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Water supply
1. Water Treatment
2019-12-25
Group Members
Subham, 169
Sudarshan, 170
Sujan , 171
Sujata, 172
Suman, 174
Sugam, 200
Tutor
Asst. Prof. Shukra Raj Paudel
Department of Civil Engineering
IOE, Tribhuwan University
2. After this presentation, students will learn
• Water treatment process and its objectives.
• Types of impurity and treatment process employed to remove it.
• Screening process and its types.
• Plain sedimentation, its theory and different types of sedimentation
tanks.
• Sedimentation with coagulation, coagulants used in this process,
devices, tanks and test used in water treatment process.
• Filtration and its theory.
2
3. Presentation Outline
6.1 Objectives of water treatment
6.2 Treatment process and impurity removal
6.3 Screening
6.4 Plain sedimentation
6.5 Sedimentation with coagulation
6.6 Filtration
3
4. 6.1 Introduction: Water Treatment
• Water Treatment is the process of making the water suitable for the
intended purpose by removing the impurities in water.
• During the treatment impurities in the water is not reduced to zero but to
a level which will not be harmful for intended use as per the standards.
• After the treatment water is expected to be palatable, safe, clear,
colourless and odourless, reasonably soft and non corrosive with low
organic content.
4
5. 5
Water Treatment
1.Unit operations =>
Contaminants are removed by
physical means.
1.Unit processes =>
Contaminants are removed by
chemical and biological means.
6. Objectives of water treatment
The objective of water treatment are:
1. To remove colour, dissolved gas and murkiness of water.
2. To remove objectionable taste and odour.
3. To remove disease producing micro-organisms.
4. To remove hardness of water and make it suitable for wide variety of
industrial works such as brewing, steam generation, etc.
6
7. 6.2 Treatment processes and impurity removal
Water treatment processes can be selected depending on the type and
concentration of impurities to be removed from water. The most widely used
treatment processes are described below:
1. Screening
Removing all the large suspended and floating matters from water.
Generally provided at intakes.
2. Plain Sedimentation
Settling out of coarse and heavy suspended particles such as sand, silt,
etc. by force of gravity.
Removes turbidity of water as well as colour, odour and taste associate
with it.
7
8. 3. Sedimentation with Coagulation
Its purpose is to remove fine suspended particles and colloidal matter
present in water.
Coagulants are added in water to coalesce with the small particles and
form larger flocs which are then removed in the sedimentation tank.
4. Filtration
It is the process of passing the water in a filter media or sand.
It removes very fine suspended impurities and colloidal matters that
may have escaped the sedimentation tank.
It also removes micro-organisms.
8
9. 5. Disinfection
Disinfection is the process of killing the pathogenic microorganisms
present in water.
It is carried out to eliminate or reduce microorganisms to safe limit and to
prevent contamination.
6. Softening
Removal of hardness of water.
7. Miscellaneous Treatment
These include aeration, removal of iron and manganese and removal of
colour, odour and taste.
Aeration removes dissolved gases such as carbon dioxide, hydrogen
sulphide etc. 9
13. Treatment Process Impurity Removal
Screening Large suspended and floating matters
Plain Sedimentation Coarse and heavy suspended particles
Sedimentation with Coagulation Fine suspended particles and colloidal
matters
Filtration Very fine suspended impurities and colloidal
matters
Disinfection Pathogenic Microorganisms
Softening Hardness
Aeration Colour, odour, taste, iron, manganese
Removal of Iron and Manganese Iron and manganese
Removal of Colour odour and
taste
Colour, odour and taste
13
14. 6.3 Screening
Screening is a process of removing suspended matter from water that may
range from floating debris such as sticks, branches, leaves, etc. by passing
water through screens.
Screen may be located at the intake structure, raw water pumping station, or
the water treatment plant itself
It serves as a protective device for remainder of the plant.
Screen may be of two types base on size of the materials removed as
follows:
1. Bar screens
2. Fine Screens
14
15. 6.3.1 Bar Screens
Bar screens are intended to intercept only grosser floating materials .
The circular bars are generally of 25 mm size in diameter.
The rectangular bars are generally 10mm* 50mm and are placed with
larger dimension parallel to flow.
The screens may be course or medium size depending upon the opening
space between the bars.
50 to 150 mm openings for coarse and 20 to 50 mm openings for
medium.
15
16. The maximum head loss through
clogged racks and screens is
generally below 80 cm.
Manufactures also recommend
dropping the channel 150 to 300
mm across bar screen to
compensate head loss in the racks
and screen
16
Fig: Bar screen
Source: Water supply engineering Prof. Dr. Bhagwan Ratna Kansak
18. The head loss through the unobstructed screens depends on the nature of
their constructions as well as the approach velocity.
h=ß(
𝑤
𝐵
)
4
3ℎ 𝑣 sin 𝝷
Where h = head loss
ℎ 𝑣 = approach velocity head
w = maximum cross-sectional width of the bars facing direction of flow
B =minimum clear opening between pairs of bars.
