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Electrocoagulation for the treatment of textile industry effluent - A review

Authors:
Review
Electrocoagulation for the treatment of textile industry efuent eA
review
V. Khandegar, Anil K. Saroha
*
Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India
article info
Article history:
Received 26 October 2012
Received in revised form
1 April 2013
Accepted 24 June 2013
Available online
Keywords:
Electrocoagulation
Textile efuent
Operating parameter
Dye removal
Efciency
abstract
Various techniques such as physical, chemical, biological, advanced oxidation and electrochemical are
used for the treatment of industrial efuent. The commonly used conventional biological treatment
processes are time consuming, need large operational area and are not effective for efuent containing
toxic elements. Advanced oxidation techniques result in high treatment cost and are generally used to
obtain high purity grade water. The chemical coagulation technique is slow and generates large amount
of sludge. Electrocoagulation has recently attracted attention as a potential technique for treating in-
dustrial efuent due to its versatility and environmental compatibility. This technique uses direct current
source between metal electrodes immersed in the efuent, which causes the dissolution of electrode
plates into the efuent. The metal ions, at an appropriate pH, can form wide range of coagulated species
and metal hydroxides that destabilize and aggregate particles or precipitate and adsorb the dissolved
contaminants. Therefore, the objective of the present manuscript is to review the potential of electro-
coagulation for the treatment of industrial efuents, mainly removal of dyes from textile efuent.
Ó2013 Elsevier Ltd. All rights reserved.
1. Introduction
The textile industry wastewater varies extensively in terms of
composition due to the regular impurity in bers and the chemicals
used in different processes. Various types of dyes are produced
worldwide and are used in various industries, such as textile,
cosmetic, paper, leather, pharmaceutical and food industry. The
dyes can be classied as acid, basic, direct, azoic colors, vat, sulphur,
reactive and metal complex dyes. There are more than 100,000
commercially available dyes with an estimated annual production
of over 7 10
5
tons (Robinson et al., 2001). 10e50% of these dyes
are lost in the efuent. In India, there are about 950e1000 number
of textile units out of which 50 are in the organized sector while
rest are the small scale units. The major pollutants in textile
wastewater are especially the organic and inorganic chemicals
such as nishing agents, surfactants, inhibitor compounds, active
substances, chlorine compounds, salts, dyeing substances, total
phosphate, dissolved solids, suspended solids and total solids.
Coloring matter is the major contaminant in the textile efuent and
has to be removed before discharging the efuent into the aqueous
ecosystem. Without proper treatment, the colored efuent creates
an aesthetic problem and its color discourages the downstream use
of wastewater. Aesthetic merit, gas solubility and water trans-
parency are affected by the presence of dyes even in small amount
or concentrations. The removal of colored material from waste-
water has been rated to be relatively more important than the
removal of soluble colorless organic substances, which usually
contribute the major fraction of biochemical oxygen demand.
Textile industry involves wide range of machineries and pro-
cesses to produce the required shape and properties of the product.
Huge amount of efuent is generated in the various processes such
as sizing, scouring, bleaching, mercerizing, dyeing, printing and
nishing. This efuent contains mainly byproducts, residual dye,
salts, acid/alkali, auxiliary chemicals and cleaning solvents. Salts
such as sodium chloride and sodium sulphate are used to assist in
exhaustion of anionic dyes and nd their way into the efuent as
byproducts of neutralization or other reactions in textile wet pro-
cessing. Typically, for a textile unit processing 400,000 pounds per
week of cotton, more than 50,000 pounds of salts are released. The
usual salt concentration in efuent is 2000e3000 ppm
(Koltuniewicz and Drioli, 2008). The salts in the efuent can lead to
the soil infertility and aquatic life damage. The characteristics of the
Abbreviation: AC, alternating current (amp); BOD, biochemical oxygen demand
(mg/L); COD, chemical oxygen demand (mg/L); DC, direct current (amp); F, Fara-
days constant (96,500 C/mol); ppm, parts per million; SS, suspended solids (mg/L);
TDS, total dissolved solids (mg/L); TFS, total xed solids (mg/L); TOC, total organic
carbon (mg/L); TSS, total suspended solids (mg/L); TVS, total volatile solids (mg/L).
*Corresponding author. Tel.: þ91 11 26591032; fax: þ91 11 26581120.
E-mail address: aksaroha@chemical.iitd.ac.in (A.K. Saroha).
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Management
journal homepage: www.elsevier.com/locate/jenvman
0301-4797/$ esee front matter Ó2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jenvman.2013.06.043
Journal of Environmental Management 128 (2013) 949e963
efuent generated from different types of the textile industry along
with the efuent discharge standards prescribed by the statutory
authorities are tabulated in supplemental Table 1. It can be noticed
that the textile industry efuent is highly polluted.
A large quantity of the dye is lost during dyeing and nishing
processes and is released into wastewater. Accordingly, dye efuent
may contain chemicals that are toxic, carcinogenic, mutagenic, or
teratogenic to various sh species. Reactive dyes, which have good
water solubility and are easily hydrolysed into insoluble forms, are
extensively used in dyeing processes, and about 20e40% of these
dyes remain in the efuent. During dyeing, dyes are not completely
exhausted from dye liquor. The effectiveness of different decolor-
ation techniques for removal of various dyes depends on chromo-
phore as well as auxochromes of the dye. The unsaturated part of
the molecule is called the chromophore, which ultimately is
responsible for the color. In textile dyes, the chromophore usually
consists of aromatic rings (anthraquinone and triphenylmethane)
or azo groups (azo benzene). Subsequent groups are known as
auxochromes, which shift the wavelengths of the light absorbed
into the visible region. Typical auxochromes such as CO, OH and
NH
2
tend to absorb light in the blue and violet region and so reect
light in the yellow, orange or red region. The dye is, therefore,
colored yellow, orange or red depending on which auxochromes
are present. Both anthraquinione and triphenylmethane absorb in
the yellow/red region and so appear blue. The color seen is not of
absorbed light but of reected light.
Removal of dyes from wastewater is a major environmental
concern as it can lead to severe contamination of surface and
ground waters in the vicinity of dyeing industries. Various studies
have been reported in the literature (Can et al., 2003, 2006) on color
removal techniques which can be classied into physical or phys-
icochemical, chemical, biological and electrochemical. The mech-
anisms of color removal are physical dye separation, breakdown of
the dyes and de-colorization by adsorption/biodegradation. The
physical or physicochemical techniques include coagulation/oc-
culation, adsorption, macrosorb and membrane separation. In
coagulation, the electrostatic attraction between oppositely
charged soluble dye and polymer molecule coagulates the efuent.
The coagulant dose depends on the type of efuent, concentration
of dye and other processing aids used. However, coagulation results
in generation of large amounts of sludge and total dissolved solids
in the efuent are further increased. Coagulation is effective for
sulphur and dispersive dyes. Acid, direct, vat and reactive dyes
coagulate but do not settle while the cationic dyes do not coagulate.
Adsorption is an effective method for lowering the concentration of
dissolved organics in the efuent. But the adsorbent regeneration is
expensive and involves the loss of adsorbent. In membrane ltra-
tion, the appropriate membrane is capable of removing all types of
dyes. The space requirements are less and there is no generation of
sludge. There is a reduction in the fresh water usage as the water
can be completely recycled and reused. But the high cost of
membranes and equipment, the lowered productivity with time
due to fouling of the membrane and the disposal of concentrates
are the limitations. Many dyes are effectively decolorized using
chemical oxidizing agents and seem to hold potential for future use
in the textile industry. But it leads to the generation of absorbable
organohalides which are toxic in nature.
