Journal of Environmental Chemical Engineering 2 (2014) 1527–1532

Carbon attrition during continuous electrolysis in carbon bed basedthree-phase three-dimensional electrode reactor: Treatment ofrecalcitrant chemical industry wastewater

Nitin Gedam, Nageswara Rao Neti *Wastewater Technology Division, CSIR-National Environmental Engineering Research Institute (CSIR-NEERI), Nagpur, Maharashtra 440020, India

A R T I C L E I N F O

Article history:Received 23 May 2014Accepted 28 June 2014

Keywords:Three-phase three-dimensional electrodeGranular activated carbonCarbon attritionChemical oxygen demandRecalcitrant wastewater treatment

A B S T R A C T

The performance of a carbon bed based three-dimensional electrode reactor (TDR) in terms of chemicaloxygen demand (COD) removal from recalcitrant chemical industry wastewater was assessed. The pHand temperature changes in the TDR during electrolysis were correlated with COD removal efficiency. Thecarbon weight loss and particle size reduction due to erosion of carbon particles during electrolysis wasalso examined. Two cases of experiments were performed; ‘case I’ employed a high surface area Indcarb-60 GAC, whereas a low surface area carbon (GAC-10) was used for ‘case II’. The other experimentalvariables are, initial COD concentration, hydraulic retention time (HRT) and the duration of electrolysis.The experimental results showed that TDR could remove COD efficiently (49 � 7%). The apparent Faradicefficiency and specific electrical energy consumption were estimated to be 3.42% and 6.59 kW h kg�1 CODfor case I and 0.78% and 28.65 kW h kg�1 COD for case II. Use of high surface area carbon in TDR is inferredto be beneficial. However, the GAC particles in TDR were found to undergo slow attrition duringelectrolysis. It is inferred that carbon attrition may prove to be a major setback for scale up attempts as itcan lead to gradual loss in liquid holding capacity of carbon bed due to stratification and filling of voids inthe carbon bed with carbon fine dust.

ã 2014 Elsevier Ltd. All rights reserved.

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1. Introduction

High strength chemical industry wastewater requires highdegree of treatment before being discharged into the water bodies.The conventional treatment incorporating chemical and biologicaltreatments alone is not adequate. More recently, advancedoxidation processes such as electrochemical oxidation (EO)treatment are being increasingly viewed as capable of providingnecessary treatment [1–4]. A variety of design options for settingup an electro oxidation reactor are available, viz., cell configuration(divided and undivided), electrode configuration (two-dimension-al and three-dimensional) and flow-types (plug flow andcontinuously stirred tank type) [5]. Our group has been workingtowards the development of three-phase three-dimensionalcarbon bed electrochemical reactor (TDR) with the aim of treatingrecalcitrant effluents from chemical industries [6–10]. Thus,leachate from a toxic solid waste disposal facility could be treatedeffectively i.e., 60–64% COD removal with >80% mineralization

* Corresponding author. Tel.: +91 712 2249885 88/2249970 72;fax: +91 712 2249900.

E-mail addresses: nn_rao@neeri.res.in, nnrao.neeri@gmail.com (N.R. Neti).

http://dx.doi.org/10.1016/j.jece.2014.06.0252213-3437/ã 2014 Elsevier Ltd. All rights reserved.

efficiency using TDR in 6 h [7]. On the other hand, caprolactamwastewater underwent poorer degradation in TDR with only �18%COD removal in 7 h [8]. While the above research clearlydemonstrates that electro oxidation of recalcitrant effluents inTDR is feasible, there is need for optimizing the operationalparameters (pH, flow rate etc.). Moreover, the electrochemicaldegradation of wastewater using such TDR in continuous mode isnot comprehensively investigated, particularly with reference tostability of carbon surface on long term use.

In this study, continuous electrolysis of chemical industrywastewater in the TDR was investigated. The performance of thereactor for consistent removal of pollutant (COD) was aimed at,while the other related issues viz., stability of carbon particles,changes in pH, and temperature were also investigated.

2. Material and methods

2.1. Effluent

The segregated high strength chemical industries wastewaterused in this study was obtained from a common effluent treatmentplant (CETP) site in Gujarat (India). The dark brown effluent withpH 8.1 contained high COD in the range 5–10 g L�1, 1.1–2.4 g L�1

1528 N. Gedam, N.R. Neti / Journal of Environmental Chemical Engineering 2 (2014) 1527–1532

TKN (Total Kjeldahl Nitrogen), 0.35–0.9 g L�1 ammonia nitrogen(NH4

+-N) and 17–22 g L�1 chloride. The wastewater used for thepresent study may be regarded as non-biodegradable highstrength wastewater (BOD/COD � 0.2).

