Filtration

The resultant water after sedimentation will not be pure, and may contain some very
fine suspended particles and bacteria in it. To remove or to reduce the remaining
impurities still further, the water is filtered through the beds of fine granular material,
such as sand, etc. The process of passing the water through the beds of such
granular materials is known as Filtration.

Filter Materials

Sand: Sand, either fine or coarse, is generally used as filter media. The size of the
sand is measured and expressed by the term called effective size. The effective size,
i.e. D10 may be defined as the size of the sieve in mm through which ten percent of
the sample of sand by weight will pass. The uniformity in size or degree of variations
in sizes of particles is measured and expressed by the term called uniformity
coefficient. The uniformity coefficient, i.e. (D60/D10) may be defined as the ratio of the
sieve size in mm through which 60 percent of the sample of sand will pass, to the
effective size of the sand.

Gravel: The layers of sand may be supported on gravel, which permits the filtered
water to move freely to the underdrains, and allows the wash water to move
uniformly upwards.

Other materials: Instead of using sand, sometimes, coal, garnet sand are used as
filter media.

Types of Filter

Slow sand filter: They consist of fine sand, supported by gravel. They capture
particles near the surface of the bed and are usually cleaned by scraping away the
top layer of sand that contains the particles.

Rapid-sand filter: They consist of larger sand grains supported by gravel and
capture particles throughout the bed. They are cleaned by backwashing water
through the bed to 'lift out' the particles.

Multimedia filters: They consist of two or more layers of different granular

materials, with different densities. Usually, anthracite coal, silica sand, garnet sand
and gravel are used. The different layers combined may provide more versatile
collection than a single sand layer. Because of the differences in densities, the layers
stay neatly separated/stratified, even after backwashing.

Principles of Slow Sand Filtration

 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.
 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 (schmutzdecke) and the layer of filter
bed sand just below. The purification mechanisms extend from the filter skin
to approx. 0.3-0.4 m below the surface of the filter bed, gradually decreasing
in activity at lower levels as the water becomes purified and contains less
organic material.
 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.

Sand Filters vs. Rapid Sand Filters

 Base material (gravel): In SSF it varies from 3 to 65 mm in size and 30 to 75

cm in depth while in RSF it varies from 3 to 40 mm in size and its depth is
slightly more, i.e. about 60 to 90 cm.
 Filter sand: In SSF the effective size ranges between 0.2 to 0.4 mm and
uniformity coefficient between 1.8 to 3.0. In RSF the effective size ranges
between 0.45 to 0.7 and uniformity coefficient between 1.2 to 1.8.
 Rate of filtration: In SSF it is small, such as 100 to 200 L/h/sq.m. of filter
area while in RSF it is large, such as 3000 to 6000 L/h/sq.m. of filter area.
 Flexibility: SSF are not flexible for meeting variation in demand whereas RSF
are quite flexible for meeting reasonable variations in demand.
 Post treatment required: Almost pure water is obtained from SSF. However,
water may be disinfected slightly to make it completely safe. Disinfection is a
must after RSF.
 Method of cleaning: Scrapping and removing of the top 1.5 to 3 cm thick
layer is done to clean SSF. To clean RSF, sand is agitated and backwashed
with or without compressed air.
 Loss of head: In case of SSF approx. 10 cm is the initial loss, and 0.8 to
1.2m is the final limit when cleaning is required. For RSF 0.3m is the initial
loss, and 2.0 to 2.5m is the final limit when cleaning is required.
Backwashing of Rapid Sand Filter

 For a filter to operate efficiently, it must be cleaned before the next filter run. If
the water applied to a filter is of very good quality, the filter runs can be very
long. Some filters can operate longer than one week before needing to be
backwashed. However, this is not recommended as long filter runs can cause
the filter media to pack down so that it is difficult to expand the bed during the
backwash.
 Treated water from storage is used for the backwash cycle. This treated water
is generally taken from elevated storage tanks or pumped in from the clear
well.
 The filter backwash rate has to be great enough to expand and agitate the
filter media and suspend the floc in the water for removal. However, if the filter
backwash rate is too high, media will be washed from the filter into the
troughs and out of the filter.
 Backwash duration : 10 min
 The water standing on the bed at the close of wash should be clear
with turbidity < 10 NTU
 Washwater < 2-3% of the amount of water filtered
 Following backwash, waste for 2 min until the turbidity <1 NTU


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; and/or
 Floc starts to break through the filter and the turbidity in the filter effluent
increases; and/or
 A filter run reaches a given hour of operation.

Operational Troubles in Rapid Gravity Filters

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. Most of this resistance
is offered by the top 10 to 15 cm sand layer. The bottom sand acts like a
vacuum, and water is sucked through the filter media rather than getting
filtered through it.
 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.

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.
250 (1+0.03)*24/23.5 = 263 cum per hour

A = 263/5 = 52.6 sqm

Provide two parallel filters

Area for each = 26.3 sqm

L:W = 1.3:1

L = 5.85 m

W = 4.5 m