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Treatment of cooling tower blowdown water containing silica, calcium and magnesium by electrocoagulation

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This research investigated the effectiveness of electrocoagulation using iron and aluminium electrodes for treating cooling tower blowdown (CTB) waters containing dissolved silica (Si(OH)(4)), Ca(2 + ) and Mg(2 + ). The removal of each target species was measured as a function of the coagulant dose in simulated CTB waters with initial pH values of 5, 7, and 9. Experiments were also performed to investigate the effect of antiscaling compounds and coagulation aids on hardness ion removal. Both iron and aluminum electrodes were effective at removing dissolved silica. For coagulant doses < or =3 mM, silica removal was a linear function of the coagulant dose, with 0.4 to 0.5 moles of silica removed per mole of iron or aluminium. Iron electrodes were only 30% as effective at removing Ca(2 + ) and Mg(2 + ) as compared to silica. There was no measurable removal of hardness ions by aluminium electrodes in the absence of organic additives. Phosphonate based antiscaling compounds were uniformly effective at increasing the removal of Ca(2 + ) and Mg(2 + ) by both iron and aluminium electrodes. Cationic and amphoteric polymers used as coagulation aids were also effective at increasing hardness ion removal.
a shows silica removal as a function of the aluminium dose for feed solutions with pH values of 5.0, 7.0 and 9.0. The results shown in Figure 2 are for samples filtered with 0.45 mm filter paper, which showed similar removals to those filtered with 0.8 mm filter paper. The supernatant and 10 mm filtered samples both showed lower levels of silica removal. This indicates that both colloidal and settleable solids were formed from the aluminium coagulating agent. As with iron electrodes, the feed solution pH value had only a minor impact on the fraction of silica removed, and final solution pH values were less than the initial values. The linear relationship between the dose and the fractional removal that was observed for iron electrodes also applies for aluminium electrodes for doses #3 mM. At higher aluminium doses, this relationship breaks down due to near depletion of silica from the solution. The data in Figure 2b show that there was no statistically significant Ca 2 þ removal in solutions with initial pH values of 5.0 and 7.0. For solutions with an initial pH value of 9.0, approximately 5 to 10% of the Ca 2 þ was removed. However, the amount of Ca 2 þ removed declined with increasing Al 3 þ dose. The Al 3 þ coagulant was also only marginally effective for Mg 2 þ removal, as shown by the data in Figure 2c. Approximately 5 to 10% of the Mg 2 þ was removed from solutions with final pH values less than 7.0, but no Mg 2 þ removal was observed from the solutions that had their final pH values increased back to 7.0.
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Treatment of cooling tower blowdown water containing
silica, calcium and magnesium by electrocoagulation
Z. Liao, Z. Gu, M. C. Schulz, J. R. Davis, J. C. Baygents and J. Farrell
ABSTRACT
Z. Liao
Z. Gu
M. C. Schulz
J. R. Davis
J. C. Baygents
J. Farrell
Department of Chemical and
Environmental Engineering,
University of Arizona,
Tucson AZ 85721,
USA
E-mail: zhliao@hotmail.com;
zgu@email.arizona.edu;
mcschulz@email.arizona.edu;
jrdavis@email.arizona.edu;
baygents@u.arizona.edu;
farrellj@email.arizona.edu
This research investigated the effectiveness of electrocoagulation using iron and aluminium
electrodes for treating cooling tower blowdown (CTB) waters containing dissolved silica (Si(OH)
4
),
Ca
2+
and Mg
2+
. The removal of each target species was measured as a function of the coagulant
dose in simulated CTB waters with initial pH values of 5, 7, and 9. Experiments were also
performed to investigate the effect of antiscaling compounds and coagulation aids on hardness
ion removal. Both iron and aluminum electrodes were effective at removing dissolved silica.
For coagulant doses #3 mM, silica removal was a linear function of the coagulant dose, with 0.4
to 0.5 moles of silica removed per mole of iron or aluminium. Iron electrodes were only 30%
as effective at removing Ca
2+
and Mg
2+
as compared to silica. There was no measurable
removal of hardness ions by aluminium electrodes in the absence of organic additives.
Phosphonate based antiscaling compounds were uniformly effective at increasing the removal
of Ca
2+
and Mg
2+
by both iron and aluminium electrodes. Cationic and amphoteric polymers
used as coagulation aids were also effective at increasing hardness ion removal.
Key words
|
blowdown, cooling tower, electrocoagulation, phosphonate, water softening
INTRODUCTION
Evaporative cooling is widely used in manufacturing
and electric power generation and is one of the major
contributors to industrial water use. Evaporative cooling
commonly employs cooling towers packed with a high
surface area material in which water and air flow in
counter-current directions. Water evaporation contributes
to cooling by an amount equal to the latent heat of water
vaporisation. Water evaporation also concentrates dis-
solved solids, which may then lead to scale formation on
heat transfer surfaces (Boerlage 2001;Jiang et al. 2002).
The hardness ions, Ca
2þ
and Mg
2þ
, and dissolved silica
(e.g., Si(OH)
4
) are the most problematic scale-forming
species in cooling tower waters (Mathie 1998). To prevent
loss in heat transfer efficiency, the concentrations of Ca
2þ
,
Mg
2þ
, and silica must be kept below levels that result in
scale formation on heat transfer surfaces. This requires the
continuous disposal, and replacement by fresh water, of a
small fraction of the water circulating in the cooling tower
loop. This water is commonly referred to as cooling tower
blowdown (CTB), and is normally discarded into the
sanitary sewer system. The amount of blowdown required
for cooling tower operation could be decreased by removing
scale-forming dissolved solids from the water.
