3. Coagulation and flocculation
In this chapter we will answer the following questions:
How do coagulation and flocculation fit into the water
Which chemical principles influence coagulation and
Which chemicals are used in coagulation?
How the optimal dose of coagulant can be determined?
What factors influence coagulation and flocculation?
To remove turbidity, color, and some bacteria from
The addition and rapid mixing of coagulants
The destabilization of colloidal and fine particles
The initial aggregation of destabilized particles
The gentle agitation to aggregate destabilized
particles to form rapid-settling floc
5. Coagulation Chemistry
The hydraulic settling values of small size particles in water
are very small
they require longer time to settle in plain sedimentation tanks
In a 3m depth sedimentation tank:
a slit particle of size 0.05mm will require about 11hrs to settle
clay particle of size 0.002mm will require about 4 days time to
Due to electrical charge, colloidal impurities remain
continuously in motion and never settle down by gravity in
6. Coagulation Chemistry
When water is turbid due to presence of such fine size
and colloidal impurities, plain sedimentation is of no use.
A chemical process which removes all fine size and
colloidal impurities within reasonable period of 2 – 3hrs.
the chemical used in the process
The objective of coagulation is :
to unit several colloidal particles together to form
bigger sized settable flocs which may settle down in
7. Coagulation Chemistry
The principle of coagulation can be explained from the
following two conditions:
1. Floc formation
When coagulants are dissolved in water and thoroughly
mixed, they produce a think gelatinous precipitate = floc.
has the property of arresting suspended impurities in water
during downward travel towards the bottom of tank.
Hence, removing fine and colloidal particles quickly.
8. Coagulation Chemistry
2. Electric charges
Most particles dissolved in water have a negative charge, so
they tend to repel each other.
They stay dispersed and dissolved or colloidal in the water.
The purpose of most coagulant chemicals is:
to neutralize the negative charges on the turbidity particles
The amount of coagulant will depend on:
a measurement of the magnitude of electrical charge surrounding
the colloidal particles.
the amount of repulsive force which keeps the particles in the
9. Coagulation Chemistry
Coagulants tend to be positively charged.
Due to their positive charge, they are attracted to the
negative particles in the water.
Positively charged coagulants attract to
negatively charged particles due to electricity
Negatively charged particles
repel each other due to electricity
10. Coagulation Chemistry
The combination of positive and negative charge results in
The next force which will affect the particles is known as
Van der Waal's forces.
Van der Waal's forces refer to the tendency of particles in
nature to attract each other weakly if they have no charge.
Neutrally charged particles attract
due to Van der Waal's forces
When enough particles have joined together, they become floc and will settle out of the water.
When enough particles have
joined together, they become
floc and will settle out of the
Particles and coagulants join
together into floc
11. Destabilization of Colloidal Dispersion
Have particle size between 0.0001 and 0.1mm
Have particle size between 0.000001 to 0.001mm
Most of non-settleable solids are colloidal particulates
Colloids do not settle by the force of gravity
Are stable in suspensions
Colloid removal requires that they should be agglomerated
in to large particles.
This requires surface charge to be destabilized.
12. Destabilization of Colloidal Dispersion
Particle destabilization can be achieved by four
1. Double layer compression
Addition of electrolyte/coagulant to water shrinks the layer
of charged ions around the particle.
If reduced enough, the attractive Van der Waals force (which
acts close to particle) can overcome Zeta potential.
Diffused double layer is created by cations attaching to
negatively charged particles (fixed layer) and cations and
anions loosely attaching in outer diffused layer.
13. Destabilization of Colloidal Dispersion
1. Double layer compression
A negatively colloidal particle with its electrostatic field
Two major forces acting on colloids:
1. Electrostatic repulsion, Zeta potential
2. Intermolecular, Van der Waals, attraction
14. Destabilization of Colloidal Dispersion
2. Adsorption and charge neutralization
Adding positively charged ions that adsorb to particle
surface can reduce surface charge and repulsion
3. Entrapment in precipitate
Al and Fe salts added at right pH will precipitate as flocs
with colloids as nuclei.
4. Particle bridging
Large organic molecules attach to multiple particles
‘bridging’ them (often used in addition to metal salts)
15. Factors affecting coagulation
1. Characteristic of water
a) Type and quantity of suspended matter
b) Temperature of water
c) pH of water
2. Type of coagulant
3. Dose of coagulant
4. Time and method of mixing
16. Factors affecting coagulation
Low temperature affects coagulation and flocculation
processes by altering:
• coagulant solubility
• increasing water viscosity
• retarding the kinetics of hydrolysis reactions and particle
Higher alkalinity waters have higher pH.
