vailable online at www.sciencedirect.com
Separation and Puriﬁcation Technology 60 (2008) 285–291
Study on the treatment of waste metal cutting
ﬂuids using electrocoagulation
M. Kobya a,∗, C. Ciftcia, M. Bayramoglu b, M.T. Sensoya
aDepartment of Environmental Engineering, Gebze Institute of Technology, 41400 Gebze, Turkey
bDepartment of Chemical Engineering, Gebze Institute of Technology, 41400 Gebze, Turkey
Received 8 May 2007; received in revised form 29 August 2007; accepted 4 September 2007
Electrocoagulation (EC) technique is applied for the treatment of waste metal cutting ﬂuids (WMCFs) characterized by high COD and TOC
concentration, discharged from metal manufacturing facilities including automotive engine, transmission, and stamping plants. The effects of initial
pH, current density and operating time on the performance of EC are investigated by using sacriﬁcial Al and Fe electrodes. Upon treatment by EC,
the COD of WMCF is reduced by 93% and the TOC is reduced by 78% for Al electrode at pH 5.0, current density of 60 A/m2and operating time
of 25 min. For Fe electrode, the reduction in COD is 92% and reduction in TOC is 82% at pH 7.0, current density of 60 A/m2and operating time
of 25 min. Under optimal operating conditions, the operating costs are calculated as 0.497$/m3(0.023 $/kg removed COD or 0.144 $/kg removed
TOC) for Fe electrode, and 0.768 $/m3(0.036 $/kg removed COD or 0.228 $/kg removed TOC) for Al electrode. Fe electrode is found to be more
efﬁcient than Al electrode in terms of parameters such as COD and TOC removal efﬁciencies and operating costs.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Waste metal cutting ﬂuids; Electrocoagulation; Aluminum electrode; Iron electrode; Operating cost
Metal cutting ﬂuids (MCFs) are widely used in metal indus-
tries, such as rolling mills, forge and metal workshops, because
these ﬂuids provide the combined cooling and lubrication
required by different metal working operations. MCF is also
used to carry away chips and ﬁnes produced in machining and
cutting operations. A MCF concentrate usually contains a min-
eral oil, a surfactant mixture, in some cases water and various
additives, which are included to meet the speciﬁcations of com-
mercial concentrates such as resistance to bacterial growth and
low corrosion capacity . Water content varies depending on
the source, but oils in this group generally contain between 3
and 60% water. A wide variety of oil contaminated waters and
spent emulsions (“white waters”) contains up to 15% mineral
oils or combinations of mineral, vegetable or animal oils [2,3].
The main problem with MCFs is that they become contam-
inated with use, losing their properties and effectiveness, and
have to be replaced by new ones, thus yielding waste metal cut-
ting ﬂuids (WMCFs), which are generally high in COD, TOC
∗Corresponding author. Tel.: +90 262 605 3214; fax: +90 262 605 3101.
E-mail address: email@example.com (M. Kobya).
and turbidity. The amount of WMCFs generated from metal
working operations increases every year, constituting a seri-
ous danger to the environment due to their high surface-active
and organic pollutant loads [4,5]. These ﬂuids require treatment
prior to disposal to meet local sewer discharge standards, which
are subject to local and state laws. Various treatment methods
are used to treat WMCFs, as microﬁltration and ultraﬁltration
[6–8], adsorption , chemical coagulation  and biological
(aerobic and anaerobic process) [11–14].
Recently, there has been considerable interest in identifying
new technologies that are capable of meeting more stringent
treatment standards. For this purpose, electrocoagulation has
a more prominent role in the treatment of WMCFs because
it provides some signiﬁcant advantages such as quite compact
and easy operation and automation, no chemical additives, a
shorter retention time, high sedimentation velocities, more eas-
ily dewatered and reduced amount of sludge due to the lower
water content. It also can prevent the production of unwanted
2. EC process description
EC is an electrolysis process with reactive (or soluble) anode
(iron or aluminum electrode). The action of the electrical current
1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
286 M. Kobya et al. / Separation and Puriﬁcation Technology 60 (2008) 285–291
between the two electrodes allows the formation of metal ions
(Al3+ or Fe3+) by anode oxidation.
