Separation and Purification Technology 112 (2013) 11–19

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Separation and Purification Technology

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A comparative study of electrocoagulation and electro-Fenton for treatmentof wastewater from liquid organic fertilizer plant

Abdurrahman Akyol a,⇑, Orhan Taner Can b, Erhan Demirbas c, Mehmet Kobya a

a Gebze Institute of Technology, Department of Environmental Engineering, 41400 Gebze, Turkeyb Bitlis Eren University, Department of Environmental Engineering, 13000 Bitlis, Turkeyc Gebze Institute of Technology, Department of Chemistry, 41400 Gebze, Turkey

a r t i c l e i n f o

Article history:Received 21 June 2012Received in revised form 20 March 2013Accepted 21 March 2013Available online 3 April 2013

Keywords:ElectrocoagulationElectro-FentonLiquid organic fertilizer wastewaterOperating cost

1383-5866/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.seppur.2013.03.036

⇑ Corresponding author. Tel.: +90 262 6053174; faxE-mail address: abdakyol@gyte.edu.tr (A. Akyol).

a b s t r a c t

Treatments of the liquid organic fertilizer manufacturing wastewater (LFW) by electrocoagulation (EC)and electro-Fenton (EF) processes using iron electrodes were carried out in a batch electrolytic reactor.Effects of operating conditions such as current density and initial pHi for the EC process, and current den-sity and initial H2O2 concentration for the EF process on removal efficiencies of total organic carbon(TOC), chemical oxygen demand (COD), color and total phosphate (TP) were investigated. Removal effi-ciencies of 79% for TOC, 83% for COD, 73% for TP and 95% for color from the EC process at the optimumoperating conditions (50 A/m2, 45 min and pHi 6) and 87% for TOC, 91% for COD, 96% for TP and 99% forcolor from the EF process at the optimum operating conditions (50 A/m2, 45 min, 25 mM H2O2 and pHi 3)respectively, were obtained. Operating costs for the EC and the EF processes were calculated as 0.74 and1.23 €/m3. As a comparison from the obtained result, the EF process was found to be more effective thanthe EC process with respect to the removal efficiencies of COD, TOC and TP. However, the EF process wasmore expensive than the EC process.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, the use of liquid organic fertilizers (LOFs) con-taining humic matters in modern agriculture industries has beenincreased considerably [1,2]. The main applications of LOFs areproducing of high-yielding plants such as healthy green lawnsand hearty plants. since they contain growth promoting macronu-trients, micronutrients, fulvic and humic acids [3,4]. Annual fertil-izer usage of the world in 2011 was 173 million tons and around5 million tons was consumed only in Turkey [5,6]. Chemical fertil-izers used with a suitable method in plant nutrition increase cropfertility in the rate of 50–80% depending on plant type and areas[7]. According to Food and Agriculture Organization of the UnitedNations (FAO), the majority of the fertilizer produced is used forthe growing of cereals (55–60%), and fruits and vegetables (17%).

Effluents from LOF production processes may include darkbrown color, chemical oxygen demand (COD), total organic carbon(TOC), total nitrogen (TN) and total phosphorus (TP) [8]. The pres-ence of the above pollutants in LOF manufacturing wastewater(LFW) leads to serious damage when discharged directly into theenvironment. TP, TN and TOC have all been responsible for encour-aging the rapid growth of algae (eutrophication) due to introducing

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phosphorus compounds into the surface water systems [9]. Humicand fulvic acids in the LFW have seriously threatened the aquaticenvironments due to high organic content and dark brown color.Humic substances also cause formation of disinfection byproductssuch as chloroform and bromo-dichloromethane, which are carcin-ogenic compounds [10,11]. Hence, LFW must be treated beforebeing discharged into the environment due to the legal restrictionsand conservation of natural life in Turkey. Legal limitations for nat-ural protected areas and recreation and various uses (includingnatural salt, bitter and soda-rich lakes) with respect to water pol-lution control regulation in Turkey are 0.1–1.0 mg/L for TN,0.005–0.1 mg/L for TP and 3–8 mg/L for COD.

Recent researches have shown that electrochemical techniquescan offer a good opportunity to prevent and remedy pollutionproblems due to strict environmental regulations. Electrocoagula-tion (EC) and electro-Fenton (EF) processes are simple and efficientmethods that have attracted a great deal of attention for the treat-ments of various industrial wastewaters such as olive mill waste-water [12] alcohol distillery wastewater [13], paper pulptreatment effluent [14], agro-industry wastewater [15], baker’syeast wastewater [16], potato chips manufacturing wastewater[17], chemical mechanical polishing wastewater [18], poultryslaughterhouse wastewater [19], tannery wastewater [20,21], elec-troplating rinse water [22] and textile wastewaters [23,24] and soon. The EC and EF processes are environmentally friendly and donot generate secondary pollutants. Some advantages of these

