- The rapid sand filter must treat 10 million liters of raw water per day, allowing 0.5% of filtered water for backwashing. - Assuming a filtration rate of 5000 liters/hour/square meter and half an hour per day for backwashing, the required filter area is calculated to be 85.5 square meters. - To allow for one filter to be taken offline for backwashing, the design calls for two filter beds, each with an area of 85.5 square meters.

1. Semester- 6, Division-C
Guided by,
Prof. H.H. Jariwala
L.D. College of Engineering, Ahmedabad-15
Filtration
[Water and Wastewater Engineering]

2. Prepared by,
Sr. No. Name of Student Roll No. Enrollment No.
1. Baloliya Payal R. 6062001 130280106007
2. Khandor Bhavya M. 6062005 130280106046
3. Merin Issac 6062007 130280106057
4. Patni Mo.Aamir Z. 6062016 130280106091
5. Shah Vrushti K. 6062023 130280106110
2

3. Content
3
Sr. No. Topic Slide No.
1. Introduction [04-05]
2. Theory of filter [06-10]
3. Filter materials [11-15]
4. Classification of Filters [16-16]
5. Slow Sand Filter [17-28]
6. Rapid Sand Filter [29-40]
7. Backwashing [41-45]
8. Comparison of SSF & RSF [46-47]
9. Design of Rapid Sand Filter [48-53]

4.  Sedimentation removes a large percentages of settable solids,
suspended solids, organic matter and small percentage of bacteria.
 But water still contains fine suspended solids, microorganisms and
color(if present).
 To remove these impurities, still further and to produce potable and
palatable water, the water is filtered through the beds of granular
materials like sand and gravel.
Introduction
4

5.  The process of passing the water
through the beds of granular
material(sand and gravel) is known
as Filtration.
 By doing filtration, we can remove
bacteria, colour, taste, odours and
produce clear and sparkling water.
Introduction
5

6.  When water is filtered through the bed of filter media, usually
consisting of clean sand, the following factors take place:
◦ (1) Mechanical straining
◦ (2) Sedimentation
◦ (3) Biological action &
◦ (4) Electrolytic action
Theory of filter
6

7. Mechanical straining
 Sand contains small pores. The suspended particles which are bigger
than the size of the voids in the sand layer, cannot pass through these
voids and get arrested.
 These arrested particles forms a mat on the top of the bed which
further helps in straining out
the impurities.
7

8.  In mechanical straining only those particles which are coarser than
void size are arrested. Finer particles are removed by sedimentation.
 The voids between sand grains of filter acts like small sedimentation
tanks.
 The colloidal matter arrested in the voids is the gelatinous mass and
therefore attract other finer particles.
Sedimentation
8

9.  Suspended impurities contain some portion of organic impurities such
as algae, plankton, etc., which are food of various types of micro-
organisms.
 These organic impurities form a layer on the top of sand bed which is
known as ‘Schmutzdecke’ or ‘dirty skin’.
 This layer further helps in absorbing and
straining out the impurities.
Biological action
9

10.  Another function of the filter is to remove the particulate matter by
electrostatic exchange.
 The charge of the filter medium neutralize the charge of floc, thereby
permitting the floc to be removed.
 During the process of ‘back washing’ of filter, the electrostatically
neutral material is removed and the charge of the filter
media is replaced.
Electrolytic action
10

11.  Sand(fine/coarse) is generally used as filter media.
 The layer of sand may be supported on gravel, which permits the
filtered water to move freely to the under drains and allows the wash
water to move uniformly upward.
 Filter materials:
◦ 1. Sand
◦ 2. Gravel
◦ 3. Anthracite
Filter materials
11

12. It should have following properties:
 Obtained from hard rock such as – Quartzite, Trap, Basalt, etc.
 Free from – Clay, Loam and Organic matter
 Uniform size and nature
 Hard and resistant
 If placed in HCl for 24 hr, it should not
loose more than 5% of its weight.
Sand
12

13.  Uniformity Coefficient(Cu):
◦ It is a measure of particle range and is
given by,
Sasasasaaaaaaaaaaaaaaaaaaaadsd
ddfsssssssssssssssds
 Uniformity coefficient shall be,
◦ 3.0-5.0 for Slow Sand Filter
◦ 1.3-1.7 for Rapid Sand Filter
Sand
 Effective size of sand(D10):
◦ Very fine sand: Clogged quickly &
reduce rate of filteration
◦ Very coarse sand: Suspended particles
and bacteria pass through the sand bed
 Effective size shall be,
0.20-0.30 mm for Slow Sand Filter
0.45-0.70 mm for Rapid Sand Filter
Depth of sand bed should be between 60 cm to 90 cm
𝐶𝑢 =
𝐷60
𝐷10
13

