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1. Environmental Engineering- I
By
Akash Padole
Department of Civil Engineering
“Treatment of Water”
Screening
Aeration
Sedimentation (Coagulation And Flocculation)

2. Anybody who can solve the problems of water
will be worthy of two noble prizes – one for peace
and one for science.
– J F K
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3. OBJECTIVES OF WATER TREATMENT
• To remove dissolved gases, murkiness & colour.
• To remove unpleasant & objectionable tastes &
odors from water.
• To kill pathogenic germs harmful to human health.
• To make water fit for domestic as well as industrial
purpose.
• To eliminate tuberculation & corrosive properties of
water that affect pipes & conduits
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4. REVIEW OF TREATMENT PROCESSES /
TYPE OF IMPURITY TO BE REMOVED
Sr. No. Type Of Impurity Removal Process
1 Floating Matter (Leaves, Dead Animals) Screening
2
Suspended Impurities
(SILT, SAND, CLAY, etc.)
Plain Sedimentation
3 Fine Suspended Matter
Sedimentation With
Coagulation
4 Micro-organisms & Colloidal Matter
Chemically Aided
Sedimentation (F+C+S)
5 Dissolved Gases, Taste & Odour
Aeration &
Chemical Treatment
6 Softening Zeolite Process
7 Pathogenic Bacteria Disinfection
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5. Typical Water Treatment Flow Diagram
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6. G W SOURCE WITH EXCESS Fe, CO2, ODOUROUS GASES
AERATION
COAGULANT
MIXING
FLOCCULATION
SEDIMENTATION
FILTRATION
POST
CHLORINATION DISTRIBUTION
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7. EXCESS HARD GROUND WATER
G. W. AERATION SOFTENING R. S. F.
POST CHLORINATION
DISTRIBUTION
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8. G. W. WITH DISSOLVED SOLIDS / DE-MINERALISATION
G. W.
SOURCE
DISTRIBUTION SYSTEM
R. S. G. F. SOFTENING
POST
CHLORINATION
ION
EXCHANGE
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9. Water Treatment Units
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10. Akash Padole 10

11. Akash Padole 11

12. Units in Water Treatment Plant
Screening
Aeration
Coagulation and Flocculation
Sedimentation
Filtration
Disinfection
Softening
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13. SCREENING
• Screening is done for removal the heavy suspended
solids like plants, pieces of cloth, paper, wood, stones,
animals from the water.
• It is generally adopted for the treatment of surface
water.
• There are two types of screens:
– Coarse Screen
– Fine Screen
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14. • Coarse Screen
o It is in the form of bars of dia. 10 to 25 mm
having the spacing of 20 to 100 mm in between
them.
o Placed at an inclination of 3-6V:1H as it helps
effective cleaning. This also increased the area of
flow.
o Thereby increases the opportunity of Suspended
particles to retain over the screens.
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16. Akash Padole 16

17. • Fine Screen
o It is in the form of wire mesh of opening size of
10mm.
o Under normal treatment of water, fine screens
are generally avoided as it get clogged and
requires frequent cleaning which increases the
operational cost.
10mm
10mm
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18. AERATION
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19. • It is the process in which the water is bought in
intimate contact with air for following reasons:
– TO ADD OXYGEN TO WATER FOR IMPARTING “FRESHNESS”
e.g. Water from underground sources devoid of or
deficient in oxygen.
– EXPULSION OF CO2, H2S & OTHER VOLATILE SUBSTANCES
CAUSING TASTE & ODOUR
e.g. Water from deeper layer of an impounding reservoir.
– TO PRECIPITATE OUT IMPURITIES LIKE Fe & Mn in certain
forms e.g. Water from underground sources.
Fe2+ + O2 + H2O Fe(OH)3 + 8H+
Mn2+ + O2 + H2O MnO2 + 4H+
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20. • Precipitate of Mn is formed very slowly if pH is less
than 9, hence external alkaline reagent like KMnO4 is
added to increase the pH of water.
• To increase area of water in contact with air
– Smaller droplets produced -----> greater contact
between droplet & water.
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21. TYPES OF AERATION SYSTEMS
1. SPRAY NOZZLE AERATOR
2. CASCADE AERATOR
3. TRICKLING BED / MULTIPLE TRAY AERATORS
4. DIFFUSED AERATION SYSTEMS
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22. 1. SPRAY NOZZLE AERATOR
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23. 2. CASCADE AERATOR
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24. 3. TRICKLING BED / MULTIPLE TRAY AERATORS
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25. 4. DIFFUSED AERATION SYSTEMS
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26. SEDIMENTATION
• It is also called as Primary Sedimentation Tank.
• It is provided to remove the Suspended solids from
the water.
• The entire process of sedimentation is based upon
the single parameter i.e., Specific Gravity.
=
𝛾𝑠
𝛾𝑤
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27. Specific Gravity
Organic solids 1-2 1.2
Inorganic solids 2.6-2.9 2.65
• Factors affecting Sedimentation:
– Velocity of flow / Turbulence
– Viscosity of water
– Size of particles
Settling velocity ∝ Diameter2
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28. Stokes’ Law
Vs=
4
3
.
𝑔
𝐶𝐷
𝐺 − 1 𝑑
For the Laminar Flow, (R < 1 and d < 0.1mm)
CD =
24
𝑅𝑒
, 𝑅𝑒 =
𝜌 .𝑉𝑠.𝑑
𝜇
Vs=
𝑔
18𝜇
. 𝜌𝑤. 𝐺 − 1 𝑑2
𝛾𝑤 = 𝜌𝑤.
𝑔
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29. Types of Settling
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30. (Velocity field overlaps)
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31. Types of Sedimentation Tank
Quiescent type tank
or
Intermittent settling tank
Continuous Flow Type
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32. Akash Padole 32

