Slow sand filters and rapid sand filters are two methods for large-scale water purification. Slow sand filters operate at a lower rate of filtration and consist of a bed of carefully selected sand over gravel. A biological layer called a schmutzdecke forms on the surface and provides mechanical, chemical, and biological filtration. Rapid sand filters require preliminary coagulation, flocculation, and sedimentation steps before water passes through finer sand at a higher rate. They occupy less space but require more skilled operation including periodic backwashing to clean the sand bed. Both methods significantly reduce bacteria in treated water, though slow sand filters provide superior removal.

1. WATER
PURIFICATION
ON
LARGE SCALE

3. Slow sand filter
• First used in 1804 in Scotland & subsequently in
London.
• During 19th Century used through out the world.
• consists of concrete rectangular basin containing
carefully selected graded sand supported on
gravel and stones.

5. Filter box
1-1.5 m
1m
0.3 m

6. Slow sand filter
1. Supernatant water above the sand bed
Depth 1-1.5 mtr

7. Slow sand filter
2. A bed of graded sand
– Most important part of the filter
– Thickness – about 1 mtr
– Sand : preferably rounded with effective diameter
of 0.2-0.3 mm
– Supported by graded gravel (30-40 cm deep)
– Water percolates through the bed and gets
filtered by
• Mechanical straining, sedimentation, adsorption,
oxidation and bacterial action
– Rate of filtration: 0.1-0.4 m3/hour/m2

8. Vital Layer
– The Heart of Slow Sand Filter
Surface of the sand bed gets covered with a slimy growth known
as ‘Schmutzdecke’, vital layer or zoological layer or biological layer
It is slimy geletinuos, consists of algae, plankton, diatoms and
bacteria.
The formation of this layer is known as RIPENING of the filter,
which may take several days to fully develop (2-3 cm)
Removes organic matter, holds back bacteria, oxidizes
ammonical nitrogen to nitrates and yield a bacteria free water
Until the vital layer is fully formed, filter only worked as a
mechanical strainer & so for the first few days filter water run to

9. Slow sand filter
3. An under-drainage system
porous of perforated pipes at the bottom of filter
– Provides an outlet to the filtered water and
– supports the filter medium

10. Slow sand filter
4. A system of filter control valves
Venturi meter -regulation system
– To maintain constant rate of filtration
– measures the resistance of the sand bad or “loss
of head”

11. Slow sand filter
• Filter cleaning
– When the bad resistance increases to such an
extent that the regulating valve has to be kept fully
open, filter bed needs to be cleaned.
– Cleaning is done by
• Draining of supernatant water
• Scrapping of the top portion of the sand bed to a depth
of 1-2 cm
– After 20-30 scrapings , new bed (filter) is required
to be constructed

12. Slow sand filter
• Advantages :
√ Simple to construct and operate
√ Cheaper
√ Physical, Chemical & Bacteriological quality of
water is very high
√ Reduces bacterial count by 99.9% & E. coli by
99%

13. Disadvantages
× Old fashioned and outdated method of water
purification (but still in use)
× Initial cost is low but maintenance cost is
much more than rapid sand filter
× These filters need a lot of space

14. Rapid sand filter
• In 1885, the first rapid sand filters were installed
1885
in USA
• 2 types
– Open/gravity type—
– Closed/presure type—
PATERSON FILTER
Candy’s Filter
• Steps of filtration
Coagulation
Rapid Mixing
Flocculation
Sedimentation


15. STEPS
• Coagulation
– Alum is added – 5-40 mg/liter depending upon turbidity, color,
pH & temperature
• Rapid mixing of Alum
– By violent agitation in a mixing chamber for few minutes
• Flocculation
– Slow & gentle stirring of the treated water for about 30 minutes
– Results in formation of thick, copious, white flocculent
precipitate of aluminium hydroxide
• Sedimentation
– For 2-6 hours
– Flocculent precipitate together with impurities and bacteria
– 95% of the flocculent needs to be removed before the water
enters the rapid sand filter

17. Filter beds –
Each unit has a surface of 80-90 m2
Supernatant water – 1 to 1.5 mtr
Depth of sand bed – 1 mtr
Effective size of the sand particle is
0.4-0.7 mm
Supported graded gravel – 30-40
cm
Filtration rate – 5-15 m3 / m2/ hr

18. