xvEPA
United States
Environmental Protection
Agency
EPA/540/S-93>!504
September 1993
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology
Summary
Electro-Pure Alternating Current
Electrocoagulation
Naomi P. Barkley, Clifton Farrell, and Trade Williams
The Superfund Innovative Technol-
ogy Evaluation (SITE) Program was au-
thorized as part of the 1986 amend-
ments to the Superfund legislation. It
represents a joint effort between the
U.S. Environmental Protection Agency's
(EPA) Office of Research and Develop-
ment and Office of Solid Waste and
Emergency Response. The program is
designed to assist and encourage the
development of waste treatment tech-
nologies that would contribute innova-
tive solutions to our hazardous waste
problems. Under the Emerging Tech-
nology portion of the SITE Program, a
2-yr research effort was conducted by
Electro-Pure Systems, Inc., to evaluate
the technical and economic feasibility
of alternating current electrocoagulation
(ACE) for remediation of aqueous waste
streams at Superfund sites.
The ACE Technology introduces low
concentrations of nontoxic aluminum
hydroxide species into the aqueous
media by the electrochemical dissolu-
tion of aluminum-containing electrodes
or pellets. The aluminum species that
are produced neutralize the electrostatic
charges on suspended material and/or
prompt the coprecipitation of certain
soluble ionic species, and thereby fa-
cilitate their removal.
Electrocoagulation has been demon-
strated to enhance the filtration and de-
watering rates for solids removed from
an effluent; such enhancements are
prompted by growth in the mean par-
ticle size from typically <0.3 p.m in di-
ameter to as much as 150 |xm, depend-
ing on the degree of electrocoagulation.
Significant reductions in the total sus-
pended solids (TSS) loading of particu-
late slurries and in the concentrations
of metals (lead, copper, zinc, chromium),
fluorides, and phosphates from aque-
ous streams can be achieved under cer-
tain pH conditions. Treatment does not
entail adding chemicals (polymers,
metal salts, polyelectrolytes) whose ac-
cumulation might inhibit reuse of the
effluent as process water. Rather, the
insoluble aluminum hydroxide result-
ing from electrocoagulation may be re-
moved by standard filtration practices.
This Summary was developed by
EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the SITE Emerging Tech-
nology Program that is fully docu-
mented in a journal article (see order-
ing information at back).
Introduction
Chemical coagulation has been used
for decades to destabilize colloidal sus-
pensions and to effect precipitation of
soluble metal species as well as* other
inorganic species from aqueous streams,
Printed on Recycled Paper
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thereby permitting their removal through
sedimentation or filtration. Alum, lime,
and/or polymers have been the chemical
coagulants used. These processes, how-
ever, tend to generate large vplumes of
sludge with a high bound-water content
that can be slow to filter and difficult to
dewater. These treatment processes also
tend to increase the total dissolved sol-
ids content of the effluent, making it un-
acceptable for reuse within industrial ap-
plications.
The ACE Technology was originally
developed in the early 1980s to break
stable aqueous suspensions of clays and
coal lines produced in the mining indus-
try. Traditionally, these effluents were
treated with conventional techniques that
made use of organic polymers and inor-
ganic salts to agglomerate and enhance
the removal of the suspended materials.
This ACE Technology was developed to
simplify effluent treatment, realize cost
savings, and facilitate recovery of fine-
grained coal.
ACE Technology is based upon colloi-
dal chemistry principles-principles using
alternating electrical power and electro-
phoretfc metal hydroxide coagulation. The
basic mechanism for the technology is
electroflocculation wherein small quanti-
ties (generally <30 mg/L) of aluminum
hydroxide species are introduced into so-
lution to facilitate flocculation. Electrofloc-
culation causes an effect similar to that
produced by the addition of chemical co-
agulants such as aluminum or ferric sul-
fate. These cationic salts destabilize col-
loidal suspensions by neutralizing nega-
tive charges associated with these par-
ticles at neutral or alkaline pHs. This
enables the particles to come together
closely enough to agglomerate under the
influence of van der Waals attractive
forces. See Figure 1 for the ACE basic
process flow.