There are several commonly used methods for removing
scale-forming dissolved solids from water, such as reverse
osmosis, nanofiltration, ion exchange and coagulation
methods. Due to concerns with scale formation on the
membranes themselves, membrane methods are not suitable
for treating waters with high concentrations of scale-forming
ions (Boerlage 2001). Both membrane and ion exchange
methods suffer from the fact that they produce brine streams
requiring disposal or further treatment by evaporation or
doi: 10.2166/wst.2009.675
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crystallisation. In contrast, coagulation methods produce
settleable solids that can easily be removed from the treated
water via clarification or microfiltration.
Ferric chloride (FeCl
3
) and alum (KAl(SO
4
)
2
z12(H
2
O))
are the most commonly used chemical coagulating agents.
Addition of ferric and aluminium salts to water results in the
formation of ferric and aluminium hydroxide precipitates.
These precipitates serve as adsorbents and complexing
agents and are capable of removing a wide range of
organic and inorganic contaminants from water. Electro-
coagulation (EC) adds Fe
2þ
or Al
3þ
to water via the
sacrificial dissolution of iron or aluminium anodes.
Subsequent oxidation of Fe
2þ
by dissolved oxygen then
produces a Fe
3þ
coagulating agent (Laksmanan et al. 2009).
EC offers two advantages over chemical coagulation. The
use of chemical coagulants results in the unwanted addition
of anions along with the metal cations that form the
precipitates. In contrast, EC introduces Al
3þ
or Fe
2þ
without the concomitant addition of anions to the water.
Additionally, precipitation of Fe(OH)
3
or Al(OH)
3
solids
results in a decline in solution pH values. In EC, this pH
decline is counterbalanced by the cathode reactions of
hydrogen evolution and oxygen reduction that remove
protons from solution (Mickley 2004;Holt et al. 2005;
Lin et al. 2005;Bayat et al. 2006)
EC has been applied to treat water containing organic
contaminants (Laridi et al. 2005), dyes (Kim et al. 2002;Can
et al. 2003), metals (Bissen & Frimmel 2003;Adhoum et al.
2004;Gao et al. 2005;Parga et al. 2005), and suspended
solids. Recently, several studies have focused on the
removal of colloidal silica via EC (Den & Huang 2006;
Kin et al. 2006;Lai & Lin 2006) and on the removal of
hardness ions (Kannan et al. 2006). However, these studies
were conducted using water streams with very different
characteristics from those normally found in cooling tower
waters. For example, studies investigating silica removal
have focused on the removal of silica nanoparticles from
very dilute solutions (Den & Huang 2006) rather than the
dissolved silica that is normally found in cooling tower
waters. Studies investigating hardness removal by EC have
focused on distillation liquors which have more than an
order of magnitude more hardness than CTB waters
(Kannan et al. 2006). In addition, cooling tower waters
usually contain antiscaling compounds that chelate Ca
2þ
and Mg
2þ
in order to prevent carbonate scale formation on
heat transfer surfaces. The impact of antiscaling compounds
on the effectiveness of EC for removing hardness ions from
water has not been previously investigated. Therefore, there
is a need for investigating the effectiveness of EC for waters
typical of those present in cooling towers.
This research investigated the use of EC for removing
scale-forming dissolved solids from mock cooling tower
water. Experiments were performed to investigate dis-
solved silica, Ca
2þ
and Mg
2þ
removal via electrocoagula-
tion using iron and aluminium electrodes. The removal of
each target compound was determined over a range of
iron and aluminium doses for inlet water pH values of 5, 7
and 9. Experiments were also performed to investigate the
effects of scale inhibitors and coagulation aids on hardness
ion removal.
MATERIALS AND METHODS
Concentrations of the target compounds in the mock CTB
were based on measurements of real CTB from a semi-
conductor processing facility in Hillsboro, Oregon. Feed
solutions containing 1.8 mM silica, 1.125 mM Ca
2þ
and
0.50 mM Mg
2þ
were prepared from sodium metasilicate
nonahydrate (Na
2
SiO
3
z9H
2
O), CaCl
2
and MgCl
2
reagents
in ultrapure water (18 MVcm). Feed solution pH values
were adjusted to 5.0, 7.0 and 9.0 using NaOH or HCl. Prior
to pH adjustments, the electrical conductivity of the mock
water was ,735 mS/cm. In order to match the electrical
conductivity of the real CTB, all feed solutions were
adjusted to 850 mS/cm using NaCl.
EC tests were performed in a rectangular, parallel-plate
flow-through reactor with an empty bed volume of 630 mL
(Powell Water Systems, Centennial, CO). The reactor
contained nine electrodes with dimensions of 34 cm £3.2
cm £0.32 cm with interelectrode gaps of 0.4 cm. The
electrodes were oriented vertically and were wetted up to
a height of 30.5 cm, yielding a total effective anode area of
800.5 cm
2
. The void volume in the reactor with nine
electrodes in place was 350 mL. A peristaltic pump
operating at 350 mL/min was used to pass the feed
solutions in an upflow direction through the eight parallel
channels between the nine electrodes. Feed solutions were
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contained in a 20 litre glass carboy and pumped into the
bottom of the reactor. Effluent from the EC reactor was
collected in either a 20 litre carboy or 1 litre polypropylene
sampling bottles.
All tests were performed under galvanostatic control
using a direct current power supply (Protek model 3005B).
The electrodes were operated in a bipolar stack arrange-
ment with power connections to the first and last electro-
des. The current densities ranged from 0.30 to 9.0 mA/cm
2
.