𝝷 = angle of the rack with horizontal; and
ß = a shape factor
18
19. =2.42 for sharp edged rectangular bars
=1.83 for rectangular bars with semi-circular upstream face
=1.79 for circular rods.
It may be calculated by means of the common orifice formula as given :
h =
1
2𝑔𝐶 𝑑
2(𝑉𝑠
2
-𝑉2
)sin 𝝷
Where, h = head loss
𝐶 𝑑 = coefficient of discharge 𝝷 = angle of rack with horizontal
𝑉𝑠 = velocity through screens g = acceleration due to gravity
V = velocity in the approach channel
19
20. 6.3.2 Fine Screens
Fine screens are used at surface water intakes.
It is generally made of wire mesh.
The size of wire mesh should be more than 6mm.
The fine screens are generally not used in water treatment as it is frequently
clogged and create difficulty in its operation and maintenance.
Velocity through screens are much higher than approach velocity V. Hence
approach velocity V is generally neglected.
20
21. The expression for head loss is:
h =
𝑉𝑠
2
sin 𝝷
2𝑔𝐶 𝑑
2
h =
1
2𝑔
(
𝑄
𝐶 𝑑 𝐴
)
2
sin 𝝷
Where h= head loss
𝑉𝑠= velocity through screens A= effective submerged open area
𝐶 𝑑= coefficient of discharge 𝝷= angle of rack with horizontal
Q= discharge through screens g= acceleration due to gravity
21
22. Fig. Fine screen Fig. Rotating drum fine screen
22Source: wrtengineering.comSource: waterworld.com
23. 6.4 Plain Sedimentation
Plain sedimentation is the process in which water is retained in a tank
or basin so that the suspended particles present in water may settle
under the action of gravity without addition of chemical.
It is suitable for water containing large amounts of suspended particles
of relatively large size.
There are mainly two types of suspended particles in water:
1. Inorganic solids having specific gravity of about 2.65
2. Organic solids having specific gravity of 1.0 to1.4
23
24. In sedimentation tank water is brought to rest so suspended particles held
by turbulence of water settles down under the action of gravity.
Plain sedimentation removes suspended particles of specific gravity 1.2 or
above.
This process is effective in removing colour, taste, odour and turbidity
associated with suspended particles.
The time for which water is retained in a sedimentation tank is known as
detention period or retention period.
The particles which are settled in the bottom of the sedimentation tank are
known as sludge.
24
25. 6.4.1 Theory of Settlement
Hydraulic subsidence is the phenomenon of settling down of particles at the
bottom of sedimentation tank.
It is affected by following factors :
i) Velocity of flow of water
ii) Size and shape of particles
iii) Specific gravity of particles
iv) Viscosity of water
v) Surface overflow rate
vi) Detention period
vii) Inlet and outlet arrangements
25
26. The particles which do not change in size, shape or weight during
settling are known as discrete particles.
The settling of a particle is caused by the effective weight of the particle
acting in vertical downward direction and is opposed by the drag acting
in vertical upward direction.
The effective weight of the particle in this case is its submerged weight
which is the actual weight of the particle minus force of buoyancy.
26
27. Effective weight is given as
We =
𝜋𝑑3
6
(𝑤𝑠 − 𝑤) where, d= diameter of particle
ws= specific weight of particle
w = specific weight of water
The drag FD acting on the particle is
FD = CD A ρ
𝑉𝑠2
2
where, CD = coeff. of drag
A = Projected area of particle
p = mass density of water
Vs = settling velocity of particle
27
28. In the limiting condition of equilibrium, equating the two forces, we get
We = FD
or,
πd3
6
(ws − w) = CD A ρ
Vs2
2
Since A =
π d2
4
,
πd3
6
(ws − w) = CD ρ
π d2
4
Vs2
2
or, Vs2
=
4
3
wd
p CD
ws
w
− 1
Now, w = ρ g and ws/ w = S, the specific gravity of particle.
28
29. Thus we obtain, 𝑉𝑠2 =
4
3
ρg d
ρCD
𝑆 − 1
or, 𝑉𝑠 =
4
3
𝑔𝑑
CD
(𝑆 − 1)
The above equation is known as Hazen’s equation in which the value of
CDis given by:
CD=
24
𝑅𝑒
+
3
𝑅𝑒
+ 0.34
29
30. Equations for Settling Velocity of Discrete Particles
SN Law and Equation
Reynolds
Number
Particle size
Sp Gr 2.65 Temp 20
C
1
Stoke’s (Laminar)
𝑉𝑠 =
𝑔
18
𝑆 − 1
𝑑2
𝑣
Up to 1 Up to 0.1 mm
2
Hazen’s (Transition)
𝑉𝑠 =
4
3
𝑔𝑑
𝐶𝐷
(𝑆 − 1)
CD=
24
𝑅𝑒
+
3
𝑅𝑒
+ 0.34
>1 to 1000 >0.1 to 1 mm
3
Newton’s (Turbulent)
𝑉𝑠 = 3.33 𝑔𝑑 (𝑆 − 1) >1000 >1 mm
30
31. Temperature Effect on Settlement
The increase in temperature decreases the kinematic viscosity of
water and in turn increases the settling velocity of the particles and
vice-versa.
