Mnemonic for Rapid sand Filter/ Methods of purification of Water.
Methods of water purification
I. Natural and
II. Artificial
Artificial
A. Large Scale and
B. Small scale
A. Large Scale includes
a. Storage
b. Filtration and
c. Disinfection
a. Filtration
i. Slow Sand Filter - ( I have already uploaded a video)
ii. Rapid Sand Filter
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Rapid Sand Filtration
- First RSF were installed in USA in 1885
Type
1. Gravity Type ( Paterson’s filter)
2. Pressure Type (Candy’s filter)
Steps of RSF
- Mnemonic
1. Coagulation
2. Mixing
3. Flocculation
4. Sedimentation
5. Filtration
6. Chlorination.
1. Coagulation
• Raw water mixed with Alum (chemical coagulant) to remove
Turbidity and colour.
2. Mixing
• Violent agitation / Rapid mixing in a Mixing Chamber for a few
minutes,
3. Flocculation
Slow Agitation (slow and gentle stirring) of treated water for‘30mts’.
4. Sedimentation
The coagulated water is detained in Sedimentation Tank for periods
vary from 2 – 6 hrs.
5. Filtration
• The partially clarified water then subjected into Rapid Sand Filtration.
Filter bed
• Each unit of a filter bed has a surface about 80 – 90 m² (about
900sq.ft.)
• Sand is the filtering media.
• The effective size of the sand particles is between 0. 4 - 0.7mm
• The depth (Thickness) of the of sand bed is usually about ‘1m’
(about 2.5 – 3 feet).
• Below the Sand bed is a layer of graded Gravel, ‘30 – 40cm’ (about 1 -
1.5 feet) deep.
• The gravel support the sand bed and permits the filtered water to
move freely towards the Under-drains
• The depth of the water on the top of the sand bed (Supernatant
water) is 1 – 1.5m ( 5 - 6 ft.)
• The Under drain at the bottom of the filter bed, collect the filtered
water.
• The Rate of filtration is ‘100-150’ LPM (5 – 15m3/ m²/ hour).
Filtration
• As filtration proceeds, the ‘alum – floc’ not removed by sedimentation is held back on the sand bed and form a Slimy layer ( Vital layer)
6. Chlorination
• Final step in the purification of water.
• Most common cost-effective means of disinfecting.
• Kills the harmful bacteria, viruses, and protozoa, but not cyst.
Backwashing
• When the thickness of Vital layer reaches 7 – 8 feet ( loss of head),
the filtration is stopped and the filters are subjected to a washing
process known as ‘backwashing’.
• Washing is stopped when clear sand is visible and the wash water is
sufficiently clear
• The whole process of washing takes about ‘15 minutes’.
• In some RSF, the compressed air is used for back washing
• Advantages – RSF over SSF
1. Rapid sand filter can deal with raw water directly (No preliminary
storage is needed).
2. The filter bed occupy less space.
3. The filtration is rapid 40 – 50 times that of the SSF
4. The washing of the filter is easy.
5. More flexibility in operation.
The difference between RSF and SSF
Thank You.
