Desalination 285 (2012) 91–99

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Desalination

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Treatment of paint manufacturing wastewater by electrocoagulation

Abdurrahman Akyol ⁎Gebze Institute of Technology, Department of Environmental Engineering, Cayırova, 41400 Gebze, Turkey

⁎ Tel.: +90 262 6053290; fax: +90 262 6053200.E-mail address: abdakyol@gyte.edu.tr.

0011-9164/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.desal.2011.09.039

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 July 2011Received in revised form 13 September 2011Accepted 16 September 2011Available online 22 October 2011

Keywords:ElectrocoagulationPaint manufacturing wastewaterOperating costToxicity

Treatability of paint manufacturing wastewater (PMW) by electrocoagulation (EC) process was investigated. Ef-fects of operating parameters for the EC process such as electrode type (Al or Fe), initial pH (2–10), current den-sity (5–80 A/m2) and operating time (0–50 min)were evaluated for optimum operating conditions. The highestremoval efficiencies for COD and TOC in PMW were obtained with 93% and 88% for Fe and 94% and 89% for Alelectrodes at the optimum conditions (35 A/m2, 15 min and pH 6.95). Operating costs for removal of PMW atthe optimum conditions were calculated for Fe and Al electrodes as 0.187€/m3 and 0.129€/m3. Toxicity testwas carried out to obtain information about toxic effect of the raw and treated wastewaters at optimum operat-ing conditions. The samples measured by respirometric method contained hardly toxicities. Performance of Alelectrode was better than that of Fe electrode in terms of removal efficiency and operating cost.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Effluents of paint manufacturing company (PMW) contain highlytoxic compounds and organic biorefractory compounds such as COD,BOD and TOC. It harms fish, wildlife, and contaminates the food chain ifpoured down a storm drain. Paint wastewaters have also adverse effectson human health occupants. If used in closed areas, its chemical compo-nents can irritate eyes, skin and lungs and causes headaches and nausea.It can also contribute to respiratory problems;muscleweakness, liver andkidney damage [1]. The PMW must be needed to discharge after treat-ment due to legal restrictions in organized industrial zone and environ-ment conservation.

There are many different techniques to remove COD, TOC and colorfrom industrial wastewater such as biodegradation, adsorption, mem-branefiltration, coagulation–flocculation, advanced oxidation processessuch as ozone, photochemical, Fenton's, electrochemical etc. [2–8].These technologies take considerable time; require an extensive set-up and they are not economically applicable for plants of producedsmall volumewastewater. Moreover, each step takes place in a separatetank and the entire treatment requires several pH adjustments as wellas the addition of acid, coagulants such as alum, ferric sulfate and chlo-ride, lime, caustic or polymeric flocculants. These conventional process-es generate a considerable quantity of secondary pollutants (chloride,sulfate in the coagulation–precipitation) and large volumes of sludgeor waste which pose serious environmental problems. The biologicalprocesses are also insufficient to treatment this kind wastewater dueto leads to the partial inhibition of biodegradation [9].

In recent years, new and novel processes for efficient and adequatetreatment of various industrial wastewaterswith relatively low operating

costs have been explored due to strict environmental regulations. Electro-coagulation (EC) process has been attracted a great attention for treat-ments of industrial wastewaters such as olive mill [10], chemicalmechanical polishing [11], poultry slaughterhouse [12], pulp and papermill [13], metal cutting [14] and textile wastewaters [15] because of theversatility and the environmental compatibility. The EC unit is environ-mentally friendly so that it does not create corrosion or any pollutants.This technique has some advantages when compared to conventionalmethods such as simple equipment, easy to operate, less retention time,reduction or absence of adding chemicals, rapid sedimentation of theelectrogenerated flocs and less sludge production [16,17].

In this study, effects of the operating parameters such as initial pH,operating time and current density using iron and aluminum elec-trodes were investigated for treatment of PMW. Operating costswere also calculated for the EC process.

2. EC mechanism removed pollutants

The EC process consists of in situ generatingMn+ ions (Fe2+, Al3+)in wastewater electrochemically dissolution of the iron (Fe) or alumi-num (Al) electrode materials (M) at the anode and simultaneous hy-drogen gas evolution occurs at the cathode according to Faraday's Law.

At the anode:

MðsÞ→MnþðaqÞ þ ne− ð1Þ

2H2O→4HþðaqÞ þ O2ðgÞ þ 4e ð2Þ

At the cathode:

nH2Oþne−→ðn=2ÞH2ðgÞþnOH−ðaqÞ ð3Þ

Fig. 1. A schematic diagram of experimental set-up.

