Coagulation and Flocculation-environmental engineering 1.pdf

coagulation and flocculation

1. CVNG2012/3007
Environmental Engineering 1
Water Supply
C&F
Lecturer: Vincent Cooper

2. Outline
Give an overview of water treatment
Discuss the importance of coagulation
Describe the coagulation process
Stress the importance of mixing
Explain how to design for coagulation in water
treatment
Do a worked example for designing a rapid mixer

3. Caroni
Water
Treatment
Plant
1. Coagulation
2. Flocculation
3. Sedimentation
4. Filtration
5. Disinfection(new addition)

4. 1 Coagulation

5. 2. Flocculation

6. 3 Sedimentation

7. 4 Filtration

8. 5 Disinfection

9. 5 Disinfection

10. INPUT
OUTPUT
WATER
TREATMENT
PLANT
PROCESSES
&
OPERATIONS
RAW WATER
•Quality
•Quantity
Surface water
Groundwater
Sea water
Reused water
Quality
•Suspended; Settleable
•Dissolved
•Microorganisms
POTABLE WATER
•Quality
•Demand
•Peak
•Average
WHO
Standards
Pathogen free
Clear
Colourless
MCL for other
substances
PHYSICAL
OPERATIONS
CHEMICAL
PROCESSES
Settling ●Filtering
pH Adjustment ●Hardness
removal ●Coagulation and
Flocculation ●Disinfection

11. Turbidity
https://www.wasa.gov.tt/WASA_Education_water_WaterTreatment.html#

12. Removal of Particles from water
•What is turbidity
•What is the problem with turbidity
•What may cause turbidity
•How to remove it
Some links
https://www.youtube.com/watch?v=5uuQ77vAV_U

13. Visual of turbidity
•What might happen if water is left to sit for
some time

14. Turbidity Removal Problems

15. Colloid Stability
•Colloidal particles are too small to settle in a
reasonable time period; and too small to be
trapped in the pores of a filter
•Colloids are stable and remain small because
of their charges
•Colloids are continually involved in Brownian
motion

16. Settling Velocity
Particlesize (mm) Size typical of:Settlingvelocity
10 Pebble 0.73 m/s
1 Coarse sand 0.23 m/s
0.1 Finesand 1.0 x 10
-2
m/s (0.6 m/min)
0.01 Silt 1.0 x 10
-4
m/s (8.6 m/d)
0.0001 Large colloid 1.0 x 10
-8
m/s (0.3 m/yr)
0.00001 Small colloid 1.0 x 10
-13
m/s (3 m/Myr)
16

17. Method of Removal

18. Colloid Destabilization
•Particles must be neutralized for
destabilization
•Can be done by the addition of ions of
opposite charge to the colloid
•Trivalent cations, such as Al
3+
and Fe
3+
are
common coagulants

19. Coagulation
Objective: To turn the small particles of colour,
turbidity and bacteria into larger flocs either as
precipitates or suspended particles.
•Flocs then conditioned so they will be readily
removed in subsequent processes
Coagulation: A method to alter the colloids so
that they will be able to approach and adhere
to each other to form larger floc particles.

20. Coagulants

21. Properties of a Coagulant
1.Must be a trivalent cation
2.Non-toxic
3.Insoluble in the neutral pH range. It must
precipitate out from solution.

22. Common Coagulants
Aluminium as alum (??????�
2????????????
43.14??????
2??????)
•Purchased as either dry or liquid alum
When added to water containing alkalinity the
following reaction occurs:
??????�
2????????????
43.14??????
2??????+6??????????????????
3
−
⇋2??????�????????????
3.3??????
2????????????+6????????????
2+8??????
2??????+3????????????
4
2−

23. Common Coagulants
Alkalinity used up and produces 6 moles of CO
2
pH decreases
With sufficient alkalinity and evolution of CO
2,
pH stays within the range.
Without alkalinity pH may fall drastically as:
??????�
2????????????
43.14??????
2??????⇋2??????�????????????
3.3??????
2????????????+3??????
2????????????
4+2??????
2??????

24. Common Coagulants
•Alum forms large complexes with water
molecules
•Complex is a large precipitate
–Many colloids removed by enmeshment

25. How Much Coagulant

26. Control of Coagulation
•pH and dose of coagulant
•Jar tests to determine optimal pH and dose
•Optimal pH: 5.5 to 6.5
–between 5 to 8 ok.

