Sand Filtration: Slow and Rapid Sand Filters | Civil Engineering Optional Notes for UPSC PDF Download

 Page 1


 
The process of passing the water through the beds of granular materials (filters) known as filtration.
Purpose of filtration :
(i) To remove very fine suspended and colloidal particles that do not settle in th sedimentation
process.
(ii) To remove dissolved impurities in water.
(iii) To remove pathogenic bacteria from water.on (iv) To remove colour, odour, turbidity in water.
Types of filters:
(i) Slow sand filters
(ii) Rapid sand filters
(iii) Pressure filter
Theory of filtration:
During filtration, the following actions take place:
(i) Mechanical straining
(ii) Sedimentation
(iii) Biological action
(iv) Electrolytic action 
 
(i)  Mechanical straining (for coarser particles)
When water passes through the filter media (sand), the suspended particles larger than the pore-
space of the filter media get trapped and removed. The trapped particles form a mat on the filter
                 
Page 2


 
The process of passing the water through the beds of granular materials (filters) known as filtration.
Purpose of filtration :
(i) To remove very fine suspended and colloidal particles that do not settle in th sedimentation
process.
(ii) To remove dissolved impurities in water.
(iii) To remove pathogenic bacteria from water.on (iv) To remove colour, odour, turbidity in water.
Types of filters:
(i) Slow sand filters
(ii) Rapid sand filters
(iii) Pressure filter
Theory of filtration:
During filtration, the following actions take place:
(i) Mechanical straining
(ii) Sedimentation
(iii) Biological action
(iv) Electrolytic action 
 
(i)  Mechanical straining (for coarser particles)
When water passes through the filter media (sand), the suspended particles larger than the pore-
space of the filter media get trapped and removed. The trapped particles form a mat on the filter
                 
media and help in straining more impurities.
 
(ii) Sedimentation (for finer particles)
The voids of the filter media acts as small sedimentation tanks and fine particles giare removed by
settling.
 
(iii) Biological action
Certain microorganisms and bacteria present in the voids of filters form coatings over the sand
grains. These organisms utilize the organic impurities in water as their food and convert them into
harmless compounds by biological metabolism. They form a layer on the filter media called
"schmutzdecke or dirty skin". This layer further helps in absorbing and straining out the impurities
in water
.
(iv) Electrolytic changes (Ionisation)
The sand grains of filter media and impurities in water are oppositely charged. When the impurities
come in contact with the sand grains, their charges get neutralised and changes the characteristics of
water making it purer. After certain period of time, the charge of sand grains gets exhausted and
should be restored by ge of s din cleaning the filters.
Filter Media:
Sand (fine or coarse) is generally used as filter media and supported on gravel.
(i) Sand:
The properties of filter sand are :
• It should be obtained from hard and resistant quartz or quartzite.
• Free from dirt and other impurities.
• It should not loose more than 5% of its weight when placed in hydrochloric acid (HCI) for 24 hrs.
• Specific gravity = 2.55 to 2.65
                 
Page 3


 
The process of passing the water through the beds of granular materials (filters) known as filtration.
Purpose of filtration :
(i) To remove very fine suspended and colloidal particles that do not settle in th sedimentation
process.
(ii) To remove dissolved impurities in water.
(iii) To remove pathogenic bacteria from water.on (iv) To remove colour, odour, turbidity in water.
Types of filters:
(i) Slow sand filters
(ii) Rapid sand filters
(iii) Pressure filter
Theory of filtration:
During filtration, the following actions take place:
(i) Mechanical straining
(ii) Sedimentation
(iii) Biological action
(iv) Electrolytic action 
 
(i)  Mechanical straining (for coarser particles)
When water passes through the filter media (sand), the suspended particles larger than the pore-
space of the filter media get trapped and removed. The trapped particles form a mat on the filter
                 
media and help in straining more impurities.
 
(ii) Sedimentation (for finer particles)
The voids of the filter media acts as small sedimentation tanks and fine particles giare removed by
settling.
 