92 A. Akyol / Desalination 285 (2012) 91–99

MnþðaqÞ þ ne−→MðsÞ ð4Þ

If iron or aluminum electrodes are used, the generated Fe2+, Fe3+

or Al3+ ions will immediately undergo further spontaneous reactionsto produce corresponding many monomeric and polymeric Al or Fespecies such as Al(OH)2+, Al(OH)2+, Al2(OH)24+, Al(OH)4−, Al6(OH)153+,Al7(OH)174+, Al8(OH)204+, Al13O4(OH)247+, Al13(OH)345+, Al(OH)3 andFe(OH)2+, Fe(OH)2+, Fe(OH)24+, Fe(OH)4−, Fe(H2O)2+, Fe(H2O)5OH2+,Fe(H2O)4(OH)2+, Fe(H2O)8(OH)24+, Fe2(H2O)6(OH)42+, Fe(OH)3 depend-ing on the pH of the aqueous medium. Mn+ and OH− ions generated byelectrode reactions (1) and (3) react to form various monomeric andpolymeric specieswhich transformfinally intoM(OH)3 according to com-plex precipitation kinetics. Freshly formed amorphous M(OH)3 (sweepflocs) have large surface areas which are beneficial for a rapid adsorptionof soluble organic compounds and trapping of colloidal particles [18,19].In this study, removal of PMW was achieved by coagulation, adsorptionand co-precipitation in the EC process.

3. Materials and methods

3.1. Wastewater source and characteristics

Wastewater was obtained from a paint manufacturing company inTurkey (Gebze) producing approximately 60 m3 of wastewater perday. The process wastewater contained a mixture of interior and ex-terior emulsions, and polyvinyl acetate. The wastewater was filteredusing a screen filter to remove large suspended solids before beingused for the subsequent studies. Characterization of the wastewaterwas shown in Table 1.

3.2. Experimental apparatus

The experimental setup was depicted in Fig. 1. Electrocoagulation re-actor was made of plexiglas with dimensions of 120 mm×110mm×110mm. Electrodes were connected to the EC reactor with monopolarparallel connection mode. Two anodes and two cathodes electrodeswere used in the EC process. Both Al (purity of 99.53%) and Fe (purityof 99.50%) electrodes in shape of rectangular plates had dimensions of45 mm×53mm×3mm. The total effective electrode area was 143 cm2

and the spacing between electrodes was 10 mm.

3.3. Experimental procedure

The EC runs were conducted at 25 °C. The impurities on the sur-faces of electrodes were removed by dipping in a solution for 1–2 min which was freshly prepared by mixing 0.1 L of HCl solution(35%) and 0.2 L of hexamethylenetetramine aqueous solution(2.80%) [20]. In each run, 0.80 L of PMW was placed into the EC reac-tor. The current density was adjusted to a desired value and the runwas started. At the end of the run, the solution was filtered, the

Table 1Characterizations of paint manufacturing wastewater.

Parameters Value (mg/L)

BOD5 2800COD 19,700SS 1100Cr+6 b0.01Total Cr 0.021Cd b0.02Pb 1.44Total Fe 4.82Zn b0.2Total cyanide 0.05Conductivity (mS/cm) 1.53pH 6.95

filtrate was centrifuged at 2000 rpm and the electrodes were washedthoroughly with water to remove any solid residues on the surfaces,dried and reweighted. The collected samples were filtered through0.45-μm cut-off Millipore membranes and analyzed for COD andTOC. The amount of the sludge produced for each parameter affectingof the EC process was determined after drying in an oven (Binder ED115) at 105 °C for 24 h (Tables 2–4).

Experiments were conducted in triplicate under identical condi-tions to confirm the results and the mean values are presented. Max-imum error was found to be 2%.

3.4. Analytical procedures

COD, TOC and pH determinations were carried out as proposed byStandard Methods [21]. COD was measured by closed reflux titrimet-ric method, TOC levels were determined through combustion of thesamples at 680 °C using a non-dispersive IR source (TekmarDohrmann Apollo 9000). The pH was adjusted to a desirable valueusing NaOH or H2SO4 and measured by a Mettler Toledo pH meter.

3.5. Toxicity testing

The activated sludge inhibition test is rarely being used as a tool forevaluation of the toxicity in the industrial wastewaters which is an im-portant criterion to evaluate ecotoxicology risk of the wastewater. Acti-vated sludge consumes oxygen rapidly in the presence of easilybiodegradable substrate (reference material) such as glucose, peptone.Oxygen consumption by activated sludge functions as live cell concentra-tion and specific respiration activity. The respiration rate of activatedsludge decreases when the wastewater contains toxicants or inhibitors.In this study, the respirometric measurements were applied withApplitek-RaCombo, BOI and Toxicity Meter at 20±2 °C. The respiromet-ric tests were started with the reach of endogenous respiration level ofactivated sludge by constant aerating. The reference material wasadded after the endogenous level (Re) was determined. Response of thesystem was monitored and maximum respiration rate (R0) was found.After the reference material was consumed completely by activatedsludge the respiration rate returned level of the endogenous respiration.Test sample (wastewater) which can be toxic probably, was then addedin a vessel. After the respiration rate returns level of the endogenous res-piration again, the same amount of reference material was added for thesecond time andmaximum respiration rate (Rt) was found. Reduction ofrespiration rate shows that microorganisms are affected by the test sam-ple. The toxicity was calculated from the following Eq. (5);

Toxicity % ¼ R0–Rtð Þ= R0−Reð Þx100½ � ð5Þ

where Ro ismaximum respiration rate at first adding of the referencema-terial (mg O2/L.h), Rt is maximum respiration rate at second time addingof the reference material (mg O2/L.h) and Re is endogenous respiration

Table 2Experimental results of PMW for Fe and Al electrodes at different pH in the EC process.