The biological treatments include anaerobic process, oxidation
ponding, trickling lters, activated sludge process, etc. Microbial
biomass is also commonly used for the treatment of industrial ef-
uents. The microorganisms such as algae, fungi, bacteria, and
yeasts are capable to degrade certain type of dyes. However, their
application is limited as biological treatment requires a large land
area, has sensitivity toward toxicity of certain chemicals and
treatment time is very high. Further, some dyes are generally toxic
and are not easily biodegraded by biological process (Hao et al.,
2000).
Advanced oxidation processes (AOPs) include wet air oxida-
tion, catalytic wet air oxidation and treatment with oxidizing
agents, such as hydrogen peroxide, ozone, UV light or their
combinations. Wet air oxidation (WAO) or thermal liquid phase
oxidation is useful for treatment of efuents containing high
Nomenclature
acost of electricity/kWh
bcost of electrode/kg electrode
BP-S bipolar electrodes in serial connections
ccost of chemical/kg of chemical
CHC chemical consumption (kg of chemical/m
3
of efuent)
CWAO catalytic wet air oxidation
EC electrocoagulation
ELC electrode consumption (kg of electrode/m
3
of efuent)
ENC energy consumption (kWh/m
3
of efuent)
Iapplied current (A)
Mrelative molar mass of the electrode (g/mol)
MP-P monopolar electrodes in parallel connections
MP-S monopolar electrodes in serial connections
MS mild steel
nnumber of electrons in oxidation/reduction reaction
SS stainless steel
St steel
telectrolysis time (h)
Uapplied voltage (V)
Vvolume of treated efuent (m
3
)
WAO wet air oxidation
Table 1
Characteristics of wastewater from textile chemical processing.
Characteristics Scouring Bleaching Mercerising Dyeing Composite Discharge limit into inland water
(Bureau of Indian Standards)
pH 10e12 8.5e11 8e10 9e11 8e10 5.6e9.0
TDS (mg/L) 12,000e30,000 2500e11,000 2000e2600 1500e4000 5000e10,000 2100
TSS (mg/L) 1000e2000 200e400 100e400 50e350 100e700 100
BOD (mg/L) 2500e3500 100e500 50e120 100e400 50e550 30
COD (mg/L) 10,000e20,000 1200e1600 250e400 400e1400 250e8000 250
Chlorides (mg/L) ee350e700 e100e500 1000
Sulphates (mg/L) ee100e350 e50e300 1000
Color eeHighly colored Strongly colored Strongly colored Colorless
V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963950
content of organic matter or toxic contaminants. WAO is carried
out at elevated temperature and pressure in which there is
generation of active oxygen species such as hydroxyl radicals.
However, the severe operating conditions result in high instal-
lation cost (material of construction of equipment to withstand
high temperature and pressure) and operating cost. The intro-
duction of catalyst in WAO, called catalytic wet air oxidation
(CWAO) facilitates the oxidation process at considerably low
temperature and pressure. CWAO is useful for treatment of ef-
uents where the concentration of organic contaminants is too
low for the incineration process to be economical. Further, the
CWAO leads to formation of carbon dioxide & water and there is
no formation of by-products. Ozone is a more powerful oxidant
than chlorine and other compounds (hydrogen peroxide, potas-
sium permanganate, chlorine dioxide and bromine). The oxida-
tion potential of ozone is 2.07 as compared to 1.36 for chlorine
and 1.78 for hydrogen peroxide. But ozone is hazardous and it
requires an ozone destruction unit to prevent ozone from
escaping from the process. Hydrogen peroxide removes the dyes
by oxidation resulting in aromatic cleavage of dye molecules. It is
an environmental friendly technique but is not effective on all
dyes as its oxidation potential is not very high. Further, the
process needs to be activated by some other means like UV light,
inorganic salts, ozone or ultrasound/sunlight. The advantages
and limitations of conventional methods have been summarized
by Hao et al. (2000).
2. Electrocoagulation
Electrocoagulation (EC) is a simple and efcient method and
has been used for the treatment of many types of wastewaters
such as electroplating wastewater (Verma et al., 2013; Adhoum
et al., 2004), laundry wastewater (Janpoor et al., 2011), restau-
rant wastewater (Chen et al., 2000) and poultry slaughterhouse
wastewater (Kobya et al., 2006c). Electrocoagulation has been
successfully used for the removal of pollutants from different
industrial wastewaters and is summarized in supplemental
Table 2. Many studies have been reported in the literature
(Yuksel et al., 2013; Khandegar and Saroha 2013a, 2013b;
Ogutveren et al., 1992) using electrocoagulation for the treat-
ment of dye and textile industry efuent and are shown in
supplemental Table 3.
Heavy metals are generally used in tanning, electroplating,
leather, textile, metal nishing, magnetic tapes, pigment products,
wood preservation and chrome plating industries. Therefore heavy
metal in wastewater has become one of the severe environmental
concerns. Majority of industrial efuents contain toxic metals such
as Cd, Cr, Cu, Ni, Zn, Pb and other compounds which are toxic to
living organisms and harmful to the environment when they are
directly discharged into the water bodies. They can be easily
absorbed by shes and vegetables due to their high solubility in the
aquatic environments and may accumulate in the human body by
means of the food chain. It is necessary to treat wastewater con-
taining toxic metals to remove their adverse effects on human and
ecology. In recent years, a variety of techniques are used for heavy
metal removal from wastewater which include ion-exchange,
adsorption, chemical precipitation, membrane ltration, occula-
tion, coagulation, otation and electrochemical methods (Fu and
Wang, 2011).
Electrocoagulation has been successfully applied to remove
soluble ionic species from solutions and heavy metals by various
investigators (Verma et al., 2013; Aji et al., 2012; Akbal and Camci,
2011a). Electrocoagulation has also been used for the removal of
arsenic (Kumar et al., 2004; Balasubramanian and Madhavan,
2001), phosphate (Bektas et al., 2004a), sulde, sulfate and sulte
(Murugananthan et al., 2004), boron (Yilmaz et al., 2005; Bektas
et al., 2004b), uoride (Mameri et al., 1998), nitrate (Kumar and
Goel, 2010) and chromate (Gao et al., 2004). Various studies re-
ported in the literature for removal of heavy metals and toxic
compounds using electrocoagulation process are summarized in
supplemental Table 4.
Combination of various techniques for the treatment of efuent
leads to a higher removal efciency as compared to the use of single
treatment method alone. Electrocoagulation in combination with
other treatment methods is a safe and effective way for the removal
of pollutants. Some studies on the combination of electro-
coagulation with other treatment techniques have been reported in
the literature and are summarized in supplemental Table 5. Studies
have been reported in the literature comparing electrocoagulation
with other conventional methods and have been summarized
in supplemental Table 6. It can be noticed from supplemental
Table 6 that electrocoagulation results in higher removal efciency
compared with other methods for the same concentration of
pollutants.
Electrocoagulation is an efcient technique because adsorp-
tion of hydroxide on mineral surfaces are 100 times greater on
in siturather than on pre-precipitated hydroxides when metal
hydroxides are used as coagulant (Mollah et al., 2004a). Since
the ocs formed by EC are relatively large which contain less
bound water and are more stable, therefore, they can be easily
removed by ltration. It is cost effective, and easily operatable. EC
needs simple equipments and can be designed for any capacity of
efuent treatment plant. Since no chemical addition is required
in this process, it reduces the possibility of generation of sec-
ondary pollutants. It needs low current, and therefore, can be
operated by green processes, such as, solar, windmills and
fuel cells (Zaroual et al., 20 06). It is an environment-friendly
technique since the electronis the main reagent and does
not require addition of the reagents/chemicals. This will mini-
mize the sludge generation to a great extent and eventually
eliminate some of the harmful chemicals used as coagulants
in the conventional efuent treatment methods. Electro-
coagulation process can effectively destabilize small colloidal
particles and generates lower quantity of sludge compared to
other processes.
