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1. Environmental Engineering- I
By
Akash Padole
Department of Civil Engineering
“Treatment of Water”
Filtration
Water Softening

2. 2

3. Filter Media
Porous medium
(Fine sand)
Raw water
Clear Filtered Water
3

4. Filtration – Objectives
 To remove suspended, colloidal and other impurities
that impart turbidity and that are untrapped by
previous processes.
 To reduce bacterial load.
 To produce sparkling and aesthetically attractive
water.
 To reduce colour and odour.
 To alter chemical characteristics of water (acceptable
side).
4

5. FILTRATION
• It is the process of separating suspended & colloidal
impurities by passage through porous media.
• Useful for removing turbidity, colour, micro-
organisms, organic matter, dissolved minerals.
5

6. CLASSIFICATION
 Based On Direction Of Flow:
Down-flow, Up-flow, Bi-flow, Radial Flow, etc.
 Based On Type Of Filter Media and Sand:
Granular Media Filter
Fabric And Mat Filters
Micro-strainers
 Method Of Flow Rate Control:
Constant Rate
Variable Rate 6

7.  Based on Driving Force:
Gravity Filters
Pressure Filters (Swimming Pools)
 Depending Upon Filtration Rate:
Slow Sand Filters
Rapid Sand Filters
7

8. Steps involved in filtration:
1. MECHANICAL STRAINING:
• Suspended solids bigger than voids in sand layer,
get trapped. Also, flocs get trapped.
• These in turn help to strained out other finer
impurities.
8

9. 2. FLOCCULATION – SEDIMENTATION:
– It is found that filters can even remove particles of
smaller size than voids.
– This is due to the fact that these voids act as if
they are tiny “coagulation-flocculation-
sedimentation” tanks.
– Finer particles get adhered to sand surface and
get removed.
9

10. 3. BIOLOGICAL CHANGES:
– Micro-organisms in water - enter voids – reside
over sand grains or may get trapped, use organic
matter as food– convert them into simpler
substances – form layer at top – “schmutzdecke /
dirty skin”.
– This layer helps for further absorption and
straining of impurities.
10

11. 4. ELECTROLYTIC ACTION:
• Sand grains and ionized matter in water carry
electrical charges of opposite nature,
• when they come in contact with each other ---- get
stuck to each other,
• when electrical charges on sand get exhausted,
• clean by backwashing.
11

12. Types of FILTER
Slow Sand Filters
Rapid Sand Filters
Dual Media Filters
Pressure Filters
12

13. Slow Sand Filters
13

14. Cross section
14

15. Plan
15

16. Under Drainage System
16

17. Purification Mechanism In Slow Sand Filter
• Water is subjected to various purification
mechanisms as it percolates through various layers .
• Impurities – removed by straining, sedimentation,
bio-chemical and biological processes.
• After start, thin slimy layer – “SCHMUTZDECKE” gets
formed – consists of variety of MO – convert OM into
simpler substances – considerable portion of inert
matter gets mechanically strained out in this layer.
17

18. • During passage of 0.4 m – 0.6 m layer of sand, water
becomes virtually free from S.S., colloidal solids,
pathogens, etc.
• Improvement in Physical, Chemical and Biological
characteristics of water
• Properly designed, constructed and operated filter:
can render filtrate that is fit for drinking (after
disinfection)
18

19. • Requirement of No. of filters depends upon the area
of filter.
19

20. Rapid Sand Filters
20

21. 60-90 cm
60 cm
Compressed Air
Wash water Storage tank
Wash water
Gutter
Central Drains
Lateral Drains
21

22. Wash water trough
Water level during filtration
22

23. 23

24. When is Backwashing Needed?
The filter should be backwashed when the following
conditions have been met:
• The head loss is so high that the filter no longer
produces water at the desired rate.
• Floc starts to break through the filter and the turbidity
in the filter effluent increases.
24

