CSE30337 Water and Waste Management
Lecture 4 – Coagulation & Flocculation
Instructor: Ben S.-Y. Leu / Waqas Qamar Zaman
Email: syleu@polyu.edu.hk
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Outline
• Definition and Importance of coagulation and flocculation processes.
• Mechanism in coagulation and flocculation processes.
• Surface properties of colloids.
• Methods of destabilization in coagulation.
• Mechanism leading to Restabilization of colloidal particles.
• Determination of optimized coagulant dosage.
• Types of coagulants
• Equipment used for coagulation and flocculation.
CSE30337 Water and Waste Management
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Coagulation/Flocculation
• Coagulation/flocculation
• goal: removal of turbidity, organic
matter, microbes
• chemicals (coagulants)
added to “destabilize” particles
• destabilized particles aggregate Loading…
• aggregated particles (flocs) settle
faster
Particle size ( m)# 0.08 1 5
Settling Velocity (cm/s) 1 x 10-6 9 x 10-5 2.2 x 10-3
Time to settle 3 m (days) 3472 38.6 0.0158
# > 1 mm: suspended particles; 0.001–1 mm: colloidal particles
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Coagulation/Flocculation
• Coagulation: chemical agent rapidly mixed to destabilize colloids
• Flocculation: slowly stirred to promote particle collisions and aggregate
formation
Principle
When certain chemicals are added to raw water containing slowly
settling/non-settleable particles, the smaller particles begin to form
lager or heavier floc can be removed by sedimentation.
I am Coagulants We are bigger
settling
faster
CSE30337 Water and Waste Management
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Coagulation/Flocculation
Relationship between Particle
Size and Settling Time
Particle Particle Type Settling
Dia Time
mm (for 1m)
10 Gravel 1 second Loading…
1.0 Sand 10 seconds
0.1 Fine Sand 2 minutes
0.01 Clay 2 hours
0.001 Bacteria 8 days
0.0001 Colloidal 2 years
Particles
0.00001 Colloidal 20 years
Particles
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Coagulation/Flocculation
Colloids
Particle diameter ( ) < 1 m Particle ( ) Number of Surface
cannot be removed from suspension by ordinary physical (mm) Particles Area:mass
separation. mm2/g
Brownian motion hinders their settlement under the effect 250 mm 1 0.00375 cm2
of gravity.
1 mm (250)3 1.00 cm2
Contribute large parts of pollution and specific cause of
1 x 10-3 mm (250 x 1000)3 1000 cm2
turbidity (higher ratio of surface area to mass).
• Surface properties ( potential and electrical charges) becomes more important than sedimentation under
gravity.
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Surface Properties of Colloids
Adsorption: Tendency to concentrate substance from the surrounding (higher
surface area/mass ratio). • colloid size (0.001-10 µm)
• negative surface charge
Application in water treatment: activated carbon is used as an absorbent to
remove taste and odor problems (due to unwanted colloidal matters) • electrical double layer
Electrokinetic Properties: Tendency to develop charge in relation to the
surrounding medium. Colloidal particles are normally charged.
Electrical charge = f (colloidal material & surrounding medium)
q= Charge per unit area (or the charge difference between the particle and the
body of the solution)
d = Thickness of the layer around the particle.
D = Dielectric constant of the liquid medium.
CSE30337 Water and Waste Management
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Surface Properties of Colloids
Zeta Potential:
A measure of both the charge on a colloidal particle & the distance into
the solution to which the effects of the charge extends.
One part of the double layer is fixed to the walls of the particle
The other is made of diffuse cloud of ions.
CSE30337 Water and Waste Management
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Surface Zeta Potential
Properties of Colloids
(Zeta potential) = The difference in the potential between the
surface separating fixed and moving parts of the liquid.
is measured using a zetameter, when the mobility of
colloidal particles across a cell, as viewed through a
microscope.
Colloidal silica minerals and most proteins are negatively
Interaction forces:
charged • electrostatic repulsion
• van der Waals attraction
Hydrous oxides of iron and aluminum are usually positively
• van der Waals force correlates
charged • positively to particle size
• negatively to square of separation distance
Colloidal Silica minerals, Hydrous oxides of Fe, Al
proteins
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PropertiesofofColloids
Surface Properties Colloids
Colloidal particles are normally charged with respect to the
surrounding medium
Hydrophobic: (e.g. Clays and Metal Oxides)
WATER HATING Colloids possess no affinity for the liquid medium.
They are readily susceptible to coagulation. Majority of inorganic colloids.
Hydrophilic: (Proteins)
WATER LOVING Exhibit a marked affinity for water. Absorb water and retards
flocculation and frequently needs special treatment to achieve effective coagulation.
Majority of organic colloids.
When colloidal particles come into contact, they join to form
large particles, flocculates and precipitates
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CoagulationProperties of Colloids
and Destabilization
Reduction of surface potential by adsorption and charge
neutralization.
Enmeshment of colloids in a precipitate. Adsorption and
Interparticle bridging.
1. Charge Neutralization:
Loading…
Adsorption of a species carrying a charge opposite to that
of colloids.
Reduction of surface potential & destabilization of colloids.
Increase in dosage of coagulant with increase in concentration of
colloids.
Overdose of coagulant cause restabilization. Optimum
coagulation when 0 mV.
