Coagulation and flocculation are important water treatment processes used to remove small particles from water. Coagulation involves adding chemicals like aluminum sulfate or ferric chloride to destabilize colloidal particles and reduce charges. This allows particles to agglomerate into larger flocs during flocculation. Jar tests are used to determine the optimum pH and coagulant dose. Mechanical and hydraulic flocculators are then used to slowly mix water and form flocs, which are removed by sedimentation. Proper design of coagulant chambers, flocculators, and clarifiers is needed for effective treatment.

1. Coagulation and Flocculation
LOKESH SAINI
M.Tech-I´st Semester
Student ID 2015PCE5271
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Why coagulation and flocculation?
Various sizes of particles in raw water
Particle diameter (mm) Type Settling velocity
10 Pebble 0.73 m/s
1 Course sand 0.23 m/s
0.1 Fine sand 0.6 m/min
0.01 Silt 8.6 m/d
0.0001 (10 micron) Large colloids 0.3 m/y
0.000001 (1 nano) Small colloids 3 m/million y
Colloids – so small: gravity settling not possible
GravItysettlIng

3. COAGULATION AND FLOCCULATION
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2TYPE SETTLING

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Typical layout of a water treatment plant

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What is Coagulation?
 Coagulation is the destabilization of colloids by addition of
chemicals that neutralize the negative charges by rapid
mixing.
 The chemicals are known as coagulants, usually higher valence
cationic salts (Al3+, Fe3+ etc.)
 Coagulation is essentially a chemical process
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6. The net resultant force is a result of:
1. attractive potential energy (mostly vander Waals forces), Va.
These forces are very strong at short separation distances
2. repulsion potential energy (electrostatic forces), VR.
(by Coulomb’s law).
a 6
1V
r

R 2
1V
r


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Colloid Stability
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Repulsion
Colloid - A Colloid - B
 Colloids have a net negative surface charge
 Electrostatic force prevents them from agglomeration
 Brownian motion keeps the colloids in suspension
H2O
Colloid
 Impossible to remove colloids by gravity settling

9. Colloidal interaction
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10. Charge reduction
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 Typical coagulants
Aluminum sulfate: Al2(SO4)3.14 H2O
Iron salt- Ferric sulfate: Fe2(SO4)3
Iron salt- Ferric chloride: Fe2Cl3
Poly aluminum chloride (PAC): Al2(OH)3Cl3

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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

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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 pH
Jar Tests – determining optimum pH

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Optimum pH: 6.3
Jar Tests – optimum pH

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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 for 15 to 20 mins

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

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What is Flocculation?
Flocculation is the agglomeration of destabilized particles into
a large size particles known as flocs by slow mixing which
can be effectively removed by sedimentation or flotation.

19. Design of Coagulant Chamber
Detention Time 't' = Volume of Tank in sec
Discharge
‘t’ is taken 30 to 60 sec
IF THE CIRCULAR TANK IS CONSIDERED
H/D may be taken 1.5
IMPELLER DIA/ TANK DIA = 0.2-0.4
VELOCITY OF TIP OF THE IMPELLER>3m/sec
Free Board= 0.3m
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DESIGN OF FLOCCULATION CHAMBER
The constant ‘G.t’ = velocity gradient X detention time
G= 20 to 75sec¯¹
Where
Gt=2 to 60000
= 1 to 15000
t = 10 to 30 min
For Al coagulant
For Fe coagulant

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MECHANICAL FLOCCULATOR DESIGN
Inlet pipe & Tank Sizing
Depth of the tank = 3 to 4.5m
Detention time ‘t” = 20 to 40 min
Total area of paddles = 10 to 25% of the cross sectional area of the tank
Velocity of flow = 0.2-0.6m/s
Peripheral velocity of blades = 0.2 to 0.6m/s
Outlet velocity = 0.15 to 0.25m/s
Water loss in de sledging = 2%
Velocity in inlet pipe = 1m/s
Free board= 0.5m
Paddle Sizing
Power input ‘P’=G².μ X vol. of tank= ½.Cd.ρ.Ap.(V-v)³
Where,
Cd= Drag coefficient, 1.8
ρ = Density of water at 25̊ c, 997Kg/m³
V = Velocity at the tip blades= 0.4m/s
v = Velocity of the water at tip of blades is 25% of V
V=2π.r.n/60 where r is the paddle length

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Clarifier sizing
SoR= 40m³/m²/day
π/4{(Dia of clf)² - (Dia of flocculator)²}= Design flow/SoR
Length of Weir= π.Dia of clf< 300 m³/day

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Hydraulic flocculators: simple
technology

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Hydraulic Flocculation: Pipe

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Hydraulic Flocculation: Pipe

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Mechanical
flocculators

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Mechanical
flocculators

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Mechanical
flocculators

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References
1. Manual on water supply and treatment, CPHEEO, Ministry of MOUD, New
Delhi, 1999;201-232:621-625
2. Peavy S. Howard, Rowe, Tchobanoglous, Environmental Engineering, 2014;
120-150
3. Weikipedia on coagulation and flocculation
4. Water treatment: Principlea and design, MWH(2005), (ISBN 04710110183)
5. Unit process in drinking water treatment W. Masschelein(1992), (ISBN
082478678 5)(635 pgs)
6. IS 3025

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THANKS