1. INTRODUCTION
One
of the major challenges facing mankind today is to provide clean water to a
vast majority of the population around the world. Rivers, canals, estuaries and
other water-bodies are being constantly polluted due to indiscriminate
discharge of industrial effluents as well as other anthropogenic activities and
natural processes. The reuse of wastewater has become an absolute necessity.
There is, therefore, an urgent need to develop innovative, more effective and
inexpensive techniques for treatment of wastewater. A host of very promising
techniques based on electrochemical technology are being developed and existing
ones improved that do not require chemical additions. These include
electrocoagulation, electro flotation, electro decantation, etc. Even though electrocoagulation,
has reached profitable commercialization, it has received very little
scientific attention. This process has the potential to extensively eliminate
the disadvantages of the classical treatment techniques.[3]
Coagulation
is a phenomenon in which the charged particles in colloidal suspension are
neutralized by mutual collision with counter ions and are agglomerated,
followed by sedimentation. The coagulant is added in the form of suitable
chemical substances. Alum is such a chemical substance which has been widely
used for ages for wastewater treatment. Coagulation is brought about primarily
by the reduction of the net surface charge to a point where the colloidal
particles, previously stabilized by electrostatic repulsion, can approach
closely enough for Vander Waal’s forces to hold them together and allow
aggregation. The reduction of the surface charge is a consequence of the
decrease of the repulsive potential of the electrical double layer by the
presence of an electrolyte having opposite charge. [3]
Electrocoagulation
is a technique involving the electrolytic addition of coagulating metal ions
directly from sacrificial electrodes. These ions coagulate with turbidity
agents in the water, in a similar manner to the addition of coagulating
chemicals such as alum and ferric chloride, and allow the easier removal of the
pollutants. In the EC process, the coagulant is generated in situ by
electrolytic oxidation of an appropriate anode material. In this process, charged
ionic species—metals or otherwise—are removed from wastewater by allowing it to
react (i) with an ion having opposite charge, or (ii) with floc of metallic
hydroxides generated within the effluent. The EC technology offers an
alternative to the use of metal salts or polymers and polyelectrolyte addition
for breaking stable emulsions and suspensions. The technology removes metals,
colloidal solids and particles, and soluble inorganic pollutants from aqueous
media by introducing highly charged polymeric metal hydroxide species. These
species neutralize the electrostatic charges on suspended solids and oil
droplets to facilitate agglomeration or coagulation and resultant separation
from the aqueous phase. The treatment prompts the precipitation of certain
metals and salts. [4]
2. MECHANISM
OF ELECTROCOAGULATION
The mechanism of EC is highly dependent on the
chemistry of the aqueous medium, especially conductivity and also on other
characteristics such as pH, particle size, and chemical
constituent concentrations. In the EC system, there are multiple
electrochemical reactions occurring simultaneously at the anodes and cathodes.
These mechanisms can be divided into the main mechanisms that cause
destabilisation of pollutants, and side reactions, such as hydrogen formation.
The most important reactions are summarised in Figure 1.
Figure 1. Schematic
representation of typical reactions during the EC treatment [2]
Electrodes
which produce coagulants into water are made from either iron or aluminium. In
addition, there can be inert electrodes, typically cathodes, which are sometimes
used as counter-electrodes in the system.
Aluminium
The
electrolytic dissolution of the aluminium anode produces the cationic monomeric
species such as Al3+ and
Al(OH)2+ at low pH, which at appropriate pH
values are transformed initially into Al(OH)3 and finally
polymerized to Aln(OH)3n according to the following reactions:
Al
→ Al3+ (aq) + 3e−
Al3+ (aq) + 3H2O → Al(OH)3 + 3H+ (aq)
n Al(OH)3
→ Aln(OH)3n
However,
depending on the pH of the aqueous medium other ionic species, such as Al(OH)2+, Al2(OH)2
4+ and Al(OH)4− may
also be present in the system. Under
appropriate conditions various forms of charged multimeric hydroxo Al3+ species
may be formed. These gelatinous charged hydroxo cationic complexes can
effectively remove pollutants by adsorption to produce charge neutralization,
and by enmeshment in a precipitate. Defluorination of water can be achieved using
aluminum electrodes.
Iron
Iron
upon oxidation in an electrolytic system produces iron hydroxide, Fe (OH)n, where
n
= 2 or 3. Two mechanisms
have been proposed for the production of Fe(OH)n.