Settling velocity can be expressed in terms of temperature as:
𝑉𝑠 = 418 𝑆 − 1 𝑑2
3𝑇 + 70
100
Vs = settling velocity of particle in mm/s
d = diameter of particle in mm
T = temperature of water in Celsius
S = specific gravity of particle
31
32. Numerical : Find the settling velocity of particles of sp gr 2.65 at 20 ̊C, if the
diameter of particles is 0.005cm (IOE, TU 2063 Poush)
Solution:
Diameter of particles d= 0.005cm = 0.05mm
Since, d<0.1 mm Stokes’ Law is applicable.
So, 𝑉𝑠 = 418 𝑆 − 1 𝑑2 3𝑇+70
100
𝑉𝑠 = 418 2.65 − 1 0.052
3 ∗ 20 + 70
100
= 2.24 m/s
Check Reynolds Number :
Take kinematic viscosity, v =1.01 mm2/s
Re =
Vs d
v
=
2.24 𝑥 0.05
1.01
= 0.11
Since Reynolds number is less than 1, the application of Stokes’ law is correct.32
33. Numerical : Find the settling velocity of particles of sp gr 2.65 at 20 ̊C, if the
diameter of particles is 0.02cm.
Solution:
Diameter of particles d= 0.02cm = 0.2mm
Since, d>0.1 mm and d<1mm Hazen’s Law is applicable.
First Trial
Approximate value for first trial is obtained from Strokes’ Law.
So, 𝑉𝑠 = 418 𝑆 − 1 𝑑2 3𝑇+70
100
𝑉𝑠 = 418 2.65 − 1 0.22
3 ∗ 20 + 70
100
= 35. 86m/s
Re =
Vs d
v
=
35.86𝑥 0.2
1.01
= 7.101
33
37. 6.4.2 Ideal Sedimentation Tank
An ideal sedimentation tank is a sedimentation tank which aims to
achieve as nearly as possible the ideal conditions of equal velocity at
all points lying in the each vertical in the settling zone during the
design.
37
38. It consists of four zones:
Inlet Zone
It uniformly distributes the incoming flow over the cross section of the tank
and enters the settling zone without causing any disturbance to the settling
particles.
Settling Zone
In this zone, the settling of particles takes place.
The flow is steady and the particles are uniformly distributed throughout the
cross section normal to the direction of flow.
38
39. Sludge Zone
This zone provides for the collection of the particles that have settled.
It is assumed that all the particles reaching this zone are effectively
removed.
Outlet Zone
It collects the clarified water coming from the settling zone and
discharges via effluent channels.
39
40. Design of ideal sedimentation tank is based on following
assumptions:
The particles will settle in the settling zone exactly in the same
manner as in a quiescent condition of equal depth without any
disturbances.
The flow of water is steady and the velocity is uniform in all
parts of the settling zone for a time equal to the detention
period.
All the particles, after entering the settling zone are uniformly
distributed on the full cross section at right angles to the flow.
40
41. Consider a rectangular sedimentation tank of length L, width B and depth H.
Every moving particle has a horizontal velocity V and a settling velocity Vs. Thus
the path of a discrete particle is given by the vector sum of the flow velocity V
and its settling velocity Vs.
From geometric considerations, it can
be seen that
V
Vs
=
L
H
or Vs=
VH
L
Substituting the value of V, we get
Vs=
Q
BH
x
H
L
=
Q
B𝐿
41
Source: Water supply engineering Prof. Dr. Bhagwan Ratna Kansakar
42. Consideration of the assumed criterion indicates that all the particle with settling
velocity Vs equals to or greater than Q/BL will settle down and will be removed.
For particles with Vs<Q/BL, it will not settle down in tank. However, if this smaller
particle enters the tank at some other level h as shown in figure, then from
geometric consideration
V
Vs′
=
L
h
or Vs’=
Vh
L
Vs’=
Q
BH
x
h
L
=
h
H
Q
B𝐿
Again consideration of the assumed criterion indicates that all the particle with
settling velocity Vs equals to or greater than
h
H
Q
B𝐿
will settle down and will be
removed.
42
43. Thus if out of x’ particles of a particular size present in water x particles settle
down and are removed, the ratio of removal of particles of the size, i.e. x/x0 may
be taken equal to h/H for assumed uniform distribution of particles as shown by
the expression below:
h
H
=
Vs′
Q
BL
Hence,
x
x′
=
h
H
=
Vs′
Q
BL
The ratio x/x’ therefore represents the removal efficiency of a sedimentation tank
for the particle of same size.
43
44. Surface Overflow Rate (SOR)
The ratio Q/BL, discharge per unit plan area of sedimentation tank, is
known as surface overflow rate or overflow rate or surface loading.
Increase in plan area reduces the overflow rate and thus increases
the settling and removal efficiency of the tank.
