.
CHEMICAL COAGULATION AND
ELECTROCOAGULATION IN TREATMENT
TECHNOLOGY FOR ENVIRONMENTAL GREEN
COMMUNITY
Ahmed Abd El Maguid Mekawy
Housing & Building National Research Centre, Egypt
Key Words: Industrial wastewater, treatment, chemical coagulation and
electrocoagulation.
ABSTRACT:
There are wide range of wastewater treatment techniques emerging
which include biological processes and physical-chemical processes. It is
also includes techniques based on electrochemical technology which
include electrocoagulation (EC). Electrocoagulation (EC) is becoming a
popular process to be used for wastewater treatment. The objective of
this study is to evaluate the EC & CC for improving wastewater quality,
such as increasing removal efficiencies of COD. Alum was used as
coagulant for chemical coagulation process at various doses and various
sedimentation time. Also the performance of the Electrocoagulation (EC)
was examined at constant operation time, various current density and
various sedimentation time. For each set of experiments COD were
analyzed for both EC and CC. Steady state COD removal was achieved
at Alum dose 60 and 120 mg/l and settling time stated from 180 min.
COD removal with the various settling time was examined by varying the
applied current between 0.4, 0.8, 1.0, 1.5 and 2.0 A with constant
operating time at 40 minutes followed be settling time till 240 min.
according to the results obtained in this study EC gave better
performance on the COD removal ratio, EC was more effective with 45%
than chemical coagulation.
1. INTRODUCTION
Calls to the treatment of industrial and domestic wastewater to
avoid environmental pollution and contamination of pure water resources
are becoming national and international issues. There is an urgent need to
develop new and more effective technologies for purifying and cleaning
wastewater before disposing into other water system. Electrocoagulation
technology presents many advantages compared to chemical coagulation
methods therefor it is becoming a popular method to be used for
wastewater treatment (Ni’am et al. 2007).
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Both types of coagulation are based on the same principle: the
addition of metallic ions (usually Fe3+, Al3+) into the treated wastewater
in order to destabilize the electrical charge of the colloidal particles from
water, leading to the formation of aggregates bigger than the initial
colloids, which determines the sedimentation and water purification
(Holt et al. 2002; Canizares et al. 2006; Canizares et al. 2007). The
difference between these two processes consists in the way metal ions are
added. In case of chemical coagulation, the reagents (FeCl3, Al2(SO4)3,
etc.) are directly added (Riera-Torres et al. 2010), while in case of
electrocoagulation, the metallic cation is supplied by an electricity stream
generated by the oxidation of the metallic electrode, producing the
corresponding metal ions.
Electrocoagulation (EC) process is an environmentally-friendly
method which has some advantages as: no need for chemicals addition;
(Daneshvar et al. 2004) requires simple equipment and less space for
installation; simple operation; (Kim et al. 2002) faster and more effective
separation of the pollutants than chemical coagulation. Furthermore, it
produces sludge with low water content in comparison with chemical
coagulation; (Bayramoglu et al. 2007). In addition, this process has
lower effluent total dissolved solids compared with chemical treatment
methods, and can remove the smallest colloidal particles; (Mollah et al.
2001; Un et al. 2009)
Extensive EC studies were carried out in the latter half of the
century in both the United States and the Soviet Union (Naje & Abbas
2013). However, EC remains practically unused in water and wastewater
treatment until the 21st century and this was mainly due to the then-high
investment and electricity costs (Koren & Syversen 1995; Chen 2004;
Holt et al. 2005). These economic facts gave other technologies an edge
over EC.
During the recent decades, researches have revealed EC as an
attractive and suitable method for the treatment of various kinds of
wastewater, by virtue of various benefits including environmental
compatibility, versatility, energy efficiency, safety, selectivity,
amenability to automation, and cost effectiveness (Mollah et al. 2001;
Chen 2004; Holt et al. 2005; Naje & Abbas 2013). This process is
characterized by simple equipment, easy operation, a shortened reactive
retention period, a reduction or absence of equipment for adding
chemicals and decreased amount of precipitate or sludge which
sediments rapidly (Bayramoglu et al. 2006).