Although the electroflocculation mecha-
nism resembles chemical coagulation in
that cationic species are responsible for
the neutralization of surface charges, the
characteristics of the electrocoagulated
floe differ dramatically from those gener-
ated by chemical coagulation. An
electrocoagulated floe tends to contain
less bound water, is more shear resis-
tant, and is more readily filterable.
Application of an AC electric field to
the electrodes induces dissolution of the
aluminum and formation of the polymeric
hydroxide species. Charge neutralization
and particle growth are initiated within
the electrocoagulation cells and continue
following discharge of the aqueous me-
dium from the apparatus. (In this way,
product separation into solids, water, and
oils may be achieved.)
The ACE Technology was tested us-
ing two designs of the ACE Separator™:
(1) a parallel electrode unit in which a
series of vertically oriented aluminum
electrodes form a series of monopolar
electrolytic cells through which the efflu-
ent passes and (2) a fluidized bed unit
with nonconductive cylinders equipped
with nonconsumable metal electrodes be-
tween which a turbulent fluidized bed of
aluminum alloy pellets is maintained. In
the fluidized bed unit, introduction of com-
pressed air into the electrocoagulation
cells assists in maintaining the turbulent
fluidized bed and in enhancing the alu-
minum dissolution efficiency by increas-
ing the anodic surface area. Typically,
the fluidized bed unit dissolves alumi-
num at least one order of magnitude
more efficiently than does the parallel
electrode unit.
Gas Outlet
Influent Liquid
Figure 1. Schematic of an ACE Separator* used in alternating current electrocoagulation.
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Electrocoagulation operating conditions
are highly dependent on the chemistry of
the aqueous medium; especially conduc-
tivity. Other influent characteristics such
as pH, particle size, and chemical con-
stituent concentrations will influence op-
erating conditions. Treatment generally
requires application of low voltage (<150
VAC) to the electrocoagulation cell elec-
trodes; current usage is typically 1 to 5
amp-min/L. The flow rate of the aqueous
medium through an electrocoagulation
cell depends on the solution chemistry
(conductivity), the nature of the entrained
suspension or emulsion, and the extent
of electrocoagulation required to achieve
the treatment objective. Retention times
as short as 5 sec are sometimes sufficient
to break a suspension. Electrocoagulation
may be accomplished in a single pass or
multiple passes (recycle mode). In the
fluidized bed unit, a mechanical scrub-
bing action is created within the
electrocoagulation cell—an action that re-
duces buildup of impermeable oxide coat-
ings on the aluminum pellets and the
inherent loss of efficiency that would re-
sult. Depending on system configuration,
maintenance of the apparatus is limited
to periodic replenishment of the alumi-
num fluidized bed material and/or elec-
trodes. For most applications, pellets for
the fluidized bed unit can be produced
from recycled aluminum scrap or bever-
age containers. Where sludge reclama-
tion is the objective, however, the use of
higher quality pellets is required to re-
duce the introduction of impurities into
the sludge.
This summary describes the research
effort associated with bench- and pilot-
scale testing of various surrogate waste-
waters for determining the optimum op-
erating conditions, treatment effective-
ness, and cost of treatment.
Experimental
EPA's SITE Program research entailed
testing the ACE Technology (in both the
parallel electrode and fluidized bed sys-
tem configurations) on various surrogate
wastes containing emulsified diesel fuel,
metals, and clays. The wastes were pre-
pared to resemble those of leaking, un-
derground storage tanks and soil wash-
ing operations. The primary testing ob-
jective of such testing was to establish
optimum operating conditions for the ACE
Separator™ to break the oil/water emul-
sion and achieve reductions in clay sus-
pended solids and soluble metal pollut-
ant loadings. Experiments were con-
ducted on'surrogate wastes prepared by
mixing 0.2 to 3.0 wt. % of the -230 mesh
(clay and silt) fraction of the EPA's Syn-
thetic Soil Matrix (SSM) with 0.5 to 1.5
wt. % Number 2 diesel fuel, with 0.05 to
0.10 wt. % of an emulsifier (Titon-100X*
or Alconox soap), and with from 10 mg/L
to 100 mg/L of one or more of the follow-
ing contaminants: copper, nickel, zinc,
orthophosphate, or fluoride. The pH of
each surrogate mixture was adjusted with
either sodium hydroxide or calcium ox-
ide to the desired value (5, 7, or 9) and
the conductivity raised to roughly 1200
microSiemens per centimeter (u.S/cm) to
1500 p,S/cm with sodium chloride to simu-
late values expected in nature.