• Mechanism 1
Anode:
4Fe(s) → 4Fe2+(aq) + 8e−
4Fe2+(aq) + 10 H2O(l) + O2(g) → 4Fe(OH)3(s) + 8H+(aq)
Cathode:
8H+(aq) + 8e− → 4H2(g)
Overall:
4Fe(s) + 10
H2O(l) + O2(g) → 4Fe(OH)3(s) + 4H2(g)
• Mechanism 2
Anode:
Fe(s) → Fe2+(aq) + 2e−
Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s)
Cathode:
2
H2O(l) + 2e− → H2(g) + 2OH−(aq)
Overall:
Fe(s) + 2
H2O(l) → Fe(OH)2(s) + H2(g)
The
Fe(OH)n(s) formed remains in the aqueous stream as a gelatinous
suspension, which can remove the pollutants from wastewater either by
complexation or by electrostatic attraction, followed by coagulation. Wastewater
containing chromium ions can be removed by the EC technique using iron as the
sacrificial anode. [3]
The
H2 produced as a result of the redox reaction may remove dissolved
organics or any suspended materials by flotation.
3. FACTORS
AFFECTING ELECTROCOAGULATION
There are various
parameters which have an effect on the efficiency of the EC in removing the
pollutants from water. The factors which are known to have an effect are:
•
Material of the
electrodes can be iron, aluminium
and/or inert material (typically cathodes). Iron and aluminium ions and
hydroxides have different chemistries and applications.
•
pH of the
solution has an effect on the
speciation of metal hydroxides in the solution and also on the zeta potential
of the colloidal particles. It also affects the dissolution of aluminium
cathodes.
•
Current density is proportional to the amount of electrochemical
reactions taking place on the electrode surface.
•
Treatment time or electric charge added per volume is proportional
to the amount of coagulants produced in the EC system and other reactions
taking place in the system.
•
Electrode
potential defines which reactions
occur on the electrode surface.
•
Concentration of
the pollutants affects the
removal efficiency because coagulation does not follow zeroth-order reaction
kinetics but rather pseudo second or first-order kinetics.
•
Concentration of
anions, such as sulphate or
fluoride, affects the composition of hydroxides because they can replace
hydroxide ions in the precipitates.
•
Temperature affects floc formation, reaction rates and
conductivity. Depending on the pollutant, increasing temperature can have a
negative or a positive effect on removal efficiency.
•
Other parameters,
such as hydrodynamic conditions and
inter-electrode distance, may have effect on efficiency of the treatment and
electricity consumption. [2]
4. APPLICATIONS OF ELECTROCOAGULATION
This
technology has been increasingly used in South America and Europe for treatment
of industrial wastewater containing metals. In North America, electrocoagulation
has been used primarily to treat wastewater from pulp and paper industries,
mining and metal-processing industries. In addition, it has been applied to
treat water containing foodstuff wastes, oil wastes, dyes, suspended particles,
chemical and mechanical polishing waste, and organic matter from landfill
leachates, defluorination of water, synthetic detergent effluent, mine wastes and
heavy metal-containing solution. [3]
The
application can be broadly divided as follows:
·
Removal of metal
ions and/or hydroxides from synthetic
solutions, groundwater or wastewaters. Typically high or complete removal could
be obtained when treatment parameters are optimised. Aluminium, iron and
combination electrodes can be used in EC system.
·
Removal of
organic material from wastewaters
or synthetic solutions. High removal (> 70%) is typically obtained with
optimum parameters. Aluminium, iron and combination electrodes can be used. In
general, iron electrodes give higher organic matter removal, whereas higher
colour removal is obtained with aluminium electrodes.
·
Purification of
surface waters from natural
organic matter, inorganic pollutants or microbes. Typically high removal of
pollutants (> 90%). Aluminium electrodes are more commonly used than iron
electrodes in these applications. [2]
5. ADVANTAGES
OF ELECTROCOAGULATION
The advantages
of electrocoagulation technology are given below:
1.
EC requires simple equipment and is easy to operate.
2.
Wastewater treated by EC gives palatable, clear, colourless and odourless
water.
3.
Sludge formed by EC tends to be readily settable and easy to de-water, because
it is composed of mainly metallic oxides/hydroxides. Above all, it is a low
sludge producing technique.
4.
Flocs formed by EC are similar to chemical floc, except that EC floc tends to
be much larger, contains less bound water, is acid-resistant and more stable,
and therefore, can be separated faster by filtration.
5.
EC produces effluent with less total dissolved solids (TDS) content as compared
with chemical treatments. If this water is reused, the low TDS level
contributes to a lower water recovery cost.
6.
The EC process has the advantage of removing the smallest colloidal particles,
because the applied electric field sets them in faster motion, thereby
facilitating the coagulation.
7.
The EC process avoids uses of chemicals, and so there is no problem of
neutralizing excess chemicals and no possibility of secondary pollution caused
by chemical substances added at high concentration as when chemical coagulation
of wastewater is used.
8.
The gas bubbles produced during electrolysis can carry the pollutant to the top
of the solution where it can be more easily concentrated, collected and
removed.
9.
The electrolytic processes in the EC cell are controlled electrically with no
moving parts, thus requiring less maintenance.
10.
The EC technique can be conveniently used in rural areas where electricity is
not vailable, since a solar panel attached to the unit may be sufficient to
carry out the process.[3]
6. DISADVANTAGES
OF ELECTROCOAGULATION
The disadvantages associated with electrocoagulation
technology are:
1.