25. Wash water Storage tank
Compressed Air
25

26. Under Drainage System in R.S.F.
26

27. Design Parameters
(RSF)
(SSF) (30times more)
• Rate of filtration:
• Eff. Size of particle:
• Coefficient of uniformity:
(RSF)
(SSF)
• Area of Single unit:
27

28. • Amount of water for back washing = 2-5% of filtered
water for a period of 15 mins.
• Washing period = 24-48 hr.
• Depth of tank =2.5 – 3.5m
• Depth of Sand particle = 60- 90 cm
• Depth of Gravel layer = 60 cm
• No. of filters required
…Q- MLD
𝑁𝑜 𝑜𝑓 𝑓𝑖𝑙𝑡𝑒𝑟𝑠 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 =
𝑇𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎
𝐴𝑟𝑒𝑎 𝑜𝑓 𝑜𝑛𝑒 𝑢𝑛𝑖𝑡
N = 1.22 𝑄
28

29. 29

30. HL = (1-n) (G-1) Depth
30

31. Difference Between Slow Sand Filter And Rapid Sand Filter
31

32. 32

33. Operational Troubles in Rapid Sand Filters
• Air Binding
• Formation of Mud Balls
• Cracking of Filters
33

34. Air Binding
• When the filter is newly commissioned, the loss of
head of water percolating through the filter is
generally very small.
• However, the loss of head goes on increasing as
more and more impurities get trapped into it.
• A stage is finally reached when the frictional
resistance offered by the filter media exceeds the
static head of water above the bed.
• The bottom sand acts like a vacuum, and water is
sucked rather than getting filtered through it.
34

35. • The negative pressure so developed, tends to release
the dissolved air and other gases present in water.
• The formation of bubbles takes place which stick to
the sand grains. This phenomenon is known as Air
Binding as the air binds the filter and stops its
functioning.
• To avoid such troubles, the filters are cleaned as soon
as the head loss exceeds the optimum allowable
value.
35

36. Formation of Mud Balls
• The mud from the atmosphere usually accumulates
on the sand surface to form a dense mat.
• During inadequate washing this mud may sink down
into the sand bed and stick to the sand grains and
other arrested impurities, thereby forming mud balls.
36

37. Cracking of Filters
• The fine sand present in the top layers of the filter
bed shrinks and causes the development of shrinkage
cracks in the sand bed.
• With the use of filter, the loss of head and pressure
on the sand bed goes on increasing, which further
goes on widening these cracks.
37

38. Rapid sand filter is to be provided in a treatment plant
for a population of 2,75,000, water demand is 200 litres
per capita per day. Rate of filtration is 15 m3/m2/hr.
Allow 5% of filtered water for storage to meet
backwashing requirement; Each backwash period is 30
minute.
Determine the number of filter required allowing one
for standby; available surface area of each filter is 10 x
4 m2.
Numerical 1:
38

39. Max. daily demand = 1.8 x 275000 x 200
= 99 MLD
5% for backwashing for 30 min
Q =
1.05 𝑥 99
23.5
= 4.423 ML/hr
Plan area =
4.42 𝑥 106 𝑥 1
1000
15
= 294.66 m2
No of filters =
294.66
10 𝑥 4
= 7.37
= 8 + 1 = 9 filters
Solution:
Rate of filtration
is 15 m3/m2/hr.
39

40. Rapid sand filter is proposed for a water treatment
plant for a town having population of 75,000 average
water supply is 150 lpcd. Rate of filtration is 100
lit/m2/min. Determine the size and the number of filter
bed. Design the lateral and manifold system of under
drain.
Given:
• Population = 75000
• Average water supply is 150 lpcd
• Rate of filtration is 100 lit/m2/min
Numerical 2:
40

41. Solution:
41

42. 42

43. 43

44. 44

45. 45

46. 46

47. 47

48. 48

49. 49

50. Dual Media Filter
• A sand-anthracite filter or dual media filter/multi-
media filter is primarily used for the removal of
turbidity and suspended solids as low as 10-20
microns.
50

