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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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