The value of surface overflow rate normally adopted for the design of
plain sedimentation tanks ranges from 15 to 30 𝑚3/ day /𝑚2 and for
design of sedimentation tanks using coagulants ranges from 30 to
40𝑚3/ day /𝑚2
44
45. Numerical :
An old tank with dimension 11mX5mX3m is available in a village. It is
proposed to use as a settling tank. At least 93%of particles with diameter
0.025mm, sp gr 2.65 is expected to be removed. What will be the overflow
rate on using that tank? Is the tank dimension enough to remove 99% of
particles with diameter 0.05m at the same conditions ?
Solution:
Diameter of particles d= 0.025mm
So, Vs = 418 𝑆 − 1 d2 3T+70
100
Vs = 418 2.65 − 1 0.0252
3 ∗ 20 + 70
100
= 0.56 m/s
45
46. V′s
Vs
= 0.93 𝑜𝑟, 𝑉𝑠 =
𝑉′ 𝑠
0.93
=
0.56
0.93
= 0.60 mm/s
Hence, over flow rate, Vs = 0.6. mm/s
Surface area of the tank, As = 11 X 5 = 55 𝑚2
Vs =
Q
As
or, Q = Vs X As =
0.60
1000
X 55 = 0.033𝑚3/s
Now , new diameter of particles, d = 0.05mm
Vs = 418 S − 1 d2
3T + 70
100
Vs = 418 2.65 − 1 0.052
3 ∗ 20 + 70
100
= 2.24 m/s
46
47. Now,
V′
s
Vs
= 0.99
or, Vs =
V′s
0.99
=
2.24
0.99
= 2.26 mm/s = 2.26 x 10−3 m/s
So,
As =
Q
Vs
=
0.033
2.266 x 10−3 = 14.60 m2
< 55 𝑚2
Thus, the given tank dimension is enough to remove 99% of particles having
diameter 0.05mm
47
48. 6.4.3 Types of Sedimentation Tank
I. I. Draw and Fill Sedimentation Tanks
The tank is filled with raw water and retained in quiescent condition
for certain time as sediments settle down.
The tank is then cleaned after the sedimentation process is
completed and the process is repeated for the next lot.
Period of retention of tank:- about 24hrs
Period of Filling, emptying and cleaning:- about 6-12hrs in each
cycle
Complete cycle of operation:- 30-36hrs
48
49. II. Continuous flow sedimentation tanks
Raw water is continuously admitted into the tank and suspension
of particles takes place as water slowly flows out continuously
from the tank.
Working principle: Reducing the velocity of flow of water a large
amount of suspended particles present in water can be settled
down.
Velocity is reduced by providing sufficient length of travel for water
in the tank.
49
52. The raw water enters the tank through the inlet provided at
one side of the tank, it passes out through a outlet provided
at the opposite side of the tank.
The baffle(if present) is provided near the entrance and
spread along the width of the tank.
Dimensions of tanks.
Length:- shorter tanks up to 30m but larger tanks up to
100m.
Length : width ratio:- 3:1 or 5:1
Width:- Limited to 12m
Depth:- 2.5-5m(usually 3m)
52
53. Removal of Sludge
o Mechanized removal of sludge
• Slope of 1% from outlet end towards inlet end
• Sludge hopper with sludge withdrawal pipe is provided
near inlet end
• Side slope of Sludge hopper:- 1.2:1 to 2:1
(Vertical:Horizontal)
• Automatic sludge removal mechanism: A slow moving
scrapper that pushes the sludge into the hopper bottom
from which withdrawal pipe either by gravity or pumping.
53
54. o Non-mechanized(Manual) removal of sludge
• Floor is provided with a cross slope of about 10% from sides towards
the longitudinal center line, and longitudinal slope of at least 5% from
the outlet ends towards the inlet end
• Supply of influent water is stopped and the washout valve located in
the sludge withdrawal pipe is opened
• The water in the tank is drained out which washes out the sludge
deposited in the sludge hopper
54
55. Function of Baffle:
o Enable the flowing water to spread evenly and thus prevent direct current.
o Provided near the outlet to prevent the floating matter and scum from
escaping with the effluent.
o Prevent the short circuiting of flow
o Induced longer path of travel
o Prevent turbulence of water
55
58. Types of Circular Tanks with Radial Flow
Circular tank with central feed
Water enters the tank at the center and leaves at its
periphery.
It is more commonly used.
Circular tank with peripheral feed
Water enters the tank from the periphery or rim and
leaves at its center.
58
60. Raw water enters continuously at the center of the tank and
emanated from multiple ports of circular well in the center of the
tank to flow radially outwards in all directions equally.
Water flows radially towards the periphery where it passes
through controlling notch or weir and into an effluent channel
and finally into an effluent pipe.
Diameter of Tank:- 30-60m
Slope of floors:- 1:12 (Vertical : Horizontal)
60
62. Dorr Clarifier
Type of Circular continuous flow tank with circular feed
Raw water enters continuously through a vertical inlet pipe at
the center and emanates from multiple ports of influent
diffuser
Circular baffle is provided to reduce velocity of incoming water
Sludge removal mechanism continuously removes the sludge
deposited at the bottom of the tank.
