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Journal of Water Process Engineering

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TOC and COD removal from instant coffee and coffee products productionwastewater by chemical coagulation assisted electrooxidation

Orhan Taner Cana,⁎, Erhan Gengecb, Mehmet Kobyac

a Bursa Technical University, Department of Environmental Engineering, 16310 Bursa, Turkeyb Kocaeli University, Department of Environmental Protection, 41285 Kartepe Kocaeli, TurkeycGebze Technical University, Department of Environmental Engineering, 41400 Gebze Kocaeli, Turkey

A R T I C L E I N F O

Keywords:Total organic carbonElectrooxidationInstant coffee wastewaterBoron doped diamondChemical coagulation

A B S T R A C T

This paper describes and discusses an investigation into the treatment performance of Boron-Doped Diamond(BDD), Pt coated Ti (Pt) and various Mixed Metal Oxide (MMO) anodes after pre-treatment chemical coagulationfor total organic carbon (TOC) and chemical oxygen demand (COD) removal from instant coffee and coffeeproducts production wastewater. Firstly, the performance of coagulants, AlCl3, Al2(SO4)3, FeCl3 and FeSO4 wereinvestigated. Secondly, the treatment performance of BDD, Pt and various MMO anodes, the influence of theapplied current density and the effect of flow rate for the best-performed electrode were investigated. Also,specific energy consumptions and anode performances were calculated and evaluated for electrooxidationprocess. At the pre-treatment, among the four coagulants, AlCl3 showed the best performance removing 29%TOC and COD 55%. At the electrooxidation process, BDD electrode achieved the best performance. At the end ofthe 6 h run period, BDD electrode achieved 95% TOC and 97% COD removal. On the other hand, Pt and MMOelectrodes just removed 13–22% TOC and 25–50% COD respectively. In this study, the BDD electrode performedmuch better than Pt and MMO electrodes.

1. Introduction

Coffee is one of the favorite beverages of the world and the secondlargest traded commodity after petroleum [1,2]. 55 countries in theworld that are involved in producing coffee as primary agriculturalproduce [3]. Coffee is cultivated in about 80 countries across the globeand entangles colossal business worldwide [1]. The demand for instantcoffee is increasing in today's fast living conditions. This demand showsthat the instant coffee production sector and the wastes from the in-dustry will grow. The wastewater of instant coffee and coffee productsproduction is primarily generated from cleaning operations, involvingapparatus cleaning and floor washing. Instant coffee wastewater in-cludes enormous amounts of macromolecules such as humic acid, lig-nins, or tannins, which causes water pollution. The untreated macro-molecules include the vast number of COD that pollutes various type ofwater resources. Hence it is needed to reduce the pollutant concentra-tion in the instant coffee wastewater before discharging it into the en-vironment [4].

In previous studies, researchers used a variety of treatment methods(Photo-Fenton (UV/Fe2+/H2O2) [5], electrocoagulation-electrooxida-tion [6], thermophilic anaerobic treatment [7], Fenton and Photo-

Fenton [8], mesophilic and thermophilic anaerobic digestion withthermophilic pre-acidification [9], chemical flocculation and advancedoxidation processes [10], electrocoagulation [11]) to remove of pollu-tants from the coffee industry wastewater.

In water and wastewater treatment, coagulation and flocculationprocess are very important. Colloidal material includes mineral sub-stances, small aggregates of precipitated and flocculated matter,viruses, silt, bacteria, plankton, biopolymers, and macromolecules.Such equipment comprises inorganic ions, molecules and polymericspecies, polyelectrolytes, organic molecules, undissociated solutes, andsmall aggregates. Chemical coagulation and flocculation in wastewatertreatment involve the addition of chemicals to change the physical stateof dissolved and suspended solids and assist their removal by sedi-mentation. Coagulation is used for removal of the wastes in a suspendedor colloidal form that does not settle out on standing or may settle bytaking an extremely long time. In water and wastewater treatment,coagulation as the pretreatment is regarded as the most popular pre-treatment [12–15]. Chemical coagulation has been used in the treat-ment of much different wastewater such as tannery wastewater [16],pulp and paper wastewater [17], textile industry wastewater [18],distillery wastewater [19], petrochemical wastewater [20], and food

https://doi.org/10.1016/j.jwpe.2019.01.002Received 3 September 2018; Received in revised form 4 January 2019; Accepted 4 January 2019

⁎ Corresponding author.E-mail addresses: ocan@gtu.edu.tr, orhan.can@btu.edu.tr (O.T. Can).

