Electrochemical Wastewater Treatment Processes

1. SUBJECT: ADVANCED
WASTEWATER TREATMENT
TECHNOLOGIES
Electrochemical
Wastewater Treatment
Processes

2. Electrochemical treatments
• Advanced wastewater treatment technologies, which
include the use of electricity
• The first water treatment using electricity was carried
out in a plant built in 1889 in the UK where sewage
treatment had been conducted by mixing the domestic
wastewater with sea wateR
• The first use of electricity in wastewater treatment in
the USA started in the late 1900s as
• The capital investment and the electricity costs
necessary for the application of this new technology
were so high that they were not widely used in that
period
• electrochemical techniques were difficult to control
which made it difficult to obtain reliable results

3. • nowadays the costs of electrochemical treatments are
comparable to the costs of other wastewater treatment
technologies.
• It should be noted that in some cases electrochemical
treatment is more efficient than other conventional
technologies.
• The process does not require additional consumption of
chemicals and only electrons are added to the processes
to stimulate reactions.
• Electrochemical processes include:
• electro-coagulation,
• electro flotation,
• electro oxidation,
• electro-disinfection,
• electroreduction etc.

4. Structure of colloids
• In wastewater, colloids generally have a negative
charge and are stable.
• The charged colloidal particles affect the ions in the
surrounding media causing oppositely charged ions to
be attracted towards the surface of the particle, and
the ions of the same charge to be repelled from the
surface of the particle.
• This separation of charges on the particle surface,
results in formation of an electrical double layer

5. What is coagulation?

6. Chemical Coagulation
• Conventional Chemical Coagulation consists of the
direct dosing of a coagulant solution to the
wastewater.
• Flocculation is a physical treatment in which the
collision of coagulated colloids is promoted in order to
make possible the formation of larger particles.
• The result of both processes is a wastewater in which
the size of the particles is enough to be separated by a
settler or a flotation unit.

7. CHEMICAL COAGULATION

8. ELECTROCOAGULATION (EC)
• The process is applied when removal of pollutant
by chemical coagulation becomes difficult or
impossible.
• An alternative to the direct use of a solution
containing the coagulant salts, is the in situ
generation of coagulants by electrolytic
oxidation of an appropriate anode material
(e.g.iron or aluminium). This process is called
electrocoagulation or electrochemically assisted
coagulation.
• Electrochemical processes involved:
 Electrodissolution
 Electrolytic generation of oxygen and hydrogen

9. Unstabilized
small
particles
flocculation
Aggregated
particles

12. • Electrocoagulation process involves applying an
electric current to sacrificial electrodes (mostly iron
and aluminum) inside a reactor tank where the current
generates a coagulating agent and gas bubbles.
• In EC, differences in electric potentials are used to
generate coagulants which can remove color,
suspended and dissolved particles in wastewater
• This technique is an indirect electrochemical method
which produces coagulant agents (Fe+3or Al+3 ) from
the electrode material (Fe or Al) in hydroxide medium.
• These species, that is, Fe (OH) 3, can remove
dissolved pollutants by precipitation or by floatation
• These complex compounds are attached to the bubbles
of H2 (gas) evolved at the cathode and transported to
the top of solution.

14. Principles of Electrocoagulation
• The electrocoagulation technique uses an electrochemical cell to treat the
water.
• In the simplest form, an electrochemical cell consists of two electrodes, the
anode and the cathode, immersed in a conducting solution or the electrolyte
and connected together via an electrical circuit which includes a current
source and control device, as shown in Figure
• The metallic cations, generated from the anode, hydrolyze to form
hydroxides, poly hydroxides and poly hydroxyl metallic compounds with a
strong affinity for dispersed particles and counter ions, thus causing
coagulation.
• Indeed, they can reduce the net surface charge of colloidal particles that are
in suspension due to the reduction of the repulsive potential of the electrical
double layer.
• As a result, the repulsive forces between the colloidal particles decrease
and this brings the particles sufficiently close so that the van der Waals
forces predominate and agglomeration of the particles occurs. It is worth
noting that in electrocoagulation the processes of coagulation and
flocculation occur simultaneously and it is not possible to distinguish

15. • Figure .Schematic diagram of an electrocoagulation cell with two electrodes

