operating cost.
1
1.Introduction Industrial metalworking processes in variety of manufacturing sectors cause significant environmental pollution [1]. The emulsionable fluids used for lubrication and refrigeration in metalworking processes are termed as Metal working fluids (MWF). A MWF concentrate usually contains mineral oils, surfactant mixtures, and various additives. Since MWFs become contaminated with usage, they become ineffective and require new replacements, resulting into generation of wastewaters with high COD, TOC and turbidity. These surface-active organic pollutants are of concern to the environment and must be treated before disposal [1,2]. Various treatment methods including; biological processes (aerobic and anaerobic treatment),
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membrane processes (microfiltration, ultrafiltration and nanofiltration), thermal processes (evaporation and incineration), electrocoagulation and chemical coagulation have been applied to treat MWW [3–8]. Due to the toxic and refractory compounds in MWW, treatment by biological processes is unsuitable as growth of microorganisms is adversely affected. Also, membrane processes are disadvantaged by the generation of membrane concentrate [9]. Thus,
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physio-chemical processes are commonly applied to treat oily industrial wastewaters [10]. Thermal process may not be economically feasible due to high energy requirements [11].
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Furthermore, chemical coagulation requires pH control coupled with addition of chemicals (coagulants), which may generate secondary pollutants depending on the applied coagulant
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[12]. Electrocoagulation (EC) process has been previously used to treat various wastewaters including metalworking oil-water emulsions [3,4,6,12]. The EC process is deemed attractive since it is easy to operate, economically viable, simple equipment and reduced amount of
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generated sludge [13,14]. The EC process usually employs sacrificial aluminium (Al) or iron (Fe) electrodes that dissolve to form their respective cations at the anode [15]. Electrolysis of water in the cathode generates hydrogen (H2) gas, which floats the generated flocs on surface
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of EC reactor as separable agglomerates [15,16]. The main anodic and cathodic reactions are as follows:
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For Al electrodes:
𝐴𝑙 → 𝐴𝑙3+ + 3𝑒 − 3𝐻2 𝑂 + 3𝑒 − →
3 2
(1) 𝐻2 + 3𝑂𝐻 −
(2)
For Fe electrodes: 𝐹𝑒(𝑠) → 𝐹𝑒 2+ + 2𝑒 −
(3)
2𝐻2 𝑂 + 2𝑒 − → 𝐻2(𝑔) + 2𝑂𝐻 −
(4)
Fe(OH)n(s) precipitates are formed from hydrolysis of dissolved iron (Fe2+ or Fe3+) 2
4𝐹𝑒 2+ + 10𝐻2 𝑂 + 𝑂2(𝑔) → 4𝐹𝑒(𝑂𝐻)3(𝑠) + 8𝐻 +
(5)
At pH greater than 10, monomeric anionic aluminium and iron species prevail as 𝐴𝑙(𝑂𝐻)− 4 and 3+ 𝐹𝑒(𝑂𝐻)− or Fe3+ cations gradually hydrolyse and transform into 4 [17]. The released Al
𝐴𝑙(𝑂𝐻)3(𝑠) and 𝐹𝑒(𝑂𝐻)3(𝑠) [18,19]. 𝐴𝑙3+ + 3𝐻2 𝑂 → 𝐴𝑙(𝑂𝐻)3(𝑠) + 3𝐻 +
(6)
𝐹𝑒 3+ + 3𝐻2 𝑂 → 𝐹𝑒(𝑂𝐻)3(𝑠) + 3𝐻 +
(7)
4+ − Monomeric and polymeric oxyhydroxides such as 𝐹𝑒𝑂𝐻 2+ , 𝐹𝑒(𝑂𝐻)+ 2 , 𝐹𝑒(𝑂𝐻)2 , 𝐹𝑒(𝑂𝐻)4 , 3+ + 4+ 2+ 𝐹𝑒(𝐻2 𝑂)+ and 2 , 𝐹𝑒(𝐻2 𝑂)6 , 𝐹𝑒(𝐻2 𝑂)5 (𝑂𝐻) , 𝐹𝑒(𝐻2 𝑂)4 (𝑂𝐻)2 , 𝐹𝑒2 (𝐻2 𝑂)8 (𝑂𝐻)2
𝐹𝑒2 (𝐻2 𝑂)6 (𝑂𝐻)2+ 4 are formed from dissolution of iron [18,19] [20]. Similarly, aluminium
ro of
9+ 4+ 4+ − oxyhydroxide like 𝐴𝑙(𝑂𝐻)2+ , 𝐴𝑙(𝑂𝐻)+ 2 , 𝐴𝑙(𝑂𝐻)4 , 𝐴𝑙2 (𝑂𝐻)2 , 𝐴𝑙6 (𝑂𝐻)15 , 𝐴𝑙7 (𝑂𝐻)17 , 7+ 5+ 𝐴𝑙8 (𝑂𝐻)4+ 20 , 𝐴𝑙13 𝑂4 (𝑂𝐻)24 and 𝐴𝑙13 (𝑂𝐻)34 etc are also generated, depending on pH of