The advantages of electrocoagulation as compared to chemical
coagulation are as follows:
EC requires no addition of chemicals and provides better
removal capabilities for the same species than chemical
coagulation
EC removes many species that chemical coagulation cannot
remove
EC produces less sludge, thus lowering the sludge disposal cost
EC sludge is more readily lterable and can be utilized as a soil
additive.
EC sludge contains metal oxides that pass the leachability test.
EC technique needs minimal startup time; the process can be
started by turning on the switch
Some of the limitations of the electrochemical coagulation are
as follows (Mollah et al., 2001, 2004a):
1. The sacricial anodes need to be replaced periodically.
2. Electrocoagulation requires a minimum solution conductivity
depending on reactor design, limiting its use with efuent
containing low dissolved solids.
3. In case of the removal of organic compounds, from efuent
containing chlorides there is a possibility of formation of toxic
chlorinated organic compounds.
V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963 951
4. An impermeable oxide lm may be formed on the cathode
which may provide resistance to the ow of electric current.
However, change of polarity and periodical cleaning of the
electrodes may reduce this interference.
5. The high cost of electricity can result in an increase in opera-
tional cost of EC.
Electrocoagulation involves many chemical and physical phe-
nomena that make use of consumable electrodes to supply ions into
the pollutant system. In EC the coagulating ions are produced in-
situ and it involves the following successive stages:
Anode dissolution,
Formation of OH
ions and H
2
at the cathode
Electrolytic reactions at electrode surfaces
Adsorption of coagulant on colloidal pollutants
Removal of colloids by sedimentation or otation.
In an electrocoagulation process, the electrode or electrode as-
sembly is usually connected to an external DC source and the
important parameter is the selection of the electrode material and
the mode of combination of anode and cathode. The electrode
material for treatment of wastewater should be non-toxic to human
Table 2
Electrocoagulation used for treatment of different types of wastewater.
Reference Type of wastewater Current density or
current
Time (min) pH Anodeecathode COD removal
efciency (%)
Mahajan et al. (2013) Hospital operation theatre 12.2 mA/cm
2
75 6.75 FeeFe, FeeAl, AleAl 100, 98, 95
Lopez-Vizcaino et al. (2012) Surfactant-aided soil-remediation
processes
7.5 mA/cm
2
e8.25 AleAl, FeeFe, FeeAl 100
Kara (2012) Transport container washing 100 A/m
2
60, 30 6, 9 AleAl, FeeFe 76.3, 79.4
Ahmed et al. (2012) Lack water 13.33 mA/cm
2
120 8.1 AleAl 99
Khandegar and Saroha (2012) Distillery spentwash 0.817 A/cm
2
120 3 AleAl, AleFe, FeeFe 81.3, 71.8, 52.4
Akyol (2012) Paint manufacturing 35 A/m
2
15 6.95 FeeFe, AleAl 93, 94
Gengec et al. (2012) Bakers yeast 80 A/m
2
, 12.5 A/m
2
30 4, 5 AleAl 48, 49
Orkun and Kuleyin (2012) Landll leachate 30 mA/cm
2
180 6.54 FeeFe 65.85
Coskun et al. (2012) Olive mill 1 A 45 4.3 AeAl, FeeFe 53.4
Bayar et al. (2011) Poultry slaughterhouse 1 mA/cm
2
30 3 AleAl 85
Bouhezila et al. (2011) Town landll leachate 500 A/m
2
30 8.2 AleAl, FeeFe 70, 68
Katal and Pahlavanzadeh (2011) Paper mill 70 mA/cm
2
30 7 FeeFe 88.4
Li et al. (2011) Landll leachate 4.96 mA/cm
2
90 6.7e7.5 FeeFe, AleAl 49.8, 32.7
Mansouri et al. (2011) Tannic acid 10 mA/cm
2
120 9 FeeFe 100
Top et al. (2011) Landll leachate 15.9 mA/cm
2
30 7 AleAl 45
Saleem et al. (2011) Raw wastewater 24.7 mA/m
2
30 8.4 FeeFe 77.2
Sridhar et al. (2011) Pulp and paper 15 mA/cm
2
20 7 AleAl 90
Yavuz et al. (2011) Dairy 15 mA/cm
2
20 7 FeeAl 79.2
Janpoor et al. (2011) Laundry 0e2 A 80 8e9AleAl 93.2
Chou et al. (2010) Chemical mechanical polishing 6.8 mA/cm
2
50 FeeAl, FeeFe, AleAl, AleFe 96.3
Hanaet al. (2010) Olive mill 250 A/m
2
15 4.2 AleAl 80
Krishna et al. (2010) Distillery spentwash 0.03 A/cm
2
120 3 AleAl 72.3
Chavalparit and Ongwandee (2009) Biodiesel e23.5 6.06 AleGraphite 55.43
El-Naas et al. (2009) Petroleum renery 13 mA/cm
2
60 9.5 AleAl, SSeAl, SSeFe 45, 43, 23
Kumar et al. (2009) Bio-digester 44.65 A/m
2
120 2e8FeeFe 50.5
Linares-Hernandez et al. (2009) Mixed wastewater 45.45 A/m
2
60 8 AleFe 69
Prasad and Srivastava (2009) Distillery spentwash 14.285 mA/cm
2
180 5 (RuO
2
eTi)eSS 37
Thakur et al. (2009) Distillery spentwash 146.75 A/m
2
130 6.75 SSeSS 63.1
Un et al. (2009) Vegetable oil renery 35 mA/cm
2
90 7 AleAl 93
Zaied and Bellakhal (2009) Black liquor 14 mA/cm
2
50 7 AleAl, FeeFe 98, 85
Kirzhner et al. (2008) Winery 2 A 40 eAleAl 98.2
Kobya and Delipinar (2008) Bakers yeast 70 A/m
2
50 6.5, 7 AleAl, FeeFe 71, 69
Ugurlu et al. (2008) Paper mill 5 mA/cm
2
7.5 7.6 AleAl, FeeFe 75, 55
Asselin et al. (2008) Oily bilgewater 1.5 A 90 7.09 FeeAl 78.1
Kurt et al. (2008) Domestic 0.12 A 5e15 7.4 FeeFe 60
Babu et al. (2007) Tannery 20 mA/cm
2
e6.8 FeeAl 52
Feng et al. (2007a) Tannery 1.0 A 60 9.8, 8.5 MSeMS, AleAl 68
Murthy et al. (2007) Restaurant e30 7 AleAl, FeeFe 50e72
Drouiche et al. (2007) Chemical mechanical polishing 125 A/m
2
320 6 FeeFe 75
Barrera-Diaz et al. (2006) Food processing 18.2 A/m
2
30 4 AleAl 88
Bayramoglu et al. (2006) Poultry slaughterhouse 150 A/m
2
25 3 AleAl, FeeFe 93, 85
Kobya et al. (2006b) Potato chips manufacturing 300 A/m
2
40 4e6AleAl 60
Sengil and Ozacar (2006) Dairy wastewater 0.6 mA/cm
2
17FeeFe 98
Zaroual et al. (2006) Textile e3 10.6 FeeFe 84
Adhoum and Monser (2004) Olive mill 75 mA/cm
2