Initially, bench-scale electroco-
agulation experiments using the parallel
electrode unit were conducted on five
aqueous-based systems that included a
metals mixture, a clay suspension, a die-
sel fuel emulsion, a soluble organic solu-
tion, and a diesel fuel/soluble organic
emulsion. The operating conditions de-
termined during the Year One SITE work
effort were used as the conditions for
these tests. Optimum treatment time was
established by examining the trend in
contaminant loadings as a function of
treatment time. To compare the results
with conventional treatment processes,
aliquots of each surrogate stock solution
were treated with alum. Sufficient alum
was added to give the aluminum equivalent
to that introduced in the electrocoagulation
experiments.
During the Year Two investigations,
operating difficulties, persistent electrode
coating and fouling, and low efficiencies
of aluminum generation prompted includ-
ing the alternative, fluidized-bed,
electrocoagulation cell design in the re-
maining bench- and pilot-scale testing
program. Three phases of laboratory ex-
periments were undertaken to evaluate
both electrocoagulation units: (1) prelimi-
nary screening experiments to demon-
strate the feasibility of reducing the con-
centration of each metal, (2) matrix ex-
periments to define the most opportune
retention time and current (or current den-
sity), and (3) optimization experiments to
define other ACE Separator™ operating
parameters to achieve the most cost-
effective removaj conditions. The pH was
adjusted to 5, 7, or 9 and the conductiv-
ity raised to approximately 1200 |j,S/cm
with sodium chloride. The conductivity of
some surrogate wastes was increased to
approximately 3000 u.S/cm and subjected
* Mention of trade names or commercial products does
not constitute endorsement or recommendation for
to electrocoagulation. Surrogate wastes
subjected to these experiments included
the five aqueous systems described
above as well as surrogate wastes con-
taining individual constituents such as
nickel, zinc, copper, fluoride, and phos-
phate.
Pilot-scale tests were performed by us-
ing both the parallel and fluidized bed
configurations of the ACE Separator™. A
12-hr experiment using the ACE Fluid-
ized Bed Separator™ was conducted on
208-L batches of surrogate waste solu-
tion containing 0.2 wt % SSM fines, 0.5
wt % diesel fuel, 0.05 wt % Alconox
surfactant, and 10 mg/L each of Cu2+,
Zn2+, PO43~, F", and Ni2*, and whose con-
ductivity and pH had been raised to 1200
|j.S/cm and 7, respectively. This surro-
gate was recycled through a 4-in.- diam-
eter, Schedule 80, PVC pipe, 24-in.-high
pilot-scale ACE Separator™, which was
equipped with two Type 316 stainless-
steel electrodes (24 in. high, 2.5 in. wide)
and whose interior was filled with 8 to
+16 mesh aluminum pellets. The unit
was powered at a constant 20 amp, and
the voltage was allowed to vary as the
electrocoagulation treatment progressed
over the 12-hr period. In this experiment,
the flow of the surrogate solution through
the ACE Separator™ was varied from 1
to 6 gpm and the quantity of compressed
air introduced into the solution feed line
ranged up to a maximum of 10 psig.
Samples of the surrogate solution were
collected at various times throughout the
experiment to document the rate of alu-
minum ion generation and the reductions
in /concentration of the metal contami-
nants, chemical oxygen demand (COD),
and TSS.