18 Egypt. J. of Appl. Sci., 36 (1) 2021
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The EC technology induces coagulation and precipitation of
contaminants by a direct current electrolytic process followed by
separation of flocculent without the addition of coagulation-inducing
chemicals. The water is pumped through a unit in which electrodes made
of iron or aluminum are installed. A direct current electric field is applied
to the electrodes to induce the electrochemical reactions needed to
achieve the coagulation. Compared with traditional flocculation–
coagulation, electrocoagulation has also the advantage of removing the
smallest colloidal particles; such charged particles have a greater
probability of being coagulated and destabilized because of the electric
field that sets them in motion. Electrocoagulation also has the advantage
of producing a relatively low amount of residue. At its simplest, an
electrocoagulation system consists of an anode and a cathode made of
metal plates, both submerged in the aqueous solution being treated. The
electrodes are usually made of aluminum, iron or stainless steel, because
these metals are cheap, readily available, proven effective and non-toxic
(Koren & Syversen 1995; Chen 2004; Bayramoglu et al. 2006;
Dohare & Sisodia 2014).
There are various treatment parameters effects on efficiency of the
Electro-coagulation in elimination of the contaminants from wastewater
are as follows (Thakur & Chauhan 2016):
1. Material of the electrodes can be Iron, Aluminum and/or inert material.
Iron and Aluminum ions and hydroxides have different chemistries
and applications.
2. PH of the solution influences the dissolution of electrodes and affects
the potential of the colloidal particles and also on the speciation of
metal hydroxides in the solution.
3. The amount of electrochemical reactions taking place on the electrode
surface is proportional to Current density.
4. Treatment time is relative to the amount of coagulants formed in the
Electro coagulation system and other reactions taking place in the
system.
5. Temperature has an effect on formation of floc, conductivity of the
solution and reaction rates. Depending on the pollutant, increasing
temperature can have either good or bad effect on removal efficiency.
6. Electrode potential defines which reactions occur on the electrode
surface.
7. Concentration of the pollutants affects the removal efficiency.
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8. Inter-electrode distance may have effect on efficiency of the treatment
and electricity consumption.
EC has been successfully tested to treat various wastewater such
as; textile wastewater, urban wastewater, landfill leachate, tar sand and
oil shale wastewater , chemical fiber plant wastewater , yeast wastewater,
food and protein wastewater , rendering wastewater, olive oil wastewater
, petrochemical wastewater , restaurant wastewater , egg process
wastewater , and oily wastewater (Can et al. 2006).
This chemical process involves destabilizing wastewater particles
so that they aggregate during chemical flocculation. Fine solid particles
dispersed in wastewater carry negative electric surface charges (in their
normal stable state), which prevent them from forming larger groups and
settling. Chemical coagulation destabilizes these particles by introducing
positively charged coagulants that then reduce the negative particles’
charge. Once the charge is reduced, the particles freely form larger
groups. Next, an anionic flocculant is introduced to the mixture. Because
the flocculant reacts against the positively charged mixture, it either
neutralizes the particle groups or creates bridges between them to bind
the particles into larger groups. After larger particle groups are formed,
sedimentation can be used to remove the particles from the mixture
(Sahu & Chaudhari 2013).
2. EXPERIMENTAL
The aim of the experimental study was to assess the performance of
EC to improve wastewater quality, through increasing removal
efficiencies of COD and various sedimentation and compare this
achievement with the traditional CC.
2.1 EC Technique
On passage of Direct Current (DC) in wastewater to be treated
causes production of metal ions at the expense of anode as sacrificing
electrode and hydroxyl ions at cathode as a result of water splitting. The
direct current provides the electromotive force to drive the chemical
reactions to produce metal hydroxides. The metal hydroxides produced
act as coagulant/flocculent for the suspended solids to convert them into
flocs of enough density to sediment under gravity (Samir 2015).