25 4e6AleAl 76
Lai and Lin (2004) Chemical mechanical polishing e30 7.3 AleFe 85
Muthukumar et al. (2004) Textile 5 A/dm
2
e12 (RuO
2
eTi)eTi 85
Bayramoglu et al. (2004) Textile 200 A/m
2
10 11 FeeFe, AleAl 76, 65
Inan et al. (2004) Olive mill 20 mA/cm
2
30 6.2 AleAl, FeeFe 52, 42
Manisankar et al. (2004) Distillery 6 A/dm
2
180 6.9e7.2 GraphiteeGraphite 85.2
Calvo et al. (2003) Soluble oil waste 7 A e6.70 AleAl 91.3
Lai and Lin (2003) Chemical mechanical polishing 0.06 A 60 8.7 AleAl 88.5
Bejankiwar (2002) Cigarette industry 10.9 A/cm
2
300 7.2 FeeFe 56
Xu et al. (2002) Egg processing e16, 24 4.5 AleAl, FeeFe, SSeSS 95, 95, 92
Chen et al. (2000) Restaurant 30e80 A/m
2
90 6e9.5 AleAl 84.1
Chiang et al. (1995) Landlls leachate 15 A/dm
2
240 8 (SnePdeRu)eSt 92
Lin and Peng (1994) Textile 92.5 A/m
2
240 10 FeeFe 51
V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963952
health and environment. The electrode materials used commonly
as electrodes are aluminum, iron, stainless steel, mild steel and
graphite as they are cheap, readily available, nontoxic and very
effective (Mollah et al., 2001, 2004a; Emamjomeh and Sivakumar,
2009). The use of different combinations (Al/Al, Al/Fe, Fe/Al, Fe/
Fe) of electrode for the treatment of same wastewater has been
reported in the literature (Khandegar and Saroha, 2012; Akyol,
2012; Bouhezila et al., 2011; Bayramoglu et al., 2006). When
aluminum and iron are used as anode material, metal ions are
released from anode and many ionic monomeric hydrolysis species
are formed, depending on the pH of the solution. The reactions
taking place at electrodes in EC are as follows:
For aluminum electrodes
Oxidation reaction takes place at the anode
Al
(s)
/Al
(aq)
3þ
þ3e
(1)
Reduction reaction takes place at the cathode
3H
2
Oþ3e
/(3/2)H
2
þ3OH
(2)
Overall reaction during electrolysis
Al
3þ
/Al(OH)
n
(3n)
/Al
2
(OH)
2
4þ
/Al
3
(OH)
4
5þ
/Al
13
complex /Al(OH)
3
(3)
For iron electrodes
Oxidation reaction takes place at the anode
Table 3
Electrocoagulation used for removal of various type of dyes.
Reference Dye Current or
current density
Anodeecathode Removal
efciency (%)
Yuksel et al. (2013) Reactive orange 84 130 A/m
2
SSeSS, FeeFe 66, 76
Khandegar and Saroha (2013a) Acid red 131, Reactive yellow 86,
Indanthrene blue RS, Basic GR 4,
Reactive yellow 145
0.0625 A/cm
2
AleAl 97
Khandegar and Saroha (2013b) Reactive black B, Orange 3R, Yellow GR 0.0625 A/cm
2
AleAl 98
Wei et al. (2012) Azo, Anthraquinone, Xanthene 0.3 A FeeSteel wool, FeeFe, FeeSS 98
Pajootan et al. (2012) Acid black 52, Acid yellow 220 40 A/m
2
AleAl 92, 95
Akbal and Kuleyin (2011) Levax brilliant blue E-B 100 A/cm
2
AleAl, FeeFe 99, 83
Korbahti et al. (2011) Acid, Reactive 4.0 mA/cm
2
FeeCarbon 95
Merzouk et al. (2011) Disperse red 20.8 mA/cm
2
AleAl 95
Parsa et al. (2011) Acid brown 14 6.329 A/m
2
AleSS 304 Bench scale: 91
Pilot scale: 80
Patel et al. (2011) Reactive black 5 7.5 mA/cm
2
FeeSS 90
Secula et al. (2011) Indigo carmine 54.57 A/m
2
MSeSS 100
Aoudj et al. (2010) Direct red 81 1.875 mA/cm
2
AleAl >98
Durango-Usugaa et al. (2010) Crystal violet 28 A/m
2
FeeAl, AleFe 100
Kobya et al. (2010) Remazol red 3B 15 mA/cm
2
FeeFe >99
Phalakornkule et al. (2010) Reactive blue 140, Direct red 1 40 A/m
2
FeeFe, AleAl >95
Mollah et al. (2010) Orange II 160 A/m
2
AleAl 94.5
Sengil and Ozacar (2009) Reactive black 5 4.575 mA/cm
2
FeeFe 98.8
Kabdasli et al. (2009c) Reactive red, Reactive yellow 145,
Reactive blue 221
22 mA/cm
2
SS 304eSS 304, AleAl 100, 58
Arslan-Alaton et al. (2009) Reactive 22 mA/cm
2
SSeSS, AleAl 99, 95
Charoenlarp and Choyphan (2009) Reactive, Basic eAleAl, FeeFe 96, 85.6
Phalakornkule et al. (2009) Direct red 23 30 A/m
2
FeeFe, AleAl >95
Aleboyeh et al. (2008) Acid Red 14 102 A/m
2
FeeSt 304 >91
Korbahti and Tanyolac (2008) Levax blue CA 35.5 mA/cm
2
FeeFe 99.5
Yildiz (2008) Bomaplex red CR-L 0.50 mA/cm
2
AleAl 99.1
Daneshvar et al. (2007) Acid yellow 23 112.5 A/m
2
AleSt 304 98
Koparal et al. (2007) Basic red 29 1 mA/cm
2
. Boron doped diamondeBoron
doped diamond
97.2
Rajkumar et al. (2007) Reactive blue 19 21.66 mA/cm
2
(TieRuO
2
eIrO
2
)eSS 100
Daneshvar et al. (2006) Basic red 46, Basic blue 3 80 A/m
2
FeeSt 304 99
Kashealasl et al. (2006) Acid yellow 36 127.8 A/m
2
FeeFe 83
Kobya et al. (2006a) Levax orange 100 A/m
2
AleAl 95
Sakalis et al. (2006) Reactive eNiobe diamondeCarbon,
(Pt/Ti)eCarbon
90
Alinsaet al. (2005) Blue reactive eAleAl 95
Yang and McGarrahan (2005) Reactive blue 19, Acid red 266, Disperse
yellow 218
1.6 A AleGraphite, FeeGraphite 98
Golder et al. (2005) Methylene blue, Eosin yellowish 16.1 mA/cm
2
MSeMS 97, 75
Fernandes et al. (2004) Acid orange 7 5 mA/cm
2
Boron doped diamondeCopper foil 98
Daneshvar et al. (2004) Acid red 14 80 A/m
2
FeeSt 304 93
Mollah et al. (2004b) II Orange 398.7 A/m
2
Carbon steeleCarbon steel 99
Can et al. (2003) Remazol red RB 133 15 mA/cm
2
AleAl 92.5
Daneshvar et al. (2003) II orange 34.62 A/m
2
FeeFe >98
Kim et al. (2002) Blue P-3R, Suncion yellow H-E4R, Suncron
blue RD-400, Suncron yellow 3GE-200,
Reactive blue 49, Reactive yellow 84, Disperse
blue 106, Disperse yellow 54
1e4.5 A/m
2
AleAl, FeeFe, SSeSS 95e99.6
Vlyssides et al. (2000) Mixed dyes e(Ti/Pt)eSS 304 100
Szpyrkowicz et al. (2000) Disperse e(Ti/PteIr)eSS 90
Vlyssides and Israilides (1998) Mixed dyes 0.89 A/cm
2
Ti/PteSS 304 96.4
Ogutveren et al. (1992) Acilan Blau 10e17.5 mA GraphiteeGraphite 98e100
V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963 953
Table 4
Removal of metals and other compounds by electrocoagulation.