51. • “Anthracite” is an excellent filter media for water
clarification in drinking or industrial use, when used
in combination with filtering sands.
• This filtering medium allows a higher flow, less
pressure drop and a better and faster backwash.
51

52. • Dual media filters provide very efficient particle
removal under the conditions of high filtration rate.
• Inside a sand-anthracite filter is a layered bed of filter
media.
52

53. 53

54. • Usually constructed of silica sand and anthracite coal
• Sand depth = 0.15 m to 0.4 m ; coal = 0.3 m to 0.6 m deep
• ANTHRACITE – larger effective size (0.9 – 1.0 mm); hence
removes large particles and flocs; particles smaller than this --
-- penetrate to sand layer (effective size = 0.5 to 0.55 mm) –
and get removed in sand layer
• Dual media filter – uses pore size of the 2 layers very
effectively.
• Disadvantage: Filter material held loosely in anthracite layer;
any sudden increase in hydraulic loading -- dislodging of
material ---- transported to sand layer ----- > rapid binding --
backing washing may be required more often.
54

55. FINE
SAND
COAL
COTTON
COTTON
PROTO-CLEAR WATER PURIFIER
By,
Danish Shaikh
Guided by,
Prof. Akash Padole
The proto clear water filter
consist of a layer of fine
sand and coal with a variety
in size and specific gravity.
This filter is designed to
remove turbidity and
suspended particle present
in the feed water.
The sample having turbidity
10.5 NTU is checked by this
filter which is come out to
be 4.5 NTU, i.e. it removes
the turbidity by nearly 60%.
55

56. FINE SAND
COAL
COTTON
COARSE
SAND
By,
Danish Shaikh
Guided by,
Prof. Akash Padole
The sample having turbidity
10.5 NTU is checked by this
filter which is come out to be
2.3 NTU, i.e. it removes the
turbidity by nearly 80%. The
reason is the tightly packed
filter media which arrest the
minute particle present in
water and decrease the
turbidity to optimum degree.
PROTO-CLEAR WATER PURIFIER
56

57. PRESSURE FILTERS
• Small rapid gravity filters placed in closed vessels and through
which water to be treated is passed under pressure
• As water is under pressure -> filters are located in air tight
vessel.
• Normal pressure range: 30 – 70 m of water (3 – 7 kg/cm2)
57

58. 58

59. • Constructional features
– May be installed in Vertical or horizontal direction; hence
classified as vertical pressure filters / horizontal pressure
filters.
– Steel cylinders are used as pressure vessels; may be
riveted or bolted.
– Diameters: 1.5 – 3 m; Lengths or heights: 3.5 to 8 m.
– Inspection windows are provided at top.
– Various control valves are provided.
– Rate of filtration = 6000 - 15000 lit/m2/hr.
59

60. • Advantages:
– Compact; full automation possible.
– Lesser space, lesser material required (as higher rate of filtration)
– No need of sedimentation and coagulation tanks.
– More flexible; rate of filtration can be adjusted by altering
pumping rate
– May not be useful for turbid waters for large flows but most
suitable for clear waters in small quantities (for isolated colonies;
small scale industries etc.)
– Water coming out has sufficient residual pressure. Hence further
conveyance does not require pumping (which may be required in
gravity filters)
60

61. • Disadvantages:
– Although rate of filtration is high, unit being small, overall output
capacity is less.
– Less efficient in removing bacteria and turbidities; hence poor
effluent quality.
– Costlier for large quantities.
– Filtration and backwashing takes place in closed container; hence
physical inspection is not possible
– Inspection, cleaning and replacement of sand and gravel is difficult
– As curved shapes (most of the times), wash water gutters can’t be
provided (they help for collecting impure b/w water)
61

62. Water Softening
• Softening is reduction or removal of hardness.
• Hardness is due to divalent metallic ions like − 𝐶𝑎+2,
𝑀𝑔+2,𝐹𝑒+2,𝑀𝑛+2,𝑆𝑟+2 etc.
• Two types of hardness.
– Temporary or carbonate hardness
– Permanent or non-carbonate hardness
• Permissible hardness: 75 –115 mg/l
• Softening may be done by the water utility at the
treatment plant or by the consumer at the point of
use.
62