14.  It should be – hard, durable, free from impurities, properly rounded
and have a density of about 1600 kg/cubic meter.
 It supports the sand and allows the filtered water to move freely
towards the underdrains.
Gravel
 It allows the wash water to move upward
uniformly on sand.
 The gravel is placed in 5 to 6 layers having
finest size on top.
14

15.  Substitute for sand
 Can be used in conjunction with sand
 Cost is more as compared to sand
Anthracite
15

16.  Filters may be classified as:
Classification of Filters
Filters
Gravity filters:
Slow sand filter
(Biological Filter)
Rapid sand filter
(Mechanical Filter)
Pressure filters:
Horizontal
pressure filter
Vertical pressure
filter
16

17.  Used in rural areas in place of a rapid gravity filter
 Filtration rate is 50 to 100 times slower than that of a rapid gravity
filter (0.1 to 0.3 m/hour - 0.2 m/hr is the typical rate)
 Used for the removal of turbidity (colloidal particles), suspended
solids and pathogens
 Replaces the coagulation-flocculation-settling, the filtration and the
disinfection by chlorination treatments in rural areas
Slow Sand Filter
17

18.  Filtered water has < 0.3 NTU turbidity (the goal is < 0.1 NTU)
 Output water may require chlorination (for quality improvement)
 A pre-treatment in roughing filters may be needed specially when the
turbidity is high (greater than 20-50 NTU)
 Oxfam filters (use of geo-textile fabric on the top of the sand layer for
straining out the suspended matter (pre-treatment!)
Slow Sand Filter
18

19. Slow Sand Filter
 Essential features:
1) Enclosure tank 2) Filter media 3) Base material
4) Under drainage system 5) Appurtenances
19

20.  SSF is open basin, rectangular shape and built below finished ground
level.
 Floor has Bed slope of 1:100 to 1:200 towards central drain
 Surface area (As) of tank varies from
50 to 1000 sqm
 Filtration rate – 100 to 200 lit/sqm/hr.
 Depth – 2.5 to 4 m
1. Enclosure tank
20

21.  Thickness of sand layer - 90 to 110 cm
 Effective size – 0.20 to 0.35 (Common value -0.3)
 Coefficient of uniformity – 2.0 to 3.0 (Common value - 2.5)
2. Filter media: Sand
21

22.  Thickness of gravel bed - 30 to 75 cm
3. Base material: Gravel
Layer Depth Size(mm)
Top most 15 cm 3 to 6
Intermediate 15 cm 6 to 20
Intermediate 15 cm 20 to 40
Bottom 15 cm 40 to 65
22

23.  Base material and filter media are supported by under drainage
system.
 Under drainage system collects filtered water and delivers it to the
reservoir
 Laterals – earthenware pipes of 7.5 to
10 cm dia.
 Spacing of laterals- 2 to 3 m c/c
4. Under drainage system
23

24. Devices are required for
 Gauge – to measure loss of head
 Vertical air vent pipe – for proper functioning of filtering layers
 Telescopic tube – to maintain constant discharge
 A meter – to measure flow
5. Appurtenances
24

25.  In a slow sand filter impurities in the water are removed by a
combination of processes: sedimentation, straining, adsorption, and
chemical and bacteriological action.
 During the first few days, water is purified mainly by mechanical and
physical-chemical processes. The resulting accumulation of sediment
and organic matter forms a thin layer on the sand surface, which
remains permeable and retains particles even smaller than the spaces
between the sand grains.
Working of Slow Sand Filter
25

26.  As this layer (referred to as “Schmutzdecke”) develops, it becomes
living quarters of vast numbers of micro-organisms which break down
organic material retained from the water, converting it into water,
carbon dioxide and other oxides.
 Most impurities, including bacteria and viruses, are removed from the
raw water as it passes through the filter skin and the layer of filter bed
sand just below.
Working of Slow Sand Filter(contd..)
26

27.  The purification mechanisms extend from the filter skin to approx.
0.3-0.4 m below surface of filter bed, gradually decreasing in activity
at lower-levels as water becomes pure & contains less organic matter.
 When the micro-organisms become well established, the filter will
work efficiently and produce high quality effluent which is virtually
free of disease carrying organisms and biodegradable organic matter.
 They are suitable for treating waters with low colors, low turbidities
and low bacterial contents.
Working of Slow Sand Filter(contd..)
27

