1. Coagulation and Flocculation

2. Why coagulation and Flocculation is needed?
Various sizes of particles in raw water
Particle diameter Type Settling velocity mm
10 Pebble m/s
1 Course sand m/s
Fine sand 0.6 m/min
Silt m/d
(10 micron) Large colloids 0.3 m/yr
(1 nano) Small colloids 3 m/million yr
G r a v I t y s e t t l I n g
Colloids - Impart color and turbidity to water – Aesthetically not acceptable
Colloids – so small- gravity settling not possible

3. Colloids
Tyndall effect: ability of a Colloid to scatter light. The beam of light can be seen through the colloid.

4. Characteristics
Solids dispersed in liquid, which will not settle by the force of gravity. When a solid colloid stays in suspension and does not settle in the system is in a stable condition
Large specific surface area: large surface area per unit volume of the particles. It makes colloids can adsorb materials such as water molecular and ions. It opposes electrostatic comparing to water.

5. Colloids 3. Hydrophilic and Hydrophobic Colloids
Focus on colloids in water.
“Water loving” colloids: hydrophilic.
“Water hating” colloids: hydrophobic.
Molecules arrange themselves so that hydrophobic portions are oriented towards each other.
If a large hydrophobic macromolecule (giant molecule) needs to exist in water (e.g. in a cell), hydrophobic molecules embed themselves into the macromolecule leaving the hydrophilic ends to interact with water.

6. Colloids
Hydrophilic and Hydrophobic Colloids

7. Hydrophilic Colloids
Colloidal solid surface has water-soluble group, such as amino , carboxyl, sulfonic , hydroxyl. These water-soluble groups can promote water of hydration of colloidal particles.
Hydrophilic colloids are general organic colloids, such as proteins and their degradation products.
Hydrophobic Colloids
Hydrophobic : There is not obvious water of hydration. Hydrophilic colloids are general inorganic colloids, such as clay.

8. - Colloid Stability Electrostatic force
H2O
Colloid
Colloids have a net negative surface charge
Electrostatic force prevents them from agglomeration
Brownian motion keeps the colloids in suspension -
Repulsion
Colloid - A
Colloid - B
Impossible to remove colloids by gravity
settling
Electrostatic force
The ionization of surface group.
The adsorption of ions from the surrounding solution.
The ion deficit within the mineral lattice.  for colloidal mineral

9. ζ, colloidal particles repulsion , colloidal particles more stable
The stability of colloidal particles in water is dependence on the electrostatic force and van der waals force.
electrostatic force and van der waals force can be represented by Zeta potential :
q = charge per unit area
d = thickness of the layer surrounding the shear surface through which the charge is effective
D = dielectric constant of the liquid
ζ, colloidal particles repulsion , colloidal particles more stable

10. Colloid Destabilization
By charge neutralization, colloids can be destabilized:
Positively charges ions (Na+, Mg2+, Al3+, Fe3+ etc.) neutralize the
colloidal negative charges and thus destabilize them.
With destalization, colloids aggregate in size and start to settle

11. Theory Reducing zeta potential to vander waal force level:
When coagulant adding to water, it will hydrolysis to form positively charged hydroxo-metallic ion complexe, These positive charges hydroxo-metallic will adsorped on the negative charge of the colloids surface, to reduce the zeta potential to destabilization point, these destabilized colloids and hydroxo-metallic complex by van der waals force adsorption and flocculation. These process can be speeded by agitation.
Particle bridge:
enmeshment of particles:
coagulant complex can be polymerized. As concentration is larger than Ksp, it will settle down.

12. What is Coagulation?
Coagulation is the destabilization of colloids by addition of
chemicals that neutralizes the negative charges
The chemicals are known as coagulants, usually higher valence
cationic salt (Al3+, Fe3+ etc.)
Coagulation is essentially a chemical process -
Rapid mixing is required
to disperse the coagulant
throughout the liquid

13. Coagulation has three forms:
electrokinetic: zeta potential reduction
perikinetic : Brownian movement to make interparticle contact
orthokinetic : agitation to make interparticle contact

14. Aluminum sulfate (Al2(SO4)3) Ferrous sulfate (FeSO4)
Coagulant
Aluminum sulfate (Al2(SO4)3)
Ferrous sulfate (FeSO4)
Ferric sulfate (Fe2(SO4)3)
Ferric chloride (FeCl3)
Polyaluminium Chloride (PAC): Al2(OH)3Cl3
Lime
Coagulant aids : to produce a quick forming, dense, rapid settling floc and to insure optimum coagulation.
Three different kinds of coagulant aids:
Alkalinity addition : CaO, Ca(OH)2, Na2CO3
Polyelectrolytes :
anionic : negative charge
cationic : positive charge
polyampholites: both negative and positive charge
Turbidity addition : Clay
Relative coagulating power:
Na+ = 1; Mg2+ = 30
Al3+ > 1000; Fe3+ > 1000

15. What is Flocculation?
Flocculation is the agglomeration of destabilized particles into
a large size particles known as flocs which can be effectively removed
by sedimentation or floatation. The addition of another reagent called
flocculant or a flocculant aid may promote the formation of the floc
Thus, flocculation is essentially a physical process that describes the transport of destabilized particles.