(iii) Biological action
Certain microorganisms and bacteria present in the voids of filters form coatings over the sand
grains. These organisms utilize the organic impurities in water as their food and convert them into
harmless compounds by biological metabolism. They form a layer on the filter media called
"schmutzdecke or dirty skin". This layer further helps in absorbing and straining out the impurities
in water
.
(iv) Electrolytic changes (Ionisation)
The sand grains of filter media and impurities in water are oppositely charged. When the impurities
come in contact with the sand grains, their charges get neutralised and changes the characteristics of
water making it purer. After certain period of time, the charge of sand grains gets exhausted and
should be restored by ge of s din cleaning the filters.
Filter Media:
Sand (fine or coarse) is generally used as filter media and supported on gravel.
(i) Sand:
The properties of filter sand are :
• It should be obtained from hard and resistant quartz or quartzite.
• Free from dirt and other impurities.
• It should not loose more than 5% of its weight when placed in hydrochloric acid (HCI) for 24 hrs.
• Specific gravity = 2.55 to 2.65
                 
• Effective size (a) 0.2 to 0.4 mm - slow sand filters.
 (b) 0.35 to 0.55 mm – rapid sand filters..
• Uniformity co-efficient
(a) 1.8 to 2.5 - slow sand filters
(b) 1.3 to 1.7 - rapid sand filters
The uniformity characteristics of sand are expressed in terms of
(a) effective size (b) Uniformity coefficient.
Effective size or diameter (D ) represents a size of sieve in mm, throuth which 10% of the
particles will pass i.e.,are finer than this size. Similarly, D size represent a size such that 60%
particles are finer than it.
Uniformity coefficient (C ) is a measure of the particle range i.e. variations in size of particles.
(ii) Anthracite - Crushed anthracite can be used as filter media separately or combined with sand
(mixed media). It is costly than sand.
(iii) Garnet sand - It has high specific gravity (4.2) and is a dense material. Due to high cost, it
cannot be used as a sole filter material. However, it can be used in mixed-media 30 so filter.
(iv) Other materials - Locally available materials such as shredded coconut husks, burned rice
husks, crushed glass, slag, metallic ores etc. can be used as filter material.
 
Slow Sand Filters (SSF)
The efficiency of slow sand filters is high and they can remove larger percentage of cy of slow and
filterich 000 oval base to siziano T suspended impurities and becteria. The efficiency of bacteria
removal is 98 to 99%.
These filters can also remove odours and tastes caused by organic impurities (algae and plankton).
They are less efficient in removing colour and can remove turbidity only upto 50mg/1.
10
60
u
                 
Page 4


 
The process of passing the water through the beds of granular materials (filters) known as filtration.
Purpose of filtration :
(i) To remove very fine suspended and colloidal particles that do not settle in th sedimentation
process.
(ii) To remove dissolved impurities in water.
(iii) To remove pathogenic bacteria from water.on (iv) To remove colour, odour, turbidity in water.
Types of filters:
(i) Slow sand filters
(ii) Rapid sand filters
(iii) Pressure filter
Theory of filtration:
During filtration, the following actions take place:
(i) Mechanical straining
(ii) Sedimentation
(iii) Biological action
(iv) Electrolytic action 
 
(i)  Mechanical straining (for coarser particles)
When water passes through the filter media (sand), the suspended particles larger than the pore-
space of the filter media get trapped and removed. The trapped particles form a mat on the filter
                 
media and help in straining more impurities.
 
(ii) Sedimentation (for finer particles)
The voids of the filter media acts as small sedimentation tanks and fine particles giare removed by
settling.
 
(iii) Biological action
Certain microorganisms and bacteria present in the voids of filters form coatings over the sand
grains. These organisms utilize the organic impurities in water as their food and convert them into
harmless compounds by biological metabolism. They form a layer on the filter media called
"schmutzdecke or dirty skin". This layer further helps in absorbing and straining out the impurities
in water
.
(iv) Electrolytic changes (Ionisation)
The sand grains of filter media and impurities in water are oppositely charged. When the impurities
come in contact with the sand grains, their charges get neutralised and changes the characteristics of
water making it purer. After certain period of time, the charge of sand grains gets exhausted and
should be restored by ge of s din cleaning the filters.
Filter Media:
Sand (fine or coarse) is generally used as filter media and supported on gravel.
(i) Sand:
The properties of filter sand are :
• It should be obtained from hard and resistant quartz or quartzite.
• Free from dirt and other impurities.
• It should not loose more than 5% of its weight when placed in hydrochloric acid (HCI) for 24 hrs.
• Specific gravity = 2.55 to 2.65
                 
