Narbaitz Ref Nyex

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Electrochemical regeneration of field spent GAC from two

water treatment plants

Roberto M. Narbaitz*, Jeff McEwen

Department of Civil Engineering, University of Ottawa, 161 Louis Pasteur Pv., Ottawa, Ontario, Canada K1N-6N5

a r t i c l e i n f o

Article history:Received 5 March 2012

Received in revised form

19 May 2012

Accepted 21 May 2012

Available online 15 June 2012

Keywords:

Activated carbon

Electrochemical regeneration

NOM

a b s t r a c t

The effectiveness of on-site thermal regeneration of field-spent granular activated carbon(GAC) from two municipal drinking water facilities was compared with bench-scale elec-

trochemical regeneration, a novel regeneration technology. The regeneration method was

evaluated using aqueous natural organic material (NOM) adsorption, iodine number

analysis, and surface area analysis. In contrast to the large electrochemical regeneration

efficiencies reported in the literature for GAC loaded with phenolics and other individual

organic compounds, the electrochemical reactor tested was only able to regenerate 8e15%

of the NOM adsorption capacity of the field spent GAC. In contrast, thermal reactivation

achieved up to 103% regeneration efficiency. To more accurately assess the efficiency of

regeneration processes for water treatment applications, GAC should be loaded in

continuous-flow columns and not batch rectors. The iodine number analysis yielded

higher efficiency values, however it did not give an accurate estimate of the regeneration

efficiency. The small changes in GAC pore size distribution were consistent with the low

electrochemical regeneration efficiencies. These low efficiencies appear to be related to thelow reversibility of NOM adsorption and to pH-induced adsorbate desorption being the

primary mechanism for this type of electrochemical regeneration system.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Granular activated carbon (GAC) is frequently utilised at water

treatment plants for the removal of taste and odour causing

compounds. Other facilities use it for the removal of synthetic

organic compounds (SOCs) and in some cases natural organicmatter (NOM), which are precursors of harmful disinfection

by-products. NOM, which is a product of the natural decay of

organic matter (i.e. vegetation, fish, and algae) in and around

the surface water source, is present in all waters and is the

highest concentration group of organic compounds within

natural waters. Although NOM removal is generally not the

primary treatment objective of GAC systems, the adsorption

of the ubiquitous NOM substantially reduces the adsorption of

capacity of the target SOCs and taste and odour compounds

(MWH, 2005). Accordingly, NOM removal is critical to the

design and the performance of GAC adsorbers. Once the GAC

in a column adsorber reaches a predetermined exhaustion

criteria, the carbon is either replaced with virgin carbon or it is

thermally reactivated. The thermal reactivation process usesgradual heating to desorb and volatilize certain contaminants

and to convert others to char, followed by pyrolysis of the char

and finishes with an oxidation step to reactivate the pores

(MWH, 2005). Thermal reactivation produces a product with

similar contaminant removal potential to that of virgin GAC,

however it has a number of drawbacks. First, the micropores

become wider as a result of contaminant burn-off reducing its

ability to remove small molecular size contaminants. Second,

* Corresponding author. Tel.:1 613 562 5800x6142; fax: 1 613 562 5173.E-mail address:[email protected](R.M. Narbaitz).

Available online atwww.sciencedirect.com

j o u r n a l h o m e p a g e : w w w . e l s e v i er . c o m / l o c a t e / wa t r e s

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0043-1354/$ e see front matter 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.watres.2012.05.046
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there is an accompanying 5e20% loss in GAC volume/mass

(Sontheimer et al., 1988;San Miguel et al., 2001;Clements and

Haarhoff, 2004). Third, it is energy intensive. In an effort to

overcome these drawbacks some researchers have investi-

gated optimizing the thermal reactivation process(Waer et al.,1992;Moore et al., 2003;San Miguel et al., 2001,2003), while

others have investigated alternative methods of GAC regen-

eration including ultrasound, microwave and electrochemical

regeneration (Lim and Okada, 2005;Ania et al., 2005; andZhou

and Lei, 2006).

