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Coagulation and
Flocculation of Water
By
Muhammad Saqib Nawaz
PhD Scholar IESE
Research Associate IESE
TVF NICE
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Conventional Surface Water
Treatment
Screening
Coagulation
Flocculation
Sedimentation
Filtration
Disinfection
Storage
Distribution
Raw water
AlumPolymers
Cl2
sludge
sludge
sludge
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Colloids?
Many of the contaminants in water and wastewater
contain matter in the colloidal form. These colloids
result in a stable suspension. In general the
suspension is stable enough so that gravity forces will
not cause precipitation of these colloidal particles. So
they need special treatment to remove them from the
aqueous phase.
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Particles which are just big enough to have a surface
which is microscopically observable or which is capable
of adsorption of another phase.
Size (arbitrary): 0.001 to 1 micron.
Surface area: ~ 1 sq yd/g of solids.
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5070 % of the organic matter in domestic wastewater is
composed of colloidal matter.
In water treatment color, turbidity, viruses, bacteria, algae
and organic matter are primarily either in the colloidal form
or behave as colloids.
Colloid Characteristics
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For colloids surface properties dominate gravity forces.
These surface properties prevent the colloids from
coming together (coagulation) to become heavy enough
to gravity settle.
Due to large surface area they adsorb substances. As anexample it would take a 1 micron colloid 1 year to
settle (by gravity) a distance of 1 foot.
These colloids are in general too small to be filtered by
standard filtration devices. Colloids will not settle or
filter until they agglomerate to a larger size.
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Colloids Classification
1. Hydrophobic ( water-hating):
These colloids are normally negatively charged and
dispersion is stabilized by electrostatic repulsion.
2. Hydrophilic ( water-loving):
This colloids have a great affinity for the solvent (usually
water in our case). The colloids usually possess a slight
charge (negative), but dispersion is stabilized by
hydration (attraction for particles of water).In water and wastewater we deal primarily with
hydrophobic colloids
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Class activity:Calculate the volume and surface area of
2nd and 3rd figure.
1 m
1 m
1 m
1 m
0.5 m
1 m
0.25 m
V = 1 m3 & A = 6 m2
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Coagulation & Flocculation
Coagulation is the addition and rapid
mixing of a coagulant, to destabilize the
colloidal particles.
Flocculation is the slow stirring or the
gentle agitation to aggregate the
destabilized particles and form a rapid
settling floc.
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ELECTRICAL DOUBLE LAYER
(EDL) The primary charge on collide attracts counter
ions (opposite charged ions) by electrostatic
attraction. This compact layer of counter ions are called
fixed layer, outside fixed layer is diffused layer.
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Zeta Potential
Zeta potential = Resultant of vander wallsattractive forces and Electrostatic repulsive
forces.
More Zeta potential, more is the repulsive
forces between colloidal particles and more
stable is the colloidal solution.
Z= 4qd / D
Where q= Charge/unit area
d= thickness of the layer surrounding shear surface through which the
charge is effective
D = dielectric constant of the liquid
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1 m
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Coagulation Tricks with Colloids:
1. Reduction of Zeta potential.
2. Aggregation of particles by
interparticulate bridging
3. Enmeshment of particles in the
precipitate floc formed (Sweep
coagulation).
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What Coagulant do?
When coagulant is added to water, it dissociates andmetallic ions undergo hydrolysis and produce hydroxo-
metallic ion complexes.
Al2 (SO4)3 Al6 (OH) 15+3 +Al7
(OH) 17+4 +Al8 (OH) 20
+4
These are highly positive charged and adsorb to the
surface of the negative colloid particles, reduce Zeta
potential to a point where colloids can be destabilized.
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Optimum Coagulant Dose?
Jar test studies.
Different concentrations are applied on
the same water and resulting decrease in
the turbidity is measured.
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Speed of Coagulation
Dilute suspension = Extremely slowcoagulation
Recycling a small portion of settled sludge
before or after rapid mixing maintainsdesired colloidal concentration.
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Polymer Coagulation
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Factors for choosing a coagulant?
1. easily available in all dry and liquid forms
2. economical
3. effective over wide range of pH
4. produces less sludges
5. less harmful for environment
6. fast or quick
7. largest surface area for polymer
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Designing factors for a coagulation
basin
Book.
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Thanks...
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