This material deals with type-II settling (Hindered settling), mechanism involved in arresting colloidal particles. Definitons, Types of mixing devices, flash mixing, and flocculators

1. COAGULATION AIDED SEDIMENTATION
Stability and destabilization of colloids, Coagulation theory,
types of coagulants, chemical feeding, flash mixing,
Clariflocculators, types of flocculators and their design
aspects
14/18/2020
Dr. Dayananda H S
Professor of Civil Engineering,
VVCE, Mysore, Karnataka

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Preamble
• Coagulation and Flocculation deals with Type II settling (i. e.,
Hindered settling)
• Particles - Silt, small and large colloids
• Types of colloids – Hydrophilic and Hydrophobic
• For colloids, ratio of surface area to mass is high, resulting in
electrostatic repulsion and hydration predominates
• Settling velocity varies from few meter/hr to meter/million year
• Hydrophilic colloids are readily dispersed in water and its
stability depends on affinity for water and are negatively
charged

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• Eg - Soaps, soluble starch & proteins, synthetic
detergents
• Stability of Hydrophobic colloids depends on possessed
electric charge, no affinity for water and positively charged
Eg - metal oxides
• Particles carry charge due to loss of atoms, abrasion,
molecular structure
• Vanderwal’s force is a resisting force and Brownian
movement favors coagulation process
• A colloidal dispersion in a liquid is known as ‘Sol’
Eg: Bentonite clay, Kaolinite

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Definitions
Coagulation is a unit process (chemical technique)
adopted for charge neutralization of colloidal particles. Ion
transfer is effected by chemical coagulation
• Floc forming chemicals are added to water for the purpose
of enmeshing the colloidal particles to form rapidly settling
aggregates
• Mixing refers to blending of coagulating chemicals with
water to create more or less homogeneous single or
multiple phase system

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Flocculation is the stirring or agitation of water containing
flocs, a slow mixing technique which promotes the
agglomeration (to gather) of the stabilized particles
• Distinction between coagulation and flocculation is
some what arbitrary
• However, combination of mixing & stirring/agitation that
enhances aggregation is known as Flocculation

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Stability of Colloids
• A colloidal suspension is said to be stable when the dispersion
do not show any tendency to aggregate
• i.e., Colloidal suspension that do not agglomerate is termed
as ‘stable’
• Stability of Colloidal Suspension is due to excessively large
surface to volume ratio due to very small size
• Surface phenomena predominates mass phenomena
• Surface Phenomena is the accumulation of electrical charges
@ the particle surface
• Electrical Conductivity is due to molecular arrangement within
crystals, loss of atoms due to abrasion of the surface or any
other factors
• In most surface waters, the surface of the colloidal particles are
negatively charged

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Destabilization of Colloids
• Two forces responsible for destabilization of a sol are
i) Vander Waal’s force and ii) Brownian movement
• Vander Waal ‘s forces - molecular cohesive forces of attraction
that increase in intensity as particles approach each other
• This force becomes very dominant when two colloids come in
close proximity
• Brownian movement is random movement of colloids caused
by molecular bombardment of the dispersion media
• This movement has destabilizing effect on a source and results
with aggregation
• In water purification, destabilization of Hydrophobic colloids is
attained by chemically coagulating using trivalent metallic salts
• Aluminum sulfate, ferric chloride into clusters which are large
enough to be removed by gravity settling

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Principle (Mechanism) of Coagulation
The process of Coagulation and Flocculation is mainly based
on two factors
i)Floc formation
When a coagulant is added to water , it dissolves in it and
forms a spongy gelatinous substance forming floc which will
arrest the suspended and colloidal matter in water, while
descending down at the bottom.
ii) Electrical charge
The colloidal particles in natural waters will be usually
negatively charged. The floc which is positively charged is
opposite in nature and hence it helps in arresting all the
colloidal particles.

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Factors affecting coagulation
1. Types of coagulant
2. Quantity of coagulant
3. Characteristics of water -Type & quantity of suspended matter,
Temperature & pH of water
4. Time, turbulence and method of mixing
Types of Coagulant
Mainly used coagulants are aluminum and iron salts
1. Aluminum sulfate
2. Chlorinated copperas
3. Ferrous sulfate and lime
4. Magnesium carbonate and lime
5. Sodium Aluminate

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1. Aluminum Sulfate (Alum)
• Aluminum sulfate is cheaper and universally used coagulant
• It contains 17% Aluminum sulfate, a dirty grey solid in form of
lumps
• Alum on addition to water, hydrolyze forming a insoluble
gelatinous precipitate of Aluminum hydroxide, Al(OH)3
• To produce the hydroxide floc, enough alkalinity should be
present in the water
• If alkalinity is not enough, then it should be added. Usually
hydrated lime is used for the purpose (optimum pH is 6.5 –
8.5)
• Under normal circumstances, dose of Alum varies from 10 to
30 mg/L of water