pH Fe electrode Al electrode

Removalefficiency (%)

pHf ELC(kg/m3)

Ws(kg/m3)

OC(€/m3)

Removalefficiency (%)

pHf ELC(kg/m3)

Ws(kg/m3)

OC(€/m3)

COD TOC COD TOC

2 35 35 4.8 0.759 4.79 0.690 53 70.5 2.5 0.091 5.02 0.1523 66 64.5 6.7 0.521 10.15 0.498 94 82.0 4.5 0.094 9.03 0.1564 91 89 5.9 0.503 11.43 0.498 94.5 91.5 4.8 0.061 14.02 0.1025 91 88.5 6.5 0.518 11.70 0.505 95 91 5.5 0.088 15.00 0.1456 93 89 6.3 0.403 12.53 0.422 94 90.5 7,1 0.058 16.23 0.0967 93.5 88.5 8.9 0.239 9.89 0.363 94.5 91 7.6 0.090 8.16 0.1508 89 89.0 9.2 0.183 15.14 0.285 93.5 90.5 7.9 0.055 9.14 0.09210 71 75.0 11.7 0.358 14.47 0.405 82 80 8.7 0.074 9.51 0.123

Table 3Experimental results of PMW for Fe and Al electrodes at different current densities in the EC process.

Current density(A/m2)

Fe electrode Al electrode

Removalefficiency (%)

pHf ELC(kg/m3)

Ws(kg/m3)

OC(€/m3)

Removalefficiency (%)

pHf ELC(kg/m3)

Ws(kg/m3)

OC(€/m3)

COD TOC COD TOC

5 21 9 7.18 0.034 1.66 0.032 34 25.5 6.87 0.011 4.57 0.02010 39.5 27.5 7.21 0.129 2.18 0.117 66 42 7.41 0.024 5.33 0.03820 90 83.5 7.63 0.141 8.84 0.144 91 85.5 7.49 0.048 6.27 0.07930 91 85 7.46 0.165 9.32 0.184 92.5 86 7.57 0.074 8.54 0.12335 93 87.5 7.54 0.145 9.63 0.187 94 89 6.77 0.078 7.73 0.12950 93.5 88.5 8.88 0.239 9.89 0.363 94.5 91 7.6 0.090 8.16 0.15065 94 90 7.8 0.621 9.91 0.737 95.5 91.5 7.1 0.153 16.15 0.25380 94 90 8.72 0.666 10.22 0.878 96 91.5 7.24 0.169 16.45 0.280

93A. Akyol / Desalination 285 (2012) 91–99

rate (mg O2/L.h), respectively. The toxicity tests were repeated threetimes and average values are considered.

3.6. Operating cost

Operating cost (OC) is very important economical parameters inthe EC process. The OC includes material cost (mainly electrodes),utility cost (mainly electrical energy), as well as labor, maintenanceand other fixed costs. In this study, energy, electrode material andchemicals costs were taken into account as major cost items in thecalculation of the OC as €/m3 for treatment of the PMW [22,23]:

Operating Cost OCð Þ ¼ aENC þ bELC þ cCC ð6Þ

where ELC, ENC and CC are consumption quantities of electrode(kg/m3), energy (kWh/m3) and chemicals (kg/m3) of wastewatertreated. Costs for ELC (kg/m3) in Eq. (9) and ENC (kWh/m3) inEq. (10) were calculated. a, b and c given for Turkish market in May2011 were electrical energy price (0.072€/kWh), electrode materialprice (1.65€/kg Al) and chemical costs (CC, 0.73€/kg for NaOH,0.29€/kg for H2SO4 and 0.15€/kg for NaCl), respectively.

Table 4Experimental results of PMW for Fe and Al electrodes at different operating times in the EC

Operating time(min)

Fe electrode

Removalefficiency (%)

pHf ELC(kg/m3)

Ws(kg/m3)

O(€

COD TOC

5 76.5 70 7.93 0.156 6.56 0.10 91.5 86.5 8.35 0.173 8.05 0.15 93 87.5 7.54 0.145 9.63 0.30 94 90 9.15 0.483 14.67 0.45 94 90 9.77 0.808 15.92 1.

4. Results and discussion

4.1. Effect of initial pH

Initial pH is an important operating factor influencing performance ofthe EC process. The influence of initial pH on COD and TOC removal effi-ciency was studied between initial pHi 2–10 at current density of50 A/m2 and operating time of 15 min for Fe and Al electrodes (Figs. 2and 3). As seen clearly in Figs. 2 and 3, CODand TOC removal efficiencieswere increased from 35% to 91% and from 35% to 89% at pHi 2–4 for Feelectrode, and from 53% to 94% at pHi 2–3 and from 71% to 92% at pHi

2–4 for Al electrode. Moreover, there wasn't much changes observed inremoval efficiencies for COD at pHi 4–7 and TOC at pHi 4–8 for Fe elec-trode and at pHi 3–8 for COD and at pHi 3–8 for TOC for Al electrode, re-spectively. There was also a decrease in removal efficiencies for CODand TOC at pHi 10whichwas 71% for Fe and 82% for Al electrodes. The op-timum COD and TOC removal efficiencies for both electrodes at pHi 2–10were obtained as 94% and 89% for Fe and 95% and 92% for Al electrodes.The rest of EC experiments were carried out at pH 6.95 which was thepH of PMW. Therefore, pH of PMW didn't need to adjust since this pHfell into optimum pH range for both electrodes which makes the EC pro-cess economically. Final effluent pHf for Fe andAl electrodeswasobserved

process.