Although electrochemical regeneration of GAC is still in

development and an optimal reactor configuration is still to

be identified, lab-scale studies are very promising (Narbaitz

and Karimi-Jashni, 2012). For the regeneration of GAC

loaded with several different organic solutes, particularly

phenolics, regeneration efficiencies ranging from 70 to 100%

have been obtained (Weng and Hsu, 2008).Berenguer et al.,

2010 found that for the regeneration of phenol-loaded GACunder optimal conditions, electrochemical regeneration

achieved similar regeneration efficiencies to thermal regen-

eration and higher surface area recoveries. In addition,

Narbaitz and Cen (1994) showed that electrochemical

regeneration achieved high efficiencies without apparent

loss of carbon mass. And based on the voltage and current

usage of their bench-scale reactor for the regeneration of

leachate loaded GAC, Weng and Hsu (2008) estimated the

electrical energy cost of electrochemical regeneration to be

only about 4% of the cost of purchasing virgin GAC. This was

based on unit costs of $0.06/kWhr and $1000/tonne GAC.

Thus, electrochemical treatment for the regeneration of GAC

spent at drinking water treatment facilities could lead to costsavings. Electrochemical regeneration research has concen-

trated on cathodic regeneration using NaCl or Na2SO4 solu-

tions as the electrolyte and it has been performed almost

exclusively using GAC loaded with single-solute organic

solutions (principally phenolics). The only study that briefly

investigated electrochemical regeneration of GAC loaded at

a water treatment plant had limited but promising results

(Narbaitz and Karimi-Jashni, 2009). The focus of this study is

to assess the suitability of electrochemical regeneration for

the drinking water industry via the direct comparison of

bench-scale electrochemically regeneration with full-scale

thermal regeneration of exhausted GAC at two full-scale

water treatment plants. Electrochemical and thermal

regenerated GAC will be compared in terms of NOM

adsorption, iodine number, and pore size distribution

analysis.

2. Experimental

2.1. Materials

This study utilized water samples and GAC (virgin, spent and

field-thermally regenerated) obtained from the Buffalo Pound

Water Treatment Plant, Regina, Sask., and the Richard Miller

Water Treatment Plant, Cincinnati, OH. Both of these facilities

use GAC within post-filtration adsorber columns and have on-

site thermal regeneration. Note that the spent (i.e., field

loaded) GAC at both locations was collected at the end of an

operational cycle (i.e., prior to regeneration) and at thebeginning of the cycles the adsorbers were filled with GAC that

had been regenerated (one or more times) plus make-up virgin

GAC. Depending on the rate of attrition GAC particle may last

approximately 7e12 regeneration cycles. At the time of this

study the Buffalo Pound Water Treatment Plant utilized

a combination of Calgon F-400 GAC and Norit 830 GAC

(denoted as Regina GAC) while the Richard Miller Water

Treatment Plant utilized Jacobi brand GAC (denoted as Cin-

cinnati GAC). GAC samples were kept under refrigeration until

used. Thermally regenerated and virgin GAC samples were

rinsed to remove fine material; oven dried at 105 C, cooled in

a desiccator then sealed in airtight containers until they were

used. The adsorbates utilized in this study were watersamples collected after granular media filtration and prior to

GAC adsorption (pre-GAC) at the Buffalo Pound Water Treat-

ment Plant (denoted as Regina water) and at the Richard Miller

Water Treatment Plant (denoted as Cincinnati water). The

water samples from the two plants were shipped in food-

grade high-density polyethylene barrels and kept in a refrig-

erator at 2 C until used. All the reagents used were ACS grade.

Ultrapure water for the desorption tests and reagent prepa-

ration was prepared by passing distilled water through a Milli-

Q water treatment system (Millipore, Bedford, MA) which

incorporates mixed bed ion exchange, activated carbon

adsorption, organic scavenging resins and an ultrafiltration

membrane.

