Filtration is used to remove remaining impurities from water after screening, sedimentation, and coagulation. There are two main types of filters: slow sand filters and rapid sand filters. Slow sand filters have a slow filtration rate but can remove a high percentage of impurities. Rapid sand filters have a faster filtration rate and are more commonly used. Both operate by passing water through sand and gravel layers that strain, flocculate, and biologically and electrically treat water to remove particles and pathogens. Proper sand size and grading is important for effective filtration. Filters are backwashed when the head loss becomes too high.

1. FILTRATION OF WATER

2. FILTRATION
™Screening and sedimentation removes a large % of SS & organic matter present
in raw supplies.
™% of removal of colloidal matter increases when coagulants added to
sedimentation.
™Even after that water is not pure, may contain fine suspended particles &
bacteria
™To remove remaining impurities still further, and to produce potable and
palatable water, the water is filtered through the beds of fine granular material,
such as sand bed
™ The process of passing the water through beds of granular materials is known as
filtration which help in removing colour, odour, turbidity & bacteria from water.
Two types of filters used in W/S system
(i)slow sand gravity filters (SSF) (ii) Rapid sand gravity filters (RSF) (iii) Pressure filter
works in same principle of RSF suitable for small colony or township but not
adopted for municipal supplies.
‰ SSF can remove large % of impurities and bacteria from the water, as compared
to RSF .But SSF yield a very slow rate of filtration & require large areas, costly SSF
are becoming obsolete & RSF are now‐a‐days universally adopted.
‰The water from the coagulation cum sedimentation plant is directly fed into RSF
& further pass through the process of disinfection which removes almost 100% of
pathogens

3. Theoy of Filtration‐ The filters purify the water under four different processes.
(i)Mechanical straining. The suspended particles present in water, and which are of bigger
size than the size of the voids in the sand layers of the filter, cannot pass through these
voids and get arrested in them.
(ii)Flocculation and sedimentation‐ Filters are able to remove even particles of size smaller
than size of the voids present in the filter. This fact may be explained by assuming that the
void spaces act like tiny coagulation‐sedimentation tanks. The colloidal matter arrested in
these voids is a gelatinous mass and, therefore, attract other finer particles. These finer
particles thus settle down in the voids and get removed.
(iii) Biological metabolism. Certain micro‐organisms present in the voids of the filters. &
they may be caught during the initial process of filtration. Nevertheless, these organisms
require organic impurities (algae, plankton, etc.) as their food for their survival. They
utilise such organic impurities and convert them into harmless compounds by the process
of biological metabolism. & harmless compounds so formed, generally form a layer on the
bed, which is called schmutzdecke or dirty skin. This top layer helps in absorbing and
straining out the impurities.
(iv) Electrolytic changes. This process is explained by the theory of ionisation of the sand
grains of the filter media and the impurities in water, carry electrical charges of opposite
nature. When these oppositely charged particles and the impurities come in contact with
each other, they neutralise each other, thereby changing the character of the water and
making it purer. After a certain interval, the electrical charges of sand grains get
exhausted and have to be restored by cleaning the filter.

4. Filter Materials
fine or coarse sand are used as filter media. layers of sand may be supported
on gravel, which permits filtered water to move freely to the under‐drains, and
allows the wash water to move uniformly upward.
Sand.
The filter sand should generally be obtained from rocks like quartzite, and
should contain the following properties: (i) free from dirt and other impurities.
(ii)uniform in nature and size (iii) hard and resistant (iv) not to loose> 5% of its
weight if placed in HCl for 24 hours
Effective size‐ It be defined as the size of the sieve in mm through which
10% of the sample of sand by weight will pass and Represented as D10
‰The selection of the correct effective size is very important, because too
smaller size will lead to very frequent clogging of filters, and will give very low
filtration rates. Similarly, too large Size will permit the suspended particles and
bacteria to pass through it, without being removed.
The uniformity in size or degree of variations in sizes of particles is measured
and expressed by the term called uniformity coefficient, UC defined as the
ratio of the sieve size in mm through which 60% of the sample of sand will pass,
to the effective size of the sand. UC = [ D60 / D 10]