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T

industry wastewater [21]. The generally used metal coagulants fall intotwo general categories: those based on iron, and those based on alu-minum. The iron coagulants include ferrous sulfate, ferric sulfate, ferricchloride sulfate, ferric chloride, polyferric sulfate, and ferric salts withorganic polymers. The aluminum coagulants contain aluminum sulfate(This is probably the most extensively used coagulant and has been inuse for water treatment for several centuries.), aluminum chlorohy-drate, aluminum chloride, sodium aluminate, polyaluminum chloride,polyaluminum sulfate chloride, polyaluminum silicate chloride, andforms of polyaluminum chloride with organic polymers. Main dis-advantages that might preclude a wholly physical-chemical solution towastewater treatment are the problems related to the highly putresciblesludge produced, and the high operating costs of chemical addition[15]. To reduce that disadvantage, coagulants were used in as smallquantities as possible in this study. The coagulant used here is intendedto remove suspended pollutants, rather than complete removal.

Electrooxidation has been used increasingly in recent years in thetreatment of various wastewaters such as textile wastewater [22], toxiccardboard plant wastewater [23], pharmaceutical industry wastewater[24], gelatin production plant wastewater [25] and landfill leachate[26]. Electrooxidation of pollutants can take place using two differentmechanisms direct anodic oxidation, where the pollutants are destroyedat the anode surface and indirect oxidation where a mediator is elec-trochemically generated to carry out the oxidation. It has to be kept inmind that during the electrooxidation of wastewater, both oxidationmechanisms may coexist [27]. Electrooxidation is an environmentallyfavorable technology capable of mineralizing non-biodegradable or-ganic matter and to eliminate nitrogen species. Two lines of in-vestigation have been followed: (1) replacement of traditional processesby electrochemical oxidation and (2) integration of electrooxidationinto a treatment process [28]. This study is using chemical coagulationas the pre-treatment to increase the performance of anode electrodesand reduce the high-energy demand of electrooxidation.

In this study, it was aimed to examine the treatment performance(removal and energy consumption) of BDD, Pt and various MMO an-odes after chemical coagulation for TOC and COD removal from instantcoffee and coffee products production wastewater (ICCPPW). Firstly,chemical coagulation was applied to the wastewater, to remove col-loidal and suspended particles, and it was followed by Electrooxidationto eliminate the remaining dissolved persistent organic compounds. Inthe chemical coagulation experiments, 4 type coagulant and theirconcentrations were tested, to study their influence on the removal ofTOC and COD. In the electrooxidation experiments the treatment per-formance of BDD, Pt and various MMO anodes, the impact of the ap-plied current density and the effect of flow rate for the best-performedelectrode and the chemical coagulation pre-treatment coagulant con-centration, was studied. Also, specific energy consumptions and anodeperformance of anodes were evaluated for electrooxidation process.

2. Materials and methods

2.1. Materials

The ICCPPW was obtained from the instant coffee and coffee pro-ducts production factory in Turkey. The ICCPPW samples were stored at4 °C accordance with the standard methods [29]. Table 1 shows thetypical characteristics of the ICCPPW. In the coagulation-flocculationprocesses, four different coagulant salts were used (FeSO4, FeCl3,Al2(SO4)3, and AlCl3). All the chemicals used in the experiments wereanalytical quality Merck products. BDD (Boron Doped Diamond), Ptand MMO (Mixed Metal Oxide) electrodes (Ti/RuO2-TiO2, Ti/RuO2-IrO2, Ti/IrO2-Ta2O5, Ti/Pt-IrO2) were used as the anode, while astainless-steel electrode was used as the cathode. Pt and MMO elec-trodes are Ti-based electrodes. BDD electrodes were supplied fromDiaCCon GmbH Germany. Pt and MMO electrodes were supplied fromBaoji Changli Special Metal Co., Ltd. China.

2.2. Experimental apparatus and procedure

Chemical coagulation runs were performed in a 2000mL Pyrex glassreactor. The reactor was equipped with a magnetic stirrer (HeidolphMR Hei-Tec model digital magnetic stirrers). 2000mL of ICCPPW wasplaced in the reactor. A rotation speed of 100 rpm was applied for2min, followed by slow mixing for 15min at 50 rpm. Then coagulatedICCPPW was let to settle for 30min and the supernatant was collectedfrom the top for the electrooxidation experiments.