33. Continuous Flow Type
Rectangular Settling Tank:
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34. Akash Padole 34

35. Q = A . V
= (H.B). Vf
Q = A . V
= (L.B). Vs
(H.B). Vf = (L.B). Vs
(H). Vf = (L). Vs
td =
𝐻
𝑉𝑠
=
𝐿
𝑉𝑓
Vs =
Design Discharge
𝑝𝑙𝑎𝑛 𝑎𝑟𝑒𝑎
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36. Akash Padole 36

37. Q:
In a continuous flow settling tank 3.5m deep and 65m
long, flow velocity of water is observed as 1.22cm/s.
what size of specific gravity 2.65 may be effectively
removed if kinematic viscosity of water is 0.01cm2/sec.
Given data:
• Depth = 3.5m
• Length = 65m
• Vf = 1.22 x 10-2 m/s
• d = ?
• G = 2.65
• v = 0.01 x (10-2)2 m2/s
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38. Given data:
• Depth = 3.5m
• Length = 65m
• Vf = 1.22 x 10-2 m/s
• d = ?
• G = 2.65
• v = 0.01 x (10-2)2 m2/s
td =
𝐻
𝑉𝑠
=
𝐿
𝑉𝑓
Vs =
3.5 𝑥 1.22 𝑥 10−2
65
= 6.57 x 10-4 m/s
Vs=
𝑔
18𝜇
. 𝜌𝑤. 𝐺 − 1 𝑑2
6.57 x 10−4
=
9.81
18 𝑥 0.01 x (10−2)−2
. 2.65 − 1 𝑑2
d = 2.72 x 10 -5 m
d = 0.027 mm
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39. Circular Settling Tank:
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41. Centrally fed sedimentation tank
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43. Akash Padole 43

44. Design Parameters
(Overflow rate)
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45. • The tanks are designed for the treatment of maximum
daily demand.
Qdesign = 1.8 x Q avg. daily
• Sludge Zone = 0.8 – 1.2 m
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46. Q:
Determine the surface area of settling tank for 0.5m3/s
of design flow using OFR as 32.5 m3/m2/day. Also find
out the depth of sedimentation tank if detention time
is 95 min. Assume length to width ratio in the range of
2:1 to 5:1 and length should not exceed 100m. Design
the tank.
Given data:
• OFR = 32.5 m3/m2/day
• t = 95 min
• L/B = 4:1
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47. • Surface area =
0.5
32.5
24 𝑥 3600
= 1329.23 m2
• Volume = Qdesign x td = 0.5 . (95 x 60) = 2850 m3
• Depth =
2850
1329.23
= 2.14 m ≅ 2.2 m
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48. Assume L/B = 4/1
L = 4B
Area = L. B
1329.23 = (4B).B
B = 18.23
As L = 4B = 4 x 18.23 = 72.9m < 100m
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49. Tube Settler
• Tube settler is a system used for clarification.
• It uses multiple tubular channels sloped at an angle
of 60° and adjacent to each other, which combine to
form an increased effective settling area.
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50. • It captures the settleable fine floc that escapes the
clarification zone beneath the tube settlers and
allows the larger floc to travel to the tank bottom in a
more settle able form.
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53. Coagulation aided Sedimentation
• The terms coagulation and flocculation used to
describe the process to remove turbidity caused by
fine suspensions and colloids.
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54. • Coagulation:
– Effect produced by addition of chemicals to a colloidal
dispersion.
– Achieved by adding appropriate chemical (coagulant) and
by rapid intense mixing.
• Flocculation:
– 2nd stage
– formation of settleable particles (i.e., flocs) from
destabilized colloidal particles
– achieved by gentle and prolonged mixing.
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55. • It is carried out in three stages:
Coagulation
Flocculation
Sedimentation
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56. Akash Padole 56