62
64. B. Vertical Flow Tanks
Square or circular shape at the top and hopper bottom.
The flow of water takes place along vertical direction.
Water enters through centrally placed inlet pipe and by
action of deflector box drops vertically.
Sludge is collected at the bottom of the tank from where it is
removed by a sludge pipe connected to a sludge pump.
Clear water flows out through a circumferential weir
discharging into a draw off channel.
64
66. 6.4.4 Design of Sedimentation Tank
Factors
Affecting the
Design
Velocity
of Flow
Detention
Period
Flowing
Through
Period
Surface
Overflow
Rate
Tank
Dimensions
Inlet and
Outlet
Arrangements
66
67. Velocity of Flow
Velocity of the flow of water in sedimentation tank should be such that:
Maximum settling of suspended particles is caused in the tank.
If the particles is settled and reached the sludge zone it should not be
scoured or lifted up.
Camp has given expression for displacement velocity to start the motion of
settling particles:
𝑉 𝑑 =
β =0.04 for unigranular sand f=friction factor(0.025-0.03)
= 0.06 for non uniform sticky S=specific gravity
8β𝑔
𝑓
S − 1 d
67
68. Detention Period
It is the theoretical time water is detained in the sedimentation tank.
In case of continuous tank it is defined as the theoretical time taken by a particle
of water to pass from entry to the exit of the tank.
If C be the capacity of the sedimentation tank and Q be the discharge detention
period to is given as
𝑡 𝑜 =
𝐶
𝑄
For rectangular tank,
𝑡 𝑜 =
𝐿𝐵𝐻
𝑄
For a circular tank with bottom slope of 1:12,
𝑡 𝑜=
݀2(0.011݀ + 0.785)ܪ
𝑄
68
69. Flowing Through Period
Actual time taken by the water to pass through a sedimentation tank.
Determined with the help of dyes and chemicals such as sodium chloride and
radioactive isotopes.
Displacement efficiency is the ratio of flowing through period to detention period.
It vary from 25% to 50%. A well designed tank should have flowing period of at
least 30% of the detention period.
69
70. Surface Overflow Rate
Quantity of water passing per unit time which is the discharge per unit plan area
of a sedimentation tank is known as SOR or overflow rate or surface loading.
The value of surface overflow rate normally adopted for the design of plain
sedimentation tanks ranges from 15 to 30 𝑚3 /day/𝑚2 .
For the design of sedimentation tanks using coagulants ranges from 30 to 40 𝑚3
/day/𝑚2
70
71. Tank Dimensions
The effective depth of the sedimentation tank should be from 2.5 to 4m.
The free board should be 0.5 to 1m.
The provision for storing of sludge until it is removed is made for a depth of 0.5 to
1m.
For rectangular sedimentation tank the maximum width is 12m with length /width
ratio of 3 to 5.
The maximum length generally do not exceed 30m, however length up to 100 m
has also been also used.
For circular sedimentation tank the maximum diameter generally do not exceed
30m,however,diameter up to 60m has also been used.
71
72. Inlet and Outlet Arrangements
Water enters through inlet or influent structure and leaves through effluent structure
Inlet structure must:
i. Uniformly distributed flow
ii. Minimize large scale turbulence
iii. Initialize longitudinal or radial flow
Outlet structure must consist weir, notches or orifices, effluent trough and outlet
pipes.
For the design of effluent trough the following equation is generally used:
72
73. ܪ = ℎ2 +
2 𝑞𝐿𝑛 2
𝑔𝑏2
ℎ
where,
H=water depth at upstream trough
h=water depth at downstream trough
q=discharge per unit length of water
b=width of trough
n=number of sides the weir receives the flow
In absence of any control device
ℎ =
3 𝑄2
𝑏2
𝑔
73
76. Example
Compute the dimensions of continuous flow sedimentation tank for a population of
30000 person with a daily per capita demand of 100litre. Assume detention period to
be 6 hours.
Here,
Flow, Q =30000*100 =3000000 l/d =3000m3 /d =125m3 /hr
Detention period, t= 6 hr
Capacity, C= Q*t =125*6 =750 m3
Assume Surface Overflow Rate, SOR = 15 m3 /m2 /d
The SOR should be 15-30 m3 /m2 /d.
Surface area, As =
𝑄
𝑆𝑂𝑅
=
30000
15
=200 m2
Depth, H =
𝐶
As
=
750
200
= 3.75 m
76
77. Assume
𝐿
𝐵
= 4. (The
𝐿
𝐵
ratio should be 3 – 5.)
As = L*B = 4B*B = 4B2 = 200 m
B =
200
4
= 7.07 m, say 7.10 m.
Length, L = 4B = 4*7.1 = 28.4 m.
Assume free board = 0.5 m. Free board should be 0.5 – 1.0 m.
Assume sludge depth = 0.75 m. Sludge depth should be 0.5 – 1.0 m.