solution [19,21]. These monomeric and polymeric oxyhydroxides complexes serve as coagulating agents in the removal of pollutants from wastewater. Several studies on treatment
-p
of oily wastewaters by EC process exist in the literature [22–26]. However, only few studies have been reported on treatment of MWW by EC process [3–6,27]. The respective operating
re
cost, COD and TOC removal efficiencies from real MWW by batch EC reactor for Fe electrode were 1.813 US $/m3, 93% and 82% at pH of 7, 80 A/m2 and 25 min. Whereas for Al electrode,
lP
these values were 1.190 US $/m3, 93% and 80% at pH of 5, 80 A/m2 and 25 min, respectively [28]. In another study, a continuous EC reactor removed 80% COD at optimum conditions (flow rate 10.5 L/h, oil content of 3000 mg/L, supporting electrolyte of 3000 mg/L NaCl, initial
na
pH 8.5, current density of 105 A/m2, and temperature of 25 oC) from oil-water emulsions using Al electrodes [4]. The above-mentioned studies mostly used batch EC processes, and only one study reported EC process in a continuous mode for treatment of synthetic MWW [4]. No
ur
literature could be found on treatment of real MWW by continuous EC process. Therefore, continuous flow EC process studies for the treatment of MCW are important for application of
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this process.
The present study was aimed at assessing the applicability of continuous EC in treatment of real wastewater from a metal working factory. The influence of process conditions like current density, EC time and flow rate on treatment of MWW by a bench-scale continuous flow EC reactor were examined. Also, operating EC conditions for maximum TOC, COD and turbidity removals were calculated, in addition to the electrode and energy consumptions, operating costs and sludge generation. 3
2. Material and methods 2.1. Characterization of metalworking wastewater The MWW was acquired from automotive engines manufacturing plant in Gebze district of Kocaeli province, Turkey. Collected samples were always utilized immediately, however, as a means of preservation against possible decomposition due to bacterial activity, samples were kept at 2◦C for not more than 5 days. The plant produces approximately two cubic metres of MWW per month. The physio-chemical characteristics of the MWW were; pH = 6.6 0.60, conductivity = 6.1 0.65 mS/cm, total suspended solids of 150 40 mg/L, COD = 17,312
ro of
1200 mg/L, TOC = 3,155 715 mg/L and turbidity = 15,350 2970 NTU.
2.2. Description of Continuous EC process and experimental procedure
Treatment of MWW was carried out in continuous mode EC process (Fig. 1). The set-up comprised of a wastewater storage tank, a feed pump, a continuous flow EC reactor, a DC
-p
power supply, and sedimentation tank. A Plexiglas EC reactor with dimensions; 19 cm length, 8 cm width, and 25 cm depth was used. The actual volume of the reactor was 4800 cm3. The
re
electrodes (two anodes and two cathodes of dimensions 22 cm × 5 cm × 0.4 cm) consisted of four pieces of iron (99.50% purity) or aluminium (99.53% purity) spaced approximately 4.5 cm
lP
apart and submerged in the EC reactor solution. The electrodes were arranged in a monopolar parallel mode, yielding a total effective electrode surface area of 660 cm2 and connected to a
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ur
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digital dc power supply (0-18 A, 0-120 V, Agilent 6675A model, galvanostatic mode).