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CoagulationProperties of Colloids
and Destabilization
How can we reduce the electrostatic repulsion?
2) Addition of salts (increase of ionic strength)
diffuse layer thickness (Na+ < Ca2+ < Al3+)
electrostatic repulsion
Explaining a natural phenomenon:
• River water usually has high turbidity, especially after heavy rain. A
massive amount of colloidal particles are stable in the river water
and transport a long distance along a high-flow river.
• However, when the river water is discharged into the ocean, a fairly
sharp line is observed. Please explain with the aid of your own
sketch.
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Properties of Colloids
Restabilization
3) Precipitate enmeshment (sweep floc)
Reaction 3: Secondary adsorption
hydrolysis of inorganic coagulant forms solid precipitate
Al3+ + 3H2O Al(OH)3↓ + 3H+
amorphous, gelatinous flocs entrap colloids and settle by gravity
Alum (hydrated aluminium sulphate) Destabilized particle Restabilized particle
Al2(SO4)3·14H2O + 6HCO3- → 2Al(OH)3(s) ↓ + 6CO2 + 14H2O If the extended segment fails to contact another
+ 3SO42- particle it may fold back and attach to other side on
optimum pH range: 5 – 7 the original surface restabilizing the particle.
Reaction 1: Adsorption Reaction 4: Initial adsorption excess polymer dosage
Polymer Particle Destabilized particle
Particle Stable particle
Excess Polymers
Reaction 2: Floc formation (no vacant site)
Reaction 5: Rupture of floc
Intense or prolonged agitation
Destabilized particle Floc particle
CSE30337 Water and Waste Management Rupture of particles if mixing is too
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high
Properties
Optimized of Colloids
Coagulation
• Jar Test
• Determine colour, turbidity, pH and alkalinity of raw water
• Measure 1 L proportion of sample into each of 6 jars of beakers
• Placed them in the stirring equipment
• Add different dosage of coagulant and coagulant aids to each jar
• Rapid mix for 1 min (100 rpm), flocculate for 20 min (30 rpm)
• Allow the flocs to settle for 30 min to 1 h
• Determine the colour, turbidity, pH and alkalinity of the supernatant
• Repeat the above with other coagulants or coagulant dosage
• Determine optimum dose for colour & turbidity removal
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Properties of Colloids
Coagulants
The most frequently coagulants: Fe(III) and Al(III)
Aluminum sulfate (Alum) Al2(SO4)3.18H2O
Ferric chloride FeCl3
➢ Mainly for economical reasons
➢ Highly pH dependent
Factors Affecting
Coagulation
➢ Turbidity
➢ Suspended solids
➢ Temperature
➢ pH
➢ Cationic & anionic
composition
➢ Agitation
➢ Coagulant dose
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➢ Coagulant aid ‹#›
Process Equipment
Coagulation in two stages:
Static
● Destabilization of Colloids. Mixers
● Particle transportation to promote collision.
● Objectives
● Neutralization/destabilization of colloids.
● The reagent has to be diffused as rapidly/quickly as possible From the plain jet, turbulence & energy
dissipation occur a large volume, with
throughout the volumes of the water. little back mixing
● The volume of reagent solution added is very small in comparison
with the main flow of the water.
Suitably placed baffles can intensify
energy disspation with some
● The Essential Features of a Good Mixing Device are : backmixing, improving mixing efficiency
● Adequate turbulence (by mixing flow direction).
● Adequate backmixing (by mixing in the direction of the flow). Lack of moving parts High head loss
Economical Very low back
● Rapid dispersion of chemicals. operationally mixing
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Low Efficiency ‹#›
ProcessPneumatic
EquipmentMixers
Pneumatic
● Mixers
Compressed air is used as a mixing device
● Introduced through submerged diffusers and causes turbulence as it rises to the surface
● Degree of turbulence/mixing is controlled by placement and type of diffusers and by the applied pressures
Coagulant
Air
imlet
Air diffuser
Outlet
Problems:
Often the flocs can get attached to the tiny air bubbles that can upset the subsequent sedimentation processes
(because bulk density of a floc particle containing air could be lesser than the density of water)
CSE30337 Water and Waste Management
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ProcessPneumatic
EquipmentMixers
Mechanical
Mixers
Turbulence is induced through high-speed rotating
impellers such as propellers, turbines and paddles.
Energy intensive process, with high O & M cost.
Batch/Continuous.
Contact Time 1 - 2 minutes/30 - 60 seconds
Mean velocity gradient G = 100 to 1000 s-1
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Coagulation/Flocculation
CSE30337 Water and Waste Management
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Summary
• Coagulation and flocculation processes are crucial in water and wastewater treatment.
• Coagulation destabilizes colloidal particles by adding coagulants, leading to flocs
formation.
• Surface properties of colloids significantly influence coagulation and flocculation
processes.
• Destabilization of colloidal particles is achieved through charge neutralization.
• However, colloidal particles can undergo restabilization due to factors such as pH changes
or the presence of dissolved organic matter.
• Optimizing coagulant dosage is essential for effective coagulation and flocculation,
determining the minimum required dosage for desired treatment goals.
• Common coagulants include metal salts of aluminum and iron.
•CSE30337
Equipment used
Water and Waste includes rapid mixers, flocculators, and settling tanks.
Management
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Questions?