The relationship between the reactor currents and the
coagulant dosing rates were determined in a previous
investigation (Gu et al. 2009). Before taking any samples,
approximately 10 reactor volumes (3.5 L) of water were
passed through the EC unit in order to achieve steady state
operation. Four 1 L samples were collected in polypropy-
lene bottles and were stirred at a moderate rate for 0.5 hours
and then allowed to settle overnight. Given that the
reported half-life of Fe
2þ
in air-equilibrated water is less
than 0.5 minutes at neutral pH (Park & Dempsey 2005), the
stirring and overnight equilibration in open containers
allowed for near complete oxidation of Fe
2þ
. The four 1
litre samples were then split into four 250 mL samples
consisting of unfiltered supernatant, and samples vacuum
filtered through 0.45, 0.8 and 10 micron cellulose nitrate
filter paper (Whatman). The different filtering procedures
were designed to test the settling properties of the
precipitates. The EC unit was drained and rinsed with
deionised water between tests.
Experiments were also performed to investigate the
effect of eight antiscaling compounds and four coagulating
aids on hardness ion removal. Five compounds commer-
cially available as antiscalants and three commodity
chemicals that are often used as antiscalants were tested.
In these experiments, the compounds listed in Table 1 were
added to the sample bottles before the 0.5 h stirring
period. Doses for the commercial antiscaling agents and
polymeric coagulating aids were based on manufacturer
recommendations.
Silica concentrations were determined with a spectro-
photometer using Hach Method 8185 based on absorbance
by silicomolybdate complexes (Knudson et al. 1940).
Calcium and magnesium concentrations were determined
using a Perkin-Elmer Optima 2100 DV inductively coupled
plasma optical emission spectrometer (ICP-OES).
RESULTS
Dose response tests
Figure 1a shows the fractional removal of silica as a
function of iron dose for feed water pH values of 5.0, 7.0
and 9.0. The fractional removals of silica were identical for
the unfiltered supernatant samples and the samples filtered
with 10, 0.8 and 0.45 mm filters. This indicates that
the precipitates that formed were all settleable solids. The
average final pH associated with feed solution pH values of
5.0, 7.0 and 9.0 was 6.2, 6.1 and 7.9, respectively. These
final solution pH values are consistent with the weak acid
properties of ferric hydroxides, which have a pK
a1
value of
7.18 and a pK
a2
value of 8.82 (Stumm 1992). Also shown in
Figure 1a are the results for a pH 7.0 feed solution whose
final pH was readjusted to 7.0 prior to filtration. Regression
analyses on fractional silica removal versus iron dose for
each feed solution are summarised in Table 2. The three
solutions with final pH values #7.0 had statistically
Table 1
|
Organic additives used in hardness removal tests
Additive/dose (w/w) Description
Antiscalants
Dequest-2000/1% Amino trimethylene
phosphonic acid
Dequest-2010/1% Hydroxyl ethylidene
(1,1-diphosphonic acid)
Dequest-2060S/1% Diethylene triamine penta
(methylene phosphonic acid)
Dequest-7000/1% 2-phosphonobutane-1,2,4-
tricarboxylic acid
Aquatreat-900A/0.5% Low molecular weight
polyacrylic acid
Sodium citrate/0.8% Na
3
C
6
H
5
O
7
Sodium alginate/0.4% NaC
6
H
7
O
6
Sodium lignosulphonate/0.1% Mixture of anionic sulphonated
polymers
Coagulation aids
Nsight-C1/0.5% Cationic high molecular
weight polymer
Nsight-A1/0.5% Anionic high molecular
weight polymer
FlocAcid-19/0.5% Amphoteric synthetic polymer
Alcoclear-CCPII/0.5% Synthetic cationic polymer
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identical regression slopes, but at the 95% confidence level
there was a greater slope for the solution with a final pH
value of 7.9.
Figures 1b and 1c show the removal data for Ca
2þ
and
Mg
2þ
by iron electrodes, and Table 2 summarises the
slopes of the regression lines for the fractional removal
versus coagulant dose. Comparing the regression slopes for
Ca
2þ
and Mg
2þ
indicates that iron coagulants remove
Mg
2þ
slightly better than they remove Ca
2þ
. However, the
removal of both these cationic species is much less than
the removal of silica.
Figure 2a shows silica removal as a function of the
aluminium dose for feed solutions with pH values of 5.0, 7.0
and 9.0. The results shown in Figure 2 are for samples
filtered with 0.45 mm filter paper, which showed similar
removals to those filtered with 0.8 mm filter paper.
The supernatant and 10 mm filtered samples both
showed lower levels of silica removal. This indicates
that both colloidal and settleable solids were formed from
the aluminium coagulating agent. As with iron electrodes,
the feed solution pH value had only a minor impact on the
fraction of silica removed, and final solution pH values were
less than the initial values. The linear relationship between
the dose and the fractional removal that was observed
for iron electrodes also applies for aluminium electrodes
for doses #3 mM. At higher aluminium doses, this
relationship breaks down due to near depletion of silica
from the solution.
The data in Figure 2b show that there was no
statistically significant Ca
2þ
removal in solutions with
initial pH values of 5.0 and 7.0. For solutions with an
initial pH value of 9.0, approximately 5 to 10% of the Ca
2þ
was removed. However, the amount of Ca
2þ
removed
declined with increasing Al
3þ
dose. The Al
3þ
coagulant
was also only marginally effective for Mg
2þ
removal, as
shown by the data in Figure 2c. Approximately 5 to 10%
of the Mg
2þ
was removed from solutions with final pH
values less than 7.0, but no Mg
2þ
removal was observed
from the solutions that had their final pH values increased
back to 7.0.
Figure 1
|
Fraction of silica (a), calcium (b) and magnesium (c) removed as a function
of iron dose for initial pH values of 5.0, 7.0, and 9.0. The average final pH
values associated with each feed solution are also shown.