The main reactions at the Al and Fe electrodes are:
anode reactions : Al(s) →Al(aq)3++3e−(1)
cathode reactions : 3H2O+3e−→(3/2)H2↑+3OH−
Al3+ and OH−ions generated by electrode reactions (1) and
(2) react to form various monomeric species such as Al(OH)2+,
Al(OH)2+, Al(OH)24+, and Al(OH)4−polymeric species such as
13O4(OH)24 7+ and
Al13(OH)34 5+, which transform ﬁnally into Al(OH)3according
to complex precipitation kinetics. Several interaction mecha-
nisms are possible between organic molecules and hydrolysis
products, and the rates of these depend on the pH of the medium
and types of ions present. Two major interaction mechanisms
have been considered in recent years: precipitation (pH 4.0–5.0
for monomeric Al species and pH 5.0–6.0 for polymeric Al
species) and adsorption, each one being proposed for a separate
pH range. Flocculation in the low pH range is explained as pre-
cipitation, while that in the higher pH range (>6.5) is explained
as adsorption. At high pH values above 9.0, Al(OH)4−is also
present in the system. Freshly formed amorphous Al(OH)3
“sweep ﬂocs” have large surface areas, which is beneﬁcial for
a rapid adsorption of soluble organic compounds and trapping
of colloidal particles. These ﬂocs polymerize as Aln(OH)3nand
they are removed easily from the aqueous medium by sedimen-
tation and by H2ﬂotation .
The following main reactions occur in the EC when Fe elec-
trode is used at different pH values:
anode reactions : Fe(s) →Fe(aq)2++2e−(3)
cathode reactions : 3H2O+3e−→(3/2)H2↑+3OH−
Similarly, ferric ions generated by electrochemical oxi-
dation of iron electrode may form monomeric species
(Fe(OH)2+, Fe(OH)2+, Fe(OH)63+ , Fe(H2O)5OH2+,
Fe(H2O)4OH2+, Fe(OH)3, and Fe(OH)4−), and polymeric
2(H2O)6OH42+), depending on
the pH of the aqueous medium in the EC process. The com-
plexes (i.e. hydrolysis products) have a pronounced tendency
to polymerize at pH 3.5–7.0. Under very acidic conditions
(pH < 2.0), Fe(OH)63+ remains in solution, but as the pH or
the coagulant concentration rises, hydrolysis occurs to form
Fe(OH)3(s). When Fe electrodes is used the amount and the
variety of hydrolysis products formed by anodic dissolution
depends signiﬁcantly on electrolysis time. The resulting metal
hydroxide polymers have amorphous structures with very large
surface and positive charges. They are hydrophobic, causing
them to sorb onto the organic anionic particle surfaces and
become insoluble. Iron has a strong tendency to form insoluble
complexes with a number of ligands, especially with polar
molecules and with oxygen-containing functional groups such
as the hydroxyl or carboxyl groups. These provide a local
negative charge, which reacts with the iron cations. Charge
neutralization leads to colloid destabilization with the conse-
quent precipitation of the iron cations and organic anions. This
induces sweep ﬂoc coagulation, the adsorption and bridging
enmeshment of both particulate organic and inorganic solids
to form large, amorphous ﬂocs. Dissolved organic compounds
are removed primarily by adsorption onto by hydroxide surface
EC treatment has been practiced for most of the 20th cen-
tury with sporadic popularity. During 20 years, the EC process
was slowed down by the high-cost of investment and the com-
petition with other chemical treatment processes. In recent
years, however, it started to regain importance with progress
of the electrochemical processes and the increase in environ-
mental restrictions on efﬂuent wastewater. This new rise of EC
is also partly due to the relative reduction in the exploitation
and capital costs. EC has the potential to be the distinct eco-
nomical and environmental choice for treatment of wastewater
and other related water management issues. The EC pro-
cess has been applied to treat textile wastewaters [20–23],
restaurant wastewater , semiconductor wastewater , dis-
tillery alcohol distillery wastewater , tannery wastewater
, dairy wastewater , poultry slaughterhouse wastewater
[29,30], olive mill wastewater [31–33], potato chips manufac-
turing wastewater , pasta and cookie processing wastewater
, electroplating wastewater , soluble oil wastes of high
COD , chemical ﬁber plant wastewater , food process
wastewater  and chemical mechanical polishing wastewater
A few literature studies have been available only for the treat-
ment of synthetic MCFs by EC [41–43]. In the present study,
the removal efﬁciencies and operating costs of COD and TOC
for the treatment of WMCF by EC process are studied with var-
ious operating conditions such as pH (3.0–8.0), current density
(20–100 A/m2) and operating time (5–30 min). The results are
evaluated for the treatment of the sewerage system discharge
standards in Turkey.
3.1. The waste metal cutting ﬂuid
WMCF is obtained from a metal heavy manufacturing
company producing of automotive engine, transmission, and
stamping plants in Gebze, Turkey. Two cubic metres of WMCF
per month is produced approximately. The waste sample taken
for the study is stored in the refrigerator. The characteristics of
WMCF used in the study are given in Table 1.