Total depth = 3.75+0.5+0.75 = 5.0 m
Provide sedimentation tank of overall dimension
28.4m*7.1m*5.0m
77
78. Example:
Find the dimension of a circular sedimentation tank for the following data.
Volume of water to be treated = 3 million liters per day
Detention period= 4 hours and velocity of flow= 10cm/min
Assume other data if necessary.
Here,
Flow, Q =3MLD =3000000 l/d =3000m3 /d =125m3 /hr
Detention period, t= 4 hr
Velocity of flow, V = 10 cm/min
Capacity, C= Q*t =125*4 =500 m3
78
79. For a circular tank with bottom slope of 1 vertical to 12 horizontal,
capacity of tank is given by
C = d2(0.011d+0.785H)
Assume depth of tank,
H = 3 m
Therefore by substitution, we get,
500 = d2(0.011d+0.785*3)
or,500 = 0.011d3 + 2.355d2
Solving the above equation by trial and error, we get,
d = 14.11 m
79
80. 6.5 Sedimentation with Coagulation
Plain sedimentation inefficient in case of raw water containing fine
suspended particles of clay and silt or light colloidal matters under
reasonable detention period.
Settling down and removal of such particles possible by chemically
assisted sedimentation called sedimentation with coagulation.
Chemicals added called coagulants.
Process of formation of floc is flocculation which is insoluble and
gelatinous.
Floc being positively charged absorbs and entrains suspended particles,
then being heavy settles down.
80
81. 6.5.1 Common Coagulants
1) ALUMINUM SULPHATE(OR ALUM)
2) IRON SALTS
3) CHLORINATED COPPERAS
4) SODIUM ALUMINATE
81
82. (1) Aluminum Sulphate Al2(SO4)3.18H2O called alum or filter alum, is most widely used
To flocculate, to which the water added shall be alkaline
Bicarbonate alkalinity generally present which gives insoluble
Al(OH)3 as floc
Al2(SO4)3 .18 H2O + 3Ca(HCO3)2 = 2Al(OH)3 + 3CaSO4 + 6CO2 +
18 H2O
Effective pH range 6.5-8.5
82
83. Incase little or no alkalinity, lime or soda ash used as
Alum + lime Al2(SO4)3 .18 H2O + 3Ca(OH)2 = 2Al(OH)3 + 3CaSO4 +
18 H2O
Alum + soda ash Al2(SO4)3 .18 H2O + 3Na2CO3 +3 H2O = 2Al(OH)3
+3Na2SO4+CO2+18H2O
Effective pH range 6.5-8.5
83
84. 2)Iron Salt
The various iron salts which are used as coagulants are:
(a) Ferrous sulphate (FeSO4.7H20)
Also known as copperas and is used as coagulant in coagulation with
lime
Chemical reaction depends upon the order in which chemicals are
added to water
(i) When ferrous sulphate is added first
FeSO4.7H2O + Ca(HCO3)2 = Fe(HCO3)2 + CaSO4 + 7H2O
Fe(HCO3)2 + 2Ca(OH)2 = Fe(OH)2 +2CaCO3 + 2H2O 84
85. (ii) When lime is added first
FeSO4.7H2O + Ca(OH)2 = Fe(OH)2 + CaSO4 + 7H2O
Finally, Fe(OH)2 is oxidized to Fe(OH)3 by dissolved oxygen Floc is
gelatinous and effective pH range is 8.5 and above
b) Ferric Chloride (FeCl3)
Used with or without lime
When used with lime, the reaction involved is:
FeCl3 + 3Ca(OH)2 = 2Fe(OH)3 + 3CaCl
85
86. When used without lime, the reaction involved is:
FeCl3 + 3H2O = 2Fe(OH)3 + 3H+ +3Cl-
Fe(OH)3 formed behave as gelatinous floc
(c) Ferric sulphate [Fe2(SO4)3]
Used in conjunction with lime
The chemical reaction involved is:
Fe2(SO4)3 + 3Ca(OH)2 = 2Fe(OH)3 + 3CaSO4
Fe(OH)3 formed behave as gelatinous floc
Effective pH range 4 to 7 and above 9
86
87. Mixture of ferric chloride and ferric sulphate
Prepared by adding chlorine to a solution of ferric sulphate in ratio 1
part of chlorine to 7.8 part of ferrous sulphate.
6[FeSO4.7H2O]+Cl2=2[FeCl3.Fe2(SO4)3]+42H2O
Forms tough floc which helps in sedimentation
Effective pH range is 3.5-6.5 and above 8.5
87
3.Chlorinated Coppers
88. Reacts with salts of calcium and magnesium to form calcium and
magnesium
aluminate
The chemical reaction involve are as follows:
Na2Al2O4 +Ca(HCO3)=CaAl2O4+Na2CO3+CO2+H2O
Na2Al2O4 +CaCl2=CaAl2O4 +2NaCl
Na2Al2O4 + CaSO4=CaAl2O4 +Na2SO
Effective pH range 6-8
88
4.Sodium Aluminate
89. Factors Determining Dosage of Coagulants
Turbidity of water
It’s color
pH value
Time of settlement
Temperature of water
The optimum dose of coagulant is determined by jar test
89
90. 6.5.2 Feeding the coagulant
Coagulant is fed to raw water either in powder form or in solution
Former one is known as dry feeding whereas the later one is known as
wet feeding
The choice between dry and wet feeding of coagulant depends upon
I. Characteristics of the coagulant and the convenience of its application
II. Dosage of coagulant
III. Size of treatment plant
90
91. 6.5.3 Mixing Devices
Success of floc formation depends on mixing of coagulants
with raw water.