4
ro of
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Fig. 1 Schematic set up of a continuous flow electrocoagulation process
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Wastewater in the feed tank was continuously stirred by a mechanical stirrer (Heidolph RZR 2021 model), and the influent was continuously fed to the reactor by a peristaltic pump (Cole-
lP
Parmer 7553-75 model) at a desired flow rate. Effluent from the EC reactor was collected in a sedimentation tank, whereas sediments settled at the bottom of sedimentation reactor after EC. Samples for analysis were collected from effluent point of the continuous flow EC reactor at
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designated time intervals. Samples collected were filtered through 0.45 m micropore membrane filter before analysis.
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2.3. Analytical details
The COD, TOC, and turbidity analysis were done in accordance with the Standard Methods
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[29]. The COD analyses were performed spectrophotometrically (Perkin Elmer Lambda 35 UV/VIS spectrophotometer, USA) using closed reflux colorimetric method. TOC was obtained by sample combustion using a non-dispersive IR source (Tekmar Dohrmann Apollo 9000) at 680 oC. The turbidity of wastewater (in NTU units) was determined by Mettler Toledo 8300 model turbidimeter, whereas total suspended solids (TSS) were determined through vacuum filtration on a Whatman model GF-C filter (1mm) and subsequently drying at 105 oC to constant weight. The pH and conductivity were measured using Mettler Toledo SevenGo duo portable
5
meter. Deionised water was used in preparation of all the reagents and all chemical used were of analytical grade. The experimental runs were performed in duplicate, and the analysis of each parameter was done in triplicate for each run.
3. Results and Discussion 3.1. Effect of current density The amount of electrochemically generated coagulant (Al3+ or Fe2+) in EC process is widely influenced by applied current density. In this work, current density was varied from 30-90 A/m2 at a flow rate Q = 0.10 L/min. The influence of these variations on COD, TOC and turbidity
min operation at different current densities. 100
100
90
70 60 50 40
2
CD (A/m ) 30 50 70 90
30 20 10
80 70 60
-p
TOC Removal Efficiency (%)
80
50 40
2
CD (A/m ) 30 50 70 90
30 20
re
COD Removal Efficiency (%)
Al electrode
Al electrode
90
ro of
removal from the MWW by EC was investigated. Figs. 2 and 3 show the data obtained for 50
10
0
0 0
5
10
15
20
25
30
35
40
45
50
55
0
5
10
15
20
25
30
35
40
45
50
55
Operating Time (min)
lP
Operating Time (min)
Fig 2 Effect of current density on COD and TOC removals from MWW by CEP process using
100 Fe electrode
80 70
ur
60 50 40 30 20
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COD Removal Efficiency (%)
90
2
CD (A/m ) 30 50 70 90
10
5
10
15
Fe electrode
90 80 70 60 50 40
2
CD (A/m ) 30 50 70 90
30 20 10
0
0
100
TOC Removal Efficiency (%)
na
Al electrode (Q = 0.10 L/min)
0 20
25
30
35
Operating Time (min)
40
45
50
55
0
5
10
15
20
25
30
35
40
45
50
55
Operating Time (min)
Fig. 3 Effect of current density on COD and TOC removals from MWW by CEP process using Fe electrode (Q = 0.10 L/min)
6
At applied current densities of 30, 50, 70, and 90 A/m2, maximum COD removal efficiencies were obtained as 75.1%, 82.3%, 88.5% and 89.5%, respectively for Fe electrode. Also, maximum TOC removal efficiencies at these current densities were obtained as 72.2%, 79.2%, 82.73% and 89.5% respectively (Fig. 2). Similarly, at 30, 50, 70, and 90 A/m2, the respective maximum COD removals with Al electrode were 68.1%, 79.0%, 83.1% and 87.0%. On the other hand, TOC removals were 55.2%, 66.9%, 74,2% and 77.7%, respectively (Fig. 3). As can be seen in Figs. 2 and 3, COD and TOC removal efficiency for both electrodes increased with current density. This may be explained by high dissolution rate of anodes at high current densities, which in turn increases coagulant production rates (Eqs. 1-7), and the amount of metal
ro of