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Sometimes sodium carbonate (Soda ash) is added to form
alkalinity, the resulting reaction is ,
Al2(SO4)3.18H2O + 3Na2CO2 = 2Al (OH)2 + 3Na2SO4 +18H2O
+ 3CO2 ….(c)
Al2(SO4)3.18H2O + 3Ca (HCO3)2 = 2Al (OH)3 + 3CaSO4 +18H2O
+ 6CO2 ….(a)
Al2(SO4)3.18H2O + 3Ca (OH)2 = 2Al (OH)3 + 3CaSO4 +18H2O
….(b)

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Advantages and disadvantages
Alum reduces taste and odor, Cheap and Easily available and
Soluble in water
The only Disadvantage is difficult to dewater the sludge

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2. Ferrous Sulfate (Chlorinated Copperas)
• The optimum pH range is 3.5 to 6.5
• At higher pH i.e. 9.5 it removes manganese
• More expensive than alum
• Effective in colour removal
• Low solubility in water

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6FeSO47H2O +3Cl2 = 2Fe2(SO4)3 + 2FeCl3 +7H2O ….(a)
• The ferric sulfate and ferric chloride produced
instantaneously are known as chlorinated copperas
• Both of these are immediately available for the formation of
ferric hydroxide floc; the resulting reactions are:
Fe2(SO4)3 +3Ca(OH)2 = 3CaSO4 + 2Fe(OH)3….(b)
2FeCl3 +3Ca(OH)2 = 3CaCl2+ 2Fe(OH)3….(c)

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3. Ferrous Sulfate and lime
• Ferrous sulphate can react with natural calcium bicarbonate
alkalinity in water, but its slow process
• Hence Lime is added in water
• Ferric hydroxide is gelatinous floc, which is heavier than floc
formed by alum
• Optimum pH range is below 7
FeSO4.7H2O + Ca(OH)2 = Fe(OH)2 + CaSO4 +7H2O ….(a)
• The ferrous hydroxide, Fe(OH)2 thus formed, though an
efficient floc, is soon oxidized by dissolved oxygen in water &
ferric hydroxide is formed
4Fe(OH)2 + O2 + 2H2O = 4Fe(OH)3 ….(b)

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4. Magnesium carbonate and lime
• When magnesium carbonate and lime are dissolved in
water, magnesium hydroxide and calcium carbonate are
formed as
• MgCO3 +Ca(OH)2 = Mg(OH)2 +CaCO3
• Byproducts of above reaction forms soluble sludge, so not
commonly used

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5. Sodium Aluminate
• Sodium aluminate (Na2Al2O4)is alkaline in nature and is used
very much less often than alum because of its cost. It reacts
with the salts of calcium and magnesium as follows;
Na2Al2O4 + Ca(HCO3)2 = CaAl2O4 +Na2CO3 +CO +H2O …(a)
Na2Al2O4 + CaCl2 = CaAl2O4 + 2NaCl …..(b)
Na2Al2O4 + CaSO4 = CaAl2O4 + Na2SO4 …..(c)
• The coagulant remove both temporary and permanent
hardness
• It is effective for a pH range of 6 to 8.50 naturally available in
water

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Comparison of Alum and Iron salt
1. Iron salts forms heavy floc as compared to alum, hence
more Solids are removed
2. Time of reaction and floc formation is less for iron salts,
hence ‘t’ reduces
3. Iron salts can work efficiently over wider pH range
4. Iron salts can remove taste and odour
5. Less mud ball formation as compared to alum
6. Under some cases iron salts are more economical

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7. Iron salts cause staining and promotes growth of iron
bacteria
8. Iron salts make water more corrosive as compared to alum
9. Handling of iron salts requires skill
10. More CO2 is formed so water becomes corrosive
11. Alum Coagulation may not be proper, if K or Na are
present in water

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Chemical feeding
• Types of Feeding device - dry feeding (Powder form)
- Wet feeding (Solution form)
Fig: Dry feed devices a) Helical screw type b) Toothed wheel type

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Fig: Solution feeding device a)Conical plug type b) Adjustable weir type

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Fig: Hydraulic mixing in water flow
a) Channel with baffles
b) Overflow weir
c) Hydraulic jump mixing

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Flash Mixing (Perikinetic Flocculation)
• Flash mixing is a process of adding coagulant chemicals to
water and the water is mixed quickly
• The purpose of flash mixing is to uniformly disperse the
chemicals throughout the volume of water
• This helps in the formation of micro flocs and results in
proper utilization of chemical preventing localization of
concentration and premature formation of hydroxides
• Flash mixing typically lasts for a minute or less
• If the water is mixed for less than thirty seconds, then the
chemicals will not be properly mixed into the water
• On the other hand, if the water is mixed beyond one minute,
the mixer blades will shear the newly forming floc into small
particles
• The coagulant is introduced at a point of high turbulence
• The rapid mixing of coagulants is carried out in a basin
known as “Flash Mixer”