Generally, three main processes occur serially during
electrocoagulation:
(a) Electrolytic reactions at electrode surfaces,
(b) Formation of coagulants in aqueous phase,
20 Egypt. J. of Appl. Sci., 36 (1) 2021
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(c) Adsorption of soluble or colloidal pollutants on coagulants, and
removal by sedimentation or floatation.
In EC with electrical current flowing between two electrodes,
coagulant is generated in situ by electrolytic oxidation of the anode
material. With an iron anode, Fe(OH)n with n = 2 or 3 is formed at the
anode (Daneshvar et al. 2003; Mollah et al. 2004).
2.2 Reactions at Sacrificial Electrodes
During EC, the following main reactions take place at the Iron
electrodes (Kuokkanen et al. 2013).
Anodic reactions
Iron Electrode Anode:
Fe → Fe2+ + 2e-
Cathode: 2H2O + 2e- → H2 + 2OH-
Overall: Fe2+ + 2H2O →H2 +Fe (OH) 2
OR
Anode: Fe →Fe3+ + 3e-
Cathode: 3H2O + 3e- → 1.5H2 +3OH
-
Overall: Fe3+ +3H2O→1.5H2 + Fe (OH)3
Electrochemically generated metal cations will react spontaneously
with OH
-
ions forming various monomeric and polymeric metal hydroxy
species. Ferric ions generated electrochemically may form monomeric
ions, ferric hydroxo complexes with OH
-
ions, and polymeric species.
These species/ions are:
Monomeric species:
Fe(OH)2+, Fe(OH)2-, Fe2(OH)24
+
Polymeric species:
Fe(OH)4
-
, Fe(H2O)5OH2
+
, Fe(H2O)4(OH)2
+
Fe(H2O)8(OH)2
4+ and Fe2(H2O)6(OH)4
2+
Which further react to form Fe(OH)3. The formation of these complexes
depends strongly on the pH of the solution. Above pH 9, Al(OH)4- and
Fe(OH)4- are the dominant species (Akbal & Camci 2010).
Iron hydrolysis products then destabilize pollutants present in the
solution, allowing agglomeration and further separation by settling or
flotation. Destabilization is achieved mainly by means of two distinct
mechanisms, i.e.
1. Charge neutralization of negatively charged colloids by cationic
hydrolysis products; and
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2. ―Sweep flocculation‖, where impurities are trapped and removed in the
amorphous hydroxide precipitate produced.
Several factors such as pH and coagulant dosage have an impact
on charge neutralization and sweep flocculation (Arega & Chavan
2018).
2.3. EXPERIMENTAL PILOT UNIT
The experimental work has been conducted as follows:
Phase (A): Studying the performance of chemical coagulation (CC) at
various alum doses and various sedimentation time, COD
was analyzed by standard methods for each set of
experiments.
Phase (B): The performance of the Electrocoagulation (EC) was
examined at constant operation time, various current density
and various sedimentation time, for each set of experiments
COD was analyzed by standard methods.
Wastewater Sample
Wastewater was collected from the automatic slaughterhouse in El
Menoufya governorate, Table-1 presents the characteristics of Poultry
Plan’s wastewater.
Table -1 Characteristics of Poultry Plant Wastewater
Character Mean
Range
Max Min
pH Value 7.45 7.88 7.02
Total solids mg/L 2000 3750 400
Suspended solids mg/L 750 1500 170
COD mg/L 1600 2650 700
BOD mg/L 900 1500 250
Total phosphorus mg/L 12 17.15 6.15
Turbidity NTU 700 1100 400
Experimental works sequences for CC:
The Alum solution was rigorously stirred at a stirring speed of
150 rpm for 3 min. in order to make certain that the chemicals are
evenly and homogeneously distributed throughout the wastewater
and followed by flocculation basin for 30 min. duration time with
different Alum dose of 45, 60, 120 mg/l.
Following flocculation process, wastewater was placed in
graduated sedimentation columns 240 min settling time, the
solution is mixed again, but this time in a slow fashion, to
encourage the formation of insoluble solid precipitates.
Chemical Oxygen Demand COD was measured at each run.