Al electrode

C/m3)

Removalefficiency (%)

pHf ELC(kg/m3)

Ws(kg/m3)

OC(€/m3)

COD TOC

146 13 16 6.86 0.035 5.17 0.059167 82 85.5 7.09 0.046 5.88 0.077187 94 89 6.77 0.078 7.73 0.129719 94.5 91.5 7.75 0.154 8.38 0.255164 95 91 8.09 0.251 8.78 0.416

2 4 6 8 1030

40

50

60

70

80

90

100C

OD

Rem

oval

Effi

cien

cy (

%)

Initial pH

Fe electrode

Al electrode

Fig. 2. Effect of initial pH on the COD removal efficiencies of the EC process.

0 10 20 30 40 50 60 70 80 900

20

40

60

80

100

CO

D R

emov

al E

ffici

ency

(%

)

Current Density (A/m2)

Fe Electrode

Al Electrode

Fig. 4. Effect of current density on the COD removal efficiencies of the EC process.

0 10 20 30 40 50 60 70 80 900

20

40

60

80

100

TO

C R

emov

al E

ffici

ency

(%

)

Current Density (A/m2)

Fe Electrode

Al Electrode

Fig. 5. Effect of current density on the TOC removal efficiencies of the EC process.

94 A. Akyol / Desalination 285 (2012) 91–99

as 8.9 and 7.6 at 15 min, respectively. On the other hand,final effluent pHf

for Fe and Al electrodes fell into limits set by discharge standards of waterpollution control regulations (pHf 6–9). Therefore, there was no second-ary treatment required for treated PMW by the EC process. COD andTOC removal efficiencies decreased in lower acidic and higher basic pHvalues since hydroxide ionswere oxidized at the anode as pHwas greaterthan 8. In addition, Fe(OH)6

3− and Fe(OH)4− ions may be present at highpH, which lacks a removing capacity [24]. At lower pH the protons inthe solution were reduced to H2 at the cathode and the same proportionof hydroxide ions couldn't be produced [24]. However, COD and TOC re-moval efficiencies decreased with aluminum electrode that could beexplained by present of some chemical species according to the solutionpH [25]. It was known that if solution pH was below 3.5, the aluminumions and Al(OH)2+ were predominant, and Al(OH)3(s) was formed inpH range 4–9whichwas responsible to the formation of electrocoagulantflocs. Finally, when pHwas higher than 9, themonomeric anion, Al(OH)4−

was formed (Eqs. (7) and (8)) and didn't exhibit any positive effect on theEC process performance [26].

2Al þ 6H2O þ 2OH−→2AlðOHÞ−4 þ 3H2 ð7Þ

AlðOHÞ3 þ OH−→AlðOHÞ−4 ð8Þ

1 2 3 4 5 6 7 8 9 10 1130

40

50

60

70

80

90

100

TO

C R

emov

al E

ffici

ency

(%

)

Initial pH

Fe electrode

Al electrode

Fig. 3. Effect of initial pH on the TOC removal efficiencies of the EC process.

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100

CO

D R

emov

al E

ffici

ency

(%

)

Operating Time (min)

Fe Electrode

Al Electrode

Fig. 6. Effect of operating time on the COD removal efficiencies of the EC process.

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100T

OC

Rem

oval

Effi

cien

cy (

%)

Operating Time (min)

Fe Electrode

Al Electrode

Fig. 7. Effect of operating time on the TOC removal efficiencies of the EC process.

95A. Akyol / Desalination 285 (2012) 91–99

The electrode consumptions were calculated for pH 2–10 as 0.183–0.759 kg Fe/m3 for Fe and 0.055–0.094 kg Al/m3 for Al electrodes re-spectively. Values of electrode consumptions for Fe electrode were2.67 times higher than that of Al electrode at pH 6.95 (Table 2). In addi-tion, difference between consumptions of electrodes was 8.34 timeshigher at pH 2.0. In especially acidic pH, while removal performancefor Fe electrode were lower than that of Al electrode, electrode con-sumptions for Fe electrodewere higher that of Al electrode.OCswas cal-culated for Fe and Al electrodes were shown in Table 2. OC forFe electrode was 2.4 times more expensive than that of Al electrode atoptimum operating conditions. Difference in these costs was relatedto high amount of electrode consumptionswhen Fe electrodewas used.

Table 5Comparison of the conventional processes with EC process for PMW.