3.2. Apparatus and instruments
The apparatus for EC consist of an electrolytic cell shown
in Fig. 1, which is a 1 l Plexiglas reactor with a dimension of
120 mm ×110 mm ×110 mm. It is equipped with thermostad
M. Kobya et al. / Separation and Puriﬁcation Technology 60 (2008) 285–291 287
Characteristics of WMCFs
Conductivity (mS/cm) 1.645
Turbidity (NTU) 15,350
COD (mg/l) 17,312
TOC (mg/l) 3155
TSS (mg/l) 110
Fig. 1. Schematic diagram of experimental setup: (1) electrochemical reactor,
(2) power supply, (3) thermostad, (4) magnetic stirrer, (5) sacriﬁcial electrodes
and (6) magnetic stirrer bar.
for the temperature control. Aluminum and cast iron plates
(45 mm ×53 mm ×3 mm) are chosen for electrodes as the
anode/cathode and the electrodes are situated 1 cm apart from
each other. Four electrodes are adopted in the electrolytic cell for
all experimental run. The electrodes are dipped in the electro-
chemical reactor to a depth of 80 mm, yielding a total effective
electrode surface area of 143 cm2. The electrodes are connected
to a digital dc power supply (Agilent 6675A model; 120 V, 18A)
and operated at galvanostatic mode.
A constant temperature of 20 ◦C and a magnetic stirring at
250 rpm is maintained for all experimental runs. The pH of
wastewater was adjusted to the required value with 0.1 M NaOH
and 0.1 M H2SO4(Merck). In each run, 800 cm3of WMCFs
is placed into the reactor. The current density is adjusted to a
desired value and the EC process is started. At the end of the
EC, the solution is ﬁltered and is analyzed. At the end of the run,
the electrodes are washed thoroughly with water to remove any
solid residues on the surfaces, dried and re-weighted.
The COD concentration is measured spectrophotometrically
(Perkin-Elmer Lambda 35 UV–vis spectrophotometer, USA),
and the analysis is conducted by the procedures described in
Standard Methods . The TOC levels are determined through
combustion of the samples at 680 ◦C using a non-dispersive IR
source (Tekmar Dohrmann Apollo 9000). The accuracy of both
determinations is estimated at 3%. The turbidity (NTU) of sam-
ples is analyzed using a Hanna LP-2000 turbidimeter. The pH is
measured using AZ 8601 model pH meter, and the conductivity
is determined with Lutron CD-4303 model conductivity meter.
4. Result and discussion
4.1. Effect of initial pH on EC of the WMCFs
The pH is an important operating factor that can inﬂuence
the EC process. In order to study its effect on the treatment, the
initial pH of the wastewater is adjusted to between 3.0 and 8.0.
Removal efﬁciencies of COD and TOC as a function of initial
pH are presented in Fig. 2(a) and (b) for Al and Fe electrodes,
respectively, with constant operating time of 25 min and current
density of 60 A/m2.
When the initial pH is adjusted to 4.0–6.0 for Al electrodes,
high removal efﬁciency of COD (93%) is achieved. The removal
efﬁciency of TOC with the initial pH, on the other hand, is not
affected substantially and only a slight change is observed (1%).
The decrease in removal efﬁciency of COD at a pH lower than 4.0
and higher than 6.0 is observed and attributed to an amphoteric
Fig. 2. Effect of initial pH on EC of the WMCFs.
288 M. Kobya et al. / Separation and Puriﬁcation Technology 60 (2008) 285–291
Fig. 3. Effect of initial pH on operating cost.
character of Al(OH)3which lead to soluble Al3+ cations, when
the initial pH is low and to monomeric anions Al(OH)4−, when
the initial pH is high [18,35,39,40,45]. This soluble species are
useless for water treatment.
The removal efﬁciencies of COD and TOC for Fe electrodes
are observed to increase with the initial pH 3.0–7.0 and a slight
decrease at pH higher than 7.0 (Fig. 2(a) and (b)). When pH
of the wastewater is acidic, the oxidation of ferrous iron (Fe2+)
to ferric iron (Fe3+) diminishes [19,42], and therefore removals
of COD and TOC decrease, since hydroxylated iron colloidal
polymers and an insoluble precipitate of hydrated ferric oxide
are formed and the removal efﬁciency is increased. This result
is quite meaningful in the application of EC for the treatment of
WMCF because it can be directly treated by EC without further
pH adjustment of wastewater. Thus, all subsequent EC experi-
ments for treatment of WMCF using Al and Fe electrodes are
conducted at the pH 5.0 and 7.0, respectively.