Nomenclature

LFW liquid organic fertilizer manufacturing wastewaterEC electrocoagulation processesEF electro-Fenton processesTOC total organic carbon (mg/L)COD chemical oxygen demand (mg/L)TP total phosphate (mg/L)TN total nitrogen (mg/L)FTIR Fourier transform infrared spectroscopyEDX energy dispersive X-ray instrumentXRD X-ray diffraction

Cenergy consumption of energy (kW h/m3)Celectrode consumption of electrodes (kg/m3)U average cell voltage (V)i current (A)MFe molecular weight of iron (g/mol)OC operating cost (€/m3)F Faraday’s constant (C/mol)

12 A. Akyol et al. / Separation and Purification Technology 112 (2013) 11–19

processes as compared to the conventional methods are easy tooperate, less retention time, lower operating costs, reduction (inthe EF process) or absence of adding chemicals (in the EC process),rapid sedimentation of the electrogenerated flocs and less sludgeproduction and requires a simple equipment.

Aim of the present study was to investigate the removal of LOFmanufacturing wastewater (LFW) by the electrocoagulation (EC)and electro-Fenton (EF) processes using iron electrodes in a batchmode operation. The effects of operating parameters such as pHi,current density, operating time and dosage of H2O2 for EC and EFprocesses on the removal efficiencies of LFW were studied to deter-mine the optimum operating conditions. Operating costs and char-acterizations of anode sludge from the EC and EF processes werealso performed.

2. Removal mechanisms of electrocoagulation and electro-Fenton

The electrocoagulation (EC) process involves generation ofcoagulants in situ by dissolving sacrificial anodes such as alumi-num or iron upon application of a direct current. When iron elec-trode is used as anodes upon oxidation in an electrolytic system,it produces iron hydroxide, Fe(OH)n where n = 2 or 3 [25,26]:

Fe! Fe2þ þ 2e� ðanodeÞ ð1Þ

2H2Oþ 2e� ! H2 þ 2OH� ðcathodeÞ ð2Þ

Fe2þ þ 2OH� ! FeðOHÞ2 ðin bulk solutionÞ ð3Þ

2Fe2þ þ5H2Oþ1=2O2!2FeðOHÞ3þ4Hþ ðin bulk solutionÞ ð4Þ

Fe3þ þ 3OH� ! FeðOHÞ3 ðin bulk solutionÞ ð5Þ

Ferric ions generated by the EC process may form monomericand polymeric hydroxyl complexes (i.e., hydrolysis products)namely, FeOH2þ; FeðOHÞþ2 ; FeðOHÞ4þ2 ; FeðOHÞ�4 ; FeðH2OÞ3þ2 ; FeðH2OÞ3þ6 ; FeðH2OÞ5OH2þ; FeðH2OÞ4ðOHÞþ2 ; FeðH2OÞ8ðOHÞ4þ2 ; Fe2ðH2

OÞ6ðOHÞ2þ4 , etc. depending on pH of the aqueous medium. The com-plexes have a pronounced tendency to polymerize at pH 3.5–7.0.Fe(OH)n remains in the aqueous stream as a gelatinous suspensionand may be removed from the wastewater by coagulation, adsorp-tion, co-precipitation and sweep flocculation [27].

In the EF process, the EC and Fenton processes are combined to-gether to increase the degradability of organic compounds presentin wastewaters [28]. In the conventional Fenton process, both H2O2

and Fe2+ are externally applied, whereas in the EF process, H2O2 isadded from outside and Fe2+ is provided from sacrificial cast ironanodes [29]. The EF process based on the employment of a sacrifi-cial iron anode delivers Fe2+ ions into the solution (Eq. (1)) whilstsimultaneously occurs the reduction of water at the cathode (Eq.

(2)); then the hydrogen peroxide is added in order to provide con-ditions for the Fenton reactions (Eqs. (6) and (8)). Moreover, Fe2+

according to Eq. (9) may be continuously regenerated at the cath-ode depending on the electrolytic cell setup [30].

Fe2þ þH2O2 ! Fe3þ þ �OHþ OH� ðin bulk solutionÞ ð6Þ

Fe2þ þ �OH! Fe3þ þ OH� ðin bulk solutionÞ ð7Þ

Fe3þ þH2O2 ! Fe2þ þHþ þHO�2 ðin bulk solutionÞ ð8Þ

Fe3þ þ e� ! Fe2þ ðcathodeÞ ð9Þ

3. Materials and methods

3.1. Wastewater source and characteristics

The wastewater was collected from organic and organic-min-eral liquid organic fertilizer manufacturing plant in Turkey (Istan-bul) producing approximately 250 m3 of wastewater per month.LFW contained organic (humic and fulvic acids, dark brown color)and inorganic compounds (nitrogen, phosphorus and potassium,etc.). The wastewater was filtered using a screen filter to removelarge suspended solids before being used for the subsequent stud-ies. Characteristics of LFW were shown in Table 1.