Abbreviations and notation

GAC granular activated carbon

NOM natural organic matter

NRCC National Research Council of Canada

RE regeneration efficiency (%)

REiodine Iodine number based regeneration efficiency (%)

SOCs synthetic organic compounds

TOC total organic carbon

Co initial liquid phase concentrations (mg TOC/L)

Ce equilibrium liquid phase concentrations (mg TOC/

L)

I#reg Iodine number of the regenerated GAC

I#virgin Iodine numbers of the virgin GAC

K Freundlich isotherm coefficient for the virgin GAC

(mg TOC/g GAC/[mg TOC/L]1/n)

M mass of GAC (g)

qe solid phase equilibrium concentration for the

virgin GAC at the same liquid equilibrium liquid-

phase concentration as the reloading test (mgTOC/g GAC)

qr solid- phase concentration after the reloading step

(mg TOC/g GAC)

V volume of adsorbate solution (L)

1/n Freundlich isotherm model exponent for the

virgin GAC (unitless)

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2.2. Methods

Total organic carbon (TOC) was used to quantify the NOM, and

its concentrations were determined using an UV-persulfate

oxidation based TOC analyser (DC-180, Dohrmann Division

Rosemount Analytical Inc., Santa Clara, CA). TOC concentra-

tions of Regina and Cincinnati waters were 3.5 and 1.5 mg/L,

respectively. All the other chemical analysis was performedfollowingStandard Methods (1995).

2.3. Isotherms and desorption tests

A series of preliminary experiments were performed to aid in

the calculations of the efficiency of the batch NOMreloadingof

regenerated GAC. These experiments included batch adsorp-

tion isotherms and batch desorption tests. Adsorption

isotherms were conducted via the bottle-point loading tech-

nique at a constant temperature. Different masses of accu-

rately weighed virgin GAC samples were combined in 500 ml

amber glass bottles with pre-GAC water from the full-scale

plants. The bottles were filled and sealed with Teflon linedcaps. The bottles were placed in an end-over-end tumbler and

rotated (toprovide mixingwith minimumabrasion) for14 days

at 21 2 C to achieve equilibrium (a kinetic study was per-

formed to establish the loading time). At theend of theloading

time, the GAC and the loading solution were separated by

vacuum filtration with 0.22 mm polyester filters (Whatman

Nuclepore, Piscataway, NJ)and thefiltrate samples were then

analysed to quantify the adsorption. The adsorption kinetics

were evaluated viabottle-point tests with a constant GAC dose

and different contact periods. The carbon dose was 2 g/L, the

same dose used in the regeneration and GAC reloading tests.

These tests established that after a 14 day contact periodthere

wasrelatively little additional adsorption, so it wasused as theloading or working equilibrium time. While 14 days may seem

a relatively short equilibrium time for commercial size GAC

particles, it should be noted that the 2 g/L GAC dose was very

high and shortened the equilibration time.

A desorption test using saturated GAC from the two water

treatment plants was conducted to assess the effect of pH on

NOM desorption. The tests were performed via the bottle-

point method with saturated GAC as the adsorbent and pH-

adjusted Milli-Q water as the desorbing solution. The pH of

the desorption solutions had pHs of 1, 7 and 13 and were

prepared using 0.2 M HCl/KCl, 0.1 M NaOH/KH2PO4, 0.2 M

NaOH/KCl buffers, respectively.

2.4. Electrochemical regeneration

The bench-scale electrochemical reactor consisted of two

glass cylindrical compartments with a 7.6 cm internal diam-

eter and a total height of 30 cm (Fig. 1). Each compartment

contained a platinum wire mesh electrode that could be used

as either the cathode or the anode. The electrodes were

maintained at a separation distance of 12 cm. The reactor was

equipped with access ports for measuring pH, for removing

accumulated gas bubbles and for filling/empting the

compartment. The electrolyte used in this study was 1% NaCl

solution asNarbaitz and Cen (1994),Zhou and Lei (2006), and

Zhang et al. (2002) found that NaCl electrolyte performed

better than several other salts when regenerating phenol-

loaded GAC, and that higher concentrations of NaCl did not

improve the regeneration efficiency. Based on the results of

Karimi-Jashni and Narbaitz (2005a)electrolyte mixing was not

incorporated into the reactor operation. All electrochemical

regeneration tests were conducted by placing the field-spent

GAC on the platinum wire mesh cathode, as several

researchers have shown that cathodic regeneration is gener-ally more effective than anodic regeneration. A cation

exchange membrane (Raipore PTFE RF4010, Electrosynthesis,

Lancaster, NY) was placed in between the electrodes to

separate the reactor into cation and anion compartments and

to control the movement of ions between the two compart-

ments. The cation exchange membrane allows the movement

of cations from the anodic compartment to the cathodic

compartment, while preventing anions from travelling from

the cathodic compartment to the anodic compartment. This

allows increases in the pH within the cathodic compartment

(where the GAC is placed) since the hydroxide ions generated

there cannot diffuse into the other compartment. One gram of

GAC was regenerated in each experiment, which resulted ina monolayer of GAC particles on the electrode, there is

therefore an average of 0.022 g GAC per cm2 of electrode area.