Every compound that contributes to COD in wastewater is
different. The COD increases when compounds react with Fe2+
or Al3+ to form soluble products and remain in solution. This
can be concluded for the pH increment since COD removal
efﬁciency depends on the ﬁnal pH. It can be expected that com-
pounds that react with both Fe2+ and Fe3+ or Al3+ to form
insoluble compounds will be completely removed.
Furthermore, the ﬁnal pH of wastewater changes during the
EC process as observed by other investigators [15–24] when the
initial pH is in the range 3.0–8.0 because of increase to hydro-
gen evolution at cathodes. The ﬁnal pH of the treated wastewater
efﬂuent was seen to increase with an increase in the initial pH and
the difference between the ﬁnal and initial pHs varies between
0.5 and 1.5 . As an example, when the initial pH is 5.0, the
ﬁnal pH reached to 6.5 allowing the efﬂuent to be directly dis-
charged into natural aquatic streams without any pH adjustment.
The electrode material has an effect on the ﬁnal pH. Final pH is
obtained between 4.7 and 8.7 for Al electrode and 6.4 and 9.5 for
Fe electrode, respectively. Various processes occurring during
EC tend to buffer the liquid medium . The pH is necessary
to adjust before being discharged into natural aquatic streams.
One of the most important parameters that affect the applica-
tion of any method of wastewater treatment greatly is the cost.
The operating cost as $/m3(or $/kg COD or $/kg TOC removed)
of wastewater treated includes material, mainly electrodes elec-
trical energy and used chemicals costs [20,22].
operating cost =aCenergy +bCelectrode +cCchemicals (6)
where Cenergy is the consumption kWh energy per m3or kg
COD or kg TOC, Celectrode the consumption kg electrode per
m3or kg COD or kg TOC and Cchemicals is the consumption kg
chemicals per m3or kg COD or kg TOC of wastewater treated.
Unit prices, a, b and c given for the Turkish Market, April 2007,
are as follows: (a) electrical energy prices 0.070 US $/kWh, (b)
electrode material price 1.8 US $/kg for aluminum and 0.30 US
$/kg for iron, respectively and (c) NaOH and H2SO4prices 0.50
and 0.20 $/kg, respectively.
The effect of the initial pH on the operating cost for Al and
Fe electrodes calculated on the basis of $/m3and $/kg COD
removed is demonstrated in Fig. 3(a) and (b). The operating
cost increases removed steadily with increasing pH 3.0–8.0 for
Al electrode as 0.393–1.543 $/m3and 0.020–0.112 $/kg COD
removed. In the case of Fe electrode, the operating cost as $/m3
shows the same trend with Al electrode in the same pH range
but almost a slight change is observed in the same pH range as
$/kg COD removed for Fe electrode (Fig. 3(b)). The operating
costs at the optimum pH (pH 5.0 for Al electrode and pH 7.0 for
Fe electrode) are calculated as 0.768 $/m3and 0.036 $/removed
kg COD for Al electrode and 0.497 $/m3and 0.023 $/removed
kg COD for Fe electrode, respectively.
4.2. Effect of current density on EC of the WMCF
It is well-known that current not only determines the coagu-
lant dosage rate but also the bubble production rate, size and the
ﬂocs growth, which can inﬂuence the treatment efﬁciency and
operating cost of the EC. The effect of current density is investi-
gated with the following experimental conditions; the operating
time of 25 min, the initial pH 5.0 for Al electrode and pH 7.0 for
Fe electrode. Fig. 4(a) and (b) shows the removal efﬁciencies of
COD and TOC as a function of current density. The removal efﬁ-
ciency of COD at current density 20–60 A/m2increase from 79
to 93% for Al electrode and from 65 to 92% for Fe electrode, and
the TOC removal efﬁciency for Al and Fe electrodes are from 77
to 78% and from 78 to 82%, respectively. Afterwards, it remains
nearly constant for removal efﬁciencies of COD and TOC for
both electrodes at current density 60–100 A/m2. According to
M. Kobya et al. / Separation and Puriﬁcation Technology 60 (2008) 285–291 289
Fig. 4. Effect of current density on EC of the WMCFs.
Fig. 5. Effect of current density on operating cost.
Faraday’s law, since the current density increases, the efﬁciency
of ion production on the anode and cathode increases. Therefore,
there is an increase in ﬂocs production in the solution and hence
an improvement in the efﬁciency of COD and TOC removals.