Treatment method Process conditions and variable Result

Fenton process Column d, 11×1,5–5×5 cm; packingmaterial size (mix, 0.3–1.2); H2O2 dose,1100–3320 mg/l; Fe, 15–105 mg/l;time (125 h); pH (1.4–3.0)

In optpH 1.4COD-R

Coagulation and microfiltration pH (7.0–7.5) COD RAlum (opt. dose; 700 mg/l.) PermePolyelectrolyte (opt. dose; 6 mg/l.) Reduc

Electrochemical oxidation Residence time; temperature (°C);Current density (5–80 A/m2);time (0–50 min)

Opt. rCOD RRE, 87

Coagulation–flocculation Coagulant type (Alum-FeSO4) CoaguCoagulant dosage COD R

Fenton oxidation H2O2 amount FentoCOD RWaste

Membrane process Membrane filtration UltrafiCoagulation Coagulant type (Alum and FeCl3) COD R

Latex-based production (High COD) COD RPVA-based paint wastewater Lime

Coagulation Coagulation agents: ferrous and aluminumsulfate and polyaluminum chloride (PACl)

COD R

Coagulant dosageCOD R

Effective pHCOD R

Electrochemical coagulation Operating parameters,Fe eleCOD R

Electrode type (Al or Fe), Al eleInitial pH (2–10), COD RCurrent density (5–80 A/m2), OC, FeOperating time (0–50 min)

Removal efficiency: RE, Operating cost: OC.a Energy cost only.

4.2. Effect of the current density

Current density is also an important parameter for controlling thereaction rate in most electrochemical processes. It is well known thatthe amount of current density determines the coagulant dosage, andsize of the bubble production, and hence affects the growth of flocs[27–29]. COD and TOC removal efficiencies at 5–20 A/m2 for 15 minwere changed from 21–90% and 9–84% for Fe electrode and from34–91% and 26–86% for Al electrode (Figs. 4 and 5). There were nodrastic changes observed for COD and TOC removal efficiencies at20–80 A/m2. Removal efficiencies of COD and TOC at 80 A/m2 were94% and 90% for Fe electrode and 96% and 92% for Al electrode, re-spectively. This can be attributed at high current densities; the extentof anodic dissolution (Faraday's law, Eq. (9)) increased positivelycharged polymeric metal species resulting in increased COD andTOC removal efficiencies.

ELC ¼ i:tEC :Mw

z:F:vð9Þ

where tEC(s) is operating time, z is the number of electrons involvedin oxidation/reduction reaction for Al, zAl=3 and zFe=2. Mw isthe atomic weight of anode material (Mw,Al=26.98 g/mol; Mw,Fe=56 g/mol), F is the Faraday's constant (96,485 C/mol) and v is the vol-ume (m3) of the wastewater in the EC reactor. It was also clear that atechnically efficient process must also be feasible economically. Majoroperating cost of the EC was associated with electrical energyconsumption during process [30]. The electrical energy (kWh/m3)COD and TOC removal efficiencies for Al and Fe electrodes were calcu-lated from Eq (10).

Cenergy ¼U:i:tEC

vð10Þ

where U is cell potential (V) in the EC reactor. The cell voltage in-creased from 2.10 to 16.36 V for Fe and from 2.22 to 14.47 V for Al

s Reference

imum condition, [31], H2O2 3320, Fe N15 mg/l, N10 g rust, mix reac.,E, 80% (70 h.)

E,74%; turbidity RE, 99.6%, [7]ate was free from microorganisms.tion in water consumption resulted in 55%esidence time 6 h, [32]E, 44,3%; Color RE, 86.2%; Turbidity.1%; OC, 42 kWh/kg CODa

lation–flocculation [33]E, 67% (alum 1000 mg/l), 45% (FeSO4 750 mg/l)n oxidation [33]E. 81% (2 M H2O2 [H2O2]/[Fe+2]:10,)sludge, CoagulationNFenton oxidationltration, Total flux decline of membrane, 73%, membrane fouling, 9%. [33]E, 20–26%(Pre-aeration) [34]E, 80% PVA-based–90% latex-based Alum and FeCl3 (2000 mg/L)was also effective at high pH as it was combined with FeCl3.E, 30–80%; Turbidity RE, 70–99% (2 g/L FeSO4 opt. pH, 9.7). [35]E, 70–95%; Turbidity RE, 90–99% (2.5 g/L Al2(SO4)3 orj. pH)E, 98%; Turbidity RE, 98%(4 g/L PACl, opt. pH 7)ctrode, (35 A/m2, 15 min, pH 6.95)

This studyE, 93%; TOC RE, 87.5%ctrodes, (35 A/m2, 15 min, pH 6.95)E, 94%; TOC RE, 89%.electrode, 0.187€/m3 OC, Al electrodes 0.129€/m3.

Table 6Comparisons of treatment performances of different wastewaters with process conditions and variables in the EC process.

Wastewater type Type of EC reactor Process conditions and Variable Results Reference

Baker's yeast production wastewater Batch reactorMonopolar parallelAl or Fe electrodes rectangular plate

Electrode type Al orFe electrodes; pH, 3–9Current density, 30–100 A/m2,Operating time, 10–60 min.

Al electrodeCOD RE, 71%; TOC RE, 53%; Turbidity RE, 90%Fe electrodeCOD RE, 69%;TOC RE, 52%; Turbidity RE, 56%OC Al, 1.54 $/m3 -0.82 $/kg; Fe, 0.51 $/m3- 0.27 $/kg.