So as to not change the cell voltage, gas bubbles that accu-

mulated under the ion exchange membrane were periodically

removed through the gas removal port using a small Teflon

tube and syringe. This study used a 5 h reactivation period as

Narbaitz and Cen (1994)found that for phenol-loaded GAC the

slightly higher regeneration efficiencies achieved with longer

regeneration periods could not justify the significant associ-

ated increases in energy consumption. The reactor was

operated galvanostatically, the power was supplied using

a controller (model 410, Electrosynthesis, Buffalo, NY) with an

accessory power supply (Model 420A, Electrosynthesis,

Fig. 1e Electrochemical regeneration reactor.

w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 4 8 5 2 e4 8 6 04854


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Buffalo, NY). After the regeneration cycle the GAC samples

were stored in a refrigerator until they were analysed.

2.5. Evaluation of regeneration performance

Electrochemically regenerated GAC in our laboratory was

compared with thermally regenerated GAC at the two water

treatment plants in terms of NOM adsorption, iodine number,and pore size distribution analysis. The NOM adsorption

regeneration efficiency required reloading the GAC with NOM,

it was performed via the same batch-bottle loading procedure

used for the isotherm, and the adsorbate was the pre-GAC

water from the corresponding water treatment plant with

the addition of a phosphate buffer (0.01 M). The buffer was

added to overcome a high pH carry-over from the regeneration

step (Karimi-Jashni, 2001).

The percent regeneration efficiency (RE) is generally

calculated as the adsorbate loading achieved after regenera-

tion and reloading divided by the original adsorbate loading.

As the loading of the field-spent GAC was not possible and the

reloading was accomplished by a batch test, RE was calculatedusing Method 3 suggested by Narbaitz and Cen (1997). It

defines RE as:

RE qrqe 100% (1)

whereqeis the solid phase equilibrium concentration for the

virgin GAC at the same liquid equilibrium liquid-phase

concentration achieved in the reloading step (mg TOC/g

GAC); and qr is the solid- phase concentration after batch-

reloading (mg TOC/g GAC), which is obtained via a TOC

mass balance. Thus if the virgin isotherm is described by the

Freundlich model the equation becomes:

RE Co CeV=M

KC1=ne 100 (2)

WhereCoand Ceare the initial and equilibrium liquid phase

concentrations during the reloading step (mg TOC/L); Vis the

volume of adsorbate solution (L); M isthemass ofGAC; K isthe

Freundlich coefficient from the virgin GAC isotherm (mg TOC/

g GAC/[mg TOC/L]1/n) and 1/n is the Freundlich isotherm

models exponent for the virgin GAC (unitless).

The electrochemical regeneration was also evaluated using

iodine number as defined by ASTM (1995). Iodine number

analysis was performed for virgin, thermally regenerated and

electrochemically regenerated samples from both the Regina

and Cincinnati water treatment plants. The procedureinvolved grinding the sample (60% pass through a 325 mesh

sieve), contacting the ground activated carbon with an iodine

solution, filtering the mixture, followed by titrating the filtrate

to determine the concentration of iodine adsorbed by the

carbon. Based on three different masses of ground GAC tested

an iodine adsorption isotherm was plotted to help determine

the iodine number for the sample. The iodine number is the

mass of iodine adsorbed per gram of carbon (determined from

the three-point isotherm) for a filtrate residual concentration

of 0.02 N. The iodine number regeneration efficiency (REiodine)

was determined using:

REiodine I#reg

I#virgin 100 (3)

Where I#regand I#virginare the iodine numbers of the regen-

erated and virgin GAC, respectively. The impact of the elec-

trochemical regeneration was further assessed by analysing

the surface chemistry, surface area and pore size distribution.