2.2. Granular activated carbon

Two different types of granular activated carbon (GAC) wereused, Indcarb-60 (IC-60) with particle size ranges from 2 to 3 mm,1100 m2g�1 surface area and GAC-10 with particle size >4 mm,surface area 600 m2g�1. The GAC was purchased from IndustrialCarbons Pvt. Ltd., Baroda, India and it was used in the threedimensional carbon bed electrochemical reactors.

2.3. Reactor setup and experimental procedure

Continuous carbon bed electrolysis of high strength chemicalindustry wastewater was carried out using three dimensionalcarbon bed electrochemical reactors described elsewhere [7,8].The schematic of the TDR is given in Fig. 1. The TDR was arectangular 2.6 L tank (18 cm � 12 cm � 12 cm) fabricated usingthe Perspex1 sheet of 6 mm thickness. In this reactor, twostainless steel plates (10 cm � 14 cm) were used as cathode andsituated 12 cm apart on opposite side walls of the reactor. In themiddle of the reactor a 12 mm carbon plate was used as anodein between SS cathodes. This carbon anode was perforated bydrilling 2 mm diameter holes to allow free flow of liquid acrossanode. Approximately 1 kg of washed and dried GAC waspacked into the space between the cathode and anode to form athree dimensional electrode (GAC bed height, 10 cm) and airwas bubbled from the bottom of the reactor through an airdistribution manifold. Current/potential was applied using aregulated direct current power supply (Aplab, LD3210). Beforeelectrolysis experiment, the TDR bed was pre-saturated bycirculating the raw effluent (10 L, pH 8.1) that was fortified with

Fig. 1. Schematics of carbon bed based thre

NaCl (2 g L�1) through the TDR bed at a rate equal to 0.1 L min�1

by using peristaltic pump (Watson-Marlow, 323 DU).Two types of experiments in continuous carbon bed electrolysis

of wastewater were performed: case I – it was carried out by usingindcarb-60, HRT of 2 h for 5 days with raw effluent (COD,10,358 mg L�1) and case II employs GAC-10, HRT of 4 h for 10 daysand raw effluent having COD, 5200 mg L�1 was used. Continuousreaction was performed at 3 A (constant current) and 5.5–7.9 V.Aliquots were taken from the outlet of the reactor at pre-selectedtime intervals and analyzed for COD reduction. GAC weight lossstudy was also performed. Experiments in continuous operationwith the different current viz., 1, 2, and 3 A were carried out usingbrine electrolyte. Fresh GAC-10 (1 kg) was packed in TDR and 50 Lof sodium chloride solution (18 g L�1) passed through in eachexperiment with constant HRT of 4 h. GAC after electrolysis waswashed, dried overnight at 110 �C and sieved (ASTM 5). GACparticle size <4 mm was collected and weighed to assess theweight loss gravimetrically. The wash water (10 L) was filteredthrough glass fiber (1.2 mm) and residue collected was driedovernight at 110 �C and weighed.

The apparent Faradic efficiency (hF) of COD removal wascalculated using following equation [7,8,10].

hF ¼ DCOD � V � F8 � I � Dt

(1)

where DCOD is the net COD removed (g L�1) after a treatment timet (h), V is the volume of treated effluent (L), F is Faraday’s constant(96,487 C equivalent�1), 8 is the equivalent weight of oxygen, I isthe applied current (A) and Dt is the treatment duration (s).

Specific energy consumption (Esp), the electric energy inkilowatt hours required to degrade a kilogram of a pollutant inwastewater, was calculated using the following equation forcontinuous mode of operation [7,8,10].

ESP ¼ P � t � 106

VðC0 � CtÞ (2)

e-dimensional electrode reactor (TDR).

N. Gedam, N.R. Neti / Journal of Environmental Chemical Engineering 2 (2014) 1527–1532 1529

where P is the rated power (kW) of the reactor, V is the volume (L)of wastewater treated in the time t (h), C0, and Ct are the initial andfinal concentrations (mg L�1) of a pollutant, and the factor of 106

converts mg to kg.