In a similar pilot-scale test using the
parallel plate unit, the surrogate waste
was composed of essentially the same
constituents as that for the fluidized bed
experiment. The notable changes were
that the conductivity of the solution was
increased to approximately 3,000 |xS/cm
and no fluoride salt was added. The other
operating parameters were based on re-
sults obtained from the bench-scale tests.
The aluminum generation and consump-
tion rates and the electrical power re-
quired to effect acceptable phase sepa-
ration as well as contaminant reductions
were monitored.
Throughout the various phases of the
experimental program, samples of the
treated effluent were collected and al-
lowed to settle for 30 min. The supernate
was removed and analyzed. The subnate,
containing the settled floe, was filtered
and the filtrate and filter cake analyzed.
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Results and Discussion
Optimum Operating Conditions
Optimum operating conditions for the
parallel electrode unit were developed
from these studies (Table 1). These con-
ditions served as the basis for the sub-
sequent pilot-scale tests.
Optimum operating conditions for par-
allel electrode unit as a result of bench-
scale tests were electrode spacing 0.5
in.; current, 4 amps; retention time, 3 to
5 m!n; frequency, 10 hertz; and fully sub-
merged.
Similarly, based on the bench-scale
testing of the fluidized bed design, the
optimum operating conditions established
were electrode spacing, 1-in.; aluminum
pellets size, 8 to +16 mesh; and, current,
20 amps.
Treatment Effectiveness
Based upon bench-scale experiments
conducted on the EPA surrogate wastes,
the following summarizes the findings:
• When compared with alum treatment,
etedrocoagulation provided approximately
83% less sludge volume arid a 76%
improvement in filtration rate
• For the fluidized bed configuration,
aluminum or stainless steel may be
used as electrode material in the
system, with comparable results
• With both increased frequency for
the AC and increased retention time,
the agglomerated particles tend to
disaggregate.
Pilot-scale tests were conducted with
the use of both the parallel and fluidized
bed configurations of the ACE
Separator™ on a 3% soil slurry contain-
ing roughly 50% clays, 1.5% diesel fuel,
and 0.1% of a strong surfactant.
Electrocoagulation reduced the TSS (222
mg/L to 4.5 mg/L) and total organic car-
bon (TOO) (130 mg/L to 6.6 mg/L). Cop-
per was reduced by 72%, cadmium by
70%, chromium by 92%, and lead by
88%. No appreciable change in total sol-
ids (TS) loadings in the supernate re-
sulted from electrocoagulation.
Table 1. OptimumOperatingCondltionsforPar-
a/fe/ Electrode Unit Based on Bench-
Scale Tests
Parameter
Value
Current
Electrode Spacing
Retention Time
Frequency
Submergence
4 Ampere
0.5 Inches
3 to 5 Minutes
10 Hertz
Fully Submerged
Particle size was enhanced by the clay
fraction as a result of electrocoagulation.
The mean particle diameters of the ACE
Separator™ treated particulates, both in
the supernate and the filtrate (188 u,m
and 230 u.m, respectively), increased by
a factor of approximately 85 and 105,
respectively, over that in the original
slurry (2.2 u.m).
Data obtained from the 12-hr, pilot-
scale, fluidized bed test showed that af-
ter 30 min of treatment, more than 90%
of the metals and phosphates were re-
moved. Aluminum generation rates were
highest when the throughput flow rate
was <4 gpm. This upper flow limit may
reflect compaction of the fluidized bed
aluminum pellets against the upper
screen of the electrocoagulation cell, thus
placing them out of the range of the
electrodes. As the emulsion is destabi-
lized, the surrogate solution most likely
becomes less resistive to ion mobility
and, thereby, improves the operational
efficiency of the ACE Separator™.
Filtration time for solids coagulated
from particulate suspensions and oily
emulsions by electrocoagulation is much
less than for solids formed by chemical
coagulant addition. Slurries tested were
treated with alum addition and with an
ACE Separator™. Electrocoagulation im-
proved the filtration rate of titanium ox-
ide by 63%. Other examples (for an oily
emulsion and for biological sludge) indi-
cate highly enhanced filtration rates for
ACE Separator™ treated waste streams
when compared with those of either un-
treated or alum-treated waste streams.