Table 2
|
Slopes with 95% confidence intervals from regression analyses of fractional
removal versus iron dose for feed solutions with initial pH values of 5.0, 7.0
and 9.0
pH Initial/Final Silica Ca
21
Mg
21
5.0/6.2 0.231 ^0.011 0.028 ^0.01 0.066 ^0.008
7.0/6.1 0.238 ^0.012 pp
9.0/7.9 0.286 ^0.013 0.063 ^0.01 0.157 ^0.012
7.0/7.0 0.230 ^0.012 0.03 ^0.004 0.034 ^0.005
pnot measured.
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Organic additive effects
Figure 3 shows the Ca
2þ
and Mg
2þ
removal efficiencies for
different organic additives for a coagulant dose of 2mM
Fe
2þ
at a pH value of 7. All four phosphonate antiscalants
increased the removal efficiency for both hardness ions by
200 to 250%. Sodium alginate increased Ca
2þ
removal by
100% and Mg
2þ
removal by 30%. The citrate antiscalant
showed mixed results with a 50% increase in Ca
2þ
removal
and a 10% decrease in Mg
2þ
removal. The polyacrylic acid
and lignosulphonate antiscalants decreased both Ca
2þ
and
Mg
2þ
removal by more than 50%. Of the four coagulating
aids tested in Figure 3, three increased both Ca
2þ
and
Mg
2þ
removal, with the Alcoclear-CCPII being the most
effective. The Alcoclear-CCPII increased the removal of
both hardness ions by more than 300%.
Figure 4 shows the effect of the organic additives on
Ca
2þ
and Mg
2þ
removal efficiencies for a coagulant dose of
2 mM Al
3þ
at a pH value of 7. Because there was no Ca
2þ
or Mg
2þ
removal without the additives, all additives
increased removal of the hardness ions. The two natural
polymers Nsight-C1 and -A1 were the most effective at
increasing hardness removal by aluminium coagulants. The
four phosphonic acid antiscalants produced similar results
for both Ca
2þ
and Mg
2þ
.
DISCUSSION
Both the iron and aluminium electrodes showed much
better performance for silica removal than for Ca
2þ
or
Mg
2þ
. This can be explained by a different removal
mechanism for silica than for Ca
2þ
and Mg
2þ
. The initial
silica concentrations used in this study were below those at
Figure 2
|
Fraction of silica (a), calcium (b) and magnesium (c) removed as a function
of aluminium dose for initial pH values of 5.0, 7.0, and 9.0. The average
final pH values associated with each feed solution are also shown. The
regression lines in part (a) are based only on data with Al concentrations
less than 3mM.
Figure 3
|
Calcium and magnesium removal efficiencies with different organic
additives and coagulation aids for an iron coagulant dose of 2mM at a pH
value of 7.0.
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which orthosilicic acid begins to form oligomeric species
(Koo et al. 2001;Sheikholeslami & Bright 2002). Therefore,
nearly all the silica was present as orthosilicic acid, Si(OH
4
).
This species forms inner-sphere complexes with ferric and
aluminium hydroxides (McPhail et al. 1972;Meng et al.
2000). The basic structural unit of ferric and aluminium
hydroxide precipitates is Fe
3þ
or Al
3þ
coordinated to six
oxygen atoms, as illustrated for iron in Figure 5a. As
illustrated in Figure 5b, these structural units are shaped
like octahedra and may aggregate to form a variety of
polymeric structures. Inner-sphere complexes between
silica and ferric hydroxide are known to form via replace-
ment of two OH
2
groups with an orthosilicate anion,
according to:
Fe2O10H14 þSiðOHÞ4!Fe2O10 H12 þSiðOHÞ2þ2H2O:ð1Þ
The final structure is often a bidentate corner-sharing
complex, as illustrated in Figure 6a. The slopes of the
regression lines in Figure 1 for silica removal by ferric
hydroxide precipitates yielded a reaction stoichiometry that
ranged from 0.8 to 1.0 Si atoms removed per dioctahedral
unit cell. For the aluminium coagulant, the reaction
stoichiometry in the linear range of the dose-response
relationship ranged from 0.45 to 0.90 Si atoms removed per
di-octahedral unit cell.
In contrast to the silica removal mechanism that
involves chemical adsorption, Ca
2þ
and Mg
2þ
removal by
iron and aluminium coagulants appears to involve only
physical adsorption, which results from electrostatic attrac-
tion by Ca
2þ
and Mg
2þ
to negatively charged sites on the
precipitates. Figure 6b illustrates adsorption of Ca
2þ
via a
weak electrostatic attraction to two oxygen atoms. The
greater the negative charge on the precipitates the greater
the expected electrostatic adsorption. Therefore, significant
Ca
2þ
and Mg
2þ
removal by ferric hydroxides is expected to
occur only at pH values above the isoelectric point, which
occurs at a pH value of ,7.2 (Stumm 1992). For aluminium
hydroxides, much higher pH values are required to obtain
negatively charged precipitates. For example, the isoelectric
point of g-AlOOH occurs at a pH value of 8.2 (Stumm
1992). Therefore, at circumneutral pH values,
g
-AlOOH
precipitates will be positively charged. This may explain
why there was no measurable removal of Ca
2þ
or Mg
2þ
by
the aluminium precipitates for the experiments depicted in
Figure 2b and 2c.
Figure 4
|
Calcium and magnesium removal efficiencies with different organic
additives and coagulation aids for an aluminium coagulant dose of 2mM at
a pH value of 7.0.
Figure 5
|
a) Di-octahedral unit cell of ferric hydroxide precipitates. b) Two Fe
3þ
atoms
are octahedrally coordinated with six oxygen atoms. Aluminium hydroxides
have similar structures. Atom key: Fe-blue; O-red; H-white. Subscribers to
the online version of Water Science and Technology can access the colour
version of this figure from http://www.iwaponline.com/wst.
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The effect of the antiscalants on Ca
2þ
and Mg
2þ
removal
indicates that multiple mechanisms play a role in hardness
ion removal. Antiscaling compounds are designed to keep
hardness ions in solution by forming highly water soluble
complexes with Ca
2þ
and Mg
2þ
ions. In 27 of the 32 tests,
the antiscalants increased hardness ion removal. This
suggests that these antiscalants also physically or chemically
adsorb to the iron and aluminium hydroxide precipitates.
The fact that both the lignosulphonate and the low molecular
weight polyacrylic acid (Aquatreat-900A) decreased both
Ca
2þ
and Mg
2þ
removal by iron but increased removal by
aluminium suggests that these polymers bind more strongly
to aluminium than iron precipitates. Because both of these
antiscalants contain many negatively charged functional
groups, this supposition is consistent with the greater
positive charge on the aluminium versus iron precipitates.
In 15 of the 16 tests, the coagulation aids increased
both Ca
2þ
and Mg
2þ
removal. The synthetic cationic
polymer (Alcoclear-CCPII) gave the greatest enhancement
for hardness ion removal by the iron precipitates. The
Nsight-C1 cationic polymer was also very effective at
enhancing hardness ion removal by the aluminium precipi-
tates. These results are difficult to explain given that cationic
polymers should not form complexes with hardness ions.
The results are also difficult to explain given that the
aluminium precipitates were positively charged under the
conditions of these experiments. One possible explanation
is that precipitation of the solids in the presence of
these polymers resulted in greater surface areas available
for hardness ion adsorption.
CONCLUSIONS
This study showed that EC is effective for removing silica
from mock CTB using both iron and aluminium electrodes.
Phosphonate based antiscaling compounds that are often
used in cooling tower waters increased hardness ion
removal. Hardness ion removal may also be enhanced by
the addition of coagulation aids. The overall practicality of
using EC for removing scale-forming species from cooling
tower waters will depend on the overall treatment costs and
the cost and availability of fresh water.
For treating 1 m
3
of water, the energy requirement
for delivering a 1 mM dose of iron coagulant ranged from
0.041 kilowatt hours (kWh) at a current density (i) of
0.35 mA/cm
2
to 0.15 kWh for i¼4.6 mA/cm
2
. For alumi-
nium, the energy requirement ranged from 0.050 kWh/m
3
(i¼0.30 mA/cm
2
) to 0.27 kWh/m
3
(i¼9.0 mA/cm
2
).
Therefore, even at high metal doses, the electrical energy
requirements were small. Further details on the effects of
electrode spacing, solution conductivity, and applied cur-
rent density on the energy requirements for EC can be
found in Gu et al. (2009). Estimates provided in that study
also indicate that the electrode cost per 1 mM of iron dose is
$0.25 per cubic metre of water treated, based on a blade
cost of $4.50/kg (Gu et al. 2009). For aluminium electrodes,
Figure 6
|
a) Chemical adsorption of orthosilicate to ferric hydroxide unit cell.
b) Electrostatic adsorption of a Ca
2þ
cation near two oxygen atoms thatcarry
a partialnegative charge.Atom key: Fe-blue;O-red; H-white;Si-tan; Ca-green.
Subscribers to the online version of Water Science and Technology can access
the colourversion of this figure fromhttp://www.iwaponline.com/wst
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a 1 mM dose costs $0.37/m
3
, based on a blade cost of
$13.50/kg. Thus, for a typical electrical energy cost of
$0.10/kWh, treating water by EC is dominated by the costs
for the consumable blades. Additional considerations are
the costs associated with separation and disposal of the
precipitated solids.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge Allen Boyce, Avi
Fuerst, Zoe Georgousis, Len Drago and the Intel Corpor-
ation, Jackie Moxley and the University of Arizona Water
Sustainability Program, and Chuck Graf and the Arizona
Water Institute for providing funding and other support for
this research.
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Treatment of cooling tower blowdown water by electrocoagulation Water Science & Technology—WST
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... investigation has a high concentration of calcium ions In literature many articles are presented dealing with (800 mg/L) and magnesium ions (400 mg/L) which always removal of calcium and magnesium from wastewater; have the tendency for scale formation and fouling through crystallization enhanced by aeration, 34% and problem during wastewater vaporization process for 51% of calcium and magnesium were removed recycling of MEG. Scale could be formed due to different respectively [3], through electrocoagulation using iron factors; supersaturation, solubility product, nucleation electrode, only 30% of both were removed [4,5], through and particle growth in MEG recovery process by precipitation of calcium cations as calcium carbonate removing water, during vaporization process, resulted in through bio-catalytic process utilizing pre-cultivated obviously increasing of salt concentrations which calcareous sludge, up to 90% of calcium ions is removed precipitate. The main problems accompanying with scale [6], through sodium hydroxide [7] and through modified formation in MEG regeneration unit include: the reducing zeolites [8]. ...
... 2). The results are higher than that obtained by[3,4,5,6, 7, 8]. 100 % removal of iron is achieved due to the high pH (10.5) where ferric hydroxide is completely precipitated. ...
... Saha et al. [10] evaluated the performance of electrochemical oxidation (EO) with a boron-doped diamond (BDD) anode to remove organic compounds (OCs) from CTB in the cooling tower. Liao et al. [11] reported that the efficiency of hardness ion removal for the cooling tower blowdown in the cooling tower can be improved by the EC using Fe and Al electrodes, and the addition of coagulation aids. ...