28. Disadvantages:
 Old fashioned and outdated
method of water purification
(but still in use)
 Initial cost is low but
maintenance cost is much more
than rapid sand filter
 These filters need a lot of space
Advantages:
 Simple to construct and operate
 Cheaper
 Physical, Chemical and
Bacteriological quality of water
is very high
 Reduces bacterial count by
99.9% & E. coli by 99%
Advantages & Disadvantages of SSF
28

29. Rapid Sand Filter
 Essential features:
1) Enclosure tank 2) Filter media 3) Base material
4) Under drainage system 5) Appurtenances
29

30.  Smaller in size, therefore can be placed under roof
 Rectangular in shape and constructed of concrete or masonry
 Depth – 2.5 to 3.5
 Surface area – 20 to 50 m2.
 L/B ratio – 1.25 to 1.35
 Designed filtration rate are 3000 to
6000 lit/sqm/hr
1. Enclosure tank
30

31.  Should be free from dirt, organic matter and other suspended solids
 It should be hard and resistant
 Depth of sand media – 0.6 to 0.9 m
 Effective size – 0.35 to 0.6 mm (Common value 0.45)
 Uniformity coefficient – 1.2 to 1.7 (Common value -1.5)
2. Filter media: Sand
31

32.  The depth of sand bed should be such that flocs should not break through the
sand bed.
 Depth varies from 60 to 90 cm
 Min depth required is given by Hudson’s formula
where,
q = Filtration rate in cum/sqm/hr [Assumed filtration rate x Factor of safety (2)]
D = sand size in mm
H = terminal head loss in m
l = depth of sand bed in m
Bi = Break through index = 4 x 10^(-4) to 6 x 10^(-3)
Estimation of depth
𝑞 ∗ 𝐷3
∗ 𝐻
𝑙
= 𝐵𝑖 ∗ 29323
32

33.  Thickness of gravel bed – 45 to 60 cm
3. Base material: Gravel
Layer Depth Size(mm)
Top most 15 cm 3 to 6
Intermediate 15 cm 6 to 12
Intermediate 15 cm 12 to 20
Bottom 15 cm 20 to 50
33

34.  To start with, a size gradation of 2 mm at top and 50 mm at bottom is
assumed.
 The required depth (l) in cm of a component of gravel layer of size d
(mm) can be computed by following equation
where,
K can be taken as 12
d = gravel size in mm
Estimation of depth
𝑙 = 2.54 ∗ 𝐾 ∗ log(𝑑)
34

35. Objectives of under drainage system
1. To collect filtered water uniformly
over the area of gravel bed
2. It provides uniform distribution of
back wash water without disturbing
or upsetting gravel layer and filter
media
4. Under drainage system
35

36. Typical devices required are,
1. Wash water troughs
2. Air compressors
3. Rate control device
5. Appurtenances
[Wash water trough]
36

37. Working of Rapid Sand Filter
37

38.  All valves are kept closed except valves A and B
 Valve A is opened to permit water from clarifier
 Valve B is opened to carry filtered water to clear water sump
 Head of 2m over sand bed is maintained
 Designed filtration rate are 3000 to 6000 lit/sqm/hr
 Filter run depends on quality of feed water
Working of Rapid Sand Filter
38

39.  Filter run may range between less than a day to several days
 Objective of backwash is to remove accumulated particles on the
surface and within the filter medium
 Backwash is performed using wash water or air scouring.
Working of Rapid Sand Filter
39

40. Disadvantages:
 Relatively high skill operation
 Costlier
 Removes bacteria by 98-99%
Advantages:
 Deal with raw water directly
 Occupies less space
 Filtration is rapid
 Washing of filter is easy
 More flexibility in operation
Advantages & Disadvantages of RSF
40

41. Rapid Sand Filter
41

42.  Filter is back washed when head loss through it has reached the
maximum permissible.
 RSF are washed by sending air and water upwards through the bed by
reverse flow through the collector system.
 2% - 4% filtered water is used for backwashing
Backwashing
42

43. 1. Close influent valve A
2. Close effluent valve B
3. Open air valve F, so that air blows at rate of 1 to 1.5 m3 free air
/min/m2 of bed area for @ 2 to 3 min. this will break up the scum
and loosen the dirt.
4. Close the air valve F and open the wash water valve E gradually to
prevent the dislodgement of finer gravel.
Steps in Back Washing
43

44. 5. Open the wastewater valve D to carry wash water to drain. Continue
backwashing till wash water appears fairly clear.
6. Close the wash water valve E. Close the wastewater valve D. wait for some
time till all matter in bed settles down.
7. Open valve A slightly, open valve C for carrying filtered water to drains for
few minutes.
8. Close the valve C and open valve B. Open valve A completely to resume
normal filtration
Steps in Back Washing(contd…)
44