Shear strength of an electroco-
agulation floe is generally much greater
than the shear strength of an alum floe.
Both sonic treatments (used to evaluate
the structural integrity of the floe) and
actual filtration tests demonstrated higher
shear strength of the electrocoagulation
floes.
Electrocoagulation of metal and phos-
phate-bearing industrial solutions indi-
cates excellent nickel, copper, and phos-
phate concentration reductions. More
than 90% (concentration basis) of phos-
phate and copper can be removed from
such solutions at low aluminum and elec-
trical power requirements. Reduction in
the nickel concentration varies between
75% and 85% (concentration basis).
Electrocoagulation of synthetic labora-
tory solutions and industrial wastewater
also confirmed the feasibility of utilizing
electrocoagulation for phosphate removal.
Treatment of effluent from a commercial
laundry reduced the phosphate concen-
tration from 45 mg/L to 5.4 mg/L after
low-intensity electrocoagulation (0.36 kW,
0.75-min retention time). Electrocoagulation
of process water from a phosphate min-
ing operation reduced the phosphate level
by 91%, from 160 mg/L to 14 mg/L (3.3
kW, 0.17 min). Finally, treatment of di-
lute phosphoric acid solutions with a
nominal 100 mg/L total phosphate con-
centration and a conductivity of approxi-
mately 2000 u.S/cm resulted in >95% re-
ductions in soluble phosphate over a
range of acidities.
Capital and Operating Costs
As part of this research effort, pro-
jected treatment cost estimates were de-
veloped. Overall treatment operating
costs (electricity, aluminum pellets, op-
eration, and maintenance) will vary up-
wards from $0.50/1000 gal, depending
on emulsion strength, unwanted compo-
nent concentration(s) (for example, emul-
sifiers) in the effluent, and its TSS. (It
should be noted that, at the outset, we
established that operating conditions ex-
ceeding $3.00/1000 gal would not be
competitive with conventional treatment
processes, and thus, they were elimi-
nated from future testing consideration.)
Estimates are based on bench- and pi-
lot-scale testing, additional considerations
may be involved in full scale operation.
Operator supervision and maintenance
would be limited to periodic replenish-
ment of the aluminum pellets, chemical
pretreatment systems (for example, salt
addition for conductivity enhancement),
and electrode replacement. Estimated
operating costs are based on laboratory
and limited pilot-scale testing of efflu-
ents, and currently these costs exceed
those for comparable traditional chemi-
cal treatment (alum or polyelectrolytes).
The lower maintenance and operator su-
pervision required for ACE Separator™
operation and the capability to use ACE
Separator™ treated water in closed-loop,
zero-discharge applications adds to its
attractiveness. Successful commercializa-
tion of the technology requires further
research to significantly improve alumi-
num dissolution efficiency. If the ACE
Separator™ can be engineered to regu-
larly generate sufficiently high aluminum
dissolution concentrations, the technol-
ogy may be applicable to industrial efflu-
ent treatment trains, as well as for some
Superfund site remediation activities.
Incorporation of an automated process
control system based on influent conduc-
tivity and solids loading, or on the dis-
charge solution's supernate turbidity, needs
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to be engineered. An automated system
for addition of replacement aluminum pel-
lets may be justified, a'tthough the frequency
of pellet addition should be predictable
after an initial period of operation. The
capital cost for a standard ACE Separa-
tor™ with nominal throughput capacity of
50 gpm is estimated at $80,000 and for a
250-gpm unit, $300,000.
Conclusions
ACE offers a promising alternative for treat-
ing waste streams containing clays, certain
metal constituents, and other soluble pollut-
ants. As an alternative to chemical condition-
ing, ACE Technology agglomerates the par-
ticles without adding any extraneous soluble
species (i.e., SO42'); the sludge it produces
has a lower bound-water content that will filter
more rapidly and has a lower susceptibility to
filter shear of the coagulated particulates and
emulsion droplets.