Electrooxidation runs were performed in a 1000mL Pyrex glass.500mL of coagulated ICCPPW was placed in the Pyrex glass reactor foreach run. One anode and one cathode in sizes of 200*60*2mm wereused in the reactor. Both the anodes and cathodes were placed verti-cally parallel to each other. The inter-electrode gap was kept constant at5mm. The total active surface area of the anodes, depending on theimmersion level in the solution was 40 cm2. Before each run, theelectrodes were wiped with phosphoric acid or acetone to remove im-purities deposited on the surface and washed with distilled water. Thereactor was equipped with a magnetic stirrer, and the stirring speed wasadjusted at 500 rpm to keep the wastewater homogeny mixed. For thispurpose, Heidolph MR Hei-Tec model digital magnetic stirrers wereused. The system powered by Agilent and Rigol brand programmable(Agilent 6674 A System; 0–60 V/0–35 A, Rigol DP832 0–30 V / 0–3 A)digital (D.C.) direct current power supply. The temperature increasecaused by the interelectrode voltage across the reactor was preventedby blowing cold air to the reactor wall from the outside. Thus, thetemperature of the solution was maintained at 25 ± 1 °C through therun, which was monitored using a laser thermometer. For continuousruns, a peristaltic pump was used. No pH adjustment was made duringthe runs. The runs were completed in 360min. TOC and COD valueswere measured by sampling 20min or every hour.

2.3. Analytical method

All the chemical analyses were carried out by the standard methodsfor the examination of water and wastewater [29]. The COD value wasdetermined by dichromate open reflux method as per Standard Method5220-B. The TOC levels were determined through combustion of thesamples at 680 °C using a non-dispersive IR source (Shimadzu, TOC-Lmodel) by non-purgeable organic carbon method. All the experimentswere repeated twice, for checking the reproducibility of the results. Themaximum experimental error was below 2%, and the average valueshave been reported. Each measurement was made with three repeti-tions, and the average of 3 results was obtained.

The TOC and COD removal efficiency, E, is calculated as

=

−

EC C

Cx(%) 100i f

i (1)

Where; Ci is initial TOC or COD concentration (mg/L) and Cf is the finalconcentration (mg/L).

On that account, specific energy consumption (SEC) based on CODand TOC removal was calculated, which is the main component of the

Table 1Typical characteristics of ICCPPW.

Parameter Amount

pH 20 °C 4.8COD (Chemical Oxygen Demand) mg/L 2380BOD5 (Biochemical Oxygen Demand) mg/L 820BOD5 / COD ratio 0.34TOC (Total Organic Carbon) mg/L 757TOC/COD ratio 0.32TSS (Total suspended solids) mg/L 1241Clˉ (Chloride) mg/L 350Physical Description Light brown, with high odorConductivity (μS/cm) 3800

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running costs of most electrochemical treatment processes. SEC is de-fined as the amount of energy consumed per unit mass of COD (CODrem)or TOC removed (TOCrem) and expressed in kWh/kg of COD or TOCremoved and is given as:

=SEC kWhkgCOD

V I t COD( ) ( * * )/ rem(2)

=SEC kWhkgTOC

V I t TOC( ) ( * * )/ rem(3)

Where; V is the voltage across the electrodes, I is the current in am-peres, t is the time in hours, CODrem is the COD removal, and TOCrem isthe TOC removal.

The anode efficiencies (η) of the electrooxidation for the wastewaterwere calculated from the following equations in milligrams (mg) ofCOD removed (CODrem), or TOC removed (TOCrem) per h per ampereper square meter. Where; (COD)exp is the experimental COD decay (orCODrem) (mg/L), (TOC)exp is the experimental TOC decay (or TOCrem)(mg/L), I is the current in amperes, t is the time in hours, S anode is theactive anode area.

=η mg COD AhmCOD VItS

( / )Δ( ) *exp s

anode

2(4)

=η mg TOC AhmTOC VItS

( / )Δ( ) *exp s

anode

2(5)

From the obtained COD data, the Instantaneous Current Efficiency(ICE, in %) for an electrolyzed (treated) wastewater at a given time (h)calculated as follows: [30,31].