Experimental works sequences for EC batch reactor
The electrocoagulation (EC) unit was cylindrical glass cell
(volume 2000 mL).
22 Egypt. J. of Appl. Sci., 36 (1) 2021
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There are two iron electrodes were used in the electrocoagulation
tank one electrode was connected as anode and one as cathode.
Electrodes dimensions were (130 mm x 50 mm x 4 mm).
The distance between both electrodes was 50 mm.
Electrodes were washed with acetone solution to remove surface
grease before each run. At the end of run, the electrodes were
washed thoroughly with water to eliminate any solid residues on
the surface, and dried.
Electrodes were placed in two liters of fluid wastewater and
connected to terminals of a Power supply.
Current density was varied from 0.4A to 2.0A and operating time 40
minutes.
After EC process, wastewater were degassed under low stirring
speed with an impeller velocity 30 rpm.
3. RESULTS AND DISCUSSION
3.1 The performance of chemical Coagulation (CC)
Chemical coagulation (CC) performance was examined at various
alum doses of 45, 60, 120 mg/l and various sedimentation time, for
each set of experiments COD were analyzed by standard methods. As
shown from firs run an increase in COD removal amount achieved with
increasing coagulation dose Figure-1 till the steady state of COD removal
at Alum dose 60 and 120 mg/l and settling time stated from 180 min. At
settling time 240 min COD removal ratio were 65.1%, 72 % & 73.2%
with Alum dose 45, 60 and 120 mg/l respectively. In this study, the
results showed that the optimum removal of COD was obtained at 60
mg/l of coagulants dosages.
Figure (1) the performance of chemical coagulation (CC)
Egypt. J. of Appl. Sci., 36 (1) 2021 23
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3.2 The performance of Electrocoagulation (EC)
The effect of current density on the reduction of metal ions from
wastewater in the batch reactor was studied with different current
densities (CD), constant operation time, and various sedimentation time.
The effect of current density on the COD removal with the various
settling time was investigated by varying the applied current between 0.4,
0.8, 1.0, 1.5 and 2.0 A with constant operating time at 40 minutes
followed be settling time till 240 min as shown in Figure 2.
The COD removal efficiencies increase as the settling time is
increased. Result showed that the efficiency of EC with current density
of 1.5 AMP was 87.0% when settling time was approximately 120
minutes with minor increasing to 90.5.0% when settling time increase to
240 min.
For the current density of 0.4, 0.8 and 1.0 AMP, the COD removal
percentage was 43, 65 and 75 % when settling time was 240 minutes.
From the above results, the more favorable percentage removal of COD
at 1.5 AMP (5.26 mA/cm2) of current density.
Figure (2) the performance of the Electrocoagulation (EC)
When the electrolysis time increases, the concentration of metal
ions and their hydroxide flocs increases; thus, the COD removal
efficiencies increase, It was found that as the value of current increased
the COD removal efficiencies increased. This behavior is due to the
applied current density that determines the coagulant dosage rate, the
bubble production rate and size of flocs growth resulting in a faster
removal of pollutants (Chavalparit & Ongwandee 2009; El-Ashtoukhy
et al. 2013). In other words by increasing the current of the cell the
24 Egypt. J. of Appl. Sci., 36 (1) 2021
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amount of hydrogen bubbles at the cathode increases, resulting in a
greater upwards flux and a faster removal of the pollutant and sludge
flotation
3.3 Technical comparison between CC and EC
The capability of EC versus CC and their consequent effect on
COD removal for industrial waste has been examined. EC exhibited
better COD removal ratio and it was more effective (42%) than chemical
coagulation. Alum showed poor COD removal, this could be due to
competitive adsorption (Zaleschi et al. 2012). Compared
electrocoagulation and chemical coagulation processes applied for
wastewater treatment and found that the best performances were obtained
for the electrocoagulation. The researchers suggested that the chemistry
behind the EC process in water in such that the positively charged ions
are attracted to the negatively charged hydroxides ions producing ionic
hydroxides with a strong tendency to attract suspended particles leading
to coagulation (Ukiwe et al. 2014).