Article
Due to the rapid economic development in recent years, water-chiller is being used more and more in hotels, restaurants, resorts, and industrial zones in Vietnam. The cooling tower’s cooling water quality is an important factor in determining the effect on electrical consumption and the coefficient of performance of a water-cooled chiller. Therefore, it is necessary to maintain the quality of cooling water during the operation time of the water-cooled chiller. The aim of this work is to investigate the effects of the pulse frequency and operational parameters such as current density, pulse duty cycle, and energy consumption on the performance of removing hardness ion of electrochemical cooling water treatment for the cooling tower of the water-cooled chiller. The results show that the highest efficiency of the total hardness removal corresponding to a pulse frequency of 1 kHz, a pulse duty cycle of 0.7, and a current density of 80A/m2.
... Dissolved silica has a very high affinity to form very stable coordination with aluminum cations. The phenomenon is widely exploited in the coagulation process to separate silica as aluminosilicates (57,58). Farajnezhad et al. (59) demonstrated a selective separation of Si from a petroleum process effluent via a chemical coagulation process using alum as a coagulant. ...
Article
Full-text available
Treating water and wastewater is energy-intensive, and traditional methods that require large amounts of chemicals are often still used. Electrocoagulation (EC), an electrochemical treatment technology, has been proposed as a more economically and environmentally sustainable alternative. In EC, sacrificial metal electrodes are used to produce coagulant in-situ, which offers many benefits over conventional chemical coagulation. However, material precipitation on the electrodes during long term operation induces a passivating effect that decreases treatment performance and increases power requirements. Overcoming this problem is considered to be the greatest challenge facing the development of EC. In this critical review, the studies that have examined the nature of electrode passivation, and its effect on treatment performance are considered. A fundamental approach is used to examine the association between passivation and faradaic efficiency, a surrogate for EC performance. In addition, the strategies that have been proposed to remove or avoid passivation are reviewed, including aggressive ion addition, AC current operation, polarity reversal, ultrasonication, and mechanical cleaning of the electrodes. It is concluded that the success of implementing each method is dependent on critical operating parameters, and careful consideration should be taken when designing an EC system based on the phenomena discussed in this article. In conclusion, this review provides insight into passivation mechanisms, delivers guidelines for sustaining high treatment performance, and offers an outlook for the future development of EC.
... Dissolved silica has a very high affinity to generate very stable coordination with Al cations. The phenomenon is largely employed in the coagulation technique to separate silica as aluminosilicates [117]. Researchers [1] proved a selective separation of Si from a petroleum process effluent via a chemical coagulation technique employing alum as a coagulant. ...
Article
Full-text available
Electrocoagulation (EC) is a very efficient process in dealing with effluent streams and separating complicated contaminants prior to the discharge of the treated water. Attention to such a technique augmented thanks to its large set of utilizations, zero-or minimal-chemical dosing demands, low waste formation, and low price. EC appears as an efficacious option to traditional water treatment techniques for the separation of a large collection of contaminants. This work examines the theories of the EC method and its application for the separation of contaminants from wastewater streams. Such a technique depends on the integration of electrochemical and coagulation methods. Basic parameters that touch the effectiveness comprise the electrode material (Fe or Al), current density, the electrical charge per unit volume, and solution pH. Electrode fouling could constitute a hard running dare even if it could be reduced by the alternating current operation. Next studies have to follow the routes of the EC technique for numerous kinds of pollutants at a set of working parameters, in particular for continuous mode, and the expansion of convenient models that could be utilized for scale-up and tech-no-economic evaluation of EC is required. Running as a destabilization agent and aiding to separate contaminants from the wastewater, the electric field should attract more attention to highlight its key contribution.
... At present, the common technologies for CTBD treatment include flocculation, membrane filtration, etc., but the problems of high investment cost, poor stability and secondary pollution have to be considered [14,15]. From both economy and effectiveness perspectives, adsorption is considered as a promising technology for simultaneous removal of organics and phosphorus [16][17][18]. ...
Article
An eco-friendly process was adopted to treat cooling tower blowdown water (CTBD) and the toxicity of correspondingly produced water/eluate was evaluated using the transcriptional effect level index (TELI) based on toxicogenomics. The objective of the work is to provide a feasible treatment loop including adsorption to remove organics and phosphorus from CTBD, electrocatalytic oxidation to improve the biodegradability of the eluate after desorption. Results showed that PANI/TiO2 was a promising adsorbent in the removal of organics and phosphorus from CTBD and exhibited a satisfied regeneration ability beyond 30 times of reuse. During the electrocatalytic oxidation process the biodegradability of desorption eluate was gradually increasing and BOD5/COD of the oxidized eluate reached 0.4 after 4.8 hours of treatment, indicating that the treated wastewater could be returned to the biological treatment loop for further processing. The analysis of the quantitative toxicogenomics assay revealed that the toxicity of CTBD was mainly caused by oxidizing biocides of trichloroisocyanuric acid (TCCA), leading to a significant membrane stress response of bacteria. And the toxicity level of CTBD decreased after adsorption treatment while the desorption eluate experienced increase and then decrease during the electrocatalytic oxidation, meaning that certain oxidation duration was needed to keep the eluate safe for biological treatment. According to economic analysis, the operation cost of treatment loop was estimated at around 0.6 dollars/m³, ensuring high reuse water quality and safe eluate for further biological treatment.
... Dissolved silica has a very high affinity to form very stable coordination with aluminum cations. The phenomenon is widely exploited in the coagulation process to separate silica as aluminosilicates (57,58). Farajnezhad et al. (59) demonstrated a selective separation of Si from a petroleum process effluent via a chemical coagulation process using alum as a coagulant. ...