Reference Metal or other compound Concentration (mg/L) or other unit Anodeecathode Removal efciency (%)
Vasudevan et al. (2013) B3e7ZneSS 93.2
Verma et al. (2013) Cr(III), Cr(VI) 887.29, 1495.27 FeeFe 100
Ganesan et al. (2013) Mn 2 MgeFe 97.2
Aji et al. (2012) Ni, Cu, Zn, Mn 250 FeeFe 96, 96, 96, 72.6
Olmez-Hanci et al. (2012) Naphthalene sulfonates K-acid 600 SSeSS 97
Tran et al. (2012) P5e50 MSeMS 97
Henriques et al. (2012) Nonylphenol polyethoxylate 40 AleAl 96
Drouiche et al. (2012) Fe25 FeeFe 56
Parga et al. (2012) Sr 62 FeeFe 87.45
Baek et al. (2012) Se(VI) 1.2 mM FeeFe 45.1
Sinha et al. (2012) Fe6, 11.6 AleAl 79, 68
Martinez et al. (2012) Au, Ag 13.25, 1357 SteSt 99.5
Vasudevan and Lakshmi (2012) B3e7MSeSS 93.2
Ghosh et al. (2011) Fe10 AleAl 90
Akbal and Camci (2011a) Ni, Cu, Cr 526, 335, 193 FeeAl 100
Akbal and Camci (2011b) Cu, Cr, Ni 45, 44.5, 394 AleFe 100
Vasudevan and Lakshmi (2011a) Cd 20 ZneZn AC: 97.8
DC: 96.9
Vasudevan et al. (2011) Fe50e20 AleAl AC: 93
DC: 91.5
Vasudevan et al. (2011b) Cd 20 AleAl AC: 97.5
DC: 96.2
Behbahani et al. (2011a) PO
4
3
400 AleAl, FeeFe 100, 84.7
Behbahani et al. (2011b) Fe25 AleAl 94.5
Lacasa et al. (2011a) PO
4
3
eP 27 mg/dm
3
AleAl, FeeFe 100
Lacasa et al. (2011b) As 20 mg/dm
3
FeeAl 99.9
Moussavi et al. (2011a) Petroleum hydrocarbon 64 SteFe Batch process: 95.1
Continuous process: 93.4
Moussavi et al. (2011b) CNe300 FeeAl, FeeFe, AleAl, AleFe 93, 87, 35, 32
Dermentzis et al. (2011a) Cr(VI) 200e800 FeeFe 100
Dermentzis et al. (2011b) Ni(II), Cu(II), Zn(II), Cr(VI) 50 AleAl >97, >97, >97, >80
Emamjomeh et al. (2011) Fe10 AleAl 92
Hanay and Hasar (2011) Cu(II), Zn(II), Mn(II) 50e200 AleAl 99.5
Keshmirizadeh et al. (2011) Cr(VI) 100 FeeAl 99
Kobya et al. (2011) As 150
m
g/L AleAl, FeeFe 93.5, 94
Murthy and Parmar (2011) Sr(II) 10 SSeSS, AleAl 93,77
Valero et al. (2011) TOC 2260 AleFe 79
Vepsalainen et al. (2011) Cu 1.25 FeeFe 97
Zodi et al. (2011) As 3.8
m
g/L AleAl, FeeFe 91.5
Ali et al. (2011) As 123 AleFe >99.6
Malakootian et al. (2011) NO
3
150 FeeFe, AleAl 90, 89.7
Parga et al. (2010) Cr 2.31e48.5 FeeFe 99
Khatibikamal et al. (2010) Fe4e6AleAl 93
Can and Bayramoglu (2010) p-Benzoquinone 50 AleSS 83
Pociecha and Lestan (2010) Pb, Zn, Cd 170, 50, 1.5 AleSS 95, 68, 66
Kumar and Goel (2010) NO
3
, As(V) 300, 1 MSeMS 84, 75
Vasudevan et al. (2010) B3e7AleAl 86.32
Malakootian and Youse(2009) Total hardness 464 AleAl 95.6
Nanseu-Njikia et al. (2009) Hg(II) 2 10
5
MAleFe 99.95
Chou et al. (2009) In(III) 20 FeeAl, AleFe, FeeFe, AleAl 78.3, 70.1, 31.4, 15.8
Vasudevan et al. (2009) Fe 25 AleSS 98.8
Kabdasli et al. (2009a) TOC, Ni, Zn 173, 248, 232 SS 304eSS 304 66, 90, 100
Kabdasli et al. (2009b) Dimethyl phthalate 100 SSeSS 100
Schulz et al. (2009) Si, Ca, Mg 40e50, 17e23, 4e6FeeFe, AleAl FeeFe ¼80, 20, 40
AleAl ¼60, 10, 20
Ghernaout et al. (2008) Bentonite suspension (Turbidity) 20 NTU FeeFe 80
Heidmann and Calmano (2008) Cr(VI) 10e50 MSeST 37 100
Modirshahla et al. (2008) 4-nitrophenol 20 St 310eSt 304 100
Ghosh et al. (2008) Fe(II) 25 AleAl 99.2
Sayiner et al. (2008) B 1000 AleFe 95
Golder et al. (2007a) Cr(III) 1711, 2235 MSeMS 90.6, 71.4
Golder et al. (2007b) Cr(VI) 50 AleAl 42
Golder et al. (2007c) Cr(III) 1000 AleAl 99.8
Gomes et al. (2007) As 1e1230 AleFe, FeeFe, AleAl 78.9e99.6
Feng et al. (2007b) Humic acid 20 AleAl 97.8
Rodriguez et al. (2007) Cu 50 AleAl 99.8
Chen et al. (2007) NH
3
40 (RuO
2
eIrO
2
eTiO
2
)eTi 100
Zhu et al. (2007) Fe5AleAl 96
Irdemez et al. (2006) PO
4
eP25 AleAl 100
Emamjomeh and Sivakumar (2006) Fe10e25 AleAl 100
Den and Huang (2005) Turbidity 70, 400 NTU FeeFe 95
Yilmaz et al. (2005) B 100 AleAl 97
Parga et al. (2005b) As(III), As(V) 2.24 FeeFe >99
Adhoum et al. (2004) Cu(II), Zn(II), Cr(VI) 50 AleAl 99, 99, 83
V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963954
Fe
(s)
/Fe
(aq)
2þ
þ2e
(4)
Fe
(s)
/Fe
(aq)
3þ
þ3e
(5)
Reduction reaction takes place at the cathode
2H
2
Oþ2e
/H
2(g)
þ2OH
(6)
3H
2
Oþ3e
/3/2H
2
þ3OH
(7)
Overall reaction during electrolysis
Fe
(aq)
2þ
þ2OH
/Fe(OH)
2(s)
(8)
Fe
(aq)
3þ
þ3OH
/Fe(OH)
3(s)
(9)
3. Batch and continuous mode of operation
It can be noticed from the literature that electrocoagulation has
been studied for the removal of a wide range of pollutants using
batch and continuous mode of operation. A continuous system
operates under steady state conditions, specially a xed pollutant
concentration and efuent ow rate. By contrast, a batch reactors
dynamic nature enables to study the range of operating conditions
and is more suited for research work. Continuous systems are
better suited to industrial processes for large efuent volumes
whereas the batch reactors are suited to laboratory and pilot plant
scale applications. The continuous mode of operation is preferred
due to its better control than the batch mode of operation. The
typical batch mode of operation and schematic diagram for
continuous mode of operation are shown in Fig. 1.