Metal coagulants are acidic, and coagulant addition
For low alkalinity (pH) waters, coagulant addition may
consume all of the available alkalinity, depressing the
pH to values too low for effective treatment.
17. Types of coagulant
Coagulant chemicals come in two main types:
neutralize the electrical charges of particles in the water which
causes the particles to clump together.
add density to slow-settling flocs
add toughness to the flocs so that they will not break up during
the mixing and settling processes
generally used to reduce flocculation time
Lime, calcium carbonate and bentonite are examples
Mainly aluminum and iron salts
Aluminum sulfate – Alum - Al2(SO4)3.18H2O
Intermediates in the
formation of Al(OH)3
These species may adsorb very strongly
onto the surface of most negative colloids
19. 1. Aluminum Sulfate - Alum
To produce the hydroxide floc, enough alkalinity should
present in the water.
If alkalinity is not enough, then it should be added. Usually
hydrated lime is used for that purpose.
Optimum pH is 6.5 – 8.5
Its dose may vary from 5 - 30mg/l (14mg/l)
Dose of coagulant depends on various factors such as
turbidity, color, taste, pH value, temperature
Al 2(SO4)3.18H2O + 3Ca (HCO3) 2 2Al(OH) 3 + 3CaSO4 + 6CO2 +18H2O
Al2(SO4)3.18H2O + 3Ca(OH)2 2Al(OH)3 + 3CaSO4 + 18H2O
Al2 (SO4)3.18H2O + 3Na2CO3 2Al(OH)3 + 3Na2SO4 + 3CO2 + 18H2O
20. 1. Aluminum Sulfate
Alum is the most widely used chemical coagulant, with
the following reason:
1. It is very cheap
2. It removes taste and color in addition to turbidity
3. It is very efficient
4. Flocs formed are more stable and heavy
5. It is not harmful to health
6. It is simple in working, doesn’t require skilled
supervision for dosing
21. 2. Sodium Aluminate -- Na2Al2O4
It can remove carbonate and non-carbonate hardness
React with calcium and magnesium salts to form
flocculent aluminates of these elements
The pH should be with in the range of 6 and 8.5
More expensive than alum
Na2Al2O4 + Ca (HCO3) 2 CaAl2O4 + Na2CO3 + CO2 + H2O
Na2Al2O4 + CaSO4 CaAL2O4 + Na2SO4
Na2Al2O4 + CaCl2 CaAl2O4 + 2NaCl
22. 3. Chlorinated Copperas
Combination of Ferric sulphate and Ferric chloride
When solution of Ferrous Sulphate is mixed with chlorine,
both Ferric sulphate and Ferric chloride are produced.
Both Ferric sulphate and Ferric chloride can be used
independently with lime as a coagulant
If alkalinity is insufficient, lime is added
Ferric chloride: effective pH range 3.5 – 6.5 or above 8.5
Ferric sulphate : 4 – 7 or above 9.
6FeSO4.7H2O + 3Cl2 2Fe3(SO4)2 + 2FeCl3 + 42H2O
2FeCl3 + 3Ca(OH)2 2Fe(OH)3 + CaCl2
Fe2(SO4)3 + 3Ca(OH)2 2Fe(OH)3 + 3CaSO4
23. 4. Polyelectrolytes (Polymeric Coagulants)
Anionic—ionize in solution to form negative sites along
the polymer molecule.
Cationic—ionize to form positive sites.
Non-ionic—very slight ionization.
Synthetic polymers (more common in coagulation)
Broaden the pH range over which satisfactory flocculation
Reduce the quantity of primary coagulant required
Dosage: usually 1mg/l
Polymer: a substance which has a molecular structure built up chiefly or completely
from a large number of similar units bonded together
24. Dose of the coagulant
Dosage is the required concentration of the chemical
within the water (mg/l).
Chemical feed is the amount of chemical you add to
the water (mg/day).
High coagulant dose destabilization of floc
Low coagulant dose insufficient
Optimal coagulant dose is requires
Fixing of the optimal coagulant dose requires
laboratory experiment --- Jar Test
25. Jar Test
It is used to predict the functioning of a large scale
Is used to determine:
Proper coagulant and coagulant aid
Proper coagulant dose
Rapid mixing – 1min (80 RPM)
Slow mixing - 15 – 20min (20 RPM)
Settling - 30min
Then measure the final turbidity in each container.
The final turbidity can be evaluate roughly by sight or more
accurately using a nephelometer.
The optimum coagulant dose is the dose which meets the
specified turbidity required on the regulatory permit.