16. Theory of Electrocoagulation
• Electrocoagulation (EC) is a complicated process involving many
chemical and physical phenomena that use consumable electrodes to supply
ions into the wastewater stream.
• In an EC process the coagulating ions are produced „in situ‟ it involves the
following successive stages
1- Formation of coagulants by electrolytic oxidation of sacrificial electrode.
2- Formation of OH ions and H2 at the cathode
3- Electrolytic reactions at the electrode surfaces
4- Destabilization of contaminants, particulate suspension and breaking of
emulsions
5- Aggregation of destabilized phases to form flocs
6- Removal of colloids by sedimentation or flotation
• The solid oxides, hydroxides and oxhydroxides provide active surfaces for
the adsorption of the polluting species. Electrocoagulation has been
successfully employed in removing metals, suspended particles, clay
minerals, organic dyes, and oil and greases from a variety of industrial
effluents.
• In this process, a potential is applied to the metal anodes, typically

17. Destabilization mechanism in EC process plays an important
role;
• 1. Compression of the diffuse double-layer around the
charged species, which is achieved by the interactions
of ions generated by dissolution of the sacrificial
electrode, due to passage of current through the
solution.
• 2. Charge neutralization of the ionic species present in
wastewater, which is caused by the counter ions,
produced by the electrochemical dissolution of the
sacrificial electrode. These counter ions reduce the
electrostatic inter-particle repulsion sufficiently so that
the van der Waals attraction predominates, thus causing
coagulation. A zero net charge results in the process.
• 3. Floc formation, the floc formed as a result of
coagulation creates a sludge blanket that entraps and

18. Mechanisms of Electrocoagulation Process
• The chemical processes occurring in the cell are oxidation and reduction reactions,
which take place at the electrode/electrolyte interface.
• The electrode at which reduction occurs is referred as the cathode, whereas the
anode is the electrode at which oxidation processes occur.
• The anode, also called the sacrificial electrode, corrodes to release active coagulant
cations, usually aluminum or iron, to the solution.
• Consequently, electrocoagulation introduces metal cationsin situ, rather than by
external dosing.
• Simultaneously, electrolytic gases are generated, typically hydrogen at the cathode.
• The current flow in the electrocoagulation cell is maintained by the flow of
electrons resulting from the driving force of the electrical source.
• The solution electrolyte allows the current to flow by the motion of its ionic
charged species.
• High conductivity is an advantage for the process, since it reduces the electrical
resistance of the solution and the electrical consumption
• Electrocoagulation process contains major reactions:
• 1. Electrolytic reactions at the electrode
• 2. Formation of coagulants
• 3. Adsorption of pollutants

19. REACTIONS AT THE ELECTRODES
• A simple electrocoagulation reactor is made up of one anode
and one cathode
• When a potential is applied from an external power source,
the anode material undergoes oxidation, while the cathode
will be subjected to reduction or reductive deposition of
elemental metals.
• The main plate electrodes that are commonly used for
electrocoagulation process are iron and aluminum, science
these two materials have been extensively used to clarify
wastewater .
• The type of coagulant produced was determined according to
the electrode materials used.
• This coagulant effects on the coagulation and the efficiency
processes
• The main reactions take place at the metal electrodes

20. Iron Electrode
Anode: Fe → Fe2+ +2e (1)
Cathode: 2H2O + 2e → H2 +2OH − (2)
Overall: Fe 2+ + 2H2O → H2 + Fe( OH)2 (3) OR
• Anode: Fe → Fe3+ +3e (4)
• Cathode: 3H2O + 3e → 3 2 H2 + 3OH −(5)
• Overall: Fe 3+ + 3H2O → 3 2 H2 + Fe( OH)3 (6)
• Aluminum Electrode
Anode: Al → Al3+ + 3e (7)
Cathode: 3H2O + 3e → 3 2 H2 + 3OH −(8)
Overall: Al 3+ + 3H2O → 3 2 H2 +Al( OH ) 3 (9)
In general:
Anode: M → Mn + + ne (10)
Cathode ∶ 2H2O + 2e → H2 +2OH(11)
Overall ∶ Mn + + nH2O → nH + + M(OH)n (12)