6. Construction of SSF
A typical section of a SSF is shown in Next figure
(i) Enclosure tank consists of an open water‐tight rectangular tank, made of
masonry/ concrete. Slope is kept at about 1 in 100 towards the central
drain. The depth vary from 2.5‐3.5 m , Plan area of the tank vary from
100‐to 2000 sq m
(ii) Filter media. The filtering media consists of sand layers about 90 to 110
cm in depth and placed over a gravel support. The effective size (D1O) of
the sand varies from 0.2 to 0.4 mm and the uniformity coefficient varies
from 1.8 to 2.5 or 3.0. The top 15 cm layer of this sand is generally kept of
finer variety than that of the rest, which is generally kept uniform in grain
size & coarsest layer should be placed near the bottom, and the finest
towards the top
(iii) Base material‐ Base material is gravel, and it supports the sand. It
consists of 30 to 75 cm thick gravels of different sizes placed in layers & 3‐
4 layers each of 15‐20 cm depth are used and Coarsest gravel is used in
the bottom most layer and the finest gravel is used in the topmost layer.
Size of gravel in the bottom‐most layer is kept 40‐65 mm ; intermediate
layers, (i) between 20‐40 mm (ii) 6‐20 mm (iii) top layer 3‐ 6 mm

7. (iv) Under‐drainage system. The gravel support is laid on top of an UD system
which consists of a central drain and lateral drains, as shown in Fig. The
laterals are open jointed pipe drains or some other kind of porous drains placed
3 to 5 m apart on the bottom floor and sloping towards a main covered central
drain. The laterals collect the filtered water and discharge it into the main drain,
which leads the water to the filtered water well. Sometimes, instead of placing it
in the centre the main drain is placed along one side of the tank, and the laterals
slope towards it. The loss of head caused by the resistance offered by the sand
grains to the flow of water through it, is usually called filter head or filtering head

8. Operation and Cleaning of Slow Sand Filters
¾ treated water from the sedimentation tank is allowed to enter inlet chamber
and get distributed uniformly over the filter bed.
¾ water percolates through sand & gets purified during the process of filtration.
¾It now enters the gravel layers & comes out as the filtered water & collected
in UD system (laterals through the open joints)
¾ finally discharge into the 'filtered water well', from where it can be taken to
the storage tanks for supplies. The rate of discharge or the rate of filtration is
kept constant
¾This is due to the fact that the dirty skin formed by the floc and carried to the
filter considerably affects the economical working of the filter
¾ Dpth of water on the filter should also be decided carefully and should not be
allowed to undergo large variations.
¾Depth should neither be tool large nor it should be too small. It is generally
kept equal to the depth of the filter sand. Hl head loss is generally limited to a
maximum value of about 0.7 to 1.2 m.
¾When this limiting value, which is roughly kept as 0.7 to 0.8 times the depth of
the filter sand, is reached, the filter unit must be put out of service and the filter
be cleaned. The cleaning of slow sand filters is not done by back washing as is
done for rapid gravity filters, but is done by scrapping & removing the 1.5 to 3
em of top sand layer

10. RAPID SAND FILTERS
It was pointed out earlier that the slow sand filters can filter water at a very slow rate,
and thus require huge areas for their installations. In addition to requiring huge
quantities of filtering materials
Construction of Rapid Gravity Filters.

11. Enclosure tank
‰ It consists of an open water‐tight rectangular tank, made of masonry or concrete.
The depth of the tank may vary from 2.5 to 3.5 m.
‰In order to achieve uniform distribution of water, the area of the filter units should
not be kept larger; and is generally limited to about 10 to 80 m2 for each unit
Filter media
The filtering media consists of sand layers about 60 to 90 cm in depth, and placed over
a gravel support. Effective size (D IO) of the sand varies from 0.35 to 0.55 mm &
uniformity coefficient ranges between 1.2 to 1.8
Base material In RSF in addition to supporting the sand, it distributes wash water.
It consists of 60 to 90 cm thick gravels of different sizes placed in layers. Generally, 5‐6
layers, each of 10 to 15 cm. The coarsest gravel (about 40 mm in size) is used in the
bottom‐most layer, and the finest gravel (about 3 mm in size) is used in the top‐most
layer.
Under Drainage system‐ In RSF UD system serves 2 (i) to receive and collect the filtered
water and (ii) to allow the back washing for cleaning of filter
A manifold and lateral system type of installation of UD system consists of about
40 cm diameter manifold pipe running lengthwise along centre of the filter bottom.
Taking off from the manifold in both directions at right angles to it would b~ 10 cm dia
laterals. Laterals are placed at about 15 to 30 cm apart