=−

×+ICE F V COD COD

I t(%) [( ) ( ) ]

8 Δ100s t t tΔ

(6)

Where; F is the Faraday constant (96,487 C/mol), Vs is the solution(wastewater) volume (L), COD( )t and +COD( )t tΔ are the COD at times tand t+Δt (g/L), respectively and 8 is a dimensional factor for unitconsistency (32 gO2mol−1 O2/4mol e−1 mol−1 O2), t is the time inseconds.

3. Results and discussion

This study investigated TOC and COD removal from productionwastewater of instant coffee and coffee products in 3 phases. In the firstphase, wastewater was subjected to coagulation-flocculation for theremoval of suspended organics using four different types of coagulantsalt to determine the most effective one. In the second phase, severaldifferent types of anode were used in the electrooxidation process forTOC and COD removal from wastewater to identify the most effectiveone. In the third phase, the most efficient anode electrode was studiedin batch and continuous mode at different current densities to in-vestigate treatment efficiencies.

3.1. The effects of coagulant dosage for TOC removal performance

When the wastewater with high rates of suspended organics wassubjected to electrooxidation process, some of the suspended organicswere converted to soluble form as a result of the circulation to whichthe wastewater in the process was exposed during the first minutes ofthe electrooxidation process. This unwanted phenomenon affects theefficiency of the electrooxidation process and operating conditionsadversely. To avoid this and to increase the effectiveness of the elec-trooxidation process, the production wastewater of coffee and coffeeproducts was subjected to coagulation-flocculation for the removal ofsuspended organics before electrooxidation process. The optimal coa-gulant was determined by comparing the coagulation-flocculationprocesses in which four different coagulant salts were used (FeSO4,FeCl3, Al2(SO4)3, and AlCl3)

Fig. 1 shows the TOC values of coffee and coffee products produc-tion wastewater after coagulation-flocculation using four differentcoagulant salts. Fig. 1 indicates that two aluminum salts achieved thebest removal. Al2(SO4)3 and AlCl3 reduced the TOC value from 757mg/L to 549mg/L and 539mg/L, respectively.

Coagulation-flocculation was performed on the wastewater by ap-plying the two aluminum coagulant salts, AlCl3 and Al2(SO4)3, at dif-ferent doses to determine optimal coagulant dosage. Fig. 2 shows TOCand COD removal efficiencies for AlCl3 and Al2(SO4)3 coagulant salts.After coagulation-flocculation, the wastewater color became sub-stantially clear and turned light yellow. The TOC and COD values forAlCl3 decreased to 539mg/L and 1064mg/L, respectively while theTOC and COD values for Al2(SO4)3 decreased to 549mg/L and1156mg/L, respectively

3.2. The effects of anode type on the TOC and COD removal efficiency

The removal efficiencies of the aluminum coagulants (Al2(SO4)3 andAlCl3) were close to one another. Therefore, the effect of the anode typewas also investigated for coagulation-flocculation using both coagu-lants to monitor how aluminum coagulants or their residues (SO4

− andCl−) would act on TOC and COD removal during electrooxidation.

To investigate the effects of anode type on the TOC and COD re-moval efficiency from the coffee and coffee products production was-tewater by electrooxidation process 6 type anode (BDD electrode, Ptand 4 MMO electrodes) was used. All runs were achieved at J 75mA/cm2 current density.

Fig. 3 shows the removal efficiencies after electrooxidation with

Fig. 1. The TOC value after coagulants dosage (Ferrous dosage: 112mg/L,Aluminum dosage: 112mg/L (1000mg/L AlCl3, 1380mg/L Al2(SO4)3).

Fig. 2. The TOC and COD removal efficiency for AlCl3 and Al2(SO4)3 coagulant(conditions: pH 6.5, Aluminum dosage: 28, 56, 112, 224 and 448mg/L).

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different electrodes applied to the wastewater partially purified frompollutants after coagulation-flocculation with AlSO4. Fig. 3(a) and (b)show TOC and COD removal efficiencies for different anode electrodes.Figs show that BDD electrode achieved much better removal than Ptand MMO electrodes. At the end of the 6 h run period, BDD electrodeachieved 91% TOC and 93% COD removal while TOC and COD removalof Pt and MMO electrodes ranged from 9 to 10% and from 17 to 23%,respectively. This result indicates that BDD electrode yields 4–10 timesbetter TOC and 4–5 times better COD removal efficiency than Pt andMMO electrodes.