21. PARAMETERS AFFECTED ON
ELECTROCOAGULATION
Effect of pH:
• IT has an effect on the speciation of metal hydroxides in the solution.
During electro coagulation the pH of the solution increases due to the
contribution of OH- ions into the solution and it enhances the efficiency of
the system.
• Initial pH between 7 to 9 gives better results and if the pH raises beyond 9
the efficiency decreases due to the formation of soluble Fe(OH)4 and
Al(OH)4 .
• Only insoluble metal hydroxides of iron can remove pollutants by
electrostatic attraction.
• The kinetics of conversion of Fe2+ to Fe3+ is strongly affected by pH.
Effect of Current Density
• It is proportional to the amount of electrochemical reactions taking place on
the electrode surface.
• When current density increases the reaction rate also increases by the more
metal ions in solution.
• But the rapid contribution of metal ions in the system cost more and further

22. Effect of Electrode Material
• Material of the electrodes can be iron, aluminum and/or inert
material (cathodes).
• Optimal material selection depends on the pollutants to be removed
and the chemical properties of the electrolyte.
• In general, aluminum seems to be superior compared to iron in most
cases, when only the efficiency of the treatment is considered.
• aluminum is more expensive compared to iron.
• Inert electrodes, such as metal oxide coated titanium, are used as
cathodes in some constructions.
• When water has significant amounts of calcium or magnesium ions,
the inert cathode material is recommended.
• For COD & phenol removal iron is good and for color and turbidity
removal aluminum give better results.
• If we are using electro-chemically inert materials like stainless steel
as cathode, we will get protection from corrosion. Besides SS
produce smaller bubbles which possess larger surface area and can
remove more impurities through flotation.

23. Effect of EC Time:
• Treatment time added per volume is proportional to the
amount of coagulants produced in EC system and other
reactions taking place in the system.
• Efficiency increases with increase in electrolysis time. But
if we increase the electrolysis time beyond some extend the
removal efficiency decreases because of the free metal ions
in the solution.
Effect of Operating Temperature:
• Temperature affects floc formation, reaction rates and
conductivity.
• Depending on the pollutant, the increasing temperature can
have a negative or a positive effect on removal efficiency.
• Normally better results are obtained at low temperature and
higher temperature results in dissolution of metal ions into
the solution.

24. The Effect of Inter Electrode Distance
• Inter electrode distance have great importance in EC.
• Very less electrode distance may cause short circuit and very high
distance results in lesser contribution of metal ions into the solution
which minimize the efficiency of the system.
• So it is better to keep this in medium according to the characteristics
of wastewater.
Effect of Electrolyte (NaCl) Concentration
• It is important to investigate the effect of electrolyte concentration
since actual wastewater usually contains certain amount of salts as
the electrolyte concentration increased, the removal efficiency
increased due to the increment of the electrical conductivity
reaching the maximum value. However, with the increase in NaCl
concentration, the removal efficiency decreased

25. Electrical Energy and Electrode
Consumption.
• Electrical energy consumption is a very
important economical parameter in the
electrocoagulation process
• The variation of electrical energy consumption
increased proportionally with contact time
Effect of Applied Voltage
• In all electrochemical processes applied
voltage is the most important parameter for
controlling the reaction rate within the

26. ADVANTAGES AND DISADVANTAGES OF EC
PROCESS
Advantages
1. Ec process is easy in operate
2. Ec process needs a simple equipment
3. It is no need chemicals
4. Ec process requires less maintenance
5. The process requires less civil works and other constructions
6. Sludge formed by EC process is readily settable and easy
de-watered
7. Flocs formed by EC process tends to be much larger
8. The gas bubbles generated at the time of electrolysis can be
separated easily
9. An alternative renewable sources energy such as solar and wind
energy can be used instead of electrical source.
10. EC process is used to removes the smallest colloidal particles
11. Ec process give clear , colorless and odorless water

27. Disadvantages
1. The sacrificial anodes need to be replaced
periodically
2. The use of electricity may be expensive in some
cases
3. High conductivity of the wastewater suspension is
required
4. The high cost of electricity can result in an
increase in operational
5. The hydrogen gas produced could be explosive
unless it is collected safely.