16. Jar Tests
The jar test – a laboratory procedure to determine the optimum pH
and the optimum coagulant dose
A jar test simulates the coagulation and flocculation processes
Determination of optimum pH
Fill the jars with raw water sample
(500 or 1000 mL) – usually 6 jars
Adjust pH of the jars while mixing
using H2SO4 or NaOH/lime
(pH: 5.0; 5.5; 6.0; 6.5; 7.0; 7.5)
Add same dose of the selected
coagulant (alum or iron) to each jar
(Coagulant dose: 5 or 10 mg/L)
Jar Test

17. Rapid mix each jar at 100 to 150 rpm for 1 minute. The rapid mix
helps to disperse the coagulant throughout each container
Reduce the stirring speed to 25 to 30 rpm and continue mixing for
15 to 20 mins
This slower mixing speed helps promote floc formation by
enhancing particle collisions which lead to larger flocs
Turn off the mixers and allow
flocs to settle for 30 to 45 mins
Then measure the final residual
turbidity in each jar
Plot residual turbidity against pH
Jar Test set-up

18. The pH with the lowest residual turbidity will be the optimum pH
Residual turbidity Versus pH
Optimum pH: 6.3

19. Determination of optimum coagulant dose
Repeat all the previous steps.
This time adjust pH of all jars at
optimum (6.3 found from first test)
while mixing using H2SO4 or
NaOH/lime
Add different doses of the selected
coagulant (alum or iron) to each jar
(Coagulant dose: 5; 7; 10; 12; 15; 20 mg/L)
Rapid mix each jar at 100 to 150 rpm for 1 minute. The rapid
mix helps to disperse the coagulant throughout each container
Reduce the stirring speed to 25 to 30 rpm and continue mixing
for 15 to 20 mins

20. This slower mixing speed helps promote floc formation by
enhancing particle collisions which lead to larger flocs
Turn off the mixers and allow flocs to settle for 30 to 45 mins
Then measure the final residual turbidity in each jar
Plot residual turbidity against coagulant dose
Coagulant Dose mg/L
Optimum coagulant dose: 12.5 mg/L
The coagulant dose with
the lowest residual
turbidity will be the
optimum coagulant dose

21. Rapid Mixing (Coagulation)
Objective
To uniformly mix the coagulant with colloidal matters present
in raw water so as to bring about colloidal destabilization
How to achieve rapid mixing? L
Horizontal baffled tank
The water flows in horizontal
direction. The baffle walls help
to create turbulence when the water
hit the surface and thus facilitate mixing W
Plan view (horizontal flow)
Vertical baffled tank
The water flows in vertical direction. The baffle walls help to create turbulence when the water hit the surface and thus facilitate mixing H L
Isometric View (vertical flow)

22. Hydraulic Jump: Hydraulic Jump creates turbulence and
thus help better mixing.
Coagulant
In-line flash mixing
Mechanical mixing
Back mix impeller flat-blade impeller
Inflow
Chemical
feeding
Chemical
feeding
Inflow

23. Design of Flocculator (Slow & Gentle mixing)
Flocculator is designed mainly to provide enough interparticle contacts to achieve particles agglomeration so that they can be effectively removed by sedimentation or floatation
Transport Mechanisms
Brownian motion: for relatively small particles
which follow random motion and collides with
other particles (Perikinetic motion)
Differential settling: Particles with different
settling velocity in the vertical alignment collides
when one overtakes the other (Orthokinetic motion)

24. Mechanical mixing: Using different types of mechanical mixers
to promote particles contacts and thus their agglomeration
(Orthokinetic motion) L H
Transverse paddle
Cross flow Flocculator (sectional view) W
Plan (top view)

26. The degree of mixing is measured by Velocity Gradient (G)
Mixing and Power
The degree of mixing is measured by Velocity Gradient (G)
Higher the G value, the intense the mixing and vice versa
Velocity Gradient is the relative velocity of the two fluid particles at a given distance
G = dv/dy = 1.0/0.1 = 10 S-1
0.1 m
1 m/s
In mixer design, the following equation is more useful:
G= velocity gradient, S-1; P = Power input, W
V = Tank volume, m3;  = Dynamic viscosity, (Pa.s)

27. G value for coagulation: 700 to 1000 S-1; 3000 to 5000 S-1 for
Mixing time: 30 to 60 S in-line blender; 1-2 sec
G value for flocculation: 20 to 80 S-1;
Mixing time: 20 to 60 min
In the flocculator design, Gt (also known Camp No.); a product of G and t is commonly used as a design parameter
Typical Gt for flocculation is 2 x
Large G and small T gives small but dense floc
Small G and large T gives big but light flocs
We need big as well as dense flocs
which can be obtained by designing
flocculator with different G values 1 2 3
G1:40
G2:30
G3:20

28. Power Calculation
How many horsepower do we need to supply to a flocculation basin to provide a G value of 100s-1 and a Gt of 100,000 for 10 MGD flow? (Given:  = 0.89 x 10-3 Pa.s; 1 hp = watts)
Solution:
Retention time, t = Gt/G = 100,000/100 = 1000 secs
Volume of Flocculation basin, V = (0.438 m3/sec) x (1000 sec) = 438 m3
P = G2 V x 
= 1002 x 438 x 0.89 x10-3 = 3898 watts
= 3898/745.7 = 5.22 hp