[36]

Poultry slaughterhouse wastewater Batch reactorAl or Fe electrodes rectangular plate

Initial pH, 2–10Current density, 30–200 A/m2,Operating time, 5–40 min.

COD RE, 93 %,(25 min. pH 3, 150 A/m2, Al electrode)Oil-grease RE, 98% for Fe electrodeOC, 0.015 $/kg COD , 0.3-0.4 $/m3 Fe electrodeOC, 0.027 $/kg COD - 0.6-0.7 $/m3 Al electrode.

[12]

Textile wastewater Batch reactor(65×65 mm×110 mm)Iron electrode (99.5%)

Initial pH (pH 6.9), Current density(10 mA/cm2), Conductivity (3,990 mS/cm)Electrolysis time (10 min)

COD RE 78 %, Turbidity RE, 92%;Energy consumptions; 0.7 kWh/kgCOD, 1.7 kWh/m3,Electrode consumptions, 0.2 kgFe/kgCOD, 0.5 kgFe/m3,

[23]

Potato chips manufacturing wastewater Batch modePlexiglas thermostated electrocoagulator,(65×65×110 mm)monopolar electrode

Electrode type, Al or Fe electrodespH, 2–8Current density, 25-300 A/m2,Retention time. 5-40 min.

Al electrodesNFe electrode (COD, SS removal rate)COD RE, 60%; Turbidity, 98%; Retention timeb40 min.Dried sludge 0.05–1.75 kg/removed kg CODOC, 0.48 - 5.42 $/m3 for 20–300 A/m2,OC, 0.62 - 6.32 $/m3 for 5–40 min,Energy consumption, 4 kWh/m3 (b8 min).

[17]

Synthetic humic acids (HA) Continuous andbatch modeAl electrodes(20×2 cm×2 mm)Active surface, 34 cm2

pH (3-7-11)Time (0–25 min)Continuous modeElectromagnetic treatment (EM)

HA RE, pH 7 (96%)-pH 3 (90%)-pH 11 (51%). (30 min)EM treatment is in continuous mode and taken alone, it doesnot have a significant effect on HA removal.EM treatment is followed by EC in batch, it has an importantcontribution to increase UV absorbance removal by EC near100% at pH 7.

[37]

Metal cutting wastewater Batch modeMonopolar parallel(25×19×8 cm)Volume 4.5 LAl or Fe electrodes

Initial pH,(4–9)Current density (30-100 Am-2)Operating time (10–40 min)

Fe electrode,COD RE, 93.0%;TOC RE, 83.0%;Turbidity RE, 99.8 %Al electrode,COD RE, 93.5%; TOC RE, 85.2%;Turbidity RE, 99.9 %OC, 0.371 €/m3 for Fe electrodeOC, 0.337 €/m3 for Al electrode.

[14]

Textile dye (orange II) Batch modeBeaker reactor(250 ml)Fe plates electrodes(Total area 0.0106 m2)

Conc. of dye (50–200 ppm), Electrodes distance(1–5 cm), Current density (0-50Am-2),pH (1–10), Time (0–350 min), Temp. (0–75 °C),Flow (2–12 l/h)

Color RE, 98% (Conc. of orange II 200 ppm)COD reduced, 84% (Conc. of orange II 200 ppm)Optimum current density, 34.62 A/m2, pH of 7.5-8.5, stirringrate 100 rpm, electrodes distance (2 cm), water temperature(30 °C), volume flow (2 l/h)

[38]

Paper mill wastewater Batch mode(240×110×110 mm)Different combination Al or Fe electrodesrectangular plateMonopolar parallel

Electrode combinationsInitial pH (2–10)Current density (10-70Am-2)Water temperature (20–60 °C)Time (10–70 min)

Optimum pH of 5–7, current density, 70 mA/cm2

High temperature, negative effect.Al-Al, high efficiency in the color removalFe-Fe, effective COD and phenol removalAl-Fe or Fe-Al combination, not effective color, COD andphenol removal.

[39]

Paint manufacturing company (PMW) Batch mode(120×110×110 mm)Al or Fe electrodes rectangular plateMonopolar parallel

Electrode type (Al or Fe),Initial pH (2–10),Current density (5–80 A/m2),Operating time (0–50 min)

Fe electrode, (35 A/m2, 15 min, pH 6.95)COD RE, 93 %; TOC RE, 87.5 %Al electrodes, (35 A/m2, 15 min, pH 6.95)COD RE, 94 %; TOC RE, 89 %.OC, Fe electrode, 0.187 €/m3

OC, Al electrodes 0.129 €/m3.

This study

Suspended Solid: Ss, Operating Cost: OC, Humic acid: HA, Removal Efficiency: RE.

96A.A

kyol/Desalination

285(2012)

91–99

200 300 400 500 600 700 800

200 300 400 500 600 700 800

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0A

bsor

banc

e

Wavelength (nm)

Raw Wastewater 5 min EC 10 min EC 15 min EC 30 min EC 45 min EC

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

Abs

orba

nce

Wavelength (nm)

Raw wastewater 5 min EC 10 min EC 15 min EC 30 min EC 45 min EC

a

b

Fig. 8. Absorbance spectra of PMW for (a) Fe and (b) Al electrodes during the EC process.