hydroxides flocs; Fe(OH)3(s) or Al(OH)3(s) available to form precipitate ions and complexes in solution. These generated metallic hydroxides in the EC process are reported to be good adsorbents for emulsified and colloidal dispersed oils [24]. Morever, the rate of bubble generation increased with increase in current density, leading to faster upward flow of colloidal molecules and Al or Fe hydroxide flocs with the sorbed organic compounds. In this case, oily
-p
and colloidal pollutants in the MWW were flocculated by the generated H2 gas bubbles at the cathode, hence separating pollutant molecules from aqueous phase. As a result, at about 30-40
re
minutes of treatment (as seen in Figs. 2 and 3), when the repulsive potential energy between charged particles from anodic dissolution of Al or Fe electrodes (Al3+ or Fe2+) is neutralised in
lP
the EC reactor, destabilisation of organic compounds in the MWW occurs. At high current densities and longer EC time (especially, 90 A/m2 and 30-40 min) for both electrodes, addition of the electrochemically dissolved Al3+ or Fe2+ ions to the solution becomes ineffective as the
na
removals of COD and TOC (>%85) approach their maximum level. Figs. 2 and 3 also indicates that the kinetic regime for COD removal with EC process shifted from mass transfer to kinetically-limited at different current density and EC time. Moreover, at the beginning of the
ur
EC process, the concentration of pollutants was more abundant; hence the rate of pollutant removal was high followed by a gradual decrease. On the other hand, the reduced concentration
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of TOC and COD at the end of the process resulted in a slower reaction rate (above 35-40 min). Similar findings have also been reported in different wastewater treatment by EC procces [3,26,30]. If the treated water is to be discharged to receiving environments such as rivers and lakes, effluent turbidity should also be taken into consideration. High levels of turbidity may obstruct light from reaching lower water depths, hence affecting aquatic life. The turbidity removal efficiencies for Al electrode at 30, 50, 70, and 90 A/m2 were 85.1% (2302 NTU), 99.3% (107 NTU), 99.4% (92 NTU) and 99.6% (61 NTU), respectively. Similarly, the turbidity removal efficiencies at these current density conditions for Fe electrode were determined as 7
88.0% (1842 NTU), 99.6% (61 NTU), 99.4% (92 NTU) and 99.9% (15 NTU), respectively. As exhibited by the COD and TOC removal trends, the turbidity removal efficiencies also
2,5 15 2,0 3
Wsludge (kg/m ) 3
1,5
ELC (kg/m ) 3 ENC (kWh/m )
10
1,0 5 0,5 0,0 20
30
40
50
60
70
80
90
0 100
2,5
8
Al electrode
2,0 6 1,5 4 3
Wsludge (kg/m )
1,0
3
ELC (kg/m ) 3 ENC (kWh/m )
0,5
2
0,0 20
30
40
50
60
70
80
90
0 100
Energy Consumption (ENC, kWh/kg COD)
3
Energy Consumption (ENC, kWh/m )
20
Al electrode
3
Sludge and Electrode Consumptions (kg/m )
3,0
Sludge and Electrode Consumptions (kg/kg COD)
increased with increased in applies current density.
2
2
ro of
Current Density (CD, A/m )
Current Density (CD, A/m )
Fig. 4 Generated sludge amount, electrode and energy consumptions at different current densities in the CEP process for Al electrode
10 3
2,0
Wsludge (kg/m )
8
3
ELC (kg/m ) 3 ENC (kWh/m )
1,5 1,0
6 4
0,5 0,0 20
30
40
50
60
70
80 2
Current Density (CD, A/m )
90
18 16
3
14 12
2 100
2
1
10 8
Wsludge (kg/kg COD)
6
ELC (kg/kg COD) ENC (kWh/kg COD)
4 2
0 20
30
40
50
60
70
80
90
Energy consumption (kWh/kg COD)
12
2,5
20
Fe electrode
-p
3,0
4
re
14
3
3,5
Energy Consumption (kWh/m )
16
Sludge and electrode consumptions (kg/kg COD)
18 Fe electrode
4,0
lP
3
Sludge and Electrode Consumptions (kg/m )
4,5
0 100
2
Current Density (CD, A/m )
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Fig. 5 Generated sludge amount, electrode and energy consumptions at different current densities in the CEP process for Fe electrode
ur
The energy consumption costs encompassing pumping, continuous EC reactor, (ENC, kWh/m3 or kWh/removed kg COD) and electrode consumptions (ELC, kg/m3 or kg/removed kg COD)
Jo
for both electrodes were calculated based on COD removed using Eqs. 8-12. Most national and international environmental regulatory authorities consider COD as the most important parameter for discharge of treated industrial wastewater. In this regards, we have also expressed costs in terms of kg COD removed per m3 treated wastewater. Figs. 4 and 5 show the generated sludge amounts, electrode and energy consumptions for the range of current densities investigated in this study.
