The document discusses the theory of filtration, detailing various types of filters such as slow sand, rapid sand, and pressure filters, including their construction, operation, and maintenance. It explains the essential mechanisms involved in filtration, such as mechanical straining, sedimentation, biological action, and electrolytic processes, as well as the design and operational challenges. Additionally, it outlines the specifics of slow and rapid sand filters, covering aspects like filter media, under drainage systems, and backwashing procedures for cleaning.

1. FILTRATION
Mechanism-theory Of Filtration, Types Of Filters,
Slow Sand, Rapid Sand And Pressure Filters
Including Construction, Operation, Cleaning,
Operational Problems In Filters, Design Of Slow &
Rapid Sand Filter Without Under Drainage System.
Dr. Dayananda H S
Professor of Civil Engg,
VVCE, Mysore
Karnataka, India
1

2. Introduction
• Filtration is a separation technique used to remove the
suspended and colloidal impurities from water by passing
through a bed of fine granular medium such as sand,
anthracite, garnet or activated carbon etc.,
• The water leaving the clarifier still contains small flocs & very
fine suspended impurities and bacteria in it
• For removal of these colloidal impurities, filtration is normally
used
• Water fills the pores (open spaces) between the sand
particles and the impurities are arrested either by clogging in
the pores or attached to the sand particles
• Filtration may help in removing color, odor, turbidity and some
pathogenic bacteria from water
• Filtration reduces the bacterial content by 98- 99%, turbidity
by 50 - 3 PPM 2

3. Theory of Filtration
When the water is filtered through beds of fine granular media,
the following actions takes place
i) Mechanical straining
ii) Sedimentation
iii) Biological action and
iv) Electrolytic action
Fig: Schematic of Straining, Flocculation &
Sedimentation actions in a granular-media filter
(Source: Slideshare.net) 3

4. 1. Mechanical straining
• This mechanism removes coarser particles larger than the
pore size
• Some colloidal particles are larger than the pore size of the
filter media
• These particles are entrapped and are removed
• This removal mechanism is termed Straining
• 2. Sedimentation
• Water flowing through the filter bed, is usually laminar
• The velocity and direction constantly changes due to the
obstruction by media grains
• In low-velocity zones, some particles are removed by
sedimentation 4

5. 3. Biological Action
• Normally, a film of micro-organisms & bacteria reside in the
uppermost layer of filter media as coatings over sand grains which
are caught during the initial filtration
• These organisms use organic impurities as food and convert them
into harmless compounds
• Such harmless compounds form a layer on the top, which is termed
as Schmutzdeck or dirty skin
• This acts as a fine mesh straining mat & this layer further, helps in
absorbing & straining out the impurities
• This process is known as Biological metabolism
• 4. Electrolytic Action
• The purifying action of filter can be explained by the theory of
ionization
• Sand grains of filter & impurities present in water possess
electrical charge of opposite nature
• When these two come in contact with each other, the charge of the
floc gets neutralized and permits the floc to be removed
• During the process of Back washing, electrostatically neutralized
material is removed, & the charge of the filter media is regained 5

6. Types of Filters
Filters are classified based on the following three parameters
1. Filtration rate: Slow sand filter & Rapid sand filter (High- rate)
2. Driving force: Gravity or pressure filter
3. Direction of flow : Down flow filters and Up-flow filter
1. Slow sand filters
• First used in 1804 in Scotland and subsequently in London
• During 19th century, through out the world
• Consists of concrete rectangular basin containing carefully
selected graded sand supported on gravel and stones
6

7. Essential parts of filter unit
• Enclosure tank
• Filter media
• Base material
• Underdrainage system
• Inlet & outlet arrangement
• Appurtenances
(Source:www.slideshare.net)
i) Enclosure tank
• An open water tight rectangular tank, made of masonry or
concrete
• Depth- 2.5 to 3.50m
• Surface area- 30 to 2000 sq. m
• The bed slope is kept at about 1 in 100 towards the central
drain 7