• Effective size (a) 0.2 to 0.4 mm - slow sand filters.
 (b) 0.35 to 0.55 mm – rapid sand filters..
• Uniformity co-efficient
(a) 1.8 to 2.5 - slow sand filters
(b) 1.3 to 1.7 - rapid sand filters
The uniformity characteristics of sand are expressed in terms of
(a) effective size (b) Uniformity coefficient.
Effective size or diameter (D ) represents a size of sieve in mm, throuth which 10% of the
particles will pass i.e.,are finer than this size. Similarly, D size represent a size such that 60%
particles are finer than it.
Uniformity coefficient (C ) is a measure of the particle range i.e. variations in size of particles.
(ii) Anthracite - Crushed anthracite can be used as filter media separately or combined with sand
(mixed media). It is costly than sand.
(iii) Garnet sand - It has high specific gravity (4.2) and is a dense material. Due to high cost, it
cannot be used as a sole filter material. However, it can be used in mixed-media 30 so filter.
(iv) Other materials - Locally available materials such as shredded coconut husks, burned rice
husks, crushed glass, slag, metallic ores etc. can be used as filter material.
 
Slow Sand Filters (SSF)
The efficiency of slow sand filters is high and they can remove larger percentage of cy of slow and
filterich 000 oval base to siziano T suspended impurities and becteria. The efficiency of bacteria
removal is 98 to 99%.
These filters can also remove odours and tastes caused by organic impurities (algae and plankton).
They are less efficient in removing colour and can remove turbidity only upto 50mg/1.
10
60
u
                 
They are not suitable for sedimented waters with high turbidity.
The rate of filtration is less (i.e. 100 to 200 litres per hour per to rapid sand filters.
They also require large area of land and are costly to install.
Their use has therefore decreased and are preferred only in smaller water treatment 
 
Construction of Slow Sand filters
A slow sand filter consists of the following parts:
(i) Enclosure tank
It is an open basin, rectangular and built below ground level. The water-tight tank is constructed of
stone/brick masonry with coating of water proof material. The floor has bed slope -1 in 100 to 1 in
200 towards the central drains. Surface area of tank varies between 50 m to 1000 m . Rate of
filtration is - 100 to 200 litres of water per square meter. Depth of tank varies between - 2.5 to 4 m.
(ii) Filter media
It consists of Sand layer, 90 to 110 cm thick placed over gravel.
Effective size - 0.2 to 0.35
Uniformity coefficient - 2 to 3
Sand is placed in layers of 15cm, with finer sand on the top layer and coarser sand
 (iii) Base material (Gravel)
2 2
                 
Page 5


 
The process of passing the water through the beds of granular materials (filters) known as filtration.
Purpose of filtration :
(i) To remove very fine suspended and colloidal particles that do not settle in th sedimentation
process.
(ii) To remove dissolved impurities in water.
(iii) To remove pathogenic bacteria from water.on (iv) To remove colour, odour, turbidity in water.
Types of filters:
(i) Slow sand filters
(ii) Rapid sand filters
(iii) Pressure filter
Theory of filtration:
During filtration, the following actions take place:
(i) Mechanical straining
(ii) Sedimentation
(iii) Biological action
(iv) Electrolytic action 
 
(i)  Mechanical straining (for coarser particles)
When water passes through the filter media (sand), the suspended particles larger than the pore-
space of the filter media get trapped and removed. The trapped particles form a mat on the filter
                 
media and help in straining more impurities.
 
(ii) Sedimentation (for finer particles)
The voids of the filter media acts as small sedimentation tanks and fine particles giare removed by
settling.
 
(iii) Biological action
Certain microorganisms and bacteria present in the voids of filters form coatings over the sand
grains. These organisms utilize the organic impurities in water as their food and convert them into
harmless compounds by biological metabolism. They form a layer on the filter media called
"schmutzdecke or dirty skin". This layer further helps in absorbing and straining out the impurities
in water
.
(iv) Electrolytic changes (Ionisation)
The sand grains of filter media and impurities in water are oppositely charged. When the impurities
come in contact with the sand grains, their charges get neutralised and changes the characteristics of
water making it purer. After certain period of time, the charge of sand grains gets exhausted and
should be restored by ge of s din cleaning the filters.
Filter Media:
Sand (fine or coarse) is generally used as filter media and supported on gravel.
(i) Sand:
The properties of filter sand are :
• It should be obtained from hard and resistant quartz or quartzite.
• Free from dirt and other impurities.
• It should not loose more than 5% of its weight when placed in hydrochloric acid (HCI) for 24 hrs.
• Specific gravity = 2.55 to 2.65
                 
• Effective size (a) 0.2 to 0.4 mm - slow sand filters.
 (b) 0.35 to 0.55 mm – rapid sand filters..
• Uniformity co-efficient
(a) 1.8 to 2.5 - slow sand filters
(b) 1.3 to 1.7 - rapid sand filters
The uniformity characteristics of sand are expressed in terms of
(a) effective size (b) Uniformity coefficient.
Effective size or diameter (D ) represents a size of sieve in mm, throuth which 10% of the
particles will pass i.e.,are finer than this size. Similarly, D size represent a size such that 60%
particles are finer than it.
Uniformity coefficient (C ) is a measure of the particle range i.e. variations in size of particles.
(ii) Anthracite - Crushed anthracite can be used as filter media separately or combined with sand
(mixed media). It is costly than sand.
(iii) Garnet sand - It has high specific gravity (4.2) and is a dense material. Due to high cost, it
cannot be used as a sole filter material. However, it can be used in mixed-media 30 so filter.
(iv) Other materials - Locally available materials such as shredded coconut husks, burned rice
husks, crushed glass, slag, metallic ores etc. can be used as filter material.
 
Slow Sand Filters (SSF)
The efficiency of slow sand filters is high and they can remove larger percentage of cy of slow and
filterich 000 oval base to siziano T suspended impurities and becteria. The efficiency of bacteria
removal is 98 to 99%.
These filters can also remove odours and tastes caused by organic impurities (algae and plankton).
They are less efficient in removing colour and can remove turbidity only upto 50mg/1.
10
60
u
                 
They are not suitable for sedimented waters with high turbidity.
The rate of filtration is less (i.e. 100 to 200 litres per hour per to rapid sand filters.
They also require large area of land and are costly to install.
Their use has therefore decreased and are preferred only in smaller water treatment 
 
Construction of Slow Sand filters
A slow sand filter consists of the following parts:
(i) Enclosure tank
It is an open basin, rectangular and built below ground level. The water-tight tank is constructed of
stone/brick masonry with coating of water proof material. The floor has bed slope -1 in 100 to 1 in
200 towards the central drains. Surface area of tank varies between 50 m to 1000 m . Rate of
filtration is - 100 to 200 litres of water per square meter. Depth of tank varies between - 2.5 to 4 m.
(ii) Filter media
It consists of Sand layer, 90 to 110 cm thick placed over gravel.
Effective size - 0.2 to 0.35
Uniformity coefficient - 2 to 3
Sand is placed in layers of 15cm, with finer sand on the top layer and coarser sand
 (iii) Base material (Gravel)
2 2
                 
The base material is gravel, which supports the sand. It consists of 30 to 75cm thick gravels placed
in three to four layers, each of 15 to 20 cm depth. The coarser gravel in the bottom layer and finest
gravel in the topmost layer. The gravel size in different layers is given below:
Top 15 cm layer – Gravel size 3 to 6 mm
Intermediate layers -  6 to 20 mm / 20 to 40 mm
Bottom most 15 cm layer - Gravel size 40 to 65 mm
(iv) Under-drainage system
Filter media (sand) and base material (gravel) are laid over the under-drainage system. It consists of
a central drain and lateral drains. Laterals are open jointed pipe or porous drains placed at 3 to 5m
spacing on the bottom floor and sloping towards the central drain. Filtered water is collected by the
laterals and discharged into the central main drain and then to the filtered water well.
(v) Inlet and Outlet arrangements
An inlet chamber admits water from the plain sedimentation tank and distributes over the filter
media.
A filtered water well on the outlet side, collects the filtered water coming out from the main drain.
An adjustable telescopic tube in the outlet chamber maintains constant discharge through the filter.
Inlets and outlets are controlled by automatic valves.
 
(vi) Other appurtenances