Table 4 (continued )
Reference Metal or other compound Concentration (mg/L) or other unit Anodeecathode Removal efciency (%)
Bektas et al. (2004a) PO
3
4
10 AleAl 100
Bektas et al. (2004b) B5 AleAl 92e96
Murugananthan et al. (2004) SO
4
2
100 AleAl, FeeFe 72, 68
Hu et al. (2003) Fe25 AleAl 100
Abuzaid et al. (2002) Turbidity 76 NTU SSeSS 95
Ciorba et al. (2002) Nonylphenol ethoxylates 0.1 g/L Al-St 30e80
Yang and Dluhy (2002) Fe16 AleAl 87.5
Mameri et al. (1998) Fe2.5 AleAl 80
Lin and Wu (1996) NO
2
eN, NH
3
eN5 FeeFe 100, 15
Table 5
Combination of electrocoagulation with other methods.
Reference Combination Efuent Result (%)
Secula et al. (2012) EC þAdsorption (granular activated carbon) Synthetic solution Indigo carmine dye (99.5)
Asaithambi et al. (2012) EC þOzonation Distillery COD (83), Color (100)
Modenes et al. (2012) EC þPhoto-fenton Tannery COD (99), Turbidity (93), TSS (70),
TFS (37), TVS (95), Cr (99)
Filho et al. (2012) EC þElectrootation þFluidized bed anaerobic
reactor
Tannery COD (90)
Orescanin et al. (2012) Ozonation þEC þMicrowaves Landll leachate Color (98.43), Turbidity (99.48),
SS (98.96), NH
3
(98.8), COD (94.17), Fe (98.56)
Bellebia et al. (2012) EC(AleAl) þAdsorption (activated carbon),
EC(FeeFe) þAdsorption (activated carbon)
Cardboard paper mill COD (98, 93)
Baudequin et al. (2011) EC þReverse osmosis Fireghting water Fluorinated surfactant (71e77)
Durante et al. (2011) EC þElectrooxidation Synthetic solution of CreEDTA Cr (99)
Mahvi et al. (2011) EC þElectrootation Synthetic solution NH
3
(98), PO
4
3
(98), COD (72)
Yahiaoui et al. (2011) EC þUltraltration Olive oil mill COD (96)
Zhao et al. (2011) EC þElectrooxidation Synthetic solution As (100), Fe(91)
Chang et al. (2010) EC þActivated carbonemicrowave regeneration Synthetic dye solution Reactive black 5 (82)
Merzouk et al. (2010) EC þElectrootation Textile COD (79.7), Turbidity (76.2), SS (85.5),
BOD (88.9), Color (93)
Rodriguez et al. (2010) EC þPhytoremediation Mixed industrial COD (91), Turbidity (98), Color (97)
Zodi et al. (2010) EC þSedimentation Textile COD (70), Turbidity (90)
Aouni et al. (2009) EC þNanoltration Textile Color (>99)
Merzouk et al. (2009) EC þElectrootation Textile COD (68), BOD (83), SS (86.5),
Turbidity (81.56), Color (92.5) Metals (95)
Boroski et al. (2009) EC þHeterogeneous photocatalysis (TiO
2
) Pharmaceutical and cosmetic COD (86), Turbidity (91)
Narayanan and Ganesan (2009) EC þAdsorption (granular activated carbon) Synthetic solution Cr(VI) (97)
Wang et al. (2009) EC þElectrootation Laundry COD (62)
Zhang et al. (2009) EC þElectrooxidation Synthetic solution Acid red 2 (98)
Raju et al. (2008) EC þElectrooxidation Synthetic solution COD (93)
He et al. (2007) EC þOzonation Synthetic solution Reactive yellow 84 (100), TOC (85)
Niam et al. (2007) EC þMagnetic eld Synthetic milk powder solution COD (75.5), SS (30.6)
Linares-Hernandez et al. (2007) EC þBiosorption Mixed industrial COD (84), BOD (78), Color (97),
Turbidity (98), Coli form (99)
Parga et al. (2005a) EC þAir injection Well water Cr (99), As (99)
Shen et al. (2003) EC þElectrootation Industrial F- (86)
Jiang et al. (2002) EC þFlotation Synthetic solution Dissolved organic carbon (67), Color (89)
Pelegrini et al. (1999) EC þPhotochemical Synthetic solution Color (98)
Lin and Chen (1997) EC þH
2
O
2
þIon-exchange Dyeing and nishing mill Cl
-
(99.8), NO
3
(99.5), SO
4
(99.9)
V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963 955
Table 6
Comparison of electrocoagulation with other methods.
Reference Efuent Removal parameter Result (%)
Verma et al. (2013) Electroplating Cr(III), Cr(VI) EC: 100 for both Cr(III) and Cr(VI)
Coagulation (Ferric chloride): Cr(III) (52.6), Cr(VI) (25.8)
Khandegar and Saroha (2013a) Textile Color EC: 97
Coagulation (Alum): 94
Khandegar and Saroha (2013b) Textile Color EC: 100
Coagulation (Alum): <5
Harif et al. (2012) Synthetic kaolin solution eElectrocoagulation ocs are more porous as compared to
ocs formed by alum coagulation
Chen and Deng (2012) Synthetic solution Humic acid EC: 32.9
Ultraltration: 26.8
Ouaissa et al. (2012) Tannery Turbidity, COD, Cr (VI) EC: Turbidity (94.2), COD (25), Cr(VI) (24.6)
EC þAdsorption: Turbidity (96.1), COD (92), Cr(VI) (75)
Adsorption: Turbidity (93.3), COD (50), Cr(VI) (77)
Farhadi et al. (2012) Pharmaceutical COD EC: 34.22
Photoelectrocoagulation (EC þUV): 37.32
Peroxi-electrocoagulation (EC þH
2
O
2
): 54.32
Peroxi-photoelectrocoagulation
(EC þUV þH
2
O
2
): 50.52
Bani-Melhem and Smith (2012) Grey water COD, Color EC þSubmersed membrane bioreactor:
COD (88.6), Color (93.7)
Submersed membrane bioreactor:
COD (86), Color (91.2)
Wang et al. (2012) Landscape Algae (Chlorophyll-a) EC: 79
Coagulation (Aluminum sulfate): 74
Coagulation (Polyaluminum chloride): 71
Zhao et al. (2012) Plug board COD EC: 30
EC þH
2
O
2
:76
Chaet al. (2011) Synthetic dye solution Orange II dye EC: 98
Coagulation (Aluminum sulfate): 53
Siles et al. (2011) Biodiesel manufacturing COD Acidication þEC þBiomethanization: 99
Acidication þCoagulationeocculation þBiomethanization: 94
El-Ashtoukhy and Amin (2010) Synthetic dye solution COD EC: 87
Electrochemical oxidation: 68
Akbal and Camci (2010) Metal plating Cu, Cr, Ni EC: 99.9 in 20 min
Coagulation (Aluminum sulfate and ferric chloride): 99.9
Kilic and Hosten (2010) Aqueous suspension of kaolinite Turbidity EC: 96
Coagulation (Aluminum sulfate): 88
Song et al. (2008) Synthetic dye solution Reactive blue 19 EC: 44
EC þOzonation: 96
Ozonation: 10
Song et al. (2007) Synthetic dye solution Reactive black 5 EC: 83
EC þOzonation: 94
Ozonation: 10
Golder et al. (2007b) Aqueous solution Cr(VI) EC: 53.5
Coagulation (Aluminum sulfate): 14.9
Coagulation (Alum): 13.8
Perng et al. (2007) Paper mill SS, COD EC: SS (99.3), COD (75)
EC þCoagulation (Polyacrylamide polymer): SS (99.6),
COD (92.7)
Roa-Morales et al. (2007) Pasta and cookie COD EC: 80
EC þH
2
O
2
:90
Martins et al. (2006) Aqueous solution Nonylphenol
polyethoxylate
EC: 95 in 15 min
EC þH
2
O
2
:95in10min
Can et al. (2006) Textile COD EC: 23
EC þAlum: 65
EC þPolyaluminum chloride: 80
Kannan et al. (2006) Distillery Turbidity EC: 99
EC þAreca catechu nut carbon: 99
Zhu et al. (2005) Synthetic solution MS2 virus EC: 4-log
Coagulation (Iron-coagulant): 2-log
Microltration: 0.5-log
Kim et al. (2004) Synthetic dye solution Disperse, reactive dyes EC þFenton oxidation: 99
Fenton oxidation: 85
Dimoglo et al. (2004) Petrochemical Turbidity EC: 88
Electrootation: 83
Lin et al. (1998) Saline wastewater COD EC: 34
EC þH
2
O
2
:70
Lin and Chen (1997) Dyeing and nishing mill COD, color EC: COD (33), Color (83)
EC þH
2
O
2
: COD (78), Color (92)
EC þH
2
O
2
þPolyaluminum chloride: COD (78), Color (90)
V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963956
Batch mode of electrocoagulation reactors exhibits time-
dependent behavior as the coagulant is continuously generated in
the reactor with the dissolution of anode. The anode material is
hydrolysed, and is capable of aggregating the pollutants. As a result,
the concentration of the pollutant, coagulant, and pH keeps on
changing with respect to time. A batch reactor has neither inow or
outow of efuent during the electrolysis time.