3.2. Experimental apparatus and procedure

The experimental set-up containing the specifications of elec-trode, reactor and power supply was reported elsewhere [16].The EC and EF reactors were made of Plexiglas with dimension of120 mm � 110 mm � 110 mm. Four plate electrodes (two anodesand two cathodes) with dimension of 45 mm � 53 mm � 3 mm(purity P 99.5%) were used in the study. The total effective elec-trode area was 143 cm2 and the spacing between electrodes was10 mm. The electrodes in monopolar connection mode were con-nected to a digital dc power supply (Agilent 6675A model; 120V,18A) and operated at galvanostatic mode.

The chemicals (H2O2, H2SO4, NaOH) were obtained from Merck.In each experiment, approximately 800 mL of LFW was placed inthe electrolytic reactor. pH of the solution was adjusted to 3 priorto the experiment and agitated with a magnetic stirrer at 200 rpm(Heidolp 3600 model) in the EF process. A desired amount of H2O2

(mM) was added to the electrolytic reactor before the electricalcurrent was turned on. A batch study was conducted to optimizeparameters such as concentration of H2O2 and current density. Be-fore each run, organic impurities and oxide layer on electrode sur-faces were removed by dipping for 2 min in a solution freshlyprepared by mixing HCl solution (35%) and

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j (A/m2) 25 50 75 100

(a)

(b)

Fig. 1. Effect of current density on (a) TOC and (b) TP removal efficiency of the ECprocess.

Table 1Characterization of effluents of the LFW.

Parameter Unit

pH 7.80Conductivity (mS/cm) 2.28Color Dark brown (Abs. 476 nm)Turbidity (NTU) 104SS (mg/L) 65COD (mg/L) 1010TOC (mg/L) 435TN (mg/L) 68TP (mg/L) 488

A. Akyol et al. / Separation and Purification Technology 112 (2013) 11–19 13

hexamethylenetetramine aqueous solution (2.8%). The electrodeswere then dried and weighed and placed into the reactor.

The voltage or current density of the EC reactor was set to a de-sired value through adjustment of the direct current power supplyand the run was started at room temperature. The duration of theexperiment was 45 min and samples were taken every 5 min inter-vals. The collected samples were filtered using a 45 lm Milliporemembrane paper. COD, TOC, TP, TN and turbidity (NTU) includingconductivity, initial and final pH for every sample were measured.At the end of the experiment, the electrodes were washed thor-oughly with water to remove any solid residues on the surfaces,dried and reweighed.

3.3. Analytical procedures

All analyses were carried out in accordance with the StandardMethods of the APHA (APHA, 1998). COD was measured by closedreflux titrimetric method [31]. TOC levels were determinedthrough combustion of the samples at 680 �C using a non-disper-sive IR source (Tekmar Dohrmann Apollo 9000). The total phospho-rus was determined with the optical emission spectrometer (ICP,PerkinElmer Optima 7000 DV). TN analyses were carried out byHach Lange IL 550 TOC-TN analyzer. Turbidity, pH and conductiv-ity (Mettler Toledo 7100e) of sample were measured with a turbi-dimeter, a pH meter (Hach Lange HQ 40d) and a conductivitymeter (Hach 2100AN), respectively.

The analysis of H2O2 was done by the permanganometric meth-od. Residual H2O2 was also measured in the supernatant to evalu-ate possible interference with COD. The obtained H2O2

concentrations (0.8 mg/L) increased only the total COD of the trea-ted wastewater by about 0.5 mg/L [32] and the interference wasfound to be negligible.

In addition, sludge generated by the EC and EF processes wasdried in the oven at 105 �C for 24 h. XRD of the sludge was mea-sured with an automated Rigaku X-ray diffractometer D-Max Rint2200 Series instrument using Cu Ka radiation at 40 kV and 40 mAover the range (2h of 5–70�). Fourier transform infrared spectros-copy (FTIR) can be used to identify chemicals from the sludge. FTIRis perhaps the most powerful tool for identifying types of chemicalbonds (functional groups). FTIR for the samples were recorded inthe range 4000–400 cm�1 on a Bio Rad FTS 175 C spectrophotom-eter for detecting the changes of compounds after the removal pro-cess. Experiments were conducted in triplicate under identicalconditions to confirm the results and the mean values were pre-sented. The maximum experimental error was below 3%.

The electrode (Celectrode, kg/m3) and energy (Cenergy, kW h/m3)consumptions in the EC and EF processes were calculated usingthe following equations:

Cenergy ¼UitEC

mð10Þ

Celectrode ¼itECMFe

zFmð11Þ

where U is the average cell voltage (V), i is the current (A), tEC is theoperating time (h), MFe is the molecular weight of Fe (55.86 g/mol),F is the Faraday’s constant (96,487 C/mol), z is the number of elec-trons involved in the oxidation/reduction reaction and v is the vol-ume of the treated solution (m3).