8
𝐸𝑁𝐶 (𝑘𝑊ℎ⁄𝑚3 ) =
𝑈×𝑖×𝑡𝐸𝐶 𝑣
𝐶𝑝𝑢𝑚𝑝 (𝑘𝑊ℎ⁄𝑚3 ) =
(8)
𝜌×𝑔×𝑄×ℎ×𝑡𝐸𝐶
(9)
1000×𝑣
𝐸𝑁𝐶 (𝑘𝑊ℎ⁄𝑘𝑔 𝐶𝑂𝐷) = 𝐸𝐿𝐶 (𝑘𝑔⁄𝑚3 ) =
+ 𝐶𝑝𝑢𝑚𝑝
𝑈×𝑖×𝑡𝐸𝐶 𝑘𝑔 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝐶𝑂𝐷
+ 𝐶𝑝𝑢𝑚𝑝
(10)
𝑖×𝑡𝐸𝐶 ×𝑀𝑤
(11)
𝑧×𝐹×𝑣
𝐸𝐿𝐶 (𝑘𝑔⁄𝑘𝑔 𝐶𝑂𝐷) =
𝑖×𝑡𝐸𝐶 ×𝑀𝑤
(12)
𝑧×𝐹×𝑘𝑔 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝐶𝑂𝐷
where U represents cell voltage (V), i represents current (A), tEC is the EC time (sec for ELC or
ro of
hour for ENC) and v represents volume (m3) of the solution, Mw represents molecular mass (Fe: 55.85 g/mol and Al: 26.98 g/mol), z represents the number of electron transferred (z for Fe = 2 and z for Al = 3) and the Faraday’s constant (F) is (96487 C/mol e-).
-p
The faradic yield or current efficiency (Al or Fe = mexp/mth) for an electrode (Al or Fe) was computed as the ratio of the experimental weight loss of the anodes (mexp) to the theretically
re
consumed amount of the anode (mth) [6,31]. The current efficiency at 30, 50, 70 and 90 A/m2 was calculated as 120%, 132%, 124% and 115% for Al electrodes and 109%, 110%, 106% and
lP
112% , respectively for Fe electrodes. The experimental electrode consumptions (ELC) for 30, 50, 70 and 90 A/m2 at 50 min was 0.189 kg/m3 (0.0342 kg/kg COD), 0.348 kg/m3 (0.0838 kg/kg COD), 0.458 kg/m3 (0.1469 kg/kg COD) and 0.546 kg/m3 (0.243 kg/kg COD), respectively for
na
Al electrode. Meanwhile, for Fe electrode the respective ELC was 0.357 kg/m3 (0.0828 kg/kg COD), 0.601 kg/m3 (0.1959 kg/kg COD), 0.810 kg/m3 (0.4068 kg/kg COD) and 1.100 kg/m3 (1.222 kg/kg COD). Energy consumption (ENC) at 30-90 A/m2 for Al electrode ranged from
ur
1.98-17.11 kWh/m3 (0.35-7.60 kWh//kg COD). For Fe electrode, ENC was calculated as 2.3117.67 kWh/m3 (0.53-19.63 kWh/kg COD) for the same range of current density. Moreover, the
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amount of sludge produced during the CEP process at 30-90 A/m2 ranged from 2.30 to 2.93 kg/m3 and 2.74 to 4.12 kg/m3 for Al and Fe electrodes respectively. Sludge production in EC process is affected by wastewater characteristics such as number of settable solids and presences of destabilized matter due to coagulation and flocculation [18]. We noted that EC time and applied current density were the major factors influencing the quantity of electrogenerated sludge, for both electrodes. This can be explained by the increase of dissolved metal (Fe or Al) anodes with time and applied current (Faraday’s law).