As a result of the pilot-scale efforts, we
compared the effectiveness of ACE
Separator™ treatment, alum addition, and
polymer coagulation. Conclusions drawn
from this comparison for various contami-
nant parameters were:
• TSS: ACE Separator™ treatment and
the polymer treatment yielded
equivalent results for the reduction of
TSS in the treated supernates. TSS
values for alum treatment were four to
five times greater than those for ACE
Separator™ treatment or polymer
treatment.
• COD: ACE Separator™ treatment
resulted in the highest COD reductions
of the three methods. Removal
efficiency for COD was from two to
four times higher than removal
efficiency for either alum treatment or
polymer treatment.
Lead: ACE Separator™ treatment
achieved approximately 6(3% removal
of lead in the high metals runs,
whereas polymer treatment showed a
slightly higher removal (71%). Because
some difficulties were experienced with
the alum treatment, these test results
were invalidated. Further ACE
Separator™ treatments of slurries with
low concentrations of metals yielded
the highest lead removal (96%).
• Copper: Copper in the supernate
achieved dramatic removal by
electrocoagulation in both the high
(90% reduction) and low (99%
reduction) metals concentration
experiments. In the former, however,
polymer and alum addition achieved
greater removal (approximately 100%
reduction).
• Chromium: ACE Separator™ treat-ment
resulted in good removal for total
chromium (87% and 94% reductions
for the high and low concentrations).
Alum and polymer addition
accomplished similar removal.
• Cadmium: Cadmium levels in the
supernates dropped as a result of ACE
Separator™ treatment: 14% in the high
metals runs and 99% in the low metals
tests. The inconsistency between
these two sets of experiments, as well
as the high concentrations remaining
in the supernates and filtrates, raises
questions about the accuracy of
experimental results involving high
concentrations of metals. For the low
metals tests, the cadmium concentrations
in both ACE Separator™ filtrates were
much lower than were the concentrations
for either alum or polymer treatment.
The following generalization on the ef-
fectiveness of the ACE treatment can be
made:
• ACE Separator™ treatment con-
sistently reduced the TS and TSS
loadings to an equivalent degree and
to approximately one-quarter the
level achieved through alum addition;
and
• Better reductions in soluble metal
concentrations are achieved with
electrocoagulation treatment than
with alum treatment.
In summary, ACE offers a promising
technically simple method for achieving
solids-liquid separations in aqueous-
based waste streams. The majority of
the nontoxic, aluminum ionic species in-
troduced will be removed in the coagu-
lated solids phase. The ACE Technology
may be particularly suitable for zero-dis-
charge applications in which the addition
of chemicals and the buildup of residual
concentrations (dissolved solids) would
adversely affect effluent quality or inhibit
effluent reuse. Other potential applica-
tions of the ACE Separator™ include: (1)
remediation of groundwater and
leachates (metals, COD/BOD removal),
(2) enhancement of clay separation from
aqueous suspensions/emulsions result-
ing from soil-washing operations, (3)
breakage of oil/water emulsions produced
in the pumping of hydrocarbon contami-
nated groundwater, and (4) removal of
TSS from stormwater runoff. Possible in-
dustrial applications are fine-grained
product recovery (pigments, PVC) and
extraction of TSS from waste streams
that contribute to high BOD and COD
loadings.
l.S. GOVERNMENT PRINTING OFFICE: 1993 - 750-071/80064
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Naomi P. Barkleyfalso the EPA Project Officer, see below) is with the Risk
Reduction Engineering Laboratory, Cincinnati, OH 45268; Clifton Farrell is
with Electro-Pure Systems, Inc., Amherst, NY 14228; and Trade Williams, a
U.S. Environmental Protection Agency research apprentice, is with the
University of Cincinnati, Cincinnati, OH 45221.
Details of the completed SITE Emerging Technology project are given in a
journal article published in Air and Waste, Vol. 43, No. 5, p. 784-789, May
1993. The journal article entitled "Alternating Current Electrocoagulation for
Superfund Site Remediation," (Order No. PB93-20S 144; Cost: $12.50,
subject to change) is also available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
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