Fig. 3(c) and (d) show the SEC values calculated depending on theamount of TOC and COD removal for each anode electrode. Fig. 3 in-dicate that the lowest SEC values were obtained for BDD electrode withthe highest removal efficiency while the highest SEC values were ob-tained for the Pt-IrO2 electrode with the lowest TOC removal efficiencyand the RuO2-TiO2 electrode with the lowest COD removal efficiency.

Fig. 4 shows the removal efficiencies after electrooxidation withdifferent electrodes applied to the wastewater partially purified frompollutants after coagulation-flocculation with AlCl3. Fig. 4(a) and (b)show TOC and COD removal efficiencies for different anode electrodes.Figs show that BDD electrode achieved much better removal than Ptand MMO electrodes. At the end of the 6 h run period, BDD electrodeachieved 95% TOC and 97% COD removal while TOC and COD removalof Pt and MMO electrodes ranged from 13 to 22% and from 25 to 50%,respectively. This result indicates that BDD electrode yields 4–7 timesbetter TOC and 2–4 times better COD removal efficiency than Pt andMMO electrodes.

Fig. 4(c) and (d) show the SEC values calculated depending on theamount of TOC and COD removal for each anode electrode. Fig. 4 in-dicate that the lowest SEC values were obtained for BDD electrode withthe highest removal efficiency while the highest SEC values were ob-tained for the RuO2-TiO2 electrode with the lowest TOC removal effi-ciency and with the lowest COD removal efficiency.

Table 2 summarizes the efficiency of the anode electrodes de-pending on the parameters of electrooxidation removal efficiency andenergy consumption. Table 2 indicates that the BDD electrode is su-perior in performance to other Pt and MMO electrodes in all para-meters. The anode efficiency (η) and instantaneous current efficiency(ICE) of BBD electrode are about 2–3 times and 2–5 times higher thanthose of other electrodes, respectively. Pt electrodes performed thesecond best. During anodic oxidation, two species of active oxygen canbe electrochemically generated on the surface of anodes. The chemi-sorbed which is responsible for the electrochemical conversion gen-erally produced on carbon, graphite and MMO anodes. While the otheris physisorbed which is responsible for electrochemical combustion,usually produced on PbO2, SnO2 and BDD anodes. During indirectoxidation, the anodically generated agents like chlorine and hypo-chlorite, hydrogen peroxide and ozone are responsible for the oxidationof pollutants (organics) [32].

The effect of coagulant (or coagulant residues) used for coagulation-flocculation before electrooxidation is most evident when MMO elec-trodes were used as the anode. While the use of AlCl3 or Al2(SO4)3coagulant before electrocoagulation slightly affected the performanceof BDD anode electrode, it had an effect on Pt and MMO electrodes in

Fig. 3. (a) The TOC removal efficiency for BDD, Pt and MMO electrodes (b) The COD removal efficiency for BDD, Pt and MMO electrodes (c) Effect of electrode typeon SEC for TOC (d) Effect of electrode type on SEC for COD after Al2(SO4)3 coagulant coagulation (conditions: initial TOC C0: 549mg/L COD C0: 1156mg/L, currentdensity: 75mA/cm2, stirring speed: 500 rpm, distance between the electrodes: 5mm, initial pH: 6.5.).

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favor of AlCl3; increases were observed in anode efficiency (η)(33–133%) and instantaneous current efficiency (ICE) (60–97%). Thispositive contribution of AlCl3 coagulant is attributed to the increase inthe activity of electrooxidation with the participation of Cl− ions,which are left in the water after chemical coagulation, in indirect oxi-dation in the electrooxidation process.

Table 3 summarizes the results of the first two stages of TOC andCOD removal from production wastewater of coffee and coffee pro-ducts. Table 3 indicates that using AlCl3 as the coagulant for

Fig. 4. (a) The TOC removal efficiency for BDD, Pt and MMO electrodes (b) The COD removal efficiency for BDD, Pt and MMO electrodes (c) Effect of electrode typeon SEC for TOC (d) Effect of electrode type on SEC for COD after AlCl3 coagulant coagulation (conditions: initial TOC C0: 539mg/L COD C0: 1064mg/L, currentdensity: 75mA/cm2, stirring speed: 500 rpm, distance between the electrodes: 5mm, initial pH: 6.5.).

Table 2Electrooxidation parameters during the TOC and COD removal run of coffee and coffee products production wastewater by different anodes.