9
3.2. Effect of flow rate The treatment efficiency of a CEP process is also affected by flow rate (Q) to the reactor. Lowering the flow rate results into more hydraulic retention time in the EC reactor, hence restabilising the generated metal hydroxides flocs and enhancing their removal mechanisms (oxidation, precipitation and adsorption). Mixing is also required during the EC process to promote growth and precipitation of flocs. The influence of flow rate on removals of COD and TOC for metalworking wastewater was analysed for variations of flow rate from 0.01-0.20 min/L ( τ of 350-17.5 min) at 90 A/m2. 100
Al electrode
90
80
60 50 40 Q (L/min) 0.01 0.05 0.10 0.20
20 10
ro of
70
30
80
TOC Removal Efficiency (%)
COD Removal Efficiency (%)
100
Al electrode
90
70 60 50 40
Q (L/min) 0.01 0.05 0.10 0.20
30 20 10
-p
0
0
0
5
10
15
20
25
30
35
40
45
50
55
0
Operating Time (min)
5
10
15
20
25
30
35
40
45
50
55
Operating Time (min)
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Fig 6 Effect of flow rate on COD and TOC removals from MWW by Al electrodes (CD = 90
100 Fe electrode
80 70
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60 50 40 30 20 10 0 0
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COD Removal Efficiency (%)
90
5
10
15
20
25
30
35
Fe electrode 90 80 70 60 50 40 Q (L/min) 0.01 0.05 0.10 0.20
30 20 10 0
40
45
50
55
0
5
10
15
20
25
30
35
40
45
50
55
Operating Time (min)
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Operating Time (min)
Q (L/min) 0.01 0.05 0.10 0.20
100
TOC Removal Efficiency (%)
lP
A/m2)
Fig. 7 Effect of flow rate on COD and TOC removals from MWW by Fe electrodes (CD = 90 A/m2)
Figs. 6 and 7 show the effect of flow rate on COD and TOC removal from MWW by CEP. For Al electrode, increase in flow rate from 0.010 to 0.20 L/min, resulted into decrease in COD and TOC removal efficiencies from 92.6% (1281 mg/L) to 71.3% (4968 mg/L) and 85.8% (448 mg/L) to 64.9% (1107 mg/L), respectively. Similarly, COD and TOC removal efficiencies 10
decreased from 97.8% (380 mg/L) to 78.6% (3704 mg/L) and 94.9% (160 mg/L) to 69.9 % (949 mg/L), respectively, for the same variations in flow rate with Fe electrode. Under the prescribed flow rates, both electrodes obtained > 77% COD and TOC removals except at 0.20 L/min. Low flow rates favoured mixing of the electrochemically generated coagulants and the pollutants in the wastewater, hence improving the rate of pollutant removal. The turbidity removal efficiencies at 50 min of CEP process with Al electrode were 99.9 % (15 NTU), 99.9 % (15 NTU), 99.6 % (61 NTU) and 88.9% (1703 NTU) at flow rates of 0.010, 0.050, 0.10 and 0.20 min/L, respectively. Whereas with Fe electrode, turbidity removals were recorded as
Wsludge (kg/m )
1,5
ELC (kg/m ) 3 ENC (kWh/m )
16
3
1,0
15
0,5 0,0 0,00
0,05
0,10
0,15
0,20
14 0,25
Flow Rate (Q, L/min)
11
2,0
10 9
1,5
1,0
8 7
Wsludge (kg/kg COD)
6
ELC (kg/kg COD) ENC (kWh/kg COD)
5
0,5
0,0 0,00
0,05
0,10
4 0,15
0,20
Energy Consumption (kWh/kg COD)
3
2,0
ro of
17
2,5
12
Al electrode
re
3
Energy Consumption (kWh/m )
3,0
2,5
-p
18 Al electrode
3
Sludge and Electrode Consumptions (kg/m )
3,5
Sludge and Electrode Consumptions (kg/kg COD)
99.9 % (15 NTU), except at 0.20 min/L (94.7%).