8. Fig: Section through slow sand filter & Plan of under drainage system
2 to 3m
8

9. 2. Filter media - Sand
• A bed of graded sand - Most important part of the filter
• Sand thickness: Normally 1m (0.9 - 1.10m)
• Preferably rounded with effective diameter of 0.2-0.4 mm
(Effective size)
• Uniformity co-efficient: 2 to 3 (Normal value,2.5)
• Finer the sand, better bacterial efficiency, but filtration rate is
low
• Should not contain more than 2% of Ca & Mg as carbonates
• After immersion in HCl for 24h, should not loose weight by
more than 5%
• Free from clay, lime, vegetable matter, organic impurities
• Supported by graded gravel (0.3-0.40m deep)
• Water percolates through the bed and gets filtered by
Mechanical straining, sedimentation, adsorption, oxidation
and bacterial action
• Rate of filtration: 0.1-0.4 m3/hour/m2
9

10. Vital Layer – The Heart of Slow Sand Filter
• Surface of the sand bed gets covered with a slimy growth
known as ‘Schmutzdecke’, vital layer or zoological layer or
biological layer
• It is slimy gelatinous, consists of algae, plankton, diatoms
and bacteria
• The formation of this layer is known as RIPENING of the
filter, which may take several days to fully develop (2-3 cm)
• Removes organic matter, holds back bacteria, oxidizes
ammonical nitrogen to nitrates and yield a bacteria free
water
• Until the vital layer is fully formed, filter only worked as a
mechanical strainer & so for the first few days filter water run
to the waste
10

11. Base material - Gravel
• The base material is gravel, which supports the sand
• Gravel thickness- 30 to 75mm, with gravels of different sizes,
placed in 3 to 4 layers
• Thickness of each layer is around 15 to 20cm
• The coarsest gravel is placed in the bottom most layer and the
finest layer is used in the top most layer
• The size of gravel in each layer should be as :
Bottom most layer : 40-65 mm
Intermediate layers : 20-40mm & 6-20mm
Top most layer : 3-6mm
11

12. Under drainage System
• Lateral drain – 750 - 100mm diameter earthenware pipes or
perforated pipes
• Spacing - 2 to 3m c/c
Appurtenances
• Vertical air pipes
• Loss of head through filter media- depth of water 1.5m above
sand media
• Adjustable telescopic tube
• Filter head- 0.10 to 0.15m for fresh clean water
• i.e., difference in water level b/n filter basin & outlet chamber
• 0.7 to 1.20m during cleaning
12

13. Inlet and outlet arrangements
• An inlet chamber is constructed for admitting the water from
clarifier without disturbing the sand layers of the filter and to
distribute it uniformly over the filter bed
• A ‘filtered water well’ is also constructed on the outlet side to
collect the filtered water from the main-under drain
• Inlets and outlets are generally governed by automatic
valves
13

14. OPERATION OF SLOW SAND FILTER
• The water from plain sedimentation tank (non-coagulated) is
allowed into inlet chamber of the filter for uniform distribution
over the filter bed
• The depth of water on filter media is kept equal to thickness of
sand
• Water percolates through the filter media and Gravel layer
and gets purified
• The water gets collected in the under drainage system
• Slow sand filter works on a combination of straining and
microbiological action
Limitations
• Rate of filtration:100 to 200 L/h/sq.m of filter area
• Filtration applicable for non- coagulated water
• Only plain sedimentation prior to filtering
14

15. CLEANING OF SLOW SAND FILTER
• The cleaning is done by scrapping and removing the top 1.5 to
3cm of sand layer and
• Cleaning is repeated until the sand depth is reduced to about
40cm or so
• The interval between two successive cleanings, depends upon
i. Nature of impurities and
ii. Size of filter media
• This interval normal ranges between one to three months
15

16. Rapid Sand Filter
• RSF was first developed in 1885 by G. W. Fuller @ Louisville,
USA
• RSFs’ of gravity type – most commonly used in Water Supply
Plants
• RSF differs from SSF in the following aspects
i) Effective size & uniformity coefficient of sand
ii) Rate of filtration & filtration head
iii) Method of cleaning & frequency of cleaning
iv) Pre-treatment
16

17. (Source:mathtab.com)
(Source:uomustansiriyah.edu.iq)
17

18. Fig: Diagrammatic section of Rapid Sand filter 18

19. Essential Features
1. Enclosure Tank
• Open water-tight rectangular tank in masonry or concrete
• Depth of Tank – 2.5 to 3.5m
• Surface area – 10-80m2 for each unit
• Length to Breadth ratio – 1.25 to 1.35
• Number of Units – Morrell & Wallace equation
N = 1.22√Q
Where, N is the number of filter units and
Q is plant capacity in MLD
19