2.4. Analyses

The wastewater was analyzed for COD according to standardmethods [11]. Since the samples contained higher concentration ofchloride which is known to effect the COD determination, thesamples were adequately diluted to obtain the concentration ofchlorides <2.0 g L�1 prior to COD determination by using openreflux method. The observed error in the determination of COD induplicate tests was about 2–5%. Residual free chlorine was alsoanalyzed in the aliquot from the TDR by DPD colorimetric method(Pocket ColorimeterTM II, HACH). Treated effluents (1 L) wascollected from outlet, stirred and allowed to settle in a measuringcylinder to measure the volume of sludge generated.

3. Results and discussion

Recalcitrant chemical industry wastewater was treated incontinuous three dimensional three phase carbon bed electrolyzer.As mentioned in Section 2.3, the continuous operation was carriedout over a period of 5 and 10 days covering two different cases: caseI with GAC Indcarb-60 as a packed bed electrode with 2 h HRT andraw effluent having 10,358 mg L�1 of COD, pH 7.7, passed throughTDR for 5 days and case II using GAC-10 as packed bed electrode,raw effluent (5200 mg L�1 COD, pH 7.95) passed for 10 days withthe HRT of 4 h.

3.1. Continuous carbon bed electrolysis

The time course variation in normalized COD representing CODremoval from the wastewater during treatment in TDR showsefficient performance with % COD removal about 49 � 7 (Fig. 2).High total dissolved solids and added sodium chloride (2 g L�1) inthe raw effluent lead to initiation of indirect oxidation in the TDR.There are two types of electrochemical degradation mechanismswhich are well known i.e., direct oxidation and indirect oxidation[5]. Both direct and indirect oxidation routes may have resulted inconsistent COD removal in this study. Some of the importantreactions that may be taking place at main feeder electrodes aswell as at the polarized carbon particles may be written as follows:

0.00

0.20

0.40

0.60

0.80

1.00

0 50 100 150 200 250

Ct/

C0

Time (h)

Case-IICase-I

Fig. 2. Time course variation in COD removal in TDR: (case I) – Indcarb-60 GAC;HRT, 2 h; run duration, 5 days; raw effluent COD (initial), 10,358 mg L�1; (case II) –

GAC-10; HRT, 4 h; run duration 10 days; raw effluent COD (initial), 5200 mg L�1.

Adsorption at carbon

(1) C + Org. Pollutants (R) ! C(R)ads

Water and chloride oxidation at anode

� H2O !�OH + H++ e�� 2Cl�! Cl2 + 2e�

Reduction of water and other reducible compounds at cathode

� 2H2O + 2e�! H2+ 2OH�

Disproportionation of electro-generated molecular chlorine

� Cl2 + H2O ! HOCl + H+ + Cl� (at pH �7.5)� HOCl ! H+ + OCl� (at pH >7.5)

Oxidation of organic compounds

� R + �OH + (Cl2/H2O) ! CO2 + H2O (COD, TOC removal)

It may be assumed that electrochemical mineralization oforganic compounds in the effluent occurs with concomitantevolution of oxygen and chlorine at anode (steps 2 and 3) andhydrogen evolution at SS cathodes (4). The OH radicals from step(2) and oxy-chloro species from steps (5 and 6) simultaneouslyoxidize organic pollutants represented by reaction step (7). Thedetailed mechanistic aspects of pollutant removal in TDR systemhave been already discussed in our earlier work [7]. The TDRcomprises a set of anodes and cathodes located in carbon bed in asimple ‘undivided’ electrochemical reactor configuration. Variousprocesses, viz., adsorption, oxidation, electrolysis can be expectedto occur simultaneously. The water electrolysis reactions (steps 2and 4) lead to significant pH changes which often effect the otherintended oxidation reactions. Therefore, the COD, pH andtemperature do not change for a uniform pattern but representoverall changes on long time scale measured in days.

The efficiency of COD removal decreased slowly with the timeof operation. The COD removal efficiency was found to decrease by0.96% daily over a period of 10 days (case II) (Fig. 2). The slowdecrease in the COD removal efficiency may be due to physicalchanges in GAC; such as slow erosion of surface due to prevailinghydrodynamic conditions and evolution of gases at electrodes. Weobserved buildup of carbon dust (suspended solids) in the treatedeffluent which indicates erosion of the carbon particles in TDR overa period of time. We have earlier reported that GAC adsorbspollutants which undergo degradation during electrolysis in TDR[7]. The weight fraction of fine carbon particles in the case ofeffluent treatment in TDR is small (2.0–2.5% of the initial weight ofcarbon in bed); therefore the fraction of pollutants that escapethrough adsorption on GAC particles can be expected to beinsignificant. Iodine value (IV) for the unused and used GAC wascompared. For the fresh GAC, this value was 907 mg g�1 which after2 days continuous use decreased by 40% (538 mg g�1) and furtherafter 4 days of use slightly increases to 595 mg g�1. The observeddecrease in iodine value signifies substantial changes in surfacecharacteristics of carbon; while the intermittent decrease andincrease in iodine value may imply slow in-situ electrochemicalregeneration of carbon. If the net decrease in COD removalefficiency exceeds 20% of the initial efficiency, the performance ofTDR may be regarded as unsatisfactory. Considering the rate ofdecrease in COD removal efficiency as 0.96% day�1, it is estimatedthat TDR would deliver satisfactory performance till 10–12 days.The efficiency can be regained by ex-situ washing of the carbon tofree the carbon dust and refilling the TDR with dried recovered

0

2

4

6

8

10

12

0 50 100 150 200 250 300

pH

Time (h)

Case-ICase-II

Fig. 3. Variation of effluent pH collected at the outlet of TDR for cases I and II;(inset) potential of zero charge (pHPZC) plots of GAC-10 and GAC, Indcarb-60.

0

10

20

30

40

50

60

70

0

2

4

6

8

10

12

0 50 100 150 200 250 300

Time (h)

% C

ODpH

pH % COD

Fig. 5. Comparison of the treated effluent pH and % COD removal in case II – GAC-10; HRT, 4 h; run duration 10 days; raw effluent COD (initial), 5200 mg L�1.

1530 N. Gedam, N.R. Neti / Journal of Environmental Chemical Engineering 2 (2014) 1527–1532

carbon. Free chlorine generation in the TDR was also estimated,about 0.7–0.9 mg L�1 of chlorine buildup was observed throughoutthe continuous operation of TDR indicating large part of it wasutilized for electrochemical oxidation. It was noted that thebiodegradability index of the treated effluent increases to >0.45.Thus, the treated effluent qualifies for biodegradation treatment indownstream of treatment plant.

3.2. Effect of effluent pH

The variation in pH (Fig. 3) of effluent collected at the outlet ofTDR shows complex behavior. The pH in both the cases I and IIdecreases gradually from alkaline to highly acidic which may bedue to formation of organic acid intermediates. In separateexperiments, we determined the pH representing potential ofzero charge (pHPZC) for the carbon used in cases I and II. This wasfound to be pH 3.2–3.3. The decrease in pH was more prominent incase II, implying that high concentration of organic acids was builtup. In this case, the pH turned acidic and remained well below 4.0.Since this is close to pHPZC (see inset Fig. 3) it may be inferred thatadsorption of organic acids on the carbon was impaired and hencetheir degradation was slowed down. However, in the case I whereIndcarb 60 carbon which has surface area twice that of GAC-10, theobserved pH remained higher above >5.0, implying the buildup ofacid intermediates was probably less. The higher surface area ofIndcarb 60 may have facilitated faster reaction of the acidintermediates. The comparison of effluent pH and % COD removalin case I study is shown in Fig. 4. This result shows that the TDR canperform for consistent COD removal (>49 � 7%) throughout the

0

10

20

30

40

50

60

70

0

2

4

6

8

10

12

0 50 100 150

Time (h)

% C

OD

pH

pH % COD

Fig. 4. Comparison of the treated effluent pH and % COD removal in case I –

Indcarb-60 GAC; HRT, 2 h; run duration, 5 days; raw effluent COD (initial),10,358 mg L�1.

operation period. Whereas the slow decrease in % COD removalwith effluent pH observed in case II (Fig. 5) implies that GAC 10 haspoorer ability to maintain initial activity. In general in electrooxidation process and particularly in the TDR process the effluentpH plays an important role in the electrochemical indirectoxidation of pollutants. It can be inferred that the observeddecrease in pH lead to slow down the indirect oxidation anddecrease in COD removal efficiency over a period of time. Thechanges in pH can substantially alter the distribution of chlorinebased radicals. For instance, in the treatment of synthetic tannerywastewater, the increase in the rate of COD removal with increasein pH was attributed to no loss of active chlorine which alsosupports the indirect oxidation [12]. If the reactor pH decreasesgaseous chlorine may escape from the TDR and hence a certainreduction in the observed COD removal efficiency can be occur asobserved in the case II.