One of the most important parameters that affect the applica-tion of any method of water and wastewater treatment greatly isthe operating cost which determines cost of the treatment process.The OCs of the EC and EF processes were calculated by includingthe material cost (mainly electrodes), utility cost (mainly electricalenergy), chemicals, as well as labor, maintenance and other fixedcosts. In this study, the energy, electrode material and chemicalscosts were taken into account as major cost items in the calcula-tion of the OC as €/m3 for the treatment of LFW [16]:

OC ¼ aCenergy þ bCelectrode þ cCchemicals ð12Þ

where Cchemicals is consumption quantities of chemicals (kg/m3) ofthe wastewater treated. a, b and c constants in Eq. (12) providedby the Turkish market in May 2012 were values of electrical energyprice (0.072 €/kW h), electrode price (0.85 €/kg) and chemical costs(1.22 €/kg for H2O2, 0.73 €/kg for NaOH and 0.29 €/kg for H2SO4),respectively.

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Operating time (min)

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j (A/m2)

25 50 75 100

(a)

(b)

Fig. 2. Effect of current density on (a) TOC and (b) TP removal efficiency in the EFprocess.

14 A. Akyol et al. / Separation and Purification Technology 112 (2013) 11–19

4. Results and discussion

4.1. Effect of the current density on the removal efficiency

Current density is an important parameter for controlling thereaction rate in most electrochemical processes such as the ECand EF processes. Current density determines the rate of coagulantand bubble generation, its flocs size and distribution, and hence af-fects the growth of flocs (Fe(OH)2(s) or Fe(OH)3(s) coagulate parti-cles) in the EC process. In the EC and EF processes, the OH�

formation rate is controlled by the applied current density duringthe electrolysis. Effects of the current density for the treatmentof LFW in the EC and EF processes were studied in the range of25–100 A/m2 at operating time of 0–45 min.

Table 2Experimental results of LFW at different current density in the EC and EF process.

j (A/m2) EC process

TOC(%)

TP(%)

pHf*

(–)Ws

(kg/m3)ELC(kg/m3)

ENC(kW h/m3)

OCEC

(€/m

25 62 69 8.1 0.85 0.40 0.78 0.4050 79 73 8.9 2.42 0.64 2.72 0.7475 80 80 10.2 3.19 0.66 4.93 0.92

100 81 85 10.4 4.61 0.93 6.73 1.27

* pHf: Values of final pH.

Fig. 1a and b indicated that the removal efficiencies of LFWwere increased gradually from 55% to 80% for TOC and from 58%to 78% for TP at operating time of 30 min when the current densitywas varied from 25 to 100 A/m2. As the operating time was in-creased further from 30 to 45 min, the removal efficiency of TPwas slightly improved from 69% to 85% and no change in the re-moval efficiency was observed for TOC. This can be attributed tohigh current densities; the extent of anodic dissolution (Faraday’slaw, Eq. (11)) increased positively charged polymeric metal species(Section 2) resulting in increased TOC and TP removal efficiencies.The optimum current density for this study was selected as 50 A/m2.

In the EF process, significant improvements in removal efficien-cies of TOC at 5 min and TP at 20 min were observed at 25–100 A/m2. The removal efficiencies were increased from 64% to 82% at5 min and from 79% to 88% at 45 min for TOC, and from 57% to100% for TP at 20 min and no noticeable changes from 20 to45 min at 50 and 75 A/m2 were observed except for 25 A/m2. Asthe current density was increased, H2O2 concentration was in-creased leading to existences of a higher concentration of hydroxyland hydroperoxyl radicals in less time (Fig. 2). Some organic pollu-tants due to refractory compounds cannot be further oxidized bythe oxidants generated in the EF process might be attributed tothe TOC removal efficiencies that was hardly changed between20 and 45 min. The optimum current density for the EF processwas also selected as 50 A/m2.

The cell voltage increased from 4.7 to 10.2 V with increasing thecurrent density from 25 to 100 A/m2. According to Eq. (11), amountof Fe2+ released from anode material depended on the electrolysistime and current density in the EC and EF processes which affectedthe removal efficiencies of TOC, TP and color, and controlling thereaction rate in the EC and EF reactors. Therefore, current densityand operating time were important parameters for the processes.As can be seen in Figs. 1a and 2a, TOC removal efficiencies duringthe first 30 min in the EC process and 5 min in the EF process wereincreased for values of the applied all current densities due to thehigher electro-regeneration of ferrous ion from ferric ion withincreasing the current density.

Amounts of the sludge were increased for the EC and EF pro-cesses as the current density was varied from 25 to 100 A/m2.Amount of the sludge produced after the removal process wasfound to be higher in the EC process (2.42 kg/m3) at 50 A/m2 andpHi 6 than that of the EF process (0.65 kg/m3) at 50 A/m2, pHi 3and 25 mM H2O2 (Table 2).