14. Back washing procedure is as follows:
™Valves 1 & 4 are closed, and valves 5 & 6 are opened out &wash water and
compressed‐air forced upward through the UD system through gravel and sand
beds
™Valve 5 is closed after supplying Air
™ The dirty water resulting from washings, overflows into the wash water
troughs, and is Removed.
™ by opening valve 2, through the inlet chamber into the wash water gutter.
™The process of washing the filters and removing the dirty water is generally
continued for a period of 3 to 5 minutes.
™ After the washing of the filter has been completed, valves 2 & 6 will be
closed, and valves 1 and 3 are opened out.
™This restores the inlet supplies through valve 1, but the filtered water, in the
beginning is not collected, and washed for a few minutes through valve 3 to the
gutter.
™This is necessary because the remains of the wash water must be removed
from the voids of the filter and a surface mat must be allowed to be formed on
sand.
™Ultimately, valve 3 is closed and valve 4 is opened to get the filtered supplies
again.

16. Loss of head and necessity of cleaning.
™The water percolating through the filter moves downward under gravity & is opposed by the
resistance offered by the sand grains and the impurities arrested in them
™ The percolating water, looses some of its head.
™Loss of head can be easily computed by knowing the water level in the filter and the pressure of
water in the outlet pipe.
™ The difference between the two head's will give the loss of head, called filter head or filtration
head.
™This loss of head is measured by inserting two piezometers, one in the water standing over the
filter and the other in the outlet pipe difference in the readings of these two piezometers will give
the loss of head.
™When the filter is newly commissioned, the loss of head is generally very small, and is of the order
of 15 to 30 cm. However, the loss of head goes on increasing as the time passes, and 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 sand bed Most of this resistance is caused by the top 10 to 15 cm
sand layer.
™The bottom sand then acts like more or less a vacuum, and water is sucked through the filter
media rather than getting filtered through it.
™ The fall of mercury level in the piezometer inserted in the outlet pipe below the centre line of the
pipe, indicates the presence and extent of negative pressure.
™ 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, and thereby seriously
affecting the working of the filter.
™The phenomenon is known as air binding, as the air binds the filter and stops its functioning,
thereby reducing the rate of filtration considerably.

17. Operational Troubles in Rapid Gravity Filters.
common troubles which are generally encountered during the operation of
rapid gravity filters, are: (1) Formation of mud balls; and (2) Cracking of
filters
Formation of mud balls.
The mud from the atmosphere usually accumulates on the sand surface, so
as to form a dense mat. During inadequate washing of the filter, this mud
may sink down into the sand bed. This mud then sticks to the sand grains
and other arrested impurities, thereby forming mud ·balls. These mud
balls slowly and steadily go on increasing in size and weight. They may
then sink down into the gravel, thus interfering with the upward
movement of wash water during cleaning. They cause turbulence around
them, and thus hinder with the uniform application of wash water.
Cracking of filters
The fine sand contained in the top layers of the filter bed, shrinks and
causes the development of shrinkage cracks in the sand bed. These cracks
are more prominent near the wall junctions. With the use of filter, the loss
ofhead and, therefore, the pressure on the sand bed goes on increasing,
which further goes on widening these cracks.

20. Pressure Filters
Pressure filters are just like small rapid gravity filters placed in closed
vessels, and through which water to be treated is passed under pressure.
Since water is forced through such filters at a pressure greater than the
atmospheric pressure, it is necessary that these filters are located in air
tight vessels. The raw water is pumped into the vessels by means of
pumps, The pressure so developed may normally vary between 30-70
meter head of water, i.e. 3 to 7 kg/cm2
Construction of Pressure Filters.
The filter vessel may be installed either in a horizontal or in a vertical
position, depending upon which, they may be classified as horizontal
pressure filters or vertical pressure filters. Typical cross-sections of both
these types of filters are shown in Figs. 9.25 and 9.26, respectively. Steel
cylinders are used as pressure vessel and may be riveted or welded. Their
diameters generally vary between 1.5 to 3 m, and their lengths or heights
may vary from 3.5 to 8 m. Inspection windows are provided at top for
inspection purposes. The positions of various valves are also clearly
shown in the figures.