3.3. Effect of reactor temperature

The reactor temperature was observed to increase slowly to40 � 4 �C, well above the room temperature. Seasonal variation inthe temperature was also recorded in the study as both the studieswere carried out in different seasons; for case I the roomtemperature was 32–34 �C and for the case II it was 25–27 �C.The temperature profile of the continuous TDR plotted vis-à-visefficiency of COD removal is shown in Figs. 5 and 6 for cases I and II,respectively. In case I study, temperature increased initially (32–41 �C) and remained almost steady (�2 �C) during the experimen-tal period (Fig. 6). On the other hand, in the case II, the temperature

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

0 50 100 150

% C

OD

Tem

pera

ture

C

Time (h)

TEMPERATURE, Case -I%COD, Case -I

Fig. 6. Comparison of the reactor temperature and % COD removal in case I –

Indcarb-60 GAC; HRT, 2 h; run duration, 5 days; raw effluent COD (initial),10,358 mg L�1.

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0 25 50 75 100 125 150 175 200 225 250

% C

OD

Tem

pera

ture

C

Time (h)

TEMPERATURE, Case -II

% CO D, Case -II

Fig. 7. Comparison of the reactor temperature and % COD removal in case II – GAC-10; HRT, 4 h; run duration 10 days; raw effluent COD (initial), 5200 mg L�1.

N. Gedam, N.R. Neti / Journal of Environmental Chemical Engineering 2 (2014) 1527–1532 1531

was found to increase gradually (27–38 �C) with time and thenstabilized during late hours of reaction period (Fig. 7). Initially atlower temperature the COD removal efficiency was quite high(50 � 5% COD); thereafter both temperature and % COD removalsattain steady levels (40 � 1 �C and <40% COD removal). We caninfer that COD removal efficiency decreases gradually as thereactor temperature increases. The variation in temperature in TDRmay also affect the COD removal efficiency by altering theavailability of Cl-based oxidants. The observed slow decrease inefficiency of COD removal may be due to self-decomposition ofreactive Cl2 species at higher temperature [12]. Various reasonscontribute to increase in temperature viz., exothermic oxidationreaction, changing electrical resistance offered by carbon bed,seasonal temperature variation etc. Earlier studies have shown thatincrease in the temperature resulted in increase of the removalefficiencies for both phenol and COD in parallel plate electrodereactor [13]. Thus, anode when utilized for synthetic tannerywastewater electrolysis, increase in temperature 20–40 �C wasobserved to favor COD removal however further increase intemperature did not help significantly [12]. It is commonlyacknowledged that direct oxidation of pollutants on the surfaceof an anode is not affected by temperature variations but theindirect oxidation involving reactions with electro generatedoxidants in liquid bulk is influenced [14].

3.4. GAC attrition

During electrolysis in TDR the effluent passes through thecarbon bed continuously. The prevailing hydrodynamic conditionsappear to lead to slow mechanical erosion of carbon from surfaceprimarily. Some contribution from electro oxidation of carboncannot be ruled out. We have observed that the main carbon anodesuffers ‘electrochemical erosion’ which can be visible uponprolonged use of TDR at high applied currents particularly. TheGAC attrition study was also carried out as per description inSection 2.2. This consisted of performing different sets of

Table 1Carbon attrition during continuous electrolysis in TDR-weight loss and particle size re