Effluent pHs of the treated LFW at 25–100 A/m2 and 45 minwere varied from 6.0 to 8.1 at 25 A/m2, 8.9 at 50 A/m2, 10.2 at75 A/m2 and 10.4 at 100 A/m2 for the EC process and from 6.0 to6.3 at 25 A/m2, 7.1 at 50 A/m2, 7.2 at 75 A/m2 and 8.7 at 100 A/m2 for the EF process, respectively. Final effluent pHf determinedas 8.9 for the EC and 7.1 for the EF at the optimum current densityfell into limit values set by discharge standards of water pollutioncontrol regulations (i.e., pHf 6–9).

Table 2 showed that the energy and electrode consumptions at25–100 A/m2 were calculated as 0.78–6.73 kW h/m3 and

EF process

3)TOC(%)

TP(%)

pHf

(–)Ws

(kg/m3)ELC(kg/m3)

ENC(kW h/m3)

OCEF

(€/m3)

79 93 6.3 0.62 0.07 0.18 1.1687 96 7.1 0.65 0.12 0.47 1.2387 97 7.1 1.39 0.17 0.86 1.3088 100 8.7 1.88 0.22 1.87 1.42

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Fig. 3. Effect of pHi on the (a) TOC and (b) TP removal efficiencies in the EC process.

A. Akyol et al. / Separation and Purification Technology 112 (2013) 11–19 15

0.40–0.93 kg/m3 for the EC process and 0.18–1.87 kW h/m3 and0.07–0.22 kg/m3 for the EF process, respectively. Values of elec-trode consumptions in the EC and EF processes at 50 A/m2 were0.64 kg/m3 and 0.12 kg/m3. This indicated that values of the elec-trode consumptions for both processes at the optimum currentdensity were almost 40% less as compared to values of them at100 A/m2. OCs at 25–100 A/m2 were calculated as 0.4–1.27 €/m3

for the EC process and 1.16–1.42 €/m3 for the EF process but valuesof the OCs at the optimum current density were 0.74 €/m3 for theEC and 1.23 €/m3 for the EF processes. However, values of energyand electrode consumptions in the EF process were found to bequite lower than that in the EC process. The OC for the EF at theoptimum current density was higher than that for the EC sincethe cost increased considerably with H2O2.

Table 3Experimental results of LFW at different pH in the EC process.

pH TOC (%) pHfa (–) Ws (kg/m3)

4.0 76 8.5 2.076.0 79 8.9 2.428.0 31 9.9 0.60

10.0 7 11.4 0.56

a pHf: Values of final pH.

4.2. Effect of initial pH on the removal efficiency

Initial pH is an important operating factor influencing perfor-mance of the EC and EF processes. Effect of the initial pH with re-spect to operating time (0–45 min) for treatment of LFW wasstudied at 50 A/m2 in the pHi range of 4.0–10.0 for the EC andpHi 3.0 for the EF processes.

Fig. 3a and b showed the influence of initial pH on the TOC andTP removal efficiencies in the EC process. The removal efficienciesof LFW at pHi 4.0–10.0 were decreased from 76% to 7% for TOC andfrom 82.5% to 54.6% for TP in the EC process. The maximum re-moval efficiency of TOC was 79% at pHi 6.0. The removal efficien-cies were higher at acidic conditions and reduced at high pHlevels. The generated iron hydroxides during the EC process mightadsorb rapidly the fertilizer compounds in LFW such as humic acidthrough electrostatic attractions. Humic acid was reported to gen-erate complexes with iron and iron oxide through various types ofcarboxylate, phenolic, and carbonyl functional groups in the humicacid [33]. Thus, the EC and EF processes were able to remove thecolor and organic contents efficiently from LFW.

Fig. 3b depicted for the removal efficiency of TP at pHi 4–10. TheTP removal efficiencies were varied from 5% to 83% at 5–45 min. AtpHi > 6.0, TOC and TP removal efficiencies decreased sharply. Theoxide surfaces exhibited a net positive charge and adsorption ofanionic phosphate was enhanced by the columbic attractions atacidic pHs. At basic pHs, the oxide surface had a net negativecharge and might incline to repulse the anionic phosphate insolutions.