Figure (3) Technical comparison between CC and EC- COD removal
ratio
4. CONCLUSION
The performances of the electrocoagulation and chemical
coagulation process are comparatively presented, as suitable wastewater
treatment processes for the improvement of water quality indicators.
1. At chemical coagulation (CC) the COD removal was increased
with increasing the Alum coagulation dose.
Egypt. J. of Appl. Sci., 36 (1) 2021 25
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2. The COD removal efficiency using chemical coagulation at
settling time 240 min were 52.1, 70.4 & 71.2 with the different
doses of Alum coagulant 45, 60 and 120 mg/l respectively.
3. It was deduced and found that the optimum and cost effective
dose of alum is 60 mg/l.
4. Using Electrocoagulation (EC) the COD removal efficiencies
increase as the settling time is increased with constant operating
time at 40 minutes.
5. It was found that the removal efficiency of COD increased with
increasing current density.
6. For the current density of 0.4, 0.8 and 1.0 AMP, the COD
removal percentage was 43, 65 and 75 % when settling time was
240 minutes.
7. COD removal efficiency using EC with current density of 1.5
AMP (5.26 mA/cm2) was 89.0% when settling time was
approximately 120 minutes with minor increasing to 91.0% when
settling time increased to 240 min.
8. The EC cell with current density of 1.5 AMP (5.26 mA/cm2) was
more effective in removing COD.
9. COD removal percentage was 91.0% at settling time 240 min
using EC process with current density of 1.5 AMP and it was 70.4
% using CC process.
10. COD effluent concentration was 730 ppm using EC process at
settling time 240 min with 1.5 AMP and was 170 ppm using CC
process.
The comparison of electrocoagulation and chemical coagulation
processes used for the treatment of wastewater demonstrated the
advantage of electrocoagulation treatment in improving wastewater
quality, through increasing removal efficiencies of COD and various
sedimentation.
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التخثير الکيميائي والتخثير الکهربي في تکنولوجيا المعالجة
لممجتمع الأخضر البيئي
احمد عبد المجيد مکاوي
استاذ مساعد في الهندسة الصحية والبيئية - المرکز القومي لبحوث الإسکان والبناء ، مصر
هناک مجموعة واسعة من تقنيات معالجة مياه الصرف الحديثة والتي تشمل العمميات
البيولوجية والعمميات الفيزيائية والکيميائية. ويشمل أيضًا التقنيات القائمة عمى التکنولوجيا
عممية شائعة )EC( أصبح التخثير الکهربي .)EC( الکهروکيميائية التي تشمل التخثير الکهربي
CC & EC لاستخدامها في معالجة مياه الصرف الصحي. الهدف من هذه الد ا رسة هو تقييم
تم استخدام الشبة کمخثر .COD لتحسين جودة مياه الصرف الصحي ، مثل زيادة کفاءة إ ا زلة
بجرعات مختمفة وأوقات ترسيب مختمفة. کما تم فحص أداء (CC) لعممية التخثير الکيميائي
في وقت التشغيل المستمر ، وکثافة التيار المختمفة ووقت الت رسيب )EC( التخثير الکهربي
تم تحقيق إ ا زلة .CC و EC لکل من COD المختمف. لکل مجموعة من التجارب تم تحميل
في الحالة الثابتة عند جرعة الشب 26 و 026 مجم / لتر وزمن الاستق ا رر المحدد من COD
مع وقت الاستق ا رر المختمف عن طريق تغيير التيار المطبق COD 086 دقيقة. تم فحص إ ا زلة
مع وقت تشغيل ثابت عند 06 دقيقة يتبعها وقت A بين 6.0 و 6.8 و 0.6 و 0.1 و 2.6
EC الاستق ا رر حتى 206 دقيقة. وفقًا لمنتائج التي تم الحصول عميها في هذه الد ا رسة ، أعطى
أکثر فعالية بنسبة 01 ٪ من التخثر الکيميائي. EC وکان ، COD أداءً أفضل عمى نسبة إ ا زلة
Egypt. J. of Appl. Sci., 36 (1) 2021 29
