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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
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 0
0043-1354/$ e see front matter 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.watres.2012.05.046
mailto:[email protected]://www.sciencedirect.com/science/journal/00431354http://www.elsevier.com/locate/watreshttp://dx.doi.org/10.1016/j.watres.2012.05.046http://dx.doi.org/10.1016/j.watres.2012.05.046http://dx.doi.org/10.1016/j.watres.2012.05.046http://dx.doi.org/10.1016/j.watres.2012.05.046http://dx.doi.org/10.1016/j.watres.2012.05.046http://dx.doi.org/10.1016/j.watres.2012.05.046http://www.elsevier.com/locate/watreshttp://www.sciencedirect.com/science/journal/00431354mailto:[email protected] -
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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.
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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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