This unit describes the concept of Coagulation and flocculation

1. Class : T.Y. B.Tech Civil
Subject: Water Treatment and distribution
Unit-III
Treatment of Water
Coagulation and flocculation
Prepared by
Dr. Madhukar V Jadhav
Professor, Department of Civil Engineering
Sanjivani College of Engineering, Kopargaon, 423603
Mail- jadhavmadhukarcivil@Sanjivani.org.in,
9552021276

2. Coagulation with Flocculation
 Very fine Suspended clay particles, electrically charged
colloidal particles and many more cannot settle down
due to gravitational force in sedimentation tank.
 When water contains such fine impurities, it becomes
necessary to apply such process which can easily
remove them from water.
 Hence to remove such impurities certain chemicals are
added to the water called as coagulant and the process is
known as coagulation.
 A coagulant is the substance (chemical) that is added to
the water to accomplish coagulation.

3. PARTICLES SETTLING VELOCITIES
Particle size, mm Type Settling velocity
10 Pebble 0.73 m/s
1
0.1
Course sand
Fine sand
0.23 m/s
0.6 m/min
0.01 Silt 8.3 m/day
0.0001 Large colloids 0.3 m/year
0.000001 Small colloids 3 m/million years

4. Coagulation: The process of mixing certain
chemicals in water to neutralize the electrical
charges on the particles and to form an insoluble,
gelatinous flocculent precipitate for absorbing
and entraining suspended and colloidal particles
of impurities is called coagulation.
Factors affecting the coagulation:
i) Type of coagulant ii) Dose of coagulant
iii)Time and method of mixing of the coagulant
iv) Character of water – pH, Temperature, nature
and quantity of impurities.

5. • Coagulation process:
i) feeding ii) mixing iii) flocculation
iv) sedimentation or clarification

6. Flocculation: flocculation is essentially an
operation designed to force agitation in the water
and induce coagulation.
Basically flocculation is a slow mixing process
or agitation process in which destabilized
colloidal particles are brought into intimate
contact in order to promote their agglomeration.
The operation , slow mixing is achieved in basin
commonly know as the Flocculator.

7. • The objective of the flocculation step is to cause the individual
destabilised colloidal particles to collide with one another and
with the precipitate formed by the coagulant in order to form
larger floc particles.
• Flocculation involves the stirring of water to which a coagulant
has been added at a slow rate, causing the individual particles to
“collide” with each other and with the flocs formed by the
coagulant. In this way the destabilised individual colloidal
particles are agglomerated and incorporated into the larger floc
particles.
• The rate of agglomeration or flocculation is depends on:
Concentration of turbidity, type of coagulant added and its dose
and mean velocity gradient (G) in the basin.

8. Flocculation is controlled through the introduction of energy into
the water (through paddles or by means of baffles in the flocculation
channel) to produce the right conditions (required velocity gradient)
for floc to grow to the optimum size and strength.
The velocity gradient “G” value is an extremely important factor
that determines the probability of particles to collide and form floc.
If the G-values are too low, the probability of collisions is low and
poor floc formation results.
If G values is too high, shear forces become large and this may
result in floc break-up.
Acceptable G-values for the coagulation process is between 400 and
100 per seconds.. For the flocculation process, it is in the order of
100 per seconds.

10. Rapid mixing Devices

13. Paddle Flocculators

14. • Commonly used coagulants: The commonly used metal
coagulants fall into two general categories: those based on
aluminum and those based on iron. The aluminum coagulants
include aluminum sulfate, aluminum chloride and sodium
aluminate. The iron coagulants include ferric sulfate, ferrous
sulfate, ferric chloride and ferric chloride sulfate. Other
chemicals used as coagulants include hydrated lime and
magnesium carbonate.
• When metal coagulants are added to water the metal ions (Al
and Fe) hydrolyze rapidly but in a somewhat uncontrolled
manner, forming a series of metal hydrolysis species.
The efficiency of rapid mixing, the pH, and the coagulant
dosage determine which hydrolysis species is effective for
treatment.

15. • COAGULANT SELECTION: The choice of coagulant
chemical depends upon the type of suspended solid to
be removed, raw water conditions, facility design, and
cost of chemical.
• Final selection of coagulant (or coagulants) should be
made with jar testing and plant scale evaluation.
Consideration must be given to required effluent
quality, effect upon down stream treatment process
performance, cost, method and cost of sludge
handling and disposal, and cost of the dose required
for effective treatment

16. • Inorganic Coagulants: Inorganic coagulants such
as aluminum and iron salts are the most commonly used.
• When added to water, these highly charged ions to
neutralize the suspended particles.
• The inorganic hydroxides that are formed produce short
polymer chains which enhance microfloc formation.
• Inorganic coagulants usually offer the lowest price per
kg, are widely available, and, when properly applied, are
effective in removing most suspended solids.
• They are also capable of removing a portion of the
organic precursors which may combine with chlorine to
form disinfection by-products.

17. • Inorganic coagulants produce large volumes of floc
which can also entrap bacteria as they settle.
• Inorganic coagulants may alter the pH of the water
since they consume alkalinity.
• When applied in a lime soda ash softening process,
alum and iron salts generate demand for lime and
soda ash. They also require corrosion-resistant
storage and feed equipment.
• It is important to note that large volumes of settled
floc must be disposed of in an environmentally
acceptable manner.
• Alum, ferric sulfate, and ferric chloride, lower the
alkalinity, and pH of water.

18. • Alum
• A12(SO4)3.18H2O+3Ca(HCO3)2---------> 2 Al(OH)3
+ 3CaSO4 + 18H2O +6 CO2.
• Ferric Sulfate
• Fe2(SO4)3 + 3 Ca(HCO3)2 ------------> 2 Fe(OH)3 +
3CaSO4 + 6 CO2.
• Ferric Chloride
• 2 Fe Cl3 + 3 Ca(HCO3)2 ------------> 2 Fe(OH)3 +
3CaCl2 + 6CO2

19. 6/14/2023 water treatment 19
Jar Tests
Determination of optimum pH
 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
 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

20. 6/14/2023 water treatment 20
Jar Test set-up
 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
 Measure the final residual
turbidity in each jar
 Plot residual turbidity against dose.
Jar Tests – determining optimum dose

21. 6/14/2023 water treatment 21
 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
Optimum coagulant dose

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