Mixing to be vigorously and thoroughly to fully disperse into
entire mass
Various devices adopted are:
I. Mixing basins with baffle walls
II. Mixing basins with mechanical means
III. Mixing channel
IV. Hydraulic jump method
V. Compressed air
VI. Centrifugal pumps 91
92. I. Mixing Basins with Baffle Walls
Rectangular basins or tanks with baffle walls
Mixes due to hindrance and disturbance in flow by
walls and its agitation vigorously.
Two types:
a) Horizontal or round the end type
b) Vertical or over and under type
92
93. 93
Horizontal or round end type
Water + coagulant enter the basin
through inlet at one end
The mixture moves horizontally for short
distance
Due to the presence of baffle wall, it take a
turn and move further
Ultimately flows out through the other end
Fig: Round the end type mixing basins
Source: Water Supply Engineering by Dr.
Bhagwan Ratna Kansakar
94. b. Vertical or over and under type
Mixture of water and coagulant flows up and down due to
the presence of baffle walls projecting alternatively
Ultimately flows out through an outlet provided at the other
end
94
Source: Water Supply Engineering by Dr. Bhagwan Ratna Kansakar
Fig: Over and under type mixing basins
95. Design consideration for mixing basins with baffle walls
Velocity of flow 0.15 -0.45 m/s
Detention period 20-50 minutes
Distance between successive baffle walls at least 0.45 m
Clear opening between the end of each baffle and basin
wall(roof or floor) be 1.5 times distance between successive
walls, minimum value of 0.675 m
95
96. ii. Mixing Basins with Mechanically Mean
Flash mixer =mixing basin + mechanically
driven impeller or paddle
High head loss and variation velocity occurs in
basin
Detention period 0.5 -1 minute
Ratio of tank height to diameter ratio is 1:1 to
3:1
Displacement capacity of impeller is greater
than maximum flow through tank
96
97. iii. Mixing channel
Narrow mixing channel with vertical baffles
projecting in inclined position from both
sides of channel.
Violent agitation.
Flume if present develops hydraulic jump
causing turbulence and measure flow.
97
Source: Water Supply Engineering by Dr.
Bhagwan Ratna Kansakar
Fig: Mixing Channel
98. iv. Hydraulic jump method
Flume with considerable slope
Creates vigorous turbulence
v. Compressed air
Water with constant coagulant fed into basin
Compressed air diffused from bottom of basin
Rising air causes vigorous mixing
vi. Centrifugal pumps
Centrifugal pumps to lift water to settling tank is introduced with coagulants in
suction line of pump.
Agitation during passage of water through impeller of pump.
Gentle agitation desired.
98
99. 6.5.4 Flocculation Tanks
Water from the mixing basins is taken to the flocculators
Slow stirring of water is brought about to permit build up and agglomeration
of the floc particles
Different types of flocculators but mechanical flocculators are mostly used
Mechanical flocculators consists of rectangular tank with paddles for stirring
water
Mechanical flocculators are further classified as
(i) longitudinal flow flocculators
(ii) vertical flow flocculators
99
100. Vertical flow flocculators Longitudinal flow flocculators
i. Consist of circular tank i. Consist of rectangular tank
ii. Consist of paddle revolving in
vertical shaft
ii. Consist of paddles revolving on a
horizontal shaft
iii. The inlet and the outlet are
provided at the opposite ends
iii. Both inlet and outlets are provided
near the top of the tank
100
101. Design Criteria of Flocculators
Depth of tank 2-4.5 m
Detention period 10-40 minutes
Velocity of flow 0.2-0.8 m/min.
Total area of paddle 10-25% of cross-sectional area of tank
Outlet flow velocity to outlet channel 0.1-0.25 m/s
101
102. 6.5.5 Clarifier
Water from flocculators is taken to clarifier after flocculation.
Retained sufficiently to permit settling down to bottom
Detention period 2-2.5 hours
Surface overflow rate 30-40cubicm/day/sq .m.
102
105. Consists all four units coagulant feed, flash mixer, flocculator, clarifier
in single compact unit
Developed by Dorr co.
Coagulant fed, thoroughly mixed , slow stirring of water allowing
agglomeration of floc , and ultimately settling down
The water then passes to filter
105
106. 6.5.7 Jar test
Consists of rotary device called multiple stirrer having rods and paddle
sat the bottom.
The dose of coagulant added for the purpose of coagulation and
sedimentation should be such that good flocs are formed.