27. Control of Coagulation

28. Jar test
results
(from Davis, Water and Wastewater Engineering)

29. How to Add Coagulant

30. Mixing for Coagulation and
Flocculation
Camp-Stein Equation
By equating velocity gradient to the power
dissipated per unit volume (P/V), the following
equation was derived:
G= velocity gradient (energy input rate), s
-1
;
P= power of mixing input to vessel, J/s
V= volume of mixing vessel, m
3
µ= dynamic viscosity, Pa
.
sV
P
G

=

31. Coagulation: Rapid Mix
Rapid mix important for dispersion of coagulant
Chemical reaction completed in 0.1 s
Mixing must be (as far as possible)
•Instantaneous;
•Complete
Methods:
•Direct addition in headworks channel
•Vertical shaft mixer;
•Within a pipe using:
–In-line blender
–Static mixer

32. Mixing for Coagulation
In-line
mechanical
mixing
Rapid mix tank
In-line static mixer

33. Today’s Class
Review Theory of Coagulation
Continue with design of Rapid Mix Tanks
Flocculation in water treatment
–Reason and method
–Design of a flocculator
Sedimentation
–Importance
–Theory for removal by sedimentation
–Design of a sedimentation tank
–Worked example

34. Addition of Coagulant
Alum addition at
Caroni Water
Treatment Plant
Hydraulic
Jump
Alum
Added
Example:
•Alum added at 1.5 L/min
•Water to be treated: 40000 m
3
/d
•Dilution ratio:
1.5??????
�??????�
:
40000�
3
1
×
1000??????
�
3
1440�??????�
??????
×
??????
1
1:18519
~1:20000

35. Mixing for Coagulation
Mechanical mixing in stirred tanks
For use when sweep coagulation predominates
Design criteria
1.Detention time: 1 to 7 s
2.G: 600 to 1000 s
-1
3.Tank size: < 8m
3
4.Geometric ratio: Next slide

36. Mixing for Coagulation
Geometric ratio:
B

38. Design of a flash mixer

39. Mixing for Coagulation
Power imparted by impeller to liquid tank
in a baffled tank:
P= power, W;
N
P= impeller constant (also called power number)
n = rotational speed, revolutions/s
D
i= impeller diameter, m
ρ= density of liquid, kg/m
3()()
53
iP
DnNP=

40. Example
Question 3, Dec 2016 Solution

41. COAGULATION: Things You Should Know
In reviewing the material, students should be able to:
1.State why coagulation is important.
2.Explain why colloidal particles are difficult to remove.
3.Describe the desired properties of a coagulant.
4.Explain why alum is the coagulant of choice.
5.Explain the procedure for finding the optimal doses for
coagulation.
6.Explain the importance of mixing in the coagulation process
7.Sketch popular devices for adding the coagulant to water.
8.Explain each term in the equation for calculating the level of
mixing.
9.Design a rapid mixer for adding the coagulant
10.Realize the quality of the treatment at the end of coagulation

42. Flocculation

43. Flocculation
Approaches to flocculation:
Axial flow impeller;
Paddle flocculator;
Baffled chamber
Basin should be divided into at least 3
compartments
Velocity gradient is tapered
G values decrease from first to last basin
Typical range: 80 –30 s
-1
Average G value of compartments is the
design value
Example, if tapering from 70 to 50 to 30, then use
50 s
-1
for the design
Peri-kinetic flocs— Brownian motion
Ortho-kinetic flocs— Induced mixing

44. Mixing for Coagulation and
Flocculation
Camp-Stein Equation
By equating velocity gradient to the power
dissipated per unit volume (P/V), the following
equation was derived:
G= velocity gradient (energy input rate), s
-1
;
P= power of mixing input to vessel, J/s
V= volume of mixing vessel, m
3
µ= dynamic viscosity, Pa
.
sV
P
G

=

45. Flocculators

46. Vertical-Turbine Flocculator with
Baffle Walls
MWH textbook

47. Paddle-wheel Flocculator with Baffle
Walls
https://www.meurerresearch.com/products/mri-horizontal-paddle-wheel-flocculators

48. Baffle Walls
https://www.meurerresearch.com/products/mri-baffles

49. Flocculators
(a) Horizontal paddle wheel arrangement and (b) and (c) vertical paddle arrangements

50. Mixing for Flocculation
Power requirements:
P= power, W;
N
P= impeller constant (also called power number)
n = rotational speed, revolutions/s
D
i= impeller diameter, m
ρ= density of liquid, kg/m
3()()
53
iP
DnNP=

51. Mixing for Flocculation
Power imparted by a paddle mixer to water:
P= power imparted, W;
C
D= coefficient of drag of paddle;
ρ= density of liquid, kg/m
3
;
A= cross-sectional area of paddles, m
2
;
v
P= relative velocity of paddles with respect to fluid, m/s()
2
3
pD
vAC
P

=

52. FLOCCULATION: Things You Should Know
In reviewing the material, students should be able to:
1.Explain the complementarity between coagulation and flocculation.
2.Describe the condition of the water particles at the end of the
coagulation process.
3.Explain the difference between peri-kinetic and ortho-kinetic
flocculation.
4.Explain the importance of the velocity gradient in the flocculation
process.
5.Explain the terms in the equation for calculating the level of mixing.
6.Describe the methods used in water treatment plants for flocculation.
7.Size the compartments for flocculation in a water treatment plant.
8.Describe how water passes from one compartment to the other.
9.Describe the condition of the water at the end of this treatment process.
10.Say why Stoke Law cannot be applied for determining the removal
efficiency of particles formed at the end of this process.