The effect of pH on the removal process depended on formationof the complexes having a tendency to polymerize at pH 3.5–7.0and insoluble metal hydroxides formed related to increase the solu-tion pH was precipitated as Fe(OH)2, Fe(OH)3, FePO4, and Fe3(PO4)2-

�8H2O [34]. Changes in influent pHi after the EC process were foundto be in the range of 8.5–11.4 (Table 2). The increase in the pHi dur-ing the EC process might be related to the electrochemical andchemical dissolution of iron electrode increased with values ofpHi. Therefore, there was no neutralization required for the treatedwastewater by the EC and EF processes. TOC removal efficienciesdecreased from 30% to 7% at pH > 8 because hydroxide ions wereoxidized at the anode. In addition, FeðOHÞ3�6 and FeðOHÞ�4 ionsmay be present at high pH, which lacked a removing capacity[35]. The protons in the solution at lower pHs were reduced to H2

at the cathode and existence of hydroxide ions was very low [35].The amount of TN at pHi 4–10 in the EC process was decreased

from 68 to 64 mg/L in 45 min which corresponded to the removalefficiency of 6% and could not be removed further successfully withthe present both EC and EF processes. The energy and electrodeconsumptions for the EC process were 3.07–4.02 kW h/m3 and0.67–0.11 kg/m3. OCs for the EC process at pHi 4–10 were variedfrom 0.79 to 0.38 €/m3. There was also hardly any differences ob-served for OCs at pH 4–6 (Table 3).

In this study, three major interaction mechanisms of the EC pro-cess were co-precipitation, adsorption and sweep coagulation [27].Each of these mechanisms was to be thought of being for a differ-ent pH range. At low pH values, metal species like Fe3+ generated at

ELC (kg/m3) ENC (kW h/m3) OC (€/m3)

0.67 3.07 0.790.64 2.72 0.740.16 3.91 0.420.11 4.02 0.38

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(mM) None 5 10 25 50

(a)

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Fig. 4. Effect of initial H2O2 concentration on the (a) TOC and (b) TP removalefficiencies by EF process.

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Col

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EF EC

Fig. 5. Effect of operating time on color removal efficiencies by EC and EF processes.

Table 5Comparison values of LFW before and after treated by EC and EF processes.

Parameter LFW EC processa EF processb

pH 7.80 8.9 7.1Conductivity (mS/cm) 2.28 1.93 2.05Color Dark brown Almost clear Almost clearTurbidity (NTU) 104 2 1SS (mg/L) 65 4 2COD (mg/L) 1010 168 87TOC (mg/L) 435 76 58TN (mg/L) 68 64 64TP (mg/L) 488 130 18

a The optimum operating conditions for EC process: 50 A/m2, 45 min and pHi 6.b The optimum operating condition for EF process: 50 A/m2, 45 min 25 mM H2O2

and pHi 3.

16 A. Akyol et al. / Separation and Purification Technology 112 (2013) 11–19

the anode binding to the anionic species, thus neutralizing theircharge and reducing their solubility. This process of removal wasnamed as precipitation. Second mechanism, adsorption operatedat a higher pH range (>6.0) and involved adsorption of organic sub-stances on amorphous metal hydroxide precipitates. Under theoptimum operating conditions, the initially formed colloidal pre-cipitate at final pH 4–6 was positively charged and colloidally sta-ble suggesting that co-precipitation played a vital role in the ECprocess. The low pH range was explained as co-precipitation whilethe higher pH range (>6) was adsorption and sweep coagulation[36,37]. At pH > 6, formed amorphous M(OH)3(s) (sweep flocs) withthe minimum solubility within the pH range of 6–7 had a largespecific surface area that could absorb some soluble organic com-pounds such as humic acid onto its surface [23]. The EC process inthe pH range 4–6 was explained as co-precipitation. Therefore, aneffective color removal of LFW was realized between these pHranges in the previous study [27].

Table 4Experimental results of LFW at different H2O2 dosages in the EF process.

H2O2 dosage (mM) TOC (%) pHf (–) Ws (kg/m

5 81 7.1 0.8410 81 7.5 0.6525 87 7.1 0.6550 80 7.8 0.78

4.3. Effect of H2O2 concentration on the removal efficiency

Determination of the optimum H2O2 concentration in the EFprocess is very important for both the removal efficiency and par-ticularly economic applicability related to the cost of H2O2. The ef-fect of initial H2O2 concentration on removal efficiencies of LFWwas studied in the range of 5–50 mM at the optimum conditions(50 A/m2 and pH 3). The removal efficiencies of TOC and TP werefound to be directly proportional to H2O2 concentration when thecurrent density applied to the EF process was the high enough(P50 A/m2).

TOC removal efficiency increased from 59% to 84% with increas-ing initial H2O2 concentration from 5.0 mM to 25.0 mM in 10 minand the removal efficiency at 25 mM was reached to 87% at45 min (Fig. 4a). But TOC removal efficiency decreased to 80% at50 mM of initial H2O2 concentration due to the undesired OH scav-enging reactions effect. This circumstance depended on Eqs. (12and 13) leading to the formation of less reactive radicals fromthe hydroxyl radical. The similar results were reported and theoptimum H2O2 concentration was 30 mM [38]. Among these

3) ELC (kg/m3) ENC (kW h/m3) OC (€/m3)

0.11 0.54 0.350.12 0.52 0.580.12 0.47 1.230.12 0.51 2.33

0

25

50

75

100

Current density (A/m2)