45. Back washing at Treatment Plant
45

46. Comparison of SSF & RSF
46

47. Comparison of SSF & RSF(contd…)
47

48. 48
Problem: Design a rapid sand filter to treat 10 million litres of raw water
per day allowing 0.5% of filtered water for backwashing. Half hour per
day is used for backwashing. Assume necessary data.
Solution: Total filtered water = 10.05 x 24 x 106 = 0.42766 Ml/h
24 x 23.5
Let the rate of filtration be 5000 l / h / m2 of bed.
Area of filter = 10.05 x 106 x 1 = 85.5 m2
23.5 5000
Provide 2 units. Each bed area 85.5/2 = 42.77. L/B = 1.3; 1.3B2 = 42.77
B = 5.75 m ; L = 5.75 x 1.3 = 7.5 m
Assume depth of sand = 50 to 75 cm.
Rapid Sand Filter Design

49. 49
Underdrainage system:
Total area of holes = 0.2 to 0.5% of bed area.
Assume 0.2% of bed area = 0.2 x 42.77 = 0.086 m2
100
Area of lateral = 2 (Area of holes of lateral)
Area of manifold = 2 (Area of laterals)
So, area of manifold = 4 x area of holes = 4 x 0.086 = 0.344 = 0.35 m2 .
So, Diameter of manifold = (4 x 0.35 /p)1/2 = 66 cm
Assume c/c of lateral = 30 cm. Total numbers = 7.5/ 0.3 = 25 on either
side.
Rapid Sand Filter Design(contd…)

50. 50
Length of lateral = 5.75/2 - 0.66/2 = 2.545 m.
C.S. area of lateral = 2 x area of perforations per lateral. Take dia of
holes = 13 mm
Number of holes: n p (1.3)2 = 0.086 x 104 = 860 cm2
4
So, n = 4 x 860 = 648, say 650
p (1.3)2
Number of holes per lateral = 650/50 = 13
Area of perforations per lateral = 13 x p (1.3)2 /4 = 17.24 cm2
Spacing of holes = 2.545/13 = 19.5 cm.
Rapid Sand Filter Design(contd…)

51. 51
C.S. area of lateral = 2 x area of perforations per lateral
= 2 x 17.24 = 34.5 cm2.
So, Diameter of lateral = (4 x 34.5/p)1/2 = 6.63 cm
Check: Length of lateral < 60 d = 60 x 6.63 = 3.98 m. l = 2.545 m (Hence
acceptable).
Rising washwater velocity in bed = 50 cm/min.
Washwater discharge per bed = (0.5/60) x 5.75 x 7.5 = 0.36 m3/s.
Velocity of flow in lateral = 0.36 = 0.36 x 10 4 =2.08 m/s (ok)
Total lateral area 50 x 34.5
Manifold velocity = 0.36 = 1.04 m/s < 2.25 m/s (ok)
0.345
Rapid Sand Filter Design(contd…)

52. 52
Washwater gutter
Discharge of washwater per bed = 0.36 m3/s. Size of bed = 7.5 x 5.75 m.
Assume 3 troughs running lengthwise at 5.75/3 = 1.9 m c/c.
Discharge of each trough = Q/3 = 0.36/3 = 0.12 m3/s.
Q =1.71 x b x h3/2
Assume b =0.3 m
h3/2 = 0.12 = 0.234
1.71 x 0.3
So, h = 0.378 m = 37.8 cm = 40 cm
= 40 + (free board) 5 cm = 45 cm; slope 1 in 40
Rapid Sand Filter Design(contd…)

53. 53
Clear water reservoir for backwashing
For 4 h filter capacity, Capacity of tank = 4 x 5000 x 7.5 x 5.75 x 2
1000
= 1725 m3
Assume depth d = 5 m. Surface area = 1725/5 = 345 m2
L/B = 2; 2B2 = 345; B = 13 m & L = 26 m.
Dia of inlet pipe coming from two filter = 50 cm.
Velocity < 0.6 m/s. Dia. of wash water pipe to overhead tank = 67.5 cm.
Air compressor unit = 1000 l of air/ min/ m2 bed area.
For 5 min, air required = 1000 x 5 x 7.5 x 5.77 x 2 = 4.32 m3 of air.
Rapid Sand Filter Design(contd…)

54. 54
 Water supply engineering by S.K.Garg,1977
 Picture Courtesy: http://www.google.com
 www.historyofwaterfilterrs.com/filteration
References

55. 55