Chapter
Full-text available
Theprocess of electrocoagulation is a highly effective method to remediate effluent streams and to separate problematic pollutants before the discharge of the treated water. Interest in this technology has increased due to its broad range of applications, zero or minimal chemical dosing requirements, low waste production, and low cost. The process of electrocoagulation is emerging as an effective alternative to conventional water treatment processes for the separation of a wide range of pollutants. This chapter explores the principles of the electrocoagulation process, and its implementation for the separation of pollutants from wastewater streams. The technology relies on the combination of electrochemical and coagulation processes. Key factors that influence the performance include the electrode material (usually iron or aluminum), current density, electrical charge per unit volume, and solution pH. Commercial electrocoagulation systems are normally operated at constant current (5-20 mA/ cm2) to ensure effective treatment. Electrode fouling can present a significant operational challenge but can be mitigated by alternating current operation. Dosing of the coagulant in the electrocoagulation process obeys Faraday's law of electrochemical dissolution (the Coulombic efficiency is typically close to 100%), which facilitates process automation and control. The electrocoagulation performance can be characterized in terms of the Coulombic efficiency (95%), electrical energy per unit volume (typically 0.5 kWhr/m3), and the separation efficiency (often 95%). Design parameters must be selected by considering economic, performance and operational factors. The interelectrode gap (typically 15 mm) must consider flow distribution, the risk of plugging due to fouling or coagulated solids, and the cell resistance (and hence energy consumption). Selection of the operating current density is dependent upon the solution conductivity (and hence energy consumption), the total area of electrode required for effective treatment, and the lifetime of the electrodes.
Chapter
The main objective of the investigation is to evaluate the effects of the flow rate of cooling water on the performance of the electrochemical water treatment for the cooling tower of the water-cooled chiller. The quality of cooling water of the cooling tower is mainly a parameter of the effect on the coefficient of performance of water-cooled chiller. Therefore, it is necessary to maintain the quality of cooling water during the operation time of the water-cooled chiller. In this study, the electrochemical water treatment for the cooling tower of the water - cooled chiller was designed and built on the campus of Ho Chi Minh City University of Technology and Education for experiments. This system consists of a reactor tank that has a volume of 2 L, three Titanium electrodes connected to DC power, a circulating water pump, a condenser, a cooling tower, and a control box. The experimental results show that the highest performance of the electrochemical water treatment system to maintain the water quality of the cooling tower of water-cooled chiller corresponds to the spacing between of three Titanium electrodes is 2 cm, the current density is 90 A/m², and the flow rate of cooling water through the reactor tank is 4 L/min.
Article
Full-text available
Electrochemical water softening has been widely used in industrial circulating cooling water systems; however, their low deposition efficiency is the main drawback that limits usage in medium to large enterprises. In this work, the effect of different parameters on the hardness removal efficiency and energy consumption of the electrochemical water softening system is experimentally studied, and the performance of water softening applied by high frequency electric fields and direct current electric fields are comparative analyzed. The impact factors of the electrochemical water softening system are as follows: initial feed concentration of solute, magnitude of voltage, inter-electrode distance, area of cathode and frequency of power supply. To improve the analysis efficiency, the L25 (55) orthogonal table is used to investigate the five different factors at five levels. The experimental results are shown that the initial feed concentration of solute is the most significant factor affecting the hardness removal efficiency. The optimal combination for water softening in the group applied by high frequency electric field and direct current electric field are A3B2C1D4E3 and A2B5C3D1 respectively. The energy utilization of the device applied by high frequency electric field is 3.2 times that applied by direct current electric field. The practice shows that direct current electric fields have a better softening effect, and are is more suitable for scaling ion removal. Particle image velocimetry (PIV) was used to observe the flow field induced by the electrolysis and found that the vertical and horizontal velocities of the flow field at low voltage are conducive to the migration of scaled ions to the cathode, and then the electrolytic reaction and deposition reaction synergy effect is the optimal. HIGHLIGHTS Orthogonal experiments were used to optimize electrochemical water softening.; The initial concentration determines the electrochemical softening rate.; PIV technology is used to analyze mass transfer process.; Water softening performance depends on the synergistic reaction of electrolysis and mass transfer.; Water softening performance is enhanced by high frequency electric field.;
Article
In recent years, capacitive deionization (CDI) technology gradually becomes a promising technology for hard water treatment. Up to now, most of work for water softening in CDI were severely limited by the inferior selectivity and electrosorption performances of carbon-based electrode in spite of combining Ca²⁺ selective ion exchange resin or membrane. Pseudocapacitive electrode materials that selectively interaction with specific ions by faradaic redox reactions or ions (de)intercalation offers an alternative strategy for highly selective electrosorption of Ca²⁺ from water because of brilliant ions adsorption capacity. Here, we firstly used copper hexacyanoferrate (CuHCF) as a pseudocapacitive electrode to methodically study the selective pseudocapacitive deionization of Ca²⁺ beyond Na⁺ and Mg²⁺. Using the hybrid CDI cell comprised of CuHCF cathode and activated carbon anode without any ion exchange membrane, the outstanding Ca²⁺ electrosorption capacity of 42.8 mg g⁻¹ and superior selectivity &(Ca²⁺/Na⁺) of 3.05 at molar ratio of 10:1 f was obtained at 1.4 V, surpassing the reported carbon-based electrodes. Finally, electrochemical measurement and molecular dynamics (MD) simulation provided in-depth understanding for the selective pseudocapacitive deionization of Ca²⁺ ions in CuHCF electrode. Our study would be helpful for developing high-efficiency selective electrosorption of target charged ions by intrinsic property of pseudocapacitive materials.