TO

C re

mov

al e

ffic

ienc

y (%

)

EC (10 min) EF (10 min)

0 25 50 75 100

0 25 50 75 1000

20

40

60

80

100

Tot

al p

hosp

horo

us re

mov

al (%

)

Current density (A/m2)

EC (10 min) EF (10 min)

(a)

(b)

Fig. 6. Effect of current density on (a) TOC and (b) TP removal efficiencies in the ECand EF processes at 10 min.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

Wavelength (nm)

LFW 5 min EC 10 min EC 15 min EC 20 min EC 30 min EC 45 min EC

200 300 400 500 600 700 800

200 300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

Wavelength (nm)

LFW 5 min EF 10 min EF 15 min EF 20 min EF 30 min EF 45 min EF

(a)

(b)

Fig. 7. Absorbance spectra of LFW for (a) EC and (b) EF processes at 5–45 min.

A. Akyol et al. / Separation and Purification Technology 112 (2013) 11–19 17

reactions, the production of hydroperoxyl radicals occurred in thecycle of Fe3+ to Fe2+ and the quenching of �OH by Fe2+ and H2O2.Oxidation potentials of hydroperoxyl radicals (1.25 eV) was lowerthan that of H2O2 (1.3 eV) [21].

Fe2þ þ �OH! Fe3þ þ OH� ð12Þ

H2O2 þ �OH! H2OþHO�2 ð13Þ

Fig. 4a also indicated that the removal efficiencies of LFW at 10,25 and 50 mM H2O2 concentrations increased rapidly for the first10 min which resulted in removal efficiencies of 78%, 84% and77% for TOC, respectively. There was also no noticeable differencesobserved for the removal efficiencies of TOC between 10 and45 min except for 5 mM H2O2 concentration due to the batch addi-tion of H2O2 at the beginning. Yavuz [13] added H2O2 continuouslyto maintain the effect of Fenton reaction in the experiment becausethe batch addition caused quenching of hydroxyl radicals whichwere shown in the following reactions:

HO� þH2O2 ! H2OþHO�2 ð14Þ

HO� þHO� ! H2Oþ 1=2O2 ð15Þ

whose rates competed with the degradation reactions of more re-calcitrant compounds:

HO� þ RH! R� þH2O

Fig. 4b indicated that the removal efficiencies of TP in 20 minwere nearly completed. After that, it remained almost the same un-til the operating time of 45 min. The optimum H2O2 concentrationfor the rest of the study was selected as 25 mM due to decrease inOC with respect to the higher removal efficiency. TP removal effi-ciency was obtained as 98.5% at the optimum H2O2 concentration.The EF process was found to be more suitable for TP removal due tofaster removal efficiency (Fig. 4a and b).

Effluent pHs of the treated water were changed as 7.1, 7.5, 7.1,7.8, respectively when influent pHi was 3.0 at 45 min in the EF pro-cess. All final effluent pHf fell into limit values set by dischargestandards of water pollution control regulations (pHf 6–9).

Table 4 showed that values of energy consumptions were calcu-lated by considering for concentration of H2O2 (5–50 mM) variedfrom 0.54 to 0.51 €/m3 in the EF process. In addition, values of en-ergy consumption almost remained with increasing H2O2 concen-tration. The electrode consumptions were almost the same as0.12 kg/m3. The OC of the treated LFW depended on initial H2O2

concentration because the OCs increased from 0.35 to 2.33 €/m3

at 5–50 mM H2O2. The OC at the optimum initial H2O2 concentra-tion was almost twice lower than the OC at 50 mM. The OC in theEF process (1.23 €/m3) was higher than that of EC process (0.74 €/m3) due to additional cost of H2O2 in the EF process. The EF processseemed to be more expensive but, LFW was removed with higherremoval efficiency and lower operating time with the EF processas compared to the EC process.

2.91

9°

6.74

2°

8.62

3°

13.2

62°

19.5

19°

35.8

41°

57.8

40°

0

10

20

30

40

50

Inte

nsity

(Cou

nts)

10 20 30 40 50 60 70

00-030-0662> Vivianite - Fe 3 (PO 4 ) 2 ·8H 2 O

3.65

9°

9.21

7°

12.3

99°

28.7

92° 35

.402

°

40.7

04°

53.4

39°

63.3

58°

0

25

50

75

100

Inte

nsity

(Cou

nts)

10 20 30 40 50 60 7000-030-0662> Vivianite - Fe 3 (PO 4 ) 2 ·8H 2 O

00-001-1106> Lawrencite - FeCl 2

Two-Theta (deg)

Fig. 8. XRD analyses of the sludges (a) EC and (b) EF processes.

500 1000 1500 2000 2500 3000 3500 4000 450070

75

80

85

90

95

100

105EF EC

Tra

nsm

itanc

e (%

)

Wavenumber cm1-

Sludge after EF processSludge after EC process

Fig. 9. FTIR of sludges from the EC and EF processes.

