Coagulation and flocculation

1. Class : T.Y. B. Tech Structural
Subject: Water Supply
Engineering
Unit-III
Sedimentation, Coagulation,
Flocculation
Prepared by
Prof. H. N. Wagh
Assistant Professor, Department of Structural Engineering
Sanjivani College of Engineering, Kopargaon, 423603
Mail- waghharshadast@sanjivani.org.in, 7588026776

2. Sedimentation
• Water contains variety of particles- mineral as well as
organic.
• Some particles may be colloidal, very fine or coarse.
• Also, the sp. gravity of the particles may range from
1.01 to 2.65.
• This indicates that different particles requires different
time for their settlement.
• Sedimentation is the process of removal of suspended
particles of impurities by gravitational settling, it
occurs when particles are heavier than water.

3. Principle of Sedimentation
• The very fundamental principle under lying the process of
sedimentation is that, the suspended matter present in water, is
having specific gravity greater than that of water.
• In still water these particles will therefore tend to settle down by
gravity, where as in a flowing water, they kept in suspension
because of turbulence.
• Hence as soon as turbulence is retarded by offering storage to the
water or reducing the flow velocity, the impurities tend to settle at
the bottom of the tank. This the principle of sedimentation.

4. • The basin in which the flow of water is retarded is called as
settling tank or sedimentation tank.
• Detention Time: The theoretical average time for which
the water is detained in the tank is called the detention
period. OR detention time of sedimentation tank may be
defined as the average theoretical time required for the
water to flow through the tank.
• Purpose of Settling:
• To remove coarse dispersed phase.
• To remove coagulated and flocculated impurities.
• To remove precipitated impurities after chemical treatment.
• To settle the sludge (biomass) after activated sludge process
/ tricking filters

5. Types of sedimentation
•Plain Sedimentation: When impurities are separated from
water by action of natural forces alone i.e. gravitational
and natural aggregation of settling particles, the process is
called plain sedimentation. Suspended particles of size
0.01mm and above and having sp. gr. greater than 1 are
removed by plain sedimentation.
•Sedimentation with coagulation: When chemicals or
other substances are added to induce or hasten aggregation
and settling of finely divided suspended matter, colloidal
substances and large molecules, the process is called
sedimentation with coagulation or simply clarification.

6. Factors Affecting Sedimentation
• Shape, Size, Density and Nature of particles
• Viscosity, density and temperature of
water/wastewater
• Surface overflow rate
• Velocity of flow
• Inlet and outlet arrangements
• Detention period
• Effective depth of settling zone

7. TYPES OF SEDIMENTATION
Type -I: Discrete particle settling
• No interaction between particles
• Settling velocity is constant for individual particles
• Dilute solid’s concentration
• Examples: pre-sedimentation in water treatment, grit
removal in wastewater.
Type –II: Flocculent settling
• Particles collide and adhere to each other resulting in
particle growth
• Dilute solid’s concentration
• Examples: coagulation/flocculation settling in water
treatment and primary sedimentation in wastewater
treatment

8. Type III: Hindered or Zone Settling
• Particles are so close together movement is restricted
• Intermediate solids concentration
• Solids move as a block rather than individual particles
• Fluidic interference causes a reduction in settling
velocity
• Distinguishable solids liquid interface
• Example: settling of secondary effluents
Type IV: Compression Settling
• Particles physically in contact
• Water is squeezed out of interstitial spaces
• Volume of solids may decrease
• High concentration of solids (sludges)

10. • Types of Sedimentation Tanks
Depending upon various factors sedimentation tanks
are classified as follows.
1. Based on methods of operation
a. Fill and draw type tank
b. Continuous flow type tank
2. Based on shape
a. Circular tank
b. Rectangular tank
c. Hopper bottom tank
3. Based on location
a. Primary tank
b. Secondary tank

11. • Fill and Draw Type of
Sedimentation Tank:
• In case of fill and draw type
sedimentation tank, water is stored
for some time in the tank after it is
filled completely. The time may be
24 hours. In that time, the
suspended particles are settled at
the bottom of the tank. After 24
hours, the water is discharged
through outlet.
• Then settled particle are removed.
This removal action requires 6-12
hours. So, one complete action of
sedimentation requires 30-40
hours in case of fill and draw type
sedimentation tank

12. • Continuous Flow Type
Sedimentation Tank
• In this type of thank, water is
not allowed to rest.
• The flow always takes place
but with a very small
velocity.
• During this flow, suspended
particles are settle at the
bottom of the tank.
• The flow may be either in
horizontal direction or
vertical direction.

13. • Circular Tank
• Circular sedimentation tanks are
mostly preferred for continuous
flow type sedimentation.
• In this case influent is sent
through central pipe of the tank
and radial flow takes place.
• Mechanical sludge scrappers are
provided to collect the sludge
and collected sludge is carried
through sludge pipe provided at
the bottom. But circular tanks
are uneconomical as compared
to rectangular tanks but they
have high clarification
efficiency.

14. • Hopper Bottom Tank
• In case of hopper bottom
tank, a deflector box is
located at the top which
deflects the influent coming
from central pipe to
downwards. Sludge is
collected at the bottom and
it is disposed through
sludge pump.

15. Rectangular sedimentation tank with
scraper arrangement

16. Circular sedimentation tank

17. Circular sedimentation tank with plan and
section

18. Inlet and Outlet Arrangement for Settling Tank

19. Vertical cross-section through a rectangular
sedimentation tank
H = depth of settling zone L = length of settling zone
B = width of settling zone V = volume of settling zone
Q = volumetric flow rate v = flow speed/velocity
T = detention time = hydraulic retention time

20. • Inlet Zone: The inlet distributes flow uniformly across the inlet to
the tank. The normal design includes baffles that gently spread the
flow across the total inlet of the tank and prevent short circuiting in
the tank.
• Settling Zone: The settling zone is the largest portion of the
sedimentation basin. This zone provides the calm area necessary for
the suspended particles to settle.
• Sludge Zone: The sludge zone, located at the bottom of the tank,
provides a storage area for the sludge before it is removed for
additional treatment or disposal. Sludge is removed for further
treatment from the sludge zone by scraper or vacuum devices which
move along the bottom.
• Outlet Zone: Outlet zone (or launder) should provide a smooth
transition from the sedimentation zone to the outlet from the tank. This
area of the tank also controls the depth of water in the basin. Weirs set
at the end of the tank control the overflow rate and prevent the solids
from rising to the weirs and leaving the tank before they settle out.

21. • Design Details
• Detention period: for plain sedimentation: 3 to 4 h, and for
coagulated sedimentation: 2 to 2.5 h.
• Velocity of flow: Not greater than 30 cm/min (horizontal
flow).
• Tank dimensions: L:B = 3 to 5:1. Generally L= 30 m
(common) maximum 100 m. Breadth= 6 m to 10 m. Circular:
Diameter not greater than 60 m. generally 20 to 40 m.
• Depth 2.5 to 5.0 m (3 m).
• Surface Overflow Rate: For plain sedimentation 12000 to
18000 L/d/m2 tank area; for thoroughly flocculated water
24000 to 30000 L/d/m2 tank area.
• Slopes: Rectangular 1% towards inlet and circular 8%

22. 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.

23. 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

24. 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.

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

26. 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.

27. • 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.

28. 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.

30. Paddle Flocculators

31. • 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.

32. • 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

33. • 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.

34. • 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.

35. • 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

36. THANK YOU