28. Electro-flotation

29. Flotation
• In wastewater treatment flotation is used for removing
suspended solid matter (colloids and flocs), oil, fats and
greases from emulsions in water.
• The removal is achieved by dissolving air or
introducing gas into the wastewater, under pressure.
• The released air forms the bubbles which adhere to the
suspended particles and float to the surface of the
wastewater.
• Then, the floated particles can be easily removed by the
skimming device or through overflow in a suitable
container.
• Flotation can separate the small solid particles
according to their varying capacities to float.
• At the water surface a layer of foam will be created
consisting of gas bubbles and the floated particles or oil
drops that can be removed by skimming.

30. • The flotation process consists of four basic steps:
(i) Introduction of gas into the wastewater and promotion of its bubbling,
(ii) Contact between gas bubbles and suspended particles, or oil drops,
(iii) Gas bubble adsorption on the surface of particles, or oil drops,
(iv) Raise of gas bubbles, along with the floc or oil drops to the surface
• The rate of flotation is a function of several parameters such as: the surface
tension among the water particles or oil drops and gas bubbles, the size of
the particles or oil drops, the diameter of gas bubbles, the temperature, pH
and the particles or oil drops distributions.
• flotation methods
• the conventional process involves blowing air through nozzles at the
bottom of the flotation tank using a compressor.
• The efficiency of the flotation technologies depends on the size of the
bubbles. The small bubbles have a large surface area per unit volume of gas
and provide a larger surface area for particles or drops to attach.
• dissolved air flotation, which gives a better bubble distribution in the
water. The air is injected into the water under pressure. The air is released
until the water is supersaturated with air
• Vacuum flotation is a method that uses vacuum to release air bubbles from
water that is saturated with air at atmospheric pressure

31. Electro-flotation
• Electroflotation refers to the process of electrolytically
forming bubbles to float pollutants to the water surfaces.
• It replaces bubbles generated from dissolve air.
• In the EC technology only hydrogen bubbles are produced at
the cathodes, whereas in the EF technology the hydrogen
bubbles are produced at the cathodes and oxygen bubbles at
the anodes.
• The oxygen gas will be generated at the anodes only at
higher current density.
• At the cathode there is also evolution of hydroxyl ions that is
an additional advantage for maintaining the pH of the
electrolytes.
• The current density can be easily regulated by controlling
the applied voltage between the anode and cathode, and the
surface area of the electrodes.
• There are three basic steps in the development of gas
bubbles; nucleation, growth and detachment from the

32. • the electrode surfaces are not ideally flat and the micro-roughness
of these surfaces have various energy-favorable places, like pits
and scratches. Because pits and scratches have a higher current
density than the rest of the electrode surface, nucleation begins in
these places.
• Growth is initiated by two mechanisms; super-saturation and
coalescence of gas in the electrolyte where expansion is provoked
by a high internal pressure and transport of dissolved gas through
the gas/liquid interference
• Coalescence occurs when two or more gas bubbles touch each
other and coalesce into a single gas bubble
• detachment of the gas bubbles from the electrode surface which
depends on the size of the bubbles and the angle of bubble contact.
• This step can be accelerated with the addition of surface active
substances in the electrolyte, in order to reduce the surface tension
between the electrolyte, the electrode surface, and the gas bubbles
• During electro-flotation with electrically soluble anodes made from
Al, as in the case of electro-flocculation/electro-flotation, on the Al
electrode, the following reactions

33. Factors affecting electro-flotation
• Electro-flotation is a continuous process and the gas
bubbles are generated as the water flows between two
opposite electrically charged electrodes.
• The three main factors which affect the efficiency of
electro-flotation are: pollutant removal, electrode
material consumption, and power consumption.
• The electrode material, current density and pH affect
the bubble size and consequently the efficiency of
electro-flotation.

34. Current density. Current density is an operational
parameter that can be controlled continuously.
• This parameter directly determines the bubbles’
generation rate, solution-mixing and mass transfer at
the electrodes.
Electrode material:
• EF electrodes are usually made from Al, Fe and
stainless steel as a one unit process electroflotation
• smallest gas bubbles are produced at the stainless steel
electrodes
pH of the solution strongly influences the size of the
formed gas bubbles. The smallest hydrogen bubbles are
produced at neutral or slightly acidic conditions
• hydrogen gas bubble size increases with increase in
temperature and the smallest ones were formed at pH 7.
• size of the oxygen bubbles increased with increase in
pH