It is also advisable to limit the current density in order to avoid
excessive oxygen evolution as well as to eliminate other adverse
effects, like heat generation. Therefore, 60 A/m2is chosen as
current density in our experiments.
Fig. 5(a) and (b) shows the operating costs as function of
the current density for both electrodes. The operating cost of
0.604–0.768 $/m3and 0.024–0.036 $/kg COD removed is not
changed for Al electrode when the current density is in the range
20–60 A/m2. After that, there is a sharp increase with the cost
of 0.768–1.35 $/m3and 0.036–0.078 $/kg COD removed at cur-
rent density 60–100 A/m2. On the other hand, the operating cost
increases rapidly with increasing current density between 20
and 100 A/m2(0.092–0.946 $/m3and 0.008–0.069 $/kg COD
removed) for Fe electrode.
4.3. Effect of operating time on EC of the WMCF
Fig. 6(a) and (b) shows removal efﬁciencies of COD and
TOC for both electrodes at different operating times (0–30 min),
60 A/m2and the optimum pH values. The removal efﬁciencies of
COD for both electrodes increase with increasing the operating
time which reaches about 93% in 25 min and then it almost
remains constant (Fig. 6(a)). For both electrodes, the removal
efﬁciencies of TOC (77%) is the same at 15 min and beyond this
Fig. 6. Effect of operating time on EC of the WMCFs.
290 M. Kobya et al. / Separation and Puriﬁcation Technology 60 (2008) 285–291
WMCFs by EC at the optimum operating conditions
Parameter EC process
Electrode materials Fe electrode Al electrode
Initial pH 7.0 5.0
Current density (A/m2)6060
Operating time (min) 25 25
COD removal efﬁciency (%) 92 93
TOC removal efﬁciency (%) 82 80
Turbidity removal efﬁciency (%) 99.9 99.8
Efﬂuent COD concentration (mg/l) 1385 1212
Efﬂuent TOC concentration (mg/l) 568 631
Efﬂuent turbidity (NTU) 15.4 30.7
$/kg COD 0.023 0.036
$/kg TOC 0.144 0.228
point, the removal efﬁciency of Fe electrode (80%) is slightly
more efﬁcient than Al electrode (79%) shown in Fig. 6(b). The
optimum operating time for this study is chosen as 25 min since
the highest removal efﬁencies of COD and TOC are observed at
The operating cost as a function of the operating time from
5 to 30 min for Al and Fe electrodes are 0.025–0.90 $/m3
and 0.023–0.05 $/kg COD removed for Al electrode and
0.01–0.79 $/m3and 0.019–0.042 $/kg COD removed, respec-
The effectiveness of Fe and Al electrodes at the optimum
operating conditions are shown in Table 2. It is clear that efﬁ-
ciency of the turbidity removals were observed over >99% for
both electrode materials.
The COD concentration is the most important parameter
which is required for the sewage discharge standards. In spite
of the fact that the water quality of efﬂuent wastewater is con-
siderably improved by EC, COD concentration usually fall for
short of the sewage discharge standard system. In other words,
the COD concentration of wastewater efﬂuent is reduced from
the initial 17,312 to 1385 mg/l for the case of Fe electrode while
the ﬁnal COD concentration is lowered to 1212 mg/l for the
case of Al electrode after EC process. It yields a reduction of
92–93% of the initial pollution, a signiﬁcant result taking into
account the inﬂuent value of the 17,312 mg COD/l. The value
of COD in WMCF after EC process is found to be nearly above
1200 mg/l which exceeds the limit set by sewage discharge stan-
dard (800 mg/l). Such a high COD value from WMCF indicates
that WMCF still contains a signiﬁcant amount of organic mat-
ter. Therefore, the treated WMCF by EC are needed to treat
further using second treatment process. In conclusion, consider-
ing the overall results from both electrodes, the current density
of 60 A/m2and the operating time of 25 min appear to yield
the optimum results in terms of removal efﬁciency of COD
and TOC. At the optimum operating conditions, the operat-
ing costs of the WMCF is calculated 0.497 $/m3(or 0.023 $/kg
removed COD or 0.144 $/kg removed TOC) for Fe electrode
and 0.768 $/m3(or 0.036 $/kg removed COD or 0.228 $/kg
removed TOC) for Al electrode, respectively. Fe electrode is
more efﬁcient than the Al electrode in terms of the operating cost.
Certainly both efﬂuent COD concentrations are not up to the
sewage discharge standard and further treatment of wastewater
efﬂuent is necessary.
This research work is a part of TUBITAK project. The authors
thank TUBITAK for their ﬁnancial support of this work under
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