18 A. Akyol et al. / Separation and Purification Technology 112 (2013) 11–19

4.4. Comparisons of removal efficiencies for EC and EF processes

The optimum experimental conditions (50 A/m2 and pH 6 forthe EC, and 50 A/m2 and 25 mM of H2O2 concentration for theEF) were applied for removal of color from LFW (Fig. 5). As seenin Fig. 5, the EF process gave a better color removal efficiency(99%) at 45 min than the EC process with removal efficiency of95% and the effluent became almost visually clear. The color spec-trum bands (400–700 nm) of LFW degraded uniformly in 45 minduring the EC and EF removal processes (Fig. 7). As seen in the fig-ure, there was some oxidation observed for the EF process in

Fig. 7b between 200 and 300 nm whereas there was very low per-centage oxidation present for the EC process in Fig. 7a.

TOC removal efficiencies in 10 min were obtained as 83% for theEF process at 50 A/m2 and 62% for the EC process at 100 A/m2.Moreover, the removal efficiencies of TP were 51% for the EC and90% for the EF processes in 10 min at 100 A/m2 (Fig. 6)).

The results in Table 2 showed that the EF process consumed fivetimes less electrode and energy consumptions but the OC in the EFprocess was 1.7 times higher as compared to the EC process. How-ever, the EF process can be envisaged as an effective removal pro-cess for LFW in terms of better removal efficiencies of TOC and TPin the wastewater. Table 5 also showed the comparison of effluentsof LFW for treated by EC and EF processes.

The different electrochemical processes in the degradation ofalcohol distillery wastewater was studied and the Fenton processwas the best for COD and TOC removal efficiencies which re-moved 92.6% for an initial COD of 4985 mg/L and 88.7% for an ini-tial TOC of 1507 mg/L. In addition, the EC process was notsuccessful at all experimental conditions studied [13]. Martinset al. also investigated with removal efficiencies of nonylphenolpolyethoxylate (NP9EO) by the EC and EF processes. More than95% of NP9EO removal was achieved by both treatments in5 min of the EF and 15 min of the EC. The organic load removalof COD reduction had not surpassed 55% in both the EC and EFprocesses [30].

4.5. Characterization of the sludge

The amount of sludge produced during LFW treatment was animportant problem due to disposal of the solid waste. The sludgeproduction was proportional to characteristics of raw wastewatersettable solids and matter destabilized by coagulation and concen-tration flocculent and was also proportional to current density and

A. Akyol et al. / Separation and Purification Technology 112 (2013) 11–19 19

residence time [39]. The sludge was collected by vacuum filtrationafter the EC and EF processes and dried at 105 �C for 24 h in theoven. Amounts of the sludge produced in the EC and EF processesincreased from 0.85 to 4.61 kg/m3 for the EC process and from 0.62to 1.88 kg/m3 for the EF process at 25–100 A/m2 (Tables 2–4).Characterizations of the sludge samples were carried out by XRDand FTIR. Fe3(PO4)2�8H2O (vivianite) in the sludge was detectedfrom both processes by XRD (Fig. 8). FeCl2 (Lawrencite) was ob-served from the sludge in the EF process.

FTIR spectra of the sludge samples from both processes wereshown in Fig. 9. FTIR spectra of sludge indicated a broad and in-tense band at 3364 cm�1 and 3224 cm�1 attributed to stretchingvibrations of AOH. The bands at 1632 cm�1, 1581 cm�1 and1383 cm�1 referred to the presence of C@O, C@C and CAH. Thestrong bands at 1009 cm�1and 1019 cm�1 showed CAOAC stretch-ing vibrations. FTIR spectra depicted hardly any significant spectro-scopic changes in the sludge samples.

5. Conclusions

In this study, effect of the operational parameters on the EC andEF processes were evaluated based on removal efficiency and oper-ating cost. The optimum operating conditions for LFW were deter-mined as 50 A/m2 and pH 6 for the EC process, and 50 A/m2 and25 mM of H2O2 concentration for the EF process at 45 min. TheEF process resulted in higher removal efficiencies (81% of TOC in10 min, 96% of TP and 99% of color in 45 min) than that of the ECprocess (79% of TOC, 73% of TP and 95% of color efficiency in45 min). Amount of the sludge produced was higher in the EC pro-cess (2.42 kg/m3) than that of the EF process (0.65 kg/m3). How-ever, the operating cost for the EC process was found to be 1.7times lower than that of the EF process since price of H2O2 was af-fected the cost drastically. The obtained results indicated clearlythat the EC and EF processes were very effective for the removalof LFW. Overall, the EF process was more successful for achievinghigher removal efficiencies in terms of TOC, TP and color thanthe EC process and the EF process is a promising technology forapplications in wastewater treatment.

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