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Hydraulic jump: Hydraulic jump creates
Turbulance & thus helps in better mixing
Mechanical Mixing
Fig: a) Back Mix Impeller b) Flat-blade Impeller

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i) Flash mixer (Mixing basin)
• The power input required for mixing is expressed
in terms of temporal mean velocity gradient, G’
• G’ = [ P/µ.V]1/2
G’ = Velocity gradient (per second)
µ = Absolute or dynamic viscosity of water in Ns/m2
V = Volume of water to which P is applied, in m3
The unit of G’ will evidently be 1/s (i.e., per second)

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Design criteria
• Impeller speed: 100 to 120 rpm
• Detention period: 30 – 60sec
• Power requirement: 2 to 5 kW/m3 /minute
• (0.041kW/1000cum.day)
• Tank shape: circular or square
• Velocity of flow: 0.9m/sec
• Velocity gradient, G’: 300 S-1
• Depth of tank: 1-3m
• Height to diameter (sides) ratio : 1:1 to 3:1
• Product G ‘T (unit less): 30 000 - 60 000
(T is the detention period)
• Impeller diameter to tank diameter ratio: 0.2 to 0.4
• Minimum Tanks: 2 Tanks

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Flocculation ( Forming Flocs)
• After destabilization (i.e., Coagulation), particles will be
ready to a tract and agglomerate and form flocs
• But this agglomeration is slow and they need help to
accelerate this agglomeration
• This help is called Flocculation “which is the slow stirring or
gentle agitation to aggregate the destabilized particles and
form a rapid settling floc”
• This gentle mixing increases the collisions between the
particles and help them to agglomerate.
(rapid mixing will destroy the flocs, hence need gentle
mixing)
• Flocculation occurs in a tank called a Flocculator or
Flocculation Basin equipped with a method for Slow Mixing.

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Fig: Flocculation process through Brownian movement

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Types of Flocculators (Slow stirring motion)
The most common types of Flocculators used,
a) Hydraulic Mixing Floculator: Hydraulic jump mixing
Horizontal baffled Channel
Vertically baffled channel
b) Mechanical Mixing Floculator:
i) Vertical shaft with impeller (turbine or propeller type blades)
ii) Paddle type with either horizontal or vertical shafts
iii) Walking Beam Flocculator
c) Pneumatic methods: Compressed air agitation,
centrifugal pump

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Fig: Rapid Mixing of coagulant in Venturi / Parshal Flume (Hydraulic Jump)

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Fig: Hydraulic Mixing Floculators
(b) Vertically baffled channel
(a) Horizontal baffled Channels

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Fig: Mechanical Flocculator (Paddle type with vertical shafts)

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Fig: Paddle type with horizontal shafts
Fig: Vertical shaft with impeller
(propeller type blades)

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Clariflocculator
• Clarifier and flocculator provided in single unit is known as
Clariflocculator
• The shapes of clariflocculators - Rectangular or Circular
• In case of circular clariflocculator, the flocculating chamber is
provided at the centre and clarifier component is formed by
the peripheral space
• Velocity of flow of water around 0.3m/min
• Mechanical scrapper provided to remove sludge

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Fig: Rectangular Clari-flocculator

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Effluent
Fig: Circular Clari-flocculator

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Design criteria of Flocculators
• Depth of tank : 3 - 4.5m
• Detention period: 10 - 40 minute (normally 30)
• Velocity of flow:0.2 - 0.8m/s, (usual 0.4)
• Total paddle area :10-25% of surface area of tank,
(usual 15%)
• Peripheral velocity of blades:0.2-0.6m/s,
(usual 0.3 to 0.4)
• Factor G’ T : 104 to 105
• Power consumption:10 - 36 kW/MLD
• Outlet flow velocity:0.15 - 0.25m/s

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FAQ
1. Explain the theory of coagulation.
2. Differentiate between coagulation and flocculation
3. List out coagulants used in water treatment and explain
any one with the help of chemical reactions.
4. Write detailed note on ‘design of flocculator’.
5. Compare Alum and iron salts as coagulants.
6. Sketch and name the components of : i) Flash mixer ii)
Floculation chamber.
7. What is coagulation? What factors affect it? Describe the Jar
test.
8. With the aid of a neat sketch, explain the different functional
components of sedimentation with coagulation unit.
9. Explain suitable device to i) Feed the coagulant ii) Mix the coagulant
10. Derive an expression for “Mean temporal Velocity” gradient in
flocculation of water.
11. With the help of chemical equation, explain the action of alum in water.
Explain with sketch, suitable device to feed the chemical to water.