3 0,25
Flow Rate (Q, L/min)
Al electrodes
3,0
3
Wsludge (kg/m )
1,5 1,0
3
ELC (kg/m ) 3 ENC (kWh/m )
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0,5
ur
2,5
0,0 0,00
0,05
0,10
0,15
Flow Rate (Q, L/min)
17,5 17,0 16,5 16,0 15,5 15,0
0,20
14,5 0,25
Sludge and Electrode Consumptions (kg/kg COD)
3,5
18,0
3
4,0
18,5
Energy Consumption (kWh/m )
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4,5
2,0
19,0
Fe electrode
3
Sludge and Electrode Consumptions (kg/m )
5,0
3,5
45 Fe electrode 40
3,0
35 2,5 2,0
Wsludge (kg/kg COD)
30
ELC (kg/kg COD) ENC (kWh/kg COD)
25 20
1,5
15 1,0 10 0,5 0,0 0,00
5 0,05
0,10
0,15
0,20
Energy Consumption (kWh/kg COD)
lP
Fig. 8 Generated sludge amount, electrode and energy consumptions at different flow rates for
0 0,25
Flow Rate (Q, L/min)
Fig. 9 Generated sludge amount, electrode and energy consumptions at different flow rates for Fe electrodes The calculated current efficiencies ( Φ Al or Fe) at flow rates of 0.010, 0.050, 0.10 and 0.20 L/min for Al electrode were 108%, 111%, 115% and 125%, respectively. Under the same flow rates, 11
the respective current efficiencies for Fe electrode were 120%, 113%, 112% and 122%. The generated sludge, energy and electrode consumptions for Al and Fe electrodes at various flow rates are presented in Figs. 8 and 9. At 0.010-0.20 L/ min, electrode and energy consumptions ranged from 0.512-0.593 kg/m3 (or 0.400-0.119 kg/kg COD) and 14.57-17.32 kWh/m3 (or 11.37-3.49 kWh/ kg COD) for Al electrode, and 1.18-1.20 kg/m3 (or 3.09-0.32 kg/kg COD) and 14.57-18.81 kWh/m3 (or 38.25-5.08 kWh/kg COD), for Fe electrode (Fig. 9). Electrogenerated sludge in the CEP process ranged from 3.22-2.45 kg/m3 for Al electrode and 4.68-2.98 kg/m3 for Fe electrode for change in flow rates from 0.010-0.20 L/min (Figs. 8 and 9). Generally, increasing flow rate resulted into a decrease in amount of produced sludge,
ro of
energy and electrode consumptions.
In all waste water treatment processes, the effluent parameters must be evaluated in accordance with the accepted environmental discharge regulation. The industrial effluent discharge standards at Gebze industrial zone are regulated by the Turkish Water Pollution Control
-p
Regulation. The sewage discharge standard for metal working effluents is < 1000 mg/L COD. In the present study, over 94.2% COD removal was obtained by Fe electrode (i.e., from 17312
re
mg/L to < 1000 mg/L), at flow rate < 0.050 L/min ( τ = 70 min), applied current density of 90 A/m2 and EC time of 40 min. The final effluent concentrations for COD, TOC and turbidity
lP
with Fe electrode at these optimum conditions were 987 mg/L, 312 mg/L and 107 (NTU), respectively.
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3.3. Evaluation of operating cost
The costs entailed in electrochemical treatment process are related to electrical energy, peristatic pump, sacrificial electrodes, sludge handling costs, maintenance and labour. The
ur
computation of operating cost (in terms of $/m3 of wastewater treated and $/kg COD removed)
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in this work considered energy consumptions and electrode consumptions as the main costs.
𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝐶𝑜𝑠𝑡 (𝑂𝐶) = 𝑎 × 𝐸𝑁𝐶 + 𝑏 × 𝐸𝐿𝐶 + 𝑐 × 𝑊𝑠𝑙𝑢𝑑𝑔𝑒 + 𝑑 × 𝐶𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙
(13)
Where; ENC (energy consumption; kWh/m3 and kWh/kg COD) and ELC (electrode consumption; kg Fe/m3 and kg Fe/kg COD) are experimentally obtained values from the treatment process. The parameter “a” represents the electrical energy cost and “b” represents cost of electrode material. As of May 2018, in Turkey, the electrical energy cost was 0.12 US $/kWh, and cost of electrode materials were 0.952 US $/kg Fe and 4.10 US $/kg Al. “c” landfill 12
disposal cost of sludge 0.152 US $/kg (sludge moisture of ~20%), “d” for chemical consumptions (CC) such as NaOH and H2SO4 (for adjustment of desired pH) with assigned prices of respectively, 1.01 US $/kg and 0.40 US $/kg.