The pore volume distribution of virgin, thermally regen-

erated and electrochemically regenerated GAC samples from

both Regina and Cincinnati were measured at the National

Research Council of Canada (NRCC) Laboratories via a N2adsorption BET surface area and pore size distribution ana-

lyser (ASAP 2000, Micromeritics, Norcross, GA). For more

detailed information on the experimental procedure refer to

McEwen (2004).

3. Results and discussion

3.1. Water quality and isotherms

Table 1 displays the water quality characteristics for the

Regina and Cincinnati pre-GAC waters, both waters are

moderately hard with considerable alkalinity and a neutral

pH. The Regina pre-GAC water has a TOC of 3 mg/L, which is

a relatively high concentration of NOM given that it was

collected after the plants deep media filters. The values

reported inTable 1are within the range of values reported by

the two water utilities.

The results of the preliminary bottle-point NOMadsorption

isotherms performed with virgin Regina and Cincinnati GAC

with their respective pre-GAC waters are shown inFig. 2. The

results indicate that the isotherms are well described by the

Freundlich model since the data follows a straight line pattern

within a log equilibrium solid phase concentration (qe) versus

log equilibrium liquid-phase concentration (Ce) graph for both

GACs. The Freundlich coefficients and the R2 values for the

linear regression of the logelog plot ofCevs. qeare shown in

Table 2. These Freundlich coefficients were used to evaluate

the regeneration efficiency of the electrochemically and

thermally regenerated GAC.

3.2. Impact of pH on desorption

Desorption experiments were conducted to confirm the effect

of pH on NOM desorption (Fig. 3). The greatest desorption

occurs with the pH 13.2 desorption solution. This seems to

suggests that high pH desorption is capable of regenerating

Table 1e Water quality characteristics of the Regina andCincinnati Pre-GAC waters.

Regina Cincinnati

Hardness (mg/L as CaCO3) 164 108

Alkalinity (mg/L as CaCO3) 182 108

pH 7.04 7.02

Total Dissolved

Solids (TDS) (mg/L)

460 110

Total Organic

Carbon (TOC) (mg/L)

3.0 1.5

Turbidity (NTU) 0.2 0.23

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NOM loaded GAC. This is consistent with the proposed use of

strong basic solutions for the regeneration of NOM-loaded

GAC (Sontheimer et al., 1988; Newcombe and Drikas, 1993).The NOM solid phase concentration for the field loaded acti-

vated carbons wasnot available. Therefore, it is not possible to

determine the percent of the NOM that is desorbed at high pH.

Due to the lack of another option, the Regina GAC adsorption

isotherm (Table 2) at an initial NOM concentration of 3.0 mg

TOC/L was used to estimate the initial NOM loading of field-

loaded Regina GAC as mg/g. Based on this estimate, pH

induced desorption (pH 13.2) is about 39% of the mass adsor-

bed. This is likely a conservative estimate given that column

adsorbers achieve lower loadings than predicted by

isotherms.

3.3. Regeneration efficiency based on NOM adsorption

The effect of increasing the current is shown inFigs. 4 and 5.

The regeneration efficiencies of Regina and Cincinnati for

NOM reloading range from 8 to 20 %. In contrast to the poor

regeneration efficiencies reported above, field thermal

regeneration achieved regeneration efficiencies ranging from

87% to 103%. These values are in the range reported by

Sontheimer et al. (1988) and Moore et al., 2003. As explainedby

Moore et al. (2010), in spite of the loss of total internal surface

area and micropore area, large regeneration efficiencies are

possible because thermal regeneration leads to an increase in

the mesopore volume, which is more suitable for the

adsorption of the relatively large NOM molecules. The regen-

eration efficiency for smaller target compounds are unlikely to

be as high (Sontheimer et al., 1988). The low electrochemical

regeneration efficiencies were surprising given that: a) for

GAC loaded with phenolics several authors observed

regeneration efficiencies of greater than 80% (Karimi-Jashni

and Narbaitz, 2005b; Zhou and Lei, 2006; Berenguer et al.,

2010); and b)Narbaitz and Karimi-Jashni (2009)found iodine-

number based regeneration efficiencies of up to 80% for GAC

loaded with NOM at a pilot-plant GAC column.