27. Example 1
Find out the quantity of alum required to treat 18 million
liters of water per day. The dosage of alum is 14mg/lit. Also
work out the amount of CO2 released per liter of treated
Ans: 252 kg of alum per day
100 kg of CO2 per day
The chemical reaction of alum is given by:
Al2(SO4)3.18H2O + 3Ca(HCO3)2 2Al(OH)3 + 3CaSO4 + 6CO2 +18H2O
28. Feeding of coagulant
In order to fed chemicals to the water regularly and
accurately, some type of feeding equipment must be
Coagulants may be put in raw water either in
i. powder form (Dry-feeding equipment)
ii. solution form (Solution-feeding equipment)
29. Feeding of coagulant
1. Dry-feed Type
• Dry powder of coagulant is filled in the conical hopper.
• Agitating plates are used to prevent arching of chemicals
• Feeding is regulated by the speed of toothed wheel or helical
• Activated carbon and lime are also added in powder form
Control of dose is difficult
Chemical feed rate of a dry chemical is
Chemical feed (mg/day)
= Flow (l/day)*Dosage (mg/l)
30. Feeding of coagulant
2. Solution-feed Type
Can be easily controlled with automatic devices
Not desirable (more labor)
There might be corrosion
First, solution of required strength of coagulant is
The solution is filled in the tank and allowed to mix
in the mixing channel in required proportion to the
quantity of water.
31. Mixing devices
The process of floc formation greatly depends upon:
the effective mixing (rapid mixing) of coagulant with the raw
Rapid mixing is used:
To disperse chemicals uniformly throughout the mixing basin
To allow adequate contact between the coagulant & particles
To the formation of microflocs
Mixing is done by mixing device.
1. Hydraulic jump
flume with considerable slope is developed
centrifugal pump is used to raise raw water
32. Mixing devices
3. Compressed air method
compressed air is diffused from bottom of the mixing tank
4. Mixing channels
Mixing of raw water and coagulant is made to pass through
the channel in which flume has been done.
Vertical baffles are also fixed at the end of the flumed part
on both sides of the channel.
5. Mechanical mixing basins
Mechanical means are used to agitate the mixture to achieve
the objective of thorough mixing.
Flash mixers and deflector plate mixers are used
A. Flash mixer
The mixing of coagulant in water
is achieved by rotating vigorously
fans fixed in the mixing basin.
The deflecting wall avoids
short circuiting and deflects
the water flow towards the fan
Chemical pipe discharges the coagulant just near the rotating
Design criteria of flash mixer
1. Detention period – 30 to 60 sec
2. Velocity of flow – 0.9m/sec
3. Depth – 1 to 3m
4. Rotation per minute of blade – 80 to 100
5. Power required – 0.041kW/1000m3/day
B. Deflector plate mixer
Mixing is achieved by diffusing water through a
Chemical pipe discharges the coagulant just near the
The formula used to determine the chemical feed rate of
liquid chemicals is shown below:
Chemical feeder setting 𝑚𝑙/𝑚𝑖𝑛
Flow (l/min) ∗ Dosage (mg/l)
Liquid chemical concentration (mg/ml)
Liquid chemical concentration 𝑚𝑔/𝑚𝑙
Dry chemical weight (mg)
Volume of water (ml)
Example 1: If you added 230 mg of polymer to 100 ml of water,
you would have a liquid polymer solution with a concentration
of 2.3 mg/ml.
Example 2: Consider a situation in which the flow of water into
the water treatment plant is 18.9MLD and the alum dosage is 10
mg/l. The liquid alum has a concentration of 520 mg/ml.
The chemical feeder setting would be determined as follows:
𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑓𝑒𝑒𝑑𝑒𝑟 𝑠𝑒𝑡𝑡𝑖𝑛𝑔 (𝑚𝑙/𝑚𝑖𝑛) =
𝐹𝑙𝑜𝑤 (𝑙/𝑚𝑖𝑛) ∗ 𝐷𝑜𝑠𝑎𝑔𝑒 (𝑚𝑔/𝑙)
𝐿𝑖𝑞𝑢𝑖𝑑 𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 (𝑚𝑔/𝑚𝑙)
𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑓𝑒𝑒𝑑𝑒𝑟 𝑠𝑒𝑡𝑡𝑖𝑛𝑔 (𝑚𝑙/𝑚𝑖𝑛) =
13143 𝑙/𝑚𝑖𝑛 ∗ 10 𝑚𝑔/𝑙
The setting on the liquid alum feeder should be 252 ml/min.