Anode Before Eox, Coagulated with CODrem TOCrem U SEC η ICE

(112mg/L Al) (%) (%) (V) (kWh/kgCODrem) (kWh/kgTOCrem) (gCOD/Ahm2) (gTOC/Ahm2) (%)

BDD AlCl3 96.51 94.62 15.70 550.40 1108.24 7.13 3.54 9.56Al2(SO4)3 92.90 91.07 15.90 533.02 1144.80 7.46 3.47 9.99

Pt AlCl3 49.50 21.52 11.60 792.91 3600.00 3.66 0.81 4.90Al2(SO4)3 23.04 10.02 11.90 1608.32 7789.09 1.85 0.38 2.48

Pt-IrO2 AlCl3 38.74 16.14 13.90 1214.04 5751.72 2.86 0.60 3.84Al2(SO4)3 22.30 9.29 14.10 1969.49 9952.94 1.79 0.35 2.40

RuO2-TiO2 AlCl3 25.03 13.17 13.00 1757.44 6591.55 1.85 0.49 2.48Al2(SO4)3 17.30 9.11 13.20 2375.57 9504.00 1.39 0.35 1.86

RuO2-IrO2 AlCl3 34.69 15.77 11.30 1102.01 4785.88 2.56 0.59 3.44Al2(SO4)3 20.04 9.11 11.60 1802.94 8352.00 1.61 0.35 2.16

IrO2-Ta2O5 AlCl3 33.11 15.40 9.50 970.86 4120.48 2.45 0.58 3.28Al2(SO4)3 18.80 8.74 11.00 1822.34 8250.00 1.51 0.33 2.02

U: average voltage between electrodes,η: Anode efficiency, t for 360min.

Table 3TOC and COD values after coagulation and electrooxidation.

PollutionParameter

Initial(mg/L)

After Coagulation with(112mg/L Al)

After Electrooxidation byBDD Anode

AlCl3 Al2(SO4)3 AlCl3 Al2(SO4)3

TOC 757 539 549 29 49COD 2380 1064 1156 37 82

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coagulation-flocculation before electrooxidation yielded better resultsin removal efficiencies. In the light of this result, coagulation-floccu-lation in the wastewater before electrooxidation was carried out usingAlCl3 coagulant in the later stages of the study.

3.3. TOC and COD removal using BDD electrode

3.3.1. Effect of current densityFig. 5(a) and (b) show TOC and COD removal efficiencies for BDD

electrode at different current densities. Fig. clearly, indicate that TOCand COD removal directly depends on current density. At the end of the6 h run period, 95–96% TOC and 100% COD removal efficiencies wereobtained for current densities of 100 and 125mA/cm2. 84% TOC and86% COD removal efficiencies were obtained for the lowest currentdensity of 25mA/cm2. These values are satisfactory in the sense thatthey were obtained at 5 times lower current density. The most apparentdifference in the removal efficiencies for all these 5 different currentdensity values was observed in the middle of the 360-min run time. Inthese minutes, the difference between the removal efficiencies can bemore than 2 times. At the 180. min., 41% TOC and 43% COD removalefficiencies were obtained for 25mA/cm2 while 92% TOC and 125%COD removal efficiencies were obtained for 125mA/cm2. These resultsindicate that oxidation and/or mineralization of pollutants in electro-oxidation depends on the pollutant composition, current density, andduration (or flow rate).

Fig. 5(c) and (d) show the SEC values calculated depending on theamount of TOC and COD removal for different current density values,indicating that the SEC values increase in parallel with the appliedcurrent density. The lowest SEC values were obtained for 25mA/cm2,while the highest SEC values were obtained for 125mA/cm2. At the endof 360min, the difference between the lowest and highest removal ef-ficiencies is around 12–14%, whereas the SEC rates can be 8 timesdifferent.