20. 2. Filter media - Graded sand
• Sand grain size distribution is selected to optimize the
passage of water, while minimizing the passage of particulate
matter
• RSF uses sand coarser than SSF
• Effective size: 0.35 – 0.6mm, normal value 0.45mm
• Uniformity coefficient: 1.3 – 1.7, normally 1.5
• Void space increases due to increase in effective size &
decrease in Uniformity Coefficient
• This increases rate of filtration
3. Base material : Graded gravel
• Garnet (6 – 8cm) layer : to check gravel upsets due to
localized high velocity during back wash
• Total depth: 0.60 – 0.90m
• Five to six layers - each 0.15m thick
• Grade size – 2-6mm, 6- 12mm, 12- 20mm and 20-40mm
20

21. 4. Under-drainage system
• UDS in RSF serves two purposes
i) Collects filtered water uniformly over the area of
gravel bed
ii) Uniform distribution of back wash water without
disturbing the gravel bed and filter media
Types of Under drainage system
• Perforated pipe system
• Pipe and strainer system
• Wheeler system
• Leopald system &
• Wagner system
21

22. Perforated Pipe System
• System consists of a main header (manifold) & several
Laterals on both sides
• Velocity thru’ laterals, size & # of orifices r determined
• Laterals spacing- 0.15 m to 0.30 m C/C
• Perforations provided underneath with 6-12mm ø openings,
making 30° angle w r t vertical
• Support for laterals on concrete blocks 4cm above filter bed
• Spacing of laterals ≈ spacing of orifices:150 -300mm
• Spacing of perforations- 80mm for 5mm ø & 200mm for
12mm ø
• Area of manifold - 1.5 to 2 times total area of laterals (to
minimize frictional losses)
• Back washing for this System requires 700 L of water per
minute per m2
22

23. Fig:Section of a lateral pipe drain with perforations
Pipe and Strainer system
• Instead of drilling holes, laterals are provided with strainers
• Strainers are usually brass or bronze & cast in umbrella
shape
• Strainers distribute the wash water without jet action
23

24. Pipe and Strainer
System
Fig: Section of manifold and lateral system (laterals are provided with strainers) 24

25. Rapid Sand Filter - Operation
• The working and back washing of rapid sand filter is regulated
by operating Six Valves, Viz.,
• Valve 1 – Inlet for Raw Water
• Valve 2 – To drain dirty water collected in wash water trough
• Valve 3 - To regulate Initial run after backwash
• Valve 4 - To treated water storage reservoir
• Valve 5 - Air compressor Valve
• Valve 6 - To regulate elevated wash water tank for backwash
• The water from clariflocculator enters the filter unit by regulating
Valve # 1
• The filtered water collected in the manifold is collected by
opening the Valve # 4
• During Filtration, Valves 1 and 4 are kept open and other
Valves are in closed position 25

26. (Source: Water treatment 2013 14 (2), Slideshare.net , Anurag Chandra shekhar & Dr. Sonal Dixit)
Air Compressor
5
Fig: Working of Rapid Sand Filter and Back washing
26

27. Backwashing
• RSF are cleaned by passing air and water backwards through
the sand. This operation is known as Backwashing
• Stored filtered water in elevated wash water tank is used for
the backwash
• The filtration is stopped and the water level is maintained
above the surface of the filter bed by closing Valve 1 and
Valve 4 (Inlet and Filtered water storage tank)
• Air valve and Wash water Valve (Valve 5 & 6) are kept open,
wash water and compressed air is forced upwards from under
drainage system through gravel and filter bed
• The backwash flow rate has to be great enough to expand
and agitate the filter media and suspend the floc in the water
for removal 27

28. • If it is too high, media will be washed from the filter into the
troughs and out of the filter
• Air valve (# 5) is closed
• The dirty water of washings flows into wash water troughs
and joins wash water gutter by opening Valve #2
• After completion of backwash, Valves 2 & 6 is closed and
Valves 1 & 3 are kept open
• After backwash, filtered water is not collected for a few
minutes and sent to drain by operating Valve # 3
• Finally, Valve 3 is closed and Valve 4 is kept open to get
clear filtered water
28