97A. Akyol / Desalination 285 (2012) 91–99

electrodes with increasing of the current density from 5 to 80 A/m2.According to Faraday's law, it was clear that Al3+ or Fe2+ dose re-leased from anode depended on the electrolysis time and currentdensity. So in the EC process, current density and operating timewere important parameters affecting the COD and TOC removal effi-ciencies and controlling the reaction rate in the EC reactor.

More sludge was also produced from Fe and Al electrodes athigher current density due to elevated dissolution rate of anode(Table 3). At a high current density, the extent of anodic dissolutionof Fe or Al increased, resulting in a greater amount of precipitateand removal of COD and TOC. Moreover, the rate of bubble-generation increased and the bubble size decreased with increasingcurrent density; both of these trends were beneficial in terms ofhigh pollutant-removal efficiency by H2 flotation. Therefore, whenhigh current densities were applied, the removal time of COD andTOC removal efficiencies improved. However, these parametersshould be kept at low level to achieve a low-cost treatment. For thatreason, a compromise of the current density and electrolysis timewere necessary to optimize the treatment efficiency with the lowestcost. Considering this operating cost factor, all further experimentswere carried out at 35 A/m2 for both electrodes.

Total OC increased with increasing current density and operatingtime in the EC process because energy and electrode consumptionswere related to these parameters. OCs at 5–80 A/m2 were changedfrom 0.032 to 0.878€/m3for Fe electrode and from 0.020 to0.280€/m3 for Al electrode (Table 3). The OCs for removal efficienciesof COD and TOC at the optimum conditions (pHi 6.95, 35 A/m2 and15 min) were 0.187€/m3 for Fe electrode and 0.129€/m3 for Al elec-trode, respectively. OC for Fe electrode used in removal of COD andTOC from PMW was 1.5 times more expensive than that of Al elec-trode. This result indicated that Al electrode was more efficient thanFe electrode in the EC process.

Amounts of sludge (Ws) generated during the EC process variedfrom 4.79 to 15.14 kg/m3 for Fe electrode and from 5.02 to16.23 kg/m3 for Al electrode at pH 2–10 due to increasing of currentdensity which is proportional to amount of sludge (Table 3). On theother hand, when current density changed from 5 to 80 A/m2, sludgemass varied from 1.66 to 10.22 kg/m3 for Fe electrode and from 4.57to 16.45 kg/m3 for Al electrode [12].

4.3. Effect of operating time

Operating time is an important parameter for economic applica-bility of the EC process. Based on the Faraday's law, the quantity ofiron or aluminum released to the EC system using Fe or Al electrodesmay affect the residence time which lead to an increase in Fe or Alions released to the system. Effect of operating time at 35 A/m2 andpH 6.95 was shown Figs. 6 and 7. The removal efficiencies of CODand TOC increased at 5–15 min from 77% to 94% and from 70% to89% for Fe electrode and from 13% to 95% and from 16% to 91% forAl electrode. The removal efficiencies for COD and TOC were notchanged much after operating time of 15 min for both electrodes.This could be related to decreasing extent of cathodic reduction andformation of new electrocoagulant flocs. The electrode consumptionswere calculated for operating time 5–45 min as 0.156–0.808 kg Fe/m3

for Fe and 0.035–0.251 kg Al/m3for Al electrodes respectively. Theelectrode consumptions for Fe electrode were 1.86 times higherthan that of Al electrode at 15 min (Table 4). OCs were varied in therange of 0.186–1.164€/m3 for Fe electrode and 0.059–0.416€/m3 forAl electrode when the operating time was 5–45 min. The optimumoperating time for both electrodes in the EC process was selected as15 min.

Table 5 was produced to show comparison of other techniqueswith the EC process of PMW removals. While COD removal efficien-cies by conventional techniques were found in the range of 44%–80%, COD removal efficiencies in the EC process 35 A/m2, 15 min,

pH 6.95 for Al electrode was obtained as 94%. The EC process was su-perior to the other techniques since conventional techniques hadsome limitations such as use of chemical, large volume of sludge,need of secondary treatment in some cases, longer operating time,difficulty in monitoring, high installation, operating cost and lower ef-ficiency in many cases.

Comparisons of different wastewaters with operating conditions inthe EC process were illustrated in Table 6.While the lowest COD removalefficiency in the EC process with current density of 200 A/m2 and operat-ing time of 30 min was obtained for potato chips manufacturing waste-water as 65%, the highest COD removal efficiency in the EC process withpHi 7.0, 33.3 A/m2 and 30 min was obtained as high as 96% in humicacid synthetic solution. It was difficult to compare the operating costs ofthe EC process since some of cost items were not given with details inmost of the published reports such as energy, electrode, chemicals, etc.In this study, CODandTOC removal efficiencies of 93%and88% for Fe elec-trode and 94% and 89% for Al electrode were obtained (Table 6).

4.4. Toxicity studies

The activated sludge inhibition test was applied to assess the changesin toxicity of raw and treated wastewaters under optimum experimental

1000 1500 2000 2500 3000 3500 400090

92

94

96

98

100

Tra

nsm

itanc

e [%

]

Wavenumber (cm-1)

Residue sludge of Al electrode Residue sludge of Fe electrode

Fig. 9. FTIR spectrum of the sludge.