Most of the efficient electrochemical regeneration studies

reported in the literature batch-loaded the GAC, as it is more

convenient than the more realistic column loading approach

used for the initial loading in the current study. Accordingly, it

was hypothesized that the lower regeneration efficiencies

observed in this study may be partly due to the loading

approach. Thus, the effect of the loading method was also

investigated by batch-loading virgin Regina GAC with Regina

water.Fig. 5shows the effect of current on the regeneration

efficiency of the field-spent Regina GAC and of the virgin

Regina GAC that waslaboratory loadedwith Regina water. The

batch-loaded GAC has regeneration efficiencies in the 79e87 %

range, the regeneration efficiency appears to decrease with

Table 2e Freundlich isotherm parameters for the twowaters using virgin GAC.

Water K [(mg/g)$(L/g)1/n] 1/n R2

Regina 8.86 1.86 1.26 0.19 0.97

Cincinnati 8.07 1.53 0.86 0.19 0.95

Fig. 2 e Adsorption isotherms for Regina and Cincinnati

GAC.

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

1.05 6.95 13.15 DW

pH of desorption Solution

TOCCon

centration(mg/L)

Fig. 3e Impact of pH on NOM desorption from field-loaded

Regina GAC.

Fig. 4 e NOM regeneration efficiency for GAC loaded at the

Cincinnati water treatment plant.

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increasing current, however this decrease may simply be due

to experimental error (i.e., regeneration efficiency 83 5%).

The most important result ofFig. 5 is that regeneration

efficiency of the laboratory batch-loaded GAC (79e87 %) was

significantly higher than that for the field-loaded GAC (8e17

%). NOM consists of a heterogeneous mixture of organic

compounds and a fraction of these organics adsorb strongly,

others adsorb less strongly, while another fraction may not

adsorb at all (i.e. non-adsorbable) (Summers and Roberts,

1988a; Kilduff et al., 1996). The strongly adsorbed fraction

may adsorb irreversibly (Narbaitz, 1986; Summers andRoberts, 1988a,b; Newcombe, 1994). Narbaitz (1986) found

that the adsorption of a river water NOM was almost

completely irreversible. Given that electrochemical regener-

ation of phenolics is primarily driven by high-pH induced

desorption (Karimi-Jashni and Narbaitz, 2005b), and that NOM

adsorption is more irreversible than that of phenol, electro-

chemical regeneration efficiency of NOM-loaded GAC is

expected to be lower than that of phenol-loaded GAC. Field-

spent GAC was loaded with NOM in a continuous flow

column adsorber, where the GAC is constantly being exposed

to some of the strongest adsorbing NOM fraction (Carter et al.,

1992). In addition, full-scale column runs last many months,

allowing the slow adsorbing NOM a greater opportunity tomigrate deeper into the pores of the GAC, and allowing more

time for oxic polymerization reactions to take place (Warta

et al., 1995; Karanfil et al., 1996). Furthermore the stronger

adsorbing NOM molecules may actually displace the more

weakly adsorbed NOM molecules through competitive

adsorption. On the other hand, laboratory batch loading

experiments used in this study had a contact time of only two

weeks. Batch-loaded GAC is exposed to a fixed volume of

water containing NOM. The strongest adsorbing NOM fraction

is adsorbed to a greater extent than the weaker adsorbing

fraction, but the mass of strongly adsorbing NOM is limited

which gives the other NOM molecules a greater opportunity to

adsorb. In the present study, the impact of batch NOM loading

is magnified by the large GAC dose (i.e., 2 g/L), which facilitates

the adsorption of the weaker adsorbing fraction of NOM.

Electrochemical regeneration seems to be able to remove

the more weakly adsorbed NOM fractions. Thus, in electro-

chemical regeneration of batch-loaded GAC, the weakly

adsorbed NOM is removed, allowing for the adsorption of

more NOM upon reloading, and thus yielding a higher regen-

eration efficiency. However, field-loaded GAC containsa much greater fraction of strongly adsorbed NOM, electro-

chemical regeneration results in very little NOM desorption

and only a small fraction of the adsorption sites available for

reloading, thus the lower regeneration efficiency. From these

results one can conclude that in the development of activated

carbon regeneration techniques, one should include testing of

using NOM-loaded GAC generated by continuous-flow column

adsorber runs, so as to avoid over optimistic assessments of

the technique.