Article
In the present research, electrocoagulation (EC) process was applied as a selected pretreatment to evaluate its effectiveness and operating cost regarding simultaneous calcium and turbidity removal in order to mitigate the scaling and colloidal fouling potential of reverse osmosis (RO) technology. The effects of the main parameters, namely, initial calcium concentration, initial turbidity, time, and current density, on EC process were assessed and optimized by employing response surface methodology (RSM). A reasonable correlation between the experimental and predicted data was found through analysis of variance (ANOVA). The highest calcium and turbidity removal efficiencies of 36 % and 93.5 %, respectively, and the minimal operating cost of 1.58 US$/m³ were obtained at the optimum time of 35.5 min and current density of 3.85 mA/cm² with the selected constant initial calcium concentration of 250 mg/L and initial turbidity of 85 NTU. In the present study, the applicability of two pretreatments, including hybrid EC-filtration and filtration-only to RO membrane fouling was also compared. The RO recovery of the hybrid EC-filtration pretreatment improved about 25 % in comparison to that of the filtration-only pretreatment, which demonstrated the effective capability of the EC process in scaling and colloidal fouling mitigation.
Book
Many cooling systems use water as cooling medium. They are found in public buildings, industrial production systems or power plants. Almost every cooling system using water is degraded by deposition, corrosion and microbiological fouling. This book identifies the whole bunch of problems due to water cooling systems and proposes specific solutions to all of them. The authors have an expertise of over 20 years solving cooling water problems. In this book, they advise all practitioners which need to plan, buy or operate cooling systems. © 2009 Springer-Verlag Berlin Heidelberg. All rights are reserved.
Chapter
In general, natural water that we have at our disposal is not ready for a desired end-use, thus, its treatment is necessary.
Article
Designing a reclaimed water system provides an economically and environmentally favorable method for disposing of wastewater. However, some critical influences on on-site reclaimed water systems, such as limited building area, often limit the effectiveness of conventional treatment methods. This work established a compact and inexpensive electrocoagulation process with a capacity of 28 m(3)/day to reclaim domestic greywater for human noncontact usage. The total unit cost of on-site domestic greywater reuse was U.S. $0.27/m(3), which was below the local potable water rate. Moreover, the treatment facility required an area of 8 m(2). Both unit cost and required area in this work are lower than those reported in the literature. The experimental results support the feasibility of the on-site reuse of greywater in high-rise buildings.
Article
The electrocoagulation process was developed to overcome the drawbacks of conventional wastewater treatment technologies. This process is very effective in removing organic pollutants including dyestuff wastewater and allows for the reduction of sludge generation. The purposes of this study were to investigate the effects of the operating parameters, such as current density, electrode number, electrolyte concentration, electrode gap, dyestuff concentration, pH of solution and inlet flow rate, on decolorization by continuous electrocoagulation. The dye removal efficiencies and reaction rate constants from the curves following the first-order relationship of electrocoagulation were calculated. In addition, from the points of power consumption, the effects of the operating parameters were also searched. Finally, the behaviors of decolorization according to dyestuff types, i.e., disperse dye and reactive dye, were also examined.
Article
Desalination plays an important role in producing pure water from brackish water. Reverse osmosis (RO) is by far the most efficient way to remove colloidal and dissolved silica, which can be found in high concentrations in brackish water. The presence of silica and its ability to foul membranes limits the use of silica bearing waters for desalination and when used, it has many economic penalties. This study examines the effect of silica polymerisation in the presence of polyvalent cations and anions in RO systems. Source of silica in the experimentation was from commercial grade sodium metasilicate (Na2O3Si.9H2O). The membranes used were polyamide and thin film manufactured by Osmonics. Use of glassware is minimised to avoid the possibility of any contribution by silica leaching into solution. The feed solution consists of silica, calcium and magnesium ions in various concentrations to determine the effect of polyvalent ions on polymerisation and the appropriate pre-treatment technology. The experiment was set up in a way as to simulate the conditions that would be encountered in a desalination plant. Concentration polarisation (C/P) in the system was experimentally determined with a simple technique that was developed and its effects on fouling are examined. In addition tests were carried out to examine the actual fouling mechanism in reverse osmosis units under various experimental conditions. Furthermore the effect of cleaning, with distilled water and with pulsations as well as with commercially available cleaners were examined. Some commercially available cleaners were capable at partially restoring the flux. Further investigation is underway to examine the effectiveness of new silica specific inhibitors.
Article
Swine wastewaters contain varied and high amounts of organic matter (proteins, antibiotic compounds, organic acids) which are difficult to oxidize biologically or chemically. The discharge of such effluents is undesirable and can cause excessive oxygen demand in the receiving water. In order to produce an effluent suitable for stream discharge, electrochemical techniques have been explored at the laboratory pilot scale, for refractory residual organic compound removal in liquid swine manure (LSM) following a biofiltration process. Two types of electrolytic cells (monopolar and bipolar electrode cells) using aluminum and mild steel electrodes were studied. Effectiveness was measured in terms of chemical oxygen demand (COD) and biological oxygen demand (BOD) reduction. The amount of residue sludge produced and energy consumed have been also considered. Results showed that the best performances of COD and BOD removal from LSM were obtained using either aluminum bipolar (Al-BP) electrodes or mild steel monopolar (Fe-MP) electrodes operated at current intensities of 0.5 and 2.0 A, respectively, through 30 min of treatment. The COD removal yields varied from 65 to 68%, whereas BOD removal reached 87%. The optimal conditions determined for organic compound removal, including energy consumption and metallic sludge disposal, involved a total cost of only $0.24 and $0.29 United States/m(3) of treated LSM. The treatment using the Fe-MP system was found to be more economical and practical than the chemical treatment using FeCl3 as a coagulating agent.
Article
The goals of the research project were 1) to demonstrate the removal capabilities of the electrocoagulation (EC) technology and 2) to define the best application of the technology within the context of providing pretreatment to reverse osmosis systems.