29. Appurtenances
i) Wash water troughs
• Provided @ the top of the filter to collect BWW
• It emerges out from sand & conveys to WW Drain
• C or RCC troughs are used & runs across the length of the
tank
• Bottom of trough is placed at least 5cm above the top level
of sand
• This prevents entry of sand during BW
• Spacing of trough: 1.5 to 2 m
29

30. Fig: Configuration of Wash-Water Troughs
30

31. • After backwashing, the sand settle back into place. The
larger particles settle first, resulting in fine sand layer on top
and coarse sand layer on the bottom
• After the air scour cycle, clean backwash water is forced
upwards through the filter bed continuing the filter bed
expansion and carrying the particles in suspension into
backwash troughs suspended above the filter surface
• Water requirement: 2 to 5 % of total amount of water treated
• Frequency of cleaning: 24 to 48 h
• Back wash duration : 15 minutes
31

32. Loss of Head & Negative Head
• Loss of head in RSP - Water level in filter & pressure of water
in the Outlet pipe
• LOH – measured by inserting two piezometers, one in water
standing over the filter & other one in the Outlet pipe
• Difference in readings of these two piezometers will give the
loss of head, known as ‘Filter head’
Fig: Loss of Head & Negative Head 32

33. Negative head
• Sand grains offer resistance to flow of water
• This loss of head caused is usually known as ‘Filter head’
• Loss of Head - very small in initial stages of filtration
• As thickness of Suspended Solids on top of sand bed
increases, Head Loss goes on increasing
• A stage comes, when frictional resistance exceeds static
head above the sand bed
• Lower portion of sand media now acts more of less like a
vacuum
• This is known as negative head 33

34. • Water is sucked through the filter media rather than being
filtered through it
• Negative Head is fall of liquid level in peizometric tube below
the centre line of Under Drainage System
• Due to Negative Head, dissolved gases & air are released in
form of bubbles, which sticks to sand grains
• This seriously affects the working of filter
• This phenomena is known as ‘Air binding’
• Air binds the filter and stops its functioning, thereby reducing
the rate of filtration
34

35. Pressure Filters
Based on position of installation, Pressure filters are classified
into two types
• Horizontal pressure filters &
• Vertical pressure filters
Vertical Pressure filter
Horizontal pressure filter
35

36. Constructional details
• Vertical pressure filter consists of steel cylinder
• Diameter varies b/n 1.5 – 3.0 m
• Height varies b/n 2.5 – 8.0 m
• Inspection windows are provided @ top
• Requires frequent cleaning
• Rate of filtration – 2.5 times higher than RSF
• Rate of filtration – 6000 to 15000 L/hr/sq. m
• Applicability – Industrial plants & swimming pools
36

37. Hydraulics of flow through porous media (FilterHydraulics)
• Filter Hydraulics falls into two categories
1. In actual filtration process, water gets cleaned
2. In Backwash operation, filter gets cleaned
• These two operations are equally important in overall filtration
process
• Flow through packed bed can be analyzed by classic
hydraulic theory
• Carmen modified Darcy-Wiesbach equation for head loss in a
pipe to reflect in a bed of porous media of uniform size
• Carmen-Kozeny equation: Hf = f ’L (1-e) Vs2
e3 gdp
Hf = friction loss thru’ bed of particles of uniform size dp, m
L = depth of filter, m
e = porosity of bed
Vs = filtering velocity, m/s
g = gravitational acceleration, m/s2
dp = diameter of filter media grains, m
37

38. • This equation is applicable only to filter beds
• Once solids start accumulating, porosity of bed decreases &
head loss increases
• HL is a function of the nature of suspension, characteristics
of media & filter operation
• BW of filter media is accomplished by reversing flow forcing
clean water upward thru’ the media
• To clean interior of bed, it is necessary to expand it
• Granules are no longer in contact with each other
• Thus exposing all surfaces for cleaning
38

39. • To hydraulically expand a porous bed, head loss must be at
least equal to buoyant weight of the particles in the fluid
• For a unit area of filter , this is expressed by
hfb = L(1- e) ῤm - ῤw
ῤw
hfb = head loss required to initiate expansion, m
L = bed depth, m
1- e= fraction of the packed bed composed of medium
ῤm = density of the medium, kg/m3
ῤw = density of the water, kg/m3
39