The ‘sacrificial electrodes’ are dissolved into wastewater streams as a result
of oxidation, and need to be regularly replaced.
2.
The use of electricity may be expensive in many places.
3.
An impermeable oxide film may be formed on the cathode leading to loss of
efficiency of the EC unit.
4.
High conductivity of the wastewater suspension is required.
5.
Gelatinous hydroxide may tend to solubilize in some cases. [3]
7. COMPARISON
OF CHEMICAL COAGULATION AND ELECTROCOAGULATION
|
CHEMICAL COAGULATION
|
ELECTROCOAGULATION
|
|
Chemicals
are used
|
No
addition of chemicals
|
|
Colour
removal not possible for all type of colours
|
Crystal
clear water can be obtained irrespective of the colour
|
|
No
BOD and COD reduction
|
BOD
and COD reduced by more than 50 %
|
|
High
volumes of sludge is produced per day
|
Very
less sludge volume
|
|
Hardness
is increased
|
Hardness
remains same or decreased
|
|
Scaling
of pipes and channels due to lime
|
No
scaling
|
|
Labour
intensive
|
Easy
to operate
|
8. CASE
STUDY
The dairy industry is
generally considered to be the largest source of food processing wastewater in
many countries. Water is used throughout all steps of the dairy industry, including
cleaning, sanitization, heating, cooling, and floor
washing; naturally, the industry’s need for water is huge. In general, wastes
from the dairy processing industry contain a high concentration of organic
material such as proteins, carbohydrates and lipids, high BOD5 and
COD, and high concentrations of suspended solids and suspended oil grease. The electrocoagulation
process using aluminium electrodes is a reliable technique for removal of
pollutants from dairy wastewaters. [1]
Dairy wastewaters are
generally treated usually using biological methods such as activated sludge
process, aerated lagoons, aerobic bioreactor, trickling filters, sequencing batch reactor (SBR), up flow anaerobic sludge blanket (UASB) reactor, up flow
anaerobic filters, and bio
coagulation. Aerobic biological processes are high energy intensive, whereas
anaerobic treatment of dairy wastewater rejects
very poor nutrient removal, and effluents treated by anaerobic biological
processes need additional treatment the other hand, the physical/chemical
methods that have been proven to be successful are coagulation/flocculation. This case study discusses the wastewater
taken from the local dairy factory in Iran with 25000 (mean value) kg milk per day
processing capacity. The setup is shown in figure 2. [1]
Figure 2. Experimental set up of the EC unit [1]
In each run, wastewater
(supernatant) after 12 h settling time was poured into the electrocoagulation
cell. The experiments were performed in a bipolar batch reactor as shown in
Figure 2, with six aluminium electrode connected in parallel. Only the outer
electrodes were connected to the power source, and anodic and cathodic
reactions occurred on each surface of the inner electrode when the current
passed through the electrodes. The temperature of each system was maintained at
25 ± 2∘C.
During the runs, the reactor unit was stirred at 150 rpm by a magnetic stirrer
to allow the chemical precipitate to grow large enough for removal. In this
study, the raw dairy wastewater was allowed to settle in a preliminary settling
tank before the electrocoagulation process. The parameters considered in the
study were COD, BOD5, TSS, TC and FC. Electrocoagulation is efficient and able to
achieve 98.84% COD removal, 97.95% BOD5 removal, 97.75% TSS removal,
and >99.9% bacterial indicators at 60 V during 60 min. The values of the
parameters before and after the treatment are shown in table 2. [1]
Table 2. Influence
of electrocoagulation process using aluminium electrodes on dairy wastewater
quality parameters [1]
|
Parameters
|
Raw waste water
|
12h settled waste water
|
Treated effluent after EC at 60 V
|
Permissible levels ( Iran standard)
|
|
COD (mg/L)
|
7855
|
6114
|
70.92
|
60
|
|
BOD5 (mg/L)
|
3486
|
2919
|
43.45
|
30
|
|
TSS (mg/L)
|
1724
|
734
|
16.52
|
60
|
|
TC(MPN/100mL)
|
4.39 x 10^6
|
3.53 x 10^6
|
361
|
1000
|
|
FC(MPN/100mL)
|
3.27 x 10^6
|
2.75 x 10^6
|
28
|
400
|
REFERENCES
1. Edris
Bazrafshan, Hossein Moein, et.al, 2013, Application of electrocoagulation process
for dairy wastewater treatment,
Journal of Chemistry
2. Mikko
Vepsäläinen, 2012, Electrocoagulation in the treatment of industrial waters and
wastewaters
3. M.
Yousuf A. Mollah, Robert Schennach, et. al, 2001, Electrocoagulation (EC)-
science and applications, Journal of Hazardous Materials 29–41
4.
Dr
N. N. Mahapatra, Electro-Coagulation Process for the
Waste Water Treatment
5.
Rahmani AR., 2008, Removal
of water turbidity by the electrocoagulation method, Journal of research in
health sciences
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