WATER SUPPLYSANITARY
ENGINEERING
B.E. CIVIL ENGINEERING
LECTURE SLIDES
CHAPTER VI
Treatment
1
of Water-Filtration
FILTRATION
Filtration:
Process of passing the water through the
bed of porous medium
for
Removing bacteria, colour, odour and
producing clear and sparkling water
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THEORY OF FILTRATION
•Mechanical straining
•Sedimentation
•Biological action
•Electrolytic action
3
FILTER MEDIA
Sand: -
should free from loam, clay, lime, vegetable matter, dirt
and other impurities.
should be uniform in nature and size.
wearing loss shouldn’t exceed 3%.
Ignition loss shouldn’t exceed 0.7%.
shouldn’t loose its weight more than 5% of its weight
when placed in HCl for 24 hours.
Effective size shall be –
a. 0.2 – 0.3 mm for slow sand filters.
b. 0.45 -0.7 mm for rapid sand filters.
The uniformity Coff. Shall be –
a. 3 to 5 for SSF
b. Not less than 1.3 nor more than 1.7
Specific gravity = 2.55 to 2.65.
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Gravel:
placed in 5 or 6 layers.
should be hard, clean, durable and rounded.
should be free from loam, clay, organic and vegetative
materials.
It shouldn’t contain flat, thin or long pieces.
It should have density 1600 kg/m3.(about)
Grading of gravel
Thickness of layer in cm Size of gravel in mm
5 – 10 2.5 - 4.75
5 – 10 4.75 – 12.75
10 – 15 12.75 – 20.0
10 -15 20 – 40
15 – 20 40 – 65
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SLOW – SAND FILTER (SSF)
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Enclosure Tank:
open water tight rectangular tank made of masonry or
concrete.
Floor has bed slope of 1 in 200 to 1 in 100 towards the
central drain.
Depth of tank = 2.5 to 4 m.
Plan area of tank = 50 - 1000m2
Filtration rate = 100 – 200 L/ m2/ hr
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Filter media:
Thickness of filter media = 90 – 110 cm.
D10 = 0.2 -0.35 mm (0.3 mm common)
Cu = 2 to 3 (2.5 common)
Base material: -
30–75 cm thick gravel of different size placed in layer.
Generally 3 – 4 layers of 15 to 20 cm depths are used. Coarse at the
bottom and finer at the top.
Depth Size
Topmost layer 15 cm 3 – 6 mm
Intermediate ″ 6 – 20 ″
″ ″ 20 – 40 ″
Bottom layer ″ 40 – 65 ″
Under drainage system:
porous drains placed 2 to 3 cm c/c of ф 7.5 – 10 cm. 8
RAPID SAND FILTER (RSF)
Enclosure tank: -
•Depth of tank = 2.5 -3.5 m.
•Surface area = 20 -80 m2, each unit, each unit arranged in series.
•L/B = 1- 25 to 1.35
•No. of units, N = 1.22√Q.
•Q = Plant capacity in million liter/day.
Filter media:
Depth of sand media = 60 – 90 cm.
D10 = 0.35 -0.6 mm (0.45 mm common),
D60 /D10 ) = 1.2 -1.7 (1.5 common)
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Base material:
Consists of 60 -90 cm thick gravel ,5 to 6 layers, each of 10 to 15
cm in depth
Layer Depth Size
Top most 15 cm 2 – 6 mm
Intermediate ″ ″ 6 -12 mm
Intermediate 15 cm 12 -20 mm
Bottom most 15 cm 20 – 50 mm.
Other appurtenances:-
•Wash water troughs:
•Air compressor: - about 4 minutes @ about 600 – 800 L/m /sq.m
of filter media.
•Rate control devices:
•Miscellaneous accessories:
a) Head loss indicator b) Different scale and reading meter.
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PRESSURE FILTER
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Pressure may vary 3 – 7 kg/ cm2. Diameter of vertical = 2 – 2.5m and
length = 2.5 – 8 m. Pressure developed = 30 – 70 m head (3 –7 kg/cm2).
Difference between SSF and RSF
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