3.4. Iodine number based regeneration efficiency

At water treatment plants, the effectiveness of regeneration is

frequently evaluated via the iodine number as it is a relatively

simple and quick test. Fig. 6 shows the effect of electro-

chemical and thermal regeneration on the iodine number for

field-loaded GAC from Regina and Cincinnati. The virgin GAC

iodine numbers for both Regina and Cincinnati are over 1000,

similar those reported in the literature (Clark and Lykins,

1989).Fig. 6also shows a drop of 11%, 38% and 43% for the

thermally regenerated, electrochemically regenerated and

spent (0 mA) for Regina GAC compared to the virgin GAC. A

similar trend was observed for the Cincinnati GAC: 25%, 46%

and 51% for the thermally, electrochemically and spent GAC,

respectively. Note that for the GAC from both plants the drop

in iodine numbers was much smaller for thermally regen-

erated GAC than for electrochemical regenerated GAC, and

electrochemical regenerated GAC was only slightly better

than the field-spent GAC. Although the drop in the percent-

ages for thermally regenerated GAC may seem a bit high, it

has to be considered that these GACs have been in operation

for several operational cycles and thermal regeneration

results in the loss of the micropores where most iodine

adsorbs.

Fig. 5e NOM regeneration efficiency for field-spent Regina

GAC.

Fig. 6e Iodine number of virgin, thermally regenerated and

electrochemically regenerated Regina and Cincinnati GAC.

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Fig. 7 compares the NOM adsorption regeneration effi-

ciency with iodine number regeneration efficiency, for field-

loaded GAC from Regina. As the current was increased the

iodine RE increased from 57 to 63%, while the NOM adsorp-

tion RE varies from 8 to 20%. Thus, calculating the regener-

ation efficiency according to the iodine number can greatly

overestimate the NOM-based regeneration efficiency. This is

not surprising given that the iodine number generallycorrelates well with the surface area available for the

adsorption of small molecules (Snoeyink and Summers, 1999)

while NOM molecules are significantly larger. Thus, the

assessment of regeneration efficiency based on the iodine

number is not recommended for water treatment

applications.

3.5. Pore size distribution

The pore size distribution is an important factor to consider

when assessing the regeneration efficiency, and the impact of

electrochemical regeneration on the pore size distribution has

not been investigated. It should be noted that the field loaded

and regenerated GACs used in this study had undergone

a number of thermal regeneration cycles, causing differences

in pore size distribution between the virgin and other GACs.

Fig. 8shows the results of the pore size distribution for Cin-

cinnati GAC. The virgin GACs (shown by the dashed line) have

a large pore volume that is mainly in the micropore region

(less than 2 nm), with little volume in the meso (2e50 nm) and

macro (greater than 50 nm) pore regions. As expected, the

spent or field-loaded GAC (shown as the thin solid line) have

a much smaller micropore volume than the virgin GAC, due to

the sorbed NOM and pore blocking by the NOM. Thegraph also

shows that thermal regeneration (shown by the thick solid

line) recovers some but not all of the micropore volume. The

loss in micropore volume relative to the virgin GAC is

accompanied by an increase in the mesopore volume. These

trends have also been observed by other researchers (Moore

et al., 2003) and have been attributed to several factors

including pore blockage and the conversion of micropores to

mesopores due to pore wall burnoff.

The electrochemically regenerated and the field-spent

GACs have essentially the same pore volume distribution.

One would not expect electrochemical regeneration to change

the GAC pores, given that regeneration seems to be driven by

high pH induced desorption. The regenerated Regina GAC

showed the same pore size distribution patterns as the Cin-

cinnati GAC (McEwen, 2004). Changing the regeneration

current did not significantly change the pore size distribution

of the electrochemically regenerated GAC (McEwen, 2004).

Another interesting observation is that the shape of thethermally, electrochemically regenerated and the field-spent

GAC pore volume distributions were similar and have

higher mesopore areas than the virgin GAC. The most likely

explanation for the similarity is that most of the spent GAC

was previously thermally regenerated. It would be of interest

to also study the pore size distribution of electrochemically

regenerated GAC that had not previously been thermally

regenerated; such samples were not available to us. It should

be noted that both GAC sources were similarly impacted. And

finally, the key result of this analysis is that electrochemical

regeneration was not able to recover pore volume, which

explains the low NOM-based regeneration efficiencies

observed.