4. Effect of various operating parameters
The efciency of the electrocoagulation process depends on
many operational parameters such as conductivity of the solution,
arrangement of electrode, electrode shape, type of power supply,
pH of the solution, current density, distance between the elec-
trodes, agitation speed, electrolysis time, initial pollutant concen-
tration, retention time and passivation of the electrode.
4.1. Solution conductivity
Conductivity of the solution is very important parameter in
electrolysis process as the removal efciency of the pollutant and
operating cost are directly related to the solution conductivity.
The solution must have some minimum conductivity for the ow
of the electric current. The conductivity of the low-conductivity
wastewater is adjusted by adding sufcient amount of salts such as
sodium chloride or sodium sulphate. There is an increase in the
current density with an increase in the conductivity of the solution
at constant cell voltage or reduction in the cell voltage at constant
current density (Bayramoglu et al., 2004; Merzouk et al., 2010). The
energy consumption is decreased with high conductivity solution.
4.2. Arrangement of electrodes
The electrode material and the connection mode of the elec-
trodes play a signicant role in the cost analysis of the electro-
coagulation process. Kobya et al. (2011) studied the treatment of
textile wastewater and compared the performances of various
electrode connection modes as a function of wastewater pH, cur-
rent density and operating time. They studied three different
modes of electrode connection shown in Fig. 2(a)e(c) and are as
follows:
Monopolar electrodes in parallel connections (MP-P): The an-
odes and cathodes are connected in parallel due to which the
current is divided between all the electrodes to the resistance of
individual cells. The parallel connection needs a lower potential
difference compared with serial connections.
Monopolar electrodes in serial connections (MP-S): In the
monopolar electrodes in serial connection, each pair of sacricial
electrodes is internally connected with each other. The addition of
the cell voltages leads to a higher potential difference for a given
current.
Bipolar electrode in serial connections (BP-S): In this connection
mode, the outer electrodes are connected to the power supply and
there is the no electrical connection between the inner electrodes.
Kobya et al. (2011) reported that MP-P mode is the most cost
effective for both aluminum and iron electrodes.
4.3. Shapes of the electrode
The shape of the electrodes affects the pollutant removal ef-
ciency in the electrocoagulation process. It is expected that the
punched holes type electrodes will result in higher removal ef-
ciency compared to the plane electrodes. Very few studies have
been reported in the literature (Kuroda et al., 20 03; Nielsen and
Andersson, 2009) describing the effect of electrode shape on the
performance of the electrostatic precipitator. Kuroda et al. (2003)
performed experiments using metallic electrodes with/without
punched holes as a barrier discharge electrode to study the effect
of electrode shape of precharger on the collector efciency in
electrostatic precipitator. They have reported higher discharge
current for the electrode with punched holes than for plane
electrode resulting in higher collection efciency with punched
electrode compared with plane electrode. The electric eld in-
tensity at the edge of punched holes type electrodes is higher (1.2
times) than at plane type electrode resulting in an increase in the
discharge current at punched type electrode. More studies are
needed to establish the effect of the electrode shape (punched
hole diameter and pitch of the holes) on the electrocoagulation
process.
4.4. Type of power supply
In the electrocoagulation process, there is an in-situ generation
of metal hydroxide ions by electrolytic oxidation of the sacricial
anode. These metal hydroxide ions act as coagulant and remove the
pollutants from the solution by sedimentation. Majority of the
studies reported in the literature have used direct current (DC) in
the electrocoagulation process. The use of DC leads to the corrosion
formation on the anode due to oxidation. An oxidation layer also
form on the cathode reducing the ow of current between the
cathode and the anode and thereby lowering the pollutant removal
efciency.
Fig. 1. Schematic diagram of batch and continuous mode of operation. Fig. 2. Different modes of electrode connections.
V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963 957
Vasudevan et al. (2011b) investigated the effect of alternating
current (AC) and DC on the removal of cadmium from water using
aluminum alloy as anode and cathode. They obtained a removal
efciency of 97.5% and 96.2% with the energy consumption of 0.454
and 1.002 kWh/kL at a current density of 0.2 A/dm
2
and pH of 7
using AC and DC respectively. The results indicate that the problem
of corrosion formation at the electrodes can be reduced by the use
of AC in place of DC in the electrocoagulation process.
4.5. pH of the solution
The pH of the solution is an important operational parameter in
electrocoagulation. The maximum pollutant removal efciency is
obtained at an optimum solution pH for a particular pollutant. The
precipitation of a pollutant begins at a particular pH. The pollutant
removal efciency decreases by either increasing or decreasing the
pH of the solution from the optimum pH.
Verma et al. (2013) studied the removal of hexavalent chromium
from synthetic solution using electrocoagulation and found that the
pH of the solution has a signicant effect on the Cr(VI) removal
efciency. They performed the experiments at different pH of the
synthetic solution and obtained the maximum chromium removal
efciency at the pH 4. They further reported that the pH of the
synthetic solution after the EC process increased with an increase in
the electrolysis time due to the generation of OH
in the EC process.
4.6. Current density
Current density is very important parameter in electro-
coagulation as it determines the coagulant dosage rate, bubble
production rate, size and growth of the ocs, which can affect the
efciency of the electrocoagulation. With an increase in the current
density, the anode dissolution rate increases. This leads to an in-
crease in the number of metal hydroxide ocs resulting in the in-
crease in pollutant removal efciency. An increase in current
density above the optimum current density does not result in an
increase in the pollutant removal efciency as sufcient number of
metal hydroxide ocs are available for the sedimentation of the
pollutant.