57. Ferrous
ALUM
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58. Jar Test Apparatus
To measure Optimum dose of coagulant:
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59. Different types of Coagulants
ALUM
COPPERAS
CHLORINATED COPPERAS
SODIUM ALUMINATE
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60. ALUM {Al2(SO4)3}
• Alum reacts with Bi-carbonate Alkalinity present in
water to form the sticky gelatinous precipitate of
Aluminum Hydroxide that attracts the fine
suspended particles over its surface thus grows in
size and get settles in the sedimentation tank
Al2(SO4)3 + 3Ca(HCO3)2 → 2Al(OH)3 + 3CaSO4 + 6CO2
Al(OH)3
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61. • 1 mole of Alum results in the formation of 2 mole of
Aluminum Hydroxide.
• Al2(SO4)3 = 27x2 + 3x(32+16x4) + 18x18 = 666 gm
• 3 Ca(HCO3)2 → 3{40 + 2x(1+12+16x3)} = 486 gm
• 2 Al(OH)3 = 2{27 + (16+1)x3} = 156 gm
1 mole Al2(SO4)3 + 3mole Ca(HCO3)2----> 2 mole Al(OH)3
666gm + 486gm  156 gm
1 gm Al2(SO4)3 + 0.73 gm Ca(HCO3)2 ------> 0.234 gm Al(OH)3
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62. • On adding alum, water imparts Hardness (as CaSO4)
& Corrosiveness (due to CO2 formed)
• Amount of alum required depends on initial turbidity
& colour of water
• Optimum alum dose determined by Jar Test in lab.
• Normal dose of Alum varies between 10 – 30 mg/l.
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63. Advantages:
• Alum- the most common coagulant used at WTP
• Cheap
• Does not require skilled manpower
• Easy to Store & Handle
• Helps to Remove Taste & colour as well
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64. Disadvantages:
• Sludge disposal is difficult.
• Favorable pH range is narrow (6.5 - 8.5); hence may
require use of other chemicals, making it costlier.
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65. COPPERAS (FeSO4; 7H2O)
• Copperas: (FERROUS SULPHATE) FeSO4. 7H2O
• Copperas is added along with lime to raw water.
• When copperas is added first, the reaction that takes
place is: Permanent Hardness
Temporary
Hardness
Fe(OH)3
Ferrous Hydroxide Fe(OH)2
Ferric Hydroxide Fe(OH)3
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66. • Extensively used for raw waters that are not coloured
• Not effective for coloured waters because Fe induces
colour into water.
• Cheaper than alum
• Works for pH >8.5
• Quantity: same as Alum.
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67. CHLORINATED COPPERAS
• When chlorine is added to copperas ->
Ferric Sulphate Fe2(SO4)3 & Ferric Chloride FeCl3 get
formed.
6(FeSO4.7H2O) + 3Cl2  2Fe2 (SO4)3 + 2FeCl3 + 42H2O
• Resultant combination of Ferric Sulphate + Ferric Chloride
is “Chlorinated Copperas”
Ferrous Sulphate
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68. 2Fe2 (SO4)3 + 3Ca(OH)2  3 CaSO4 + 2 Fe(OH)3
Ferric Sulphate + Hydrated Lime  Ferric Hydroxide
PPT
2FeCl3 + 3Ca(OH)2  3 CaCl2 + 2 Fe(OH)3
Ferric Chloride + Hydrated Lime  Ferric Hydroxide
PPT
• Resulting Ferric Hydroxide forms floc & helps in
sedimentation
When Lime is added:
Fe(OH)3
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69. • Ferric Sulphate is quite effective in pH range of 4 to 7
& above 9.
• Ferric Chloride is effective in pH range of 3.5 to 6.5
& above 8.5.
• Therefore combination of the two is effective for
wide pH range.
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70. SODIUM ALUMINATE (Na2Al2O4)
• Besides Alum & Iron Salts, Sodium Aluminate
(Na2Al2O4) is used as coagulant.
• It dissolves in water & reacts with salts of Ca & Mg
present in raw water, resulting in formation of ppt of
Ca or Mg Aluminate.
Na2Al2O4 + Ca(HCO3)2  CaAl2O4 + Na2CO3 + CO2 + H2O
(Calcium Aluminate)
Na2Al2O4 + CaCl2  CaAl2O4 + 2NaCl
Na2Al2O4 + CaSO4  CaAl2O4 + Na2SO4
CaAl2O4
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71. • Also helps to reduce temporary as well as permanent
hardness.
• However very effective if natural alkalinity in water is
less; No need to add alkalinity in water.
• Hence widely used for treating Boiler Fed Waters,
which permit low values of hardness.
• Costlier than Alum; hence not very common.
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72. Poly Aluminium Chloride (PAC)
• Poly Aluminium chloride (PAC) is manufactured in
both liquid and powder form.
• The product is used as a flocculent in water
purification, in treatment of drinking.
• The flocs formed are more dense and fast settling
than Alum.
• Its not mostly been used for normal treatment
purpose.
Al13(OH)20(SO4)3.Cl15
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73. Flash Mixer (Fast Mixing)
• In order to neutralize the negative protective charge
over the impurities during coagulation, Threshold
Energy is provided by inducing fast mixing in the
water which can be achieved by following methods.
a. Mixing Basin
b. Mechanical mixer
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74. a. Mixing Basin
• Gravitational flocculation: Baffle type mixing
basins are examples of gravitational flocculation.
• Water flows by gravity and baffles are provided in
the basins which induce the required velocity
gradients for achieving floc formation.
– Around and End type
– Over and Under type
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75. Around and End type
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76. Akash Padole 76