Among the variables that are usually modified in electrooxidationprocesses, the current density may be the term most frequently pointedto because it controls the reaction rate. It should be highlighted that anincrease in current density does not necessarily result in an increase inthe oxidation efficiency or oxidation rate and that for a given anodeelectrode material, the effect of current density on the treatment (orremoval) efficiency depends on the specification of the effluent (oraqueous media) to be treated. Though the use of higher current den-sities generally results in higher operating costs due to the increase inenergy consumption [28]. Table 4 summarizes the efficiency of thecurrent density depending on the parameters of electrooxidation re-moval efficiency and energy consumption, indicating that as the currentdensity increases, the removal efficiencies increase, however, energyconsumption and performance parameters are affected negatively bythis. When the current density is increased by 5 times, the anode effi-ciency (η) and instantaneous current efficiency (ICE) of BBD electrodedecrease 4.3–4.4 times and 4.3 times, respectively. This result indicates

Fig. 5. (a) The TOC removal efficiency for BDD electrodes (b) The COD removal efficiency for BDD electrodes (c) Effect of current density on SEC for TOC (d) Effect ofcurrent density on SEC for COD after AlCl3 coagulant coagulation (conditions: initial TOC C0: 539mg/L COD C0: 1064mg/L, current density: 75mA/cm2, stirringspeed: 500 rpm, distance between the electrodes: 5mm, initial pH: 6.5.).

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that operating at low current density is more profitable regarding en-ergy efficiency and anode performance.

Direct oxidation during the electrooxidation is controlled by thediffusion of the pollutants (organics) towards the electrode surface,where the ⦁OH is produced, and the current efficiency is favored by ahigh mass-transport coefficient, high pollutant (organic) concentration,and low current density. Achieving electrolysis in optimized conditions,without diffusion limitation, the current efficiency reaches 100%. Forhigh pollutant (organic) concentrations and low current densities, theCOD decreased linearly and the ICE remained about 100%, while forlow pollutant (organic) concentrations or high-current densities, the

COD decreased exponentially and the ICE started to decrease due to themass-transport limitation and the side reactions of oxygen evolution[33].

3.3.2. Effect of flowEffect of flow rate on the TOC and COD removal from the coffee and

coffee products production wastewater was studied in a range of300mL/h (5mL/min.), 600mL/h (10mL/min.) and 1200mL/h(20mL/min.) at a constant current density of 75mA/cm2 (Fig. 6a andb) and 125mA/cm2 (Fig. 6c and d).

Table 4Electrooxidation parameters during the TOC and COD removal run of coffee and coffee products production wastewater by BDD anode.

Current Density Before Eox, Coagulated with CODrem TOCrem U SEC η ICE

mA/cm2 (112mg/L Al) (%) (%) (V) (kWh/kgCODrem) (kWh/kgTOCrem) (gCOD/Ahm2) (gTOC/Ahm2) (%)

25 AlCl3 86.10 84.42 10.00 130.98 263.74 19.09 9.48 25.5850 AlCl3 91.78 89.98 13.00 319.49 643.30 10.17 5.05 13.6375 AlCl3 96.51 94.62 15.70 550.40 1108.24 7.13 3.54 9.56100 AlCl3 100.00 95.18 17.00 766.92 1590.64 5.54 2.67 7.43125 AlCl3 100.00 96.10 18.30 1031.95 2119.69 4.43 2.16 5.94

U: average voltage between electrodes,η: Anode efficiency, t for 360min.

Fig. 6. (a) The TOC removal efficiency for 75mA/cm2 at three different flow (b) The COD removal efficiency for 75mA/cm2 at three different flow (c) The TOCremoval efficiency for 125mA/cm2 at three different flow (d) The COD removal efficiency for 125mA/cm2 at three different flow after AlCl3 coagulant coagulation(conditions: initial TOC C0: 539mg/L COD C0: 1064mg/L, current density: 75mA/cm2, stirring speed: 500 rpm, distance between the electrodes: 5mm, initial pH:6.5.).

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4. Conclusion

Electrooxidation process successfully applied to the treatment ofICCPPW after pre-treatment chemical coagulation. Pre-treatment withchemical coagulation removed the suspended organics and helped tomake electrooxidation successful. In the pre-treatment stage, 29% TOCand COD 55% removed from wastewater. In the electrooxidation stage,all the remaining COD and almost all the TOC were removed from thewastewater. TOC and COD removal on the electrooxidation directlydepends on current density and anode electrode performance. BDDelectrode performed 74–82% TOC and 47–72% COD removal efficiencybetter than the other electrodes. When BDD anode was used 95–96%TOC, and 100% COD removal efficiencies were obtained for currentdensities of 100 and 125mA/cm2 at the end of the 6 h run period. Thisstudy showed that this sequential process could be an essential alter-native for the treatment of such wastewater when the BDD electrode isused in the process.

Acknowledgment

The authors wish to extend their sincere gratitude to all who sup-ported this work.

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