4. Conclusions

1. The electrochemical reactor and conditions used in this

study were ineffective for the regeneration of GAC from

post-filtration adsorbers at two full-scale water treatment

plants. The low regeneration efficiencies were attributed to

the limited NOM desorption and to the principal cathodic

electrochemical GAC regeneration mechanism, i.e., high-

pH induced desorption.

2. Electrochemical regeneration of laboratory batch-loaded

GAC with water from the same treatment plants yielded

Fig. 7eIodine number and NOM regeneration efficiency for

field-spent Regina GAC.

Fig. 8 e GAC pore volume distribution for Cincinnati GAC.

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much higher regeneration efficiencies than that for field-

loaded GAC. Thus, for more realistic assessments of the

effectiveness of electrochemical regeneration of GAC from

water treatment plants, loading and reloading to assess

regeneration efficiency should be conducted via column

tests and not batch tests.

3. Iodine number was not a good indicator of the electro-

chemical regeneration efficiency.4. The pore size analysis of the electrochemically regenerated

field-loaded GAC was consistent with the low NOM regen-

eration efficiencies observed.

This study showed that electrochemical regeneration of

commercial GAC field loaded with NOM is not very effective,

however before abandoning it, a few points bear consider-

ation. First, as NOM removal generally is not the primary

objective of water treatment GAC systems, the efficiency of

electrochemical regeneration needs to be evaluated in terms

of the regeneration efficiency of the target compound, such as

a taste and odour compound or pesticide which is being

adsorbed in competition with the NOM. Second, other types ofelectrochemical regeneration systems may prove more

effective for the task at hand. For example, some systems are

more adsorbate oxidation-controlled, rather than desorption

controlled like the one in this study (Garca-Oton et al., 2005;

Zhou and Lei, 2006). Third, the adsorption-electrochemical

regeneration system developed by Brown, Roberts and co-

workers (Brown et al., 2004; Brown and Roberts, 2007;

Mohammed et al., 2011) may workwellbecause they are based

on a low-porosity high-conductivity carbonaceous adsorbent

that should be less impacted by desorption. Third, the litera-

ture seems indicate good results for wastewater-type appli-

cations (Weng and Hsu, 2008; Wang and Balasubramanian,

2009). Fourth, due to the early stage of the development ofthis technology it is not possible to compare the full-scale

costs of electrochemical and thermal regeneration. The

energy requirements for small flow applications are compa-

rable for both types of systems, but the carbon replacement

costs are less for electrochemical systems. The electrical

energy consumption for the typical electrochemical system

tested (i.e., a mass of GAC 1.0 g, i 0.1 A, V 3.6 V and

time 5 h) was 1800 kWh/tonne of GAC. This is comparable to

the estimated energy usage for thermal regeneration in plants

with flows less than 2000 m3/d (Karimi-Jashni, 2001). Weng

and Hsu (2008)estimated the electrical energy consumption

of their electrochemical regeneration system to be 650 kWh/

tonne of GAC, so its cost is approximately 4% of replacing theGAC with virgin GAC. In considering costs, one must

remember that thermal regeneration systems may have to

replace up to 20% of the GAC per cycle, which may cost

significantly more than the energy required (Karimi-Jashni,

2001). Thus, electrochemical GAC regeneration shows

promise, but the development of an effective reactor which

regenerates large quantities of GAC remains an important

challenge. Preferably, research should be conducted to opti-

mize the materials and the column configurations for

adsorption and regeneration steps to take place in the same

vessel. This would reduce capital costs and eliminate GAC

transport losses. As direct contact between the GAC particles

and the electrodes was shown to improve regeneration

efficiencies, electrochemical regeneration in fluidized bed and

pulsed bed regeneration reactors, should also be investigated.

Acknowledgements

This work was supported by the Natural Science and Engi-neering Research Council of Canada (NSERC) Discovery Grant

Program. We are greatly indebted to Ben Boots of the Buffalo

Pound Water Treatment Plant, Regina and Morris McCormick

of the Richard Miller Water Treatment Plant, Cincinnati for

their cooperation.

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