Operating costs were calculated according to energy consumptions as indicated in Eqs.8-10, experimental electrode consumptions as indicated in Eqs. 11-12, sludge handling operating and use of chemical. 4,9
2,25
1,50
3,0
1,25
2,5
1,00
2,0
0,75 3
1,5
20
30
40
50
60
70
80
90
4,5
2,0 1,5
4,4
1,0
4,3 0,00
0,25 100
2,5
3
$/m $/kg COD
4,6
0,50
$/m $/kg COD
1,0
3,0 4,7
Operating Cost ($/kg COD)
3,5
3
1,75
Operating Cost ($/m )
4,0
3,5
4,8
Operating Cost ($/kg COD)
2,00
3
Operating Cost ($/m )
4,5
4,0 Al electrode
Al electrode
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5,0
0,05
2
0,10
0,15
0,5 0,25
0,20
Flow Rate (Q, L/min)
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Current Density (CD, A/m )
4,0
3,90
Fe electrode
7 6
3,80
2,5
2,0
2,0
1,5
1,5
1,0
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3
1,0
$/m $/kg COD
0,5 20
30
40
50
60
70
80
2
0,0 100
5
3,75 4
3,70 3
3,65
2 3
3,60 3,55 0,00
$/m $/kg COD 0,05
0,10
1
0,15
0,20
0 0,25
Flow Rate (Q, L/min)
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Current Density (CD, A/m )
90
0,5
3
2,5
Operating Cost ($/m )
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Operating Cost ($/kg COD)
3,85
3,0
3,0
3
Operating Cost ($/m )
8
Fe electrode
3,5
3,5
Operating Cost ($/kg COD)
4,0
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Fig.10 Effect of current density and flow rates on the operating cost for Al electrode
Fig.11 Effect of current density and flow rates on the operating cost for Fe electrode
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The operating cost with Al and Fe elecrodes at different current densities and flow rates are shown in Fig. 10 and Fig. 11. At 30-90 A/m2 with Al electrode, operating costs were calculated as 1.37-4.74 $/m3 or 0.399-2.140 $/kg removed COD. In addition, for 0.010-0.20 min/L ( τ of 350-17.5 min) flow rate, operating costs with Al electrode were 4.34-4.88 $/m3 or 3.34-1.11 $/kg removed COD (Fig. 10). While for Fe electodes at 30-90 A/m2, the operating costs were 1.03-3.80 $/m3 or 0.43-3.97 $/kg removed COD. For 0.010-0.20 min/L flow rate, operating costs with Fe electrode were 3.58-3.85 $/m3 or 7.73-1.41 $/kg COD removed (Fig. 11). Both 13
electrodes showed a tremedously increase in operating cost for applied current density increase from 30 to 90 A/m2. In general, operating costs for Fe electrode were lower than those for Al electrode. Moreover, Fe electrode presented better COD, TOC and turbidity removal efficiencies, making it more suitable for treatment of metal working wastewaters.
4. Conclusions Continuous EC process was used to treat wastewater from a metal working factory. Operating parameters like applied current density, flow rate and EC operating time were analysed to determine their influence on treatment of MWW by a bench-scale continuous flow EC reactor.
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Optimum conditions to obtain maximum TOC, COD and turbidity removals in the EC process were determined. In addition, evaluation of economic aspects of the treatment process was also done. For Al electrode at optimums of EC time of 50 min, flow rate of 0.010 L/min ( τ of 350 min), and current density of 90 A/m2, the obtained removals of COD, TOC and turbidity were
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92.6% (1281 mg/L), 85.8% (448 mg/L) and 99.9% (16 NTU), respectively, and the calculated operating costs were 4.34 $/m3 (3.33 $/ kg COD). On other hand, optimum operating conditions
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for Fe electrode were determined as EC time of 40 min, flow rate of 0.05 L/min ( τ of 70 min) and applied current density of 90 A/m2. Removal efficiencies of COD, TOC and turbidity at
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these optimums were 94.3% (987 mg/L), 90.1% (312 mg/L) and 99.3% (107 NTU), respectively, with operating costs of 3.09 $/m3 (2.63 $/kg COD). Consequently, the continuous EC process results with Fe electrodes were most suitable with respect to the legal sewage COD
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discharge limit of 1000 mg/L for metal working wastewater. Also, the costs entailed in EC treatment in terms of $/m3 treated wastewater or $/kg removed COD were lesser for Fe electrode than for Al electrode. Increase in current density and flow rate resulted into increase in cost of
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EC treatment.
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Acknowledgements
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