Parameter EffluenCurrent (A) 3

Initial weight of GAC (kg) in TDR 1

Initial GAC particle size (mm) >4

Total weight of recovered GAC from TDR (kg) 1.137

Weight of carbon (size <4 mm, kg) in recovered GAC 0.182

Weight of carbon dust associated with GAC wash water (kg) 0.023

Carbon weight loss (%) during TDR experiment 20.5

experiments in continuous mode using brine as circulatingelectrolyte and then determining the weight loss of carbon atdifferent applied currents (1, 2, and 3 A). Optionally, the raweffluent was also used in one experiment to enable comparison.When the influent free from suspended solid (SS) was passedthrough the TDR a certain concentration of SS in the treatedeffluent was observed. The weight loss in carbon during brineelectrolysis in the TDR at different applied currents was calculatedand presented in Table 1. The weight loss was the lowest at 1 A (8%),but increased with magnitude of the applied current, �14.0–15.0%at 2 and 3 A. When the brine is replaced with the raw effluent, theobserved weight loss was 20.5%. It seems that the effluentcharacteristics also influence the extent of carbon attrition inaddition to the magnitude of the applied current. After thecontinuous treatment of effluent in TDR (case I) the quantity of SSgenerated in the outlet effluent was determined. The volume ofsettled sludge was 25–30 mL L�1 which amounted to 0.307 g L�1 bydry weight. When the dried sludge was subjected to heating at550 �C for an hour, approximately 30% reduction in its weight wasnoticed. This can be equated to the amount of carbon dust in thesettled sludge while the balance 70% accounts for inorganic solids.In these experiments, the initial size of GAC particles was >4 mm.The entire quantity of GAC was recovered after the experiment andsieved through (ASTM 5) which allows passage of particles havingsize <4 mm. The fraction of GAC having <4 mm size was; 7.5% at1 A; �14.0% at 2 and 3 A; and 18.2% at 3 A (raw effluent). The datasuggests that carbon particles in TDR undergo slow erosion anddiminish in size gradually with experimental period. Further, someof the carbon powder (dust) generated through erosion stratifies tothe bottom of TDR and occupies void spaces while some lightparticles escape into effluent outlet. The light particles have size inthe range 200–300 mm. This phenomenon is akin to attrition ofgraphite anodes during electrolysis operation [15]. The graphiteanodes in conventional brine electrolysis underwent attritionresulting in graphite particles that created serious operationalproblems. The graphite anodes were eventually replaced withmore stable platinum plated titanium anodes. Nevertheless, someresearch groups [1,2,16–23] including ours [7,8,10] have publishedconsiderable research highlighting the usefulness of three-dimensional carbon bed electrodes systems.

3.5. Current efficiency and energy consumption-COD removal in TDR

The apparent Faradic efficiency (hF) for COD removal wascalculated using Eq. (1) by substituting DCOD (5108 mg L�1), V(1.2 L), F (96,487 C equivalent�1), I (3 A), Dt (2 h) for case I, whereasin case II, DCOD (2338 mg L�1), V (1.2 L), F (96,487 C equivalent�1), I(3 A), Dt (4 h). The hF for COD removal under EO with the pre-saturated carbon bed under continuous mode of operation inexperiments represented by cases I and II were 3.42 and 0.78%,respectively. Similar current efficiency for COD removal fromlandfill leachate using TDR at 3 A current was reported earlier [7].The specific energy consumption (Esp) which is the electric energy

duction and current dependence.

NaCl

t (5.5–7.5 V) (4–6.5 V) (2–3.5 V)3 2 11 1 1>4 >4 >40.944 1.010 0.9850.133 0.140 0.0750.010 0.009 0.00514.3 14.9 8

1532 N. Gedam, N.R. Neti / Journal of Environmental Chemical Engineering 2 (2014) 1527–1532

(kW h) required to degrade 1 kg of a pollutant (COD) from theeffluent was calculated using the formula for continuous operation(Eq. (2)). The Esp for the reduction of COD in cases I and II was 6.59and 28.65 kW h kg�1COD, respectively. Similar Faradic efficiencyand specific energy consumptions were observed for COD removalfrom landfill leachate, dye and caprolactam wastewater using TDR[7,8,10]. The reactor that used high surface area carbon showedfour times higher current efficiency and at the expense of fourtimes lesser electrical energy and with low buildup of acidintermediates. The GAC 10 used in TDR in case II experimentappears to offer greater ohmic resistance requiring application ofhigher voltage.

4. Conclusions

The experimental results from this study demonstrate that theTDR could remove COD from high strength wastewater efficientlyand consistently over a period of more than 10 days. The effluentpH at the outlet decreases during electro oxidation in TDR. It seemsadvantageous to use well defined high surface area carbon (e.g.,Indcarb 60) in TDR to achieve satisfactory continuous performanceas it does not allow buildup of acid intermediates. However, GACparticles in TDR undergo slow attrition during electrolysis ofeffluents. This may prove to be a major setback for scale upattempts as this can translate into serious engineering issues suchas gradual loss in liquid holding capacity of carbon bed due tostratification and filling of voids in the carbon bed with carbon finedust. This will demand for additional operations such as removaland washing of carbon bed or in-line washing system for removingthese fine particles continuously.

Acknowledgements

The authors wish to thank Dr. S. R. Wate, Director, CSIR-NEERIfor encouragement. NG thanks CSIR, New Delhi for Senior ResearchFellowship..

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