4.7. Distance between the electrodes
The inter-electrode distance plays a signicant role in the
electrocoagulation as the electrostatic eld depends on the dis-
tance between the anode and the cathode. The maximum pollutant
removal efciency is obtained by maintaining an optimum distance
between the electrodes. At the minimum inter-electrode distance,
the pollutant removal efciency is low. This is due to the fact that
the generated metal hydroxides which act as the ocs and remove
the pollutant by sedimentation get degraded by collision with each
other due to high electrostatic attraction (Daneshvar et al., 2004).
The pollutant removal efciency increases with an increase in the
inter-electrode distance from the minimum till the optimum dis-
tance between the electrodes. This is due to the fact that by further
increasing the distance between the electrodes, there is a decrease
in the electrostatic effects resulting in a slower movement of the
generated ions. It provides more time for the generated metal hy-
droxide to agglomerate to form the ocs resulting in an increase in
the removal efciency of the pollutant in the solution. On further
increasing the electrode distance more than the optimum electrode
distance, there is a reduction in the pollutant removal efciency.
This is due to the fact that the travel time of the ions increases with
an increase in the distance between the electrodes. This leads to a
decrease in the electrostatic attraction resulting in the less forma-
tion of ocs needed to coagulate the pollutant.
4.8. Effect of agitation speed
The agitation helps to maintain uniform conditions and avoids
the formation of concentration gradient in the electrolysis cell.
Further, the agitation in the electrolysis cell imparts velocity for the
movement of the generated ions. With an increase in agitation
speed upto the optimum agitation speed, there is an increase in the
pollutant removal efciency. This is due to the fact that with an
increase in the mobility of the generated ions, the ocs are formed
much earlier resulting in an increase in the pollutant removal ef-
ciency for a particular electrolysis time. But with a further increase
in the agitation speed beyond the optimum value, there is a
decrease in the pollutant removal efciency as the ocs get
degraded by collision with each other due to high agitation speed
(Modirshahla et al., 2008).
4.9. Electrolysis time
The pollutant removal efciency is also a function of the elec-
trolysis time. The pollutant removal efciency increases with an
increase in the electrolysis time. But beyond the optimum elec-
trolysis time, the pollutant removal efciency becomes constant
and does not increase with an increase in the electrolysis time. The
metal hydroxides are formed by the dissolution of the anode. For a
xed current density, the number of generated metal hydroxide
increases with an increase in the electrolysis time. For a longer
electrolysis time, there is an increase in the generation of ocs
resulting in an increase in the pollutant removal efciency. For an
electrolysis time beyond the optimum electrolysis time, the
pollutant removal efciency does not increase as sufcient
numbers of ocs are available for the removal of the pollutant.
4.10. Initial concentration of pollutant
The pollutant removal efciency decreases with an increase in
the initial concentration of the pollutant for a constant current
density. This is due to the fact that the number of metal hydroxide
ocs formed may be insufcient to sediment the greater number of
pollutant molecules at higher initial pollutant concentrations
(Kobya et al., 2006a; Daneshvar et al., 2006).
4.11. Retention time
After the completion of the electrocoagulation process for a
particular electrolysis time, the solution is kept for xed period
(retention time) to allow settling of the coagulated species. As the
retention time is increased the removal efciency of pollutant in-
creases. This is due to the fact that with an increase in retentiontime,
all coagulated species settle down easily to give clear supernatant
liquid and the sludge. But providing a retention time more than the
optimum retention time results in the reduction of pollutant
removal efciency as the adsorbed pollutant desorbs back into the
solution (Daneshvar et al., 2003; Khandegar and Saroha (2013b)).
4.12. Electrode passivation
Electrode passivation is the accumulation of an inhibiting layer
(usually an oxide) on the electrodes surface. Passivation is unde-
sirable for anode dissolution and electrocoagulation operation. The
primary control of passivation is the galvanostatic mode of opera-
tion. The current and the potential are dependent on the systems
overall resistance. Any resistance from a passivating layer increases
the cell potential but does not affect either the coagulant or bubble
production rates. The use of deionised water minimizes the pres-
ence of contaminants such as carbonates, which can easily
V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963958
passivate the electrodes. The electrodes should be mechanically
cleaned periodically to remove any passivating material. This
maintains the integrity of the electrodes and ensures anodic
dissolution at a constant rate. Also these impermeable layers pre-
vent the effective current transport between the anode and cath-
ode. Corrosion formation on the electrodes can be removed by
using AC in place of DC in the electrocoagulation.
4.13. Cost analysis
Cost analysis plays an important role in industrial wastewater
treatment technique as the wastewater treatment technique
should be cost attractive. The costs involved in electrocoagulation
include, the cost of energy consumption, cost of the dissolved
electrode and the cost of addition of any external chemical (for
increasing the solution conductivity or varying the pH of the so-
lution). The operating cost using electrocoagulation can be calcu-
lated by following equations.
Electrode consumptionðENCÞkg=m
3
¼kg of electrode dissolved=m
3
of effluent
¼ðItMÞ=nFV(10)
Energy consumptionðELCÞkWh=m
3
¼ðUItÞ=V(11)
Chemical consumptionðCHCÞkg of chemical=m
3
¼Chemicals used=m
3
of effluent (12)
Operating costcost=m
3
¼aENC þbELC þcCHC (13)
The detailed calculation of operating cost for the treatment of
uoride-containing drinking water using electrocoagulation has
been reported by Ghosh et al. (2011).
Espinoza-Quinones et al. (2009), studied the removal of organic
and inorganic pollutants from a wastewater of lather nishing in-
dustrial process using electrocoagulation. They found the electro-
coagulation to be cheaper compared to the conventional method.
The operational cost for the electrocoagulation was found to be US $
1.7 per m
3
of the treated tannery efuent as compared to the cost of
US $ 3.5 per m
3
of the treated efuent for conventional methods.
Similarly Bayramoglu et al. (2007) have reported that the operating
cost of chemical coagulation is 3.2 times as high as that of elec-
trocoagulation for the treatment of textile wastewater.
5. Conclusions
The rapid urbanization and industrialization in the developing
countries are creating high levels of water pollution due to harmful
industrial effects and sewage discharges. The characteristics of in-
dustrial efuents in terms of nature of contaminates, their con-
centrations, treatment technique and required disposal method
vary signicantly depending on the type of industry. Further, the
choice of an efuent treatment technique is governed by various
parameters such as the contaminates, their concentration, volume
to be treated and toxicity to microbes. Electrocoagulation is an
attractive method for the treatment of various kinds of wastewater,
by virtue of various benets including environmental capability,
versatility, energy efciency, safety, selectivity and cost effective-
ness. The process is characterized by simple equipment, easy
operation, less operating time and decreased amount of sludge
which sediments rapidly and retain less water. However, further
studies needs to be performed to study the effect of shape and
geometry of the electrodes (punched hole and pitch of the holes) to
possibly improve the pollutant removal efciency. Efforts should be
made to study the phenomena of electrode passivation to reduce
the operating cost of the electrocoagulation process. Most of the
studies reported in the literature have been carried out at the
laboratory scale using synthetic solutions. Efforts should be made
to perform electrocoagulation experiments at pilot plant scale us-
ing real industrial efuent to explore the possibility of using EC for
treatment of real industrial efuent.
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V. Khandegar, A.K. Saroha / Journal of Environmental Management 128 (2013) 949e963 961