98 A. Akyol / Desalination 285 (2012) 91–99

conditions. The biomass used in the activated sludge inhibition test wasfed with glucose and aerated well after being washed with tap water.

In the first step of toxicity analyses, two liters of the activated sludgewere placed in respirometer for each test sample. Toxicity in 0.1 L ofraw or treated wastewater was determined with an on-line respirome-ter. For raw wastewaters, values of Re, Ro and Rt were 15.1 mg/L.h,59.8 mg/L.h and 59.5 mg/L.h. Values of Re, Ro and Rt for treated waste-waters were 16.2 mg/L.h, 58.7 mg/L.h and 63.3.2 mg/L.h. According toEq. (3), while toxicity of rawwastewater was 0.67%, no toxicity of trea-ted wastewater was observed. These results proved that microorgan-isms were hardly affected by raw and treated wastewaters in the EC

Fig. 10. XRF spectra of sludge produced for (a) Fe

process. Interestingly, slightly positive effect of treated wastewater onmicroorganisms was observed.

4.5. Assessment of absorbance analysis

The progress in the absorbance spectra of the wastewater duringthe EC was also monitored with respect to operating time at optimumconditions (pH 6.95, 15 min, 35 A/m2). UV–vis spectra of the raw andelectrochemically treated wastewater were shown in Fig. 8a–b. Therewere continuous signal curves in the region around 200–800 nm inthe spectra indicating for treatment of the wastewater. The intensityof the curves decreased after the EC process. These results indicatedthat there was a significant color reduction of the raw wastewaterwith the EC process [25].

4.6. Characterization of the sludge from the EC

Characterizations of the sludge obtained from the EC were carriedout by Fourier transform infrared spectroscopy (FTIR) and X-ray fluo-rescence (XRF). The sludge was collected by vacuum filtration afterthe EC and dried at 105 °C in the oven. As seen in Fig. 9, FTIR spectrumof the sludge (iron and aluminum hydroxide flocs) showed hardly anysignificant spectroscopic changes in PMW which consisted of mixtureof phenol, formaldehyde and polyvinyl acetate. FTIR spectrum of sludgeshowed a broad and intense band at 3340 cm−1 attributed to stretchingvibrations of \OH. The bands at 2958 and 1451 cm−1, and 1728 cm−1

indicated the presence of C\H and C_O. The strong bands at 1160 and1065 cm−1 were referred to C\O stretching vibrations. The observedvariations in the sludge using were most likely due to the adsorptionof some functional groups in PMW.

and (b) Al electrodes during the EC process.

Table 7XRF analysis of produced sludge in EC at optimum conditions.

Element andcompounds

Sludge with Feelectrode (%w/w)

Sludge withAl electrode(%w/w)

S 0.0347 0.0338Cr 0.0574 0.0259SiO2 17.8670 18.0344TiO2 8.7068 8.9374Al2O3 0.0000 34.3124Fe2O3 32.4577 1.2806CaO 7.0454 5.8249MgO 0.0000 0.4779P2O5 0.2124 0.2159Mn 0.1423 0.1166

99A. Akyol / Desalination 285 (2012) 91–99

XRF was used to determine the chemical components of the pro-duced sludge. Fig. 10a–b and Table 7 presented for existence and per-centage quantity of these components in the sludge. XRF analysisprovided direct evidence that raw material used in the paint produc-tion (Table 7). Other components detected in the sludge came fromadsorption of the conducting electrolyte [40].

5. Conclusions

The EC process was successfully applied to PMW. The optimum oper-ating conditions were determined as pH 6.95, current density of 35 A/m2

and operating time of 15 min. Removal efficiencies of COD and TOC inPMW were 93% and 88% for Fe and 94% and 89% for Al electrodes at15 min. Above 35 A/m2, no significant increases for the removal efficien-cies of COD and TOC were observed. The COD and TOC removal efficien-cies were hardly changed at higher current densities and operatingtimes. Operating costs for removal of PMW at the optimum conditionswere calculated for Fe and Al electrodes as 0.187€/m3 and 0.129€/m3.As electrode material, aluminum electrode performs better than Fe elec-trode in terms of removal efficiency and operating cost. Amounts ofsludge generated during the EC process were 9.63 kg/m3 for Fe electrodeand 7.73 kg/m3 for Al electrode at optimum conditions. Characterizationsof the sludge obtained from the EC were carried out by Fourier transforminfrared spectroscopy (FTIR) andX-rayfluorescence (XRF). FTIR spectrumof the sludge (iron and aluminum hydroxide flocs) showed hardly anysignificant spectroscopic changes in PMW. XRF analysis shows thatmaghemite (Fe2O3), alumina (Al2O3) and raw material used in the paintproduction were the major components in the sludge. The toxicity testwas carried out to obtain information about the toxic effect of the rawand treated wastewaters at optimum operating conditions. The samplesmeasured by respirometric method contained hardly any toxicities. Ab-sorbance spectra of raw and treatedwastewaterswere studied to observefor decolorization which increased sharply with increasing of operatingtime. These results indicated that the EC process would be very usefulfor treatment of similar kinds of wastewaters.

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