77. b. Mechanical mixer
• The mixing is induced mechanically by the vigorous
agitation of coagulant in the water.
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78. Akash Padole 78

79. • The intensity of mixing depends upon a parameter
termed as G (TEMPORAL MEAN VELOCITY GRADIENT)
which denotes the relative velocity between the two
particles situated at a distance at a particular
distance from each other.
𝐺 = 𝑉1
−𝑉2
𝑥
V- velocity of a particle (m/s)
x – distance between the particles (m)
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80. • Intensity of mixing: G =
𝑃
𝜇 . 𝑉𝑜𝑙
• Detention period= 30-60 sec
• G is generally kept to be more than 300 sec-1
• Power supplied= 1 -3 watts/unit discharge (m3/hr)
𝑑𝑒𝑝𝑡ℎ
𝑤𝑖𝑑𝑡ℎ 𝑜𝑟 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟
= 1: 1 𝑡𝑜 3: 1
Power (Nm/s)
Dynamic viscosity
(Ns/m2)
Volume (m3)
• The speed of shaft should be greater than 3 m/s
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81. Flocculation (Slow Mixing)
• It is the process in which suspended particles are
brought in intimate contact with each other so as to
promote agglomeration resulting in the formation of
increased sized flocs which can get easily settle in the
following sedimentation tank.
• In order to increase the opportunity to come in
contact with each other, slow mixing is induced in the
tank.
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82. • The rate of flocculation depends upon
– Turbidity
– Type and dose of coagulant
– Mixing (G)
• A parameter “G.td” represents the conjugation
opportunity i.e., no. of collision of particles in the
tank.
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83. IDEAL
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84. • Since large dense flocs get easily settled in the
sedimentation tank, it is advantageous to vary the
value of G along the length of flocculation tank.
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85. • Small dense flocs formed in the initial section of the
tank combines with the large light flocs results in the
formation of LARGE DENSE FLOCS.
• Hence G at inlet is kept twice at G at outlet
Ginlet = 2 x Goutlet
• G = 10- 75 sec-1
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86.  Depth of tank = 3- 3.5 m
 Detention time = 10 -30min
 Velocity of flow = 0.2- 0.8 m/s (0.4m/s)
 Total area of paddles = 10- 25% of tank plan area
 G.td :
 20,000- 60,000 for Alum
 1 – 1.5 lakh for Iron Salts
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87. Akash Padole 87

88. Numerical:
𝜌𝑤
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89. Akash Padole 89