63. • There are two forms of hardness,
1. Carbonate or temporary hardness associated with
carbonates and bicarbonates of calcium and
magnesium and
2. Non-carbonate or permanent hardness associated
with sulphates(SO4), chlorides(Cl) and Nitrate(NO3)
of calcium and magnesium.
63

64. 64

65. • Why to do softening?
– Reduction of soap consumption
– Lowered cost in the maintenance of plumbing
– Improved taste in foods prepared
65

66. Removal of Temporary Hardness
• Temporary hardness is due to carbonates and
bicarbonates of Calcium and Magnesium.
• It can be removed either by
– Boiling
– Adding lime to the water
66

67. Boiling
• 𝐶𝑎𝐶𝑂3 is slightly soluble in water.
• So it usually exists in water as a bicarbonate.
• Boiling will lead to the precipitation of 𝐶𝑎𝐶𝑂3 and
release of 𝐶𝑂2
𝐶𝑎(𝐻𝐶𝑂3)2 + Heat ------------> 𝐶𝑎𝐶𝑂3 + 𝐶𝑂2 + 𝐻2O
• 𝐶𝑎𝐶𝑂3 is precipitated.
• Magnesium carbonates and bicarbonates are not
satisfactorily removed.
• 𝑀𝑔𝐶𝑂3 is fairly soluble.
67

68. Addition of lime
• Hydrated lime is added to the water.
• Efficient in removal of both Calcium and Magnesium
Carbonates.
68

69. Removal of Permanent Hardness
• Lime Soda Ash – Chemical precipitation
Sodium Carbonate (Soda Ash)
69

70. • Caustic Soda – Chemical precipitation
• All forms of hardness can be converted to the
precipitates
70

71. Ion Exchange (Zeolite Method)
• Base exchange or Cation Exchange
• Calcium and Magnesium can be replaced by a non-
hardness cation usually Sodium.
• Use of zeolite (a naturally occurring Sodium -Alumino
Silicate) –Green sand.
• “Zeolite” is a natural or synthetic base or cation
exchange Hydrated Silicate of Sodium and Aluminum.
71

72. • When water is passed thought the zeolite bed,
Multivalent Cation causing the hardness are replaced
by Univalent Cation of zeolite leading to the removal
of hardness from the water.
72

73. Regeneration of Zeolite:
73

74. Advantages of Ion exchange:
• No problem of incrustation of pipes as there in lime
soda process.
• 𝐹𝑒+2, 𝑀𝑛+2 can also be removed but make the
process costly.
• No sludge is formed.
• Water of varying quality can also be treated.
• Very useful in textile industries, boilers etc. as Zero
Hardness can be achieved.
74

75. Demineralization
• Removal of minerals from water.
• Even to obtain mineral free water which is as pure as
distilled water.
Cation Exchange
Resin
Anion Exchange
Resin
(Acidity)
Exhausted Resin
75

76. • It is again regenerated by passing water through a
bed of cation exchange resins, and then through a
bed of anion exchange resins
• This method also gives zero hardness water and it
does not lead to the formation of sludge. 76

77. 77

78. Numerical:
𝑄𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑜𝑓 𝐿𝑖𝑚𝑒 =
𝐺𝑖𝑣𝑒𝑛 𝑊𝑒𝑖𝑔ℎ𝑡
𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑊𝑒𝑖𝑔ℎ𝑡
𝑥 74 … 𝑓𝑜𝑟 𝐿𝐼𝑀𝐸
𝑄𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑜𝑓 𝑆𝑜𝑑𝑎 =
𝐺𝑖𝑣𝑒𝑛 𝑊𝑒𝑖𝑔ℎ𝑡
𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑊𝑒𝑖𝑔ℎ𝑡
𝑥 106 … 𝑓𝑜𝑟 𝑆𝑂𝐷𝐴
78

79. 79