The dose of coagulant at which turbidity minimum called optimum dose
of coagulant.
106
110. 6.6 Filtration
Filtration is the process of passing water through thick layers of porous
media which is most of the cases is a layer of sand supported on a
bed of gravel.
Filtration is generally adopted in the purification of the suspended
matter , fine flocs etc which are not effectively removed by coagulation.
6.6.1 Theory of Filtration
1. Mechanical straining
2. Sedimentation and adsorption
3. Biological metabolism
4. Electrolytic action
110
111. 1.Mechanical straining
The particles of suspended matter that are of lager size or voids
between the sand grains are arrested and removed by the action of
mechanical straining.
It cannot remove colloidal matter or bacteria too small to strained out.
2. Sedimentation and adsorption
By the action of sedimentation and adsorption colloids, small particles of
suspended matter and bacteria are removed.
Due to physical attraction between sand and suspended particles,
suspended particles get adhered to sand grains.
111
112. 3.Biological metabolism
The growth and life processes of the living cells are known as biological
metabolism.
The adsorbed bacteria utilizes organic impurities such as algae, plankton etc.
present in water and convert them into harmless compounds by the complex
biochemical reactions.
The layer of harmless compound deposited over sand layer is called
“schmutzdecke”(dirty skin).
112
113. 4. Electrolytic Action
It states that when two substances with opposite charges comes in
contact then electric charge become neutral forming new substances.
It is observed that some of the sand grains of filters are charged with
electricity of some polarity.
When particles of suspended and dissolved matter having electricity of
opposite polarity come in contact with sand grains, they neutralize each
other and it result in changing the chemical characteristics of water.
113
114. 6.6.2. Types of Filter
On the basis of the filtration rate and the driving force to overcome
the frictional resistance encountered by the water flowing through
filter, the filters are classified as
Slow sand filters (gravity type)
Rapid sand filters (gravity type)
Pressure filters
114
115. Elements of Slow Sand Filter
Enclosure tank
Rectangle tank of stone, brick and concrete.
Depth: 2.5-4m Area: 50-100sq.m
Filtration rate: 100-200lit/hour/sq.m
Filter media
Sand layer: 90-100cm thick with 0.25-0.35mm sand size.
Cu: 3-5
Finer the and more the efficiency of filtration.
Sand shouldn’t contain more than 2% of Ca and Mg as carbonate.
Base media
Gravel bed of 30-75 cm thick to support filter media.
This bed is laid in layers 15 cm thickness.
115
116. Under drainage system
It collects the filtered water and delivers it to the clean water reservoir.
It consists of central drain or manifold and lateral strains.
Lateral drains are placed at a distance of 2 to 3m
Lateral drains may consist of earthenware pipe or perforated pipes of
7.5 to 10 cm diameter laid with open joints or patented drain devices.
Appurtenance
For effective working, vertical air pipes , depth controlling
Devices , head loss measuring device , flow regulator , etc. are installed.
116
118. Working of Slow Sand Filter
Water of
sedimentation
Inlet
chamber
Filter
media
Clean water
storage
tank
Outlet
chamber
Drainage
system
118
119. FIG: CLEANING OF SLOW SAND
FILTER
Water in filter tank is drained off
Top layer of sand is scrap manually up to
depth 15-30mm
Removed sand are dried
Dried sand is put back in
119
120. Efficiency of Slow Sand Filter
1.Bacterial filtration : highly efficient.
2.Turbidity: to the extent about 50 ppm can be removed.
3.Color: removes 20-25%
4.Colloidal matter: not much efficient.
120
121. 1. Average water consumption rate is 150 lpcd in an urban area. Design
a slow sand filter having a population of 10000 at the base year 2068.
Solution:
Base year=2068
Population at the base year , P=10000
Per capita demand of water=150 lpcd
Assume design period =15 years
Annual population growth rate , r=1.5%
121
Numerical 1
122. Design year=Base year + Design year = 2068 +15 =2083
Population in 2083 ,
P2083 =P( 1+ r/100 )
=10000( 1+1.5/100 ) =12502
Water demand in design year ,
Q=Population x per capita demand
=12502 x 150 =1875300 liters/day =78137.50 liters/hr
Assume filtration rate=150 liters/hr/m2 .
122
123. Filtration rate should be 100-200 liters/hr/m2
Surface area, As = (Q/filtration rate)=(78137.50/150)=520.92 m2
Provide 3 units of slow sand filters including one stand by unit.
Surface area of each unit , As = 520.92 / 2 = 260.46 m2
Assume L/b =2
As = L x B =2B =260.46 m2
B=√(260.46/2) =11.41 m , say 11.50 m
L= 2B=2 x 11.50 =23.0 m
123
124. Provide depth as follows
Free board = 0.5 m
Water depth = 1.0 m
Sand depth = 1.0 m
Gravel depth = 0.6 m
Depth for under drain pipe = 0.2 m
Total depth = 3.3 m
Provide 3 nos of slow sand filters of 23.0 m x 11.5 m x 3.3 m
124