After adding the coagulant to the raw water:
rapid agitation is developed in the mixture to obtain a
thorough mixing. (for about 1 to 3 min)
next to rapid mixing, mixture is kept slowly agitated for
about 30 to 45min
Slow mixing process in which particles are brought into
contact in order to promote their agglomeration is
The tank/basin in which flocculation process is carried
out is called
Flocculation occurs by:
1. Brownian motion
the random motion of particles suspended in a fluid
resulting from their collision
important for small particles (< 0.5m) --- Perikinetic
mechanical stirring strong enough to cause particle
collisions but not so strong as to break up particles. ---
3. Differential settlement
larger, faster particles catch up with smaller, slower
The velocity of flow in the flocculation chamber is kept
between 0.12 – 0.18m/sec.
Activated carbon in powder form can be used to speed
up the flocculation.
The rate of agglomeration or flocculation is dependent
Type and concentration of turbidity
Type of coagulant and its dose
Temporal mean velocity gradient caused by mixing – G in
Mean Velocity Gradient (G)
√ The velocity of water flowing through the flocculation
basin must be within a very specific range, designed to
gently mix the water without breaking apart of the floc.
√ G is the rate of change of velocity per unit distance
normal to the section - (T-1).
√ Measurement of the intensity of mixing in the chamber.
√ Determines how much the water is agitated in the tank.
√ Determines how much energy is used to operate the
The value of G can be computed in terms of power input
by the following equation:
Where P – power dissipated (watt)
µ - absolute viscosity (Ns/m2)
V - the volume to which P is applied (m3)
G - temporal mean velocity gradient caused by mixing (s-1)
Power can also be calculated by:
Where: P = power input, Watt or Nm/s
FD = drag force on paddles, N
vp = velocity of paddles (velocity relative to the water), m/s
𝐏 = 𝑭𝒐𝒓𝒄𝒆 ∗ 𝐕𝐞𝐥𝐨𝐜𝐢𝐭𝐲 = 𝑭𝑫
CD = coefficient of drag, 1.8 for flat blades
Ap = area of paddle blades, m2
𝑤 = density of water, kg/m3)
Rotating paddle wheels or vertical mounted turbines are
the most commonly used flocculation technique.
The design criteria of a horizontal continuous flow rectangular
i. Depth of tank : 3 – 4.5m
ii. Detention time (t) : 30 – 45 minute
iii. Velocity of flow : 0.12 – 0.18m/s
iv. Total area of paddles : 10 – 25% of cross-
sectional area of the tank
v. Peripheral velocity of blades : 0.2 – 0.6m/s
vi. Velocity gradient (G) : 10 – 75 s-1
vii. Factor G*t : 104 - 105
viii. Power consumption : 49 – 196 kW/Mm3/d
ix. Outlet flow velocity : 0.15 – 0.25 m/s
45. Secondary sedimentation tank/ Clarifier
After flocculation, water enters the settling tank which is
normally called a clarifier.
Water is retained in the tank for a sufficient period to
permit the settlement of the floc to the bottom.
Principles of clarifier design:
the same as for plain sedimentation basin except that its
detention period is lower.
The detention period: 2 - 2.5hrs
Overflow rate: 1 - 1.2 m/hr (24 – 28.8) m3/m2/d
- Flash mixer - Flocculator
- Settling basin
Section of sedimentation unit consisting of
46. Example 1
1. Design a settling tank (coagulation–sedimentation)
with continuous flow for treating water for a
population of 48,000 persons with an average daily
consumption of 135 lit/head. Take detention period of
2.5 hrs. and maximum day factor of 1.8.
47. Example 2
A mechanical flocculator is used to treat 38,000 m3/day
water with detention time 20 minutes.
a. Design the dimension of the tank if L : W : H = 4 : 1 : 2
b. Find the power required when velocity gradient is 55s-1
and dynamic viscosity 1.002*10-3 Ns/m2
c. If the tank have 3 paddles and every paddle have 4 plate
with relative velocity of paddles is 0.38m/s and
coefficient of drag is 1.8, find the area of one plate.
48. Example 2
a) ∀ = 𝑄 ∗ 𝑡
= 38,000 m3/day * 20 min
= 527.8 m3
∀ = 𝐿 ∗ 𝑊 ∗ 𝐻
= 527.8m3 = 8W3
W3 = 527.8 /8
W = 4.04m
L = 4 * 4.04 = 16.16m
H = 8.08 m
49. Example 2
= 552 *(1.002 *10-3)*527.8
= 1599.8 Watt
• So, area of one plate
3 ∗ 4
𝐏 = 𝑮𝟐
2 ∗ 1599.8
1.8 ∗ 0.383 1000