40. Operational problems in filters
Following filter troubles are commonly observed
1. Cracking and clogging of filter bed
2. Formation of mud balls
3. Air binding
4. Sand Incrustation
5. Jetting and Sand boils and
6. Sand leakage
40

41. 1. Cracking and clogging of filter bed
• Surface clogging and cracking are usually caused by rapid
accumulation of solids on the top of filter media
• Cracks are more at wall junctions
2. Formation of mud balls
• Mud balls are formed because of conglomeration of
turbidity, floc, sand and other binders
. • Formed because of insufficient washing of sand grains
• Size may be pea size to 2 to 5 cm or more in dia
3. Air binding
• It is caused by release of dissolved gases and air from
water to form bubbles
41

42. • These bubbles occupy void space of the filter media sand
and drainage system
• It is caused by negative head loss, warm water and
increased DO in water
• It can be minimized by avoiding excess head loss, warming
of water, control of algal growth and avoiding super saturation
of water with air
4. Sand Incrustation
• It occurs due to accumulation of sticky gelatinous material or
crystallization of calcium carbonate
• Sand grains enlarge in size and effective size changes
• Carbonization of water can be done to prevent this problem
• Some times, Sodium hexa-meta Phosphate can be added to
keep calcium carbonate in dissolved state 42

43. 5. Jetting and Sand boils
• These are produced when during backwashing water
follows path of least resistance and break through to the
scattered points due to small differences in porosity and
permeability
• Jetting can be avoided by surface wash or air scour. •Use
of 8 cm thick layer of coarse garnet is also recommended
6. Sand leakage
• It results when smallest gravels are displaced during
backwashing
• Water will enter the under-drainage system unfiltered
• It can be reduced by properly proportioning of sand and
gravel layer
• In between sand and gravel garnet layer can be used to
tackle this type of problem 43

44. Remedial Measures to Prevent Cracking of Filters and
Formation of Mud Balls
• Breaking the top fine mud layer with rakes and washing off
the particles
• Washing the filter with a solution of caustic soda
• Removing, cleaning and replacing the damaged filter sand
44

45. Design of Slow sand filter (without under drainage system)
Design criteria Recommended level
Design period 10-15 years
Period of operation 24 h/d
Filtration rates in the filters 0.1-0.2 m/h
Filter bed area 5-200 m2 per filter, minimum of 2
units
Height of filter bed:
Initial
Minimum
0.8 -0.9 m
0.5-0.6 m
Specification of sand:
Effective size:
Uniformity coefficient
0.15-0.30 mm
<5, preferably below 3
Height of under-drains including
gravel
layer
0.3-0.5 m
Height of supernatant water 1m
45

46. Design of Rapid sand filter
Parameter Rapid sand filter High-rate filter
Number of Filters 0.1 Q0.5 0.1 Q0.5
Minimum number of filters 4 4
Filter bed area, m2 25-100 25-80
Filtration rate, m/h 5 15
Filter bed depth, m 3-6 4-7.5
Length to width ratio 2:1 -4:1 2:1 -4:1
Filter width, m 3-6 3-6
Total head loss, m 2-4 2.5 -4.5
Water backwash rate, m/h 35-50 40-60
Filte run length, h 12-72 8-80
Q – flow rate in m3/h
46

47. Table: Comparison between Slow sand filter and Rapid sand filter
47

48. FAQ
1. Explain the mechanism of filtration.
2. Differentiate between slow sand filters and rapid sand filters.
3. With the aid of neat sketch, explain the working of a rapid
sand filter.
4. Distinguish between slow sand filter and rapid sand filter
under the following heads: i) Interval and method of cleaning
ii) Bacterial removal efficiency iii) pretreatment iv) rate of
filtration
5. Draw the section of a pressure filter and name the parts.
6. Write a note on Pressure filter.
7. What is loss of head and negative head in a rapid sand filter.
With a neat sketch, explain the same.
8. What is meant by backwashing. Explain the procedure
followed to achieve th same.
9. Enumerate and discuss the operational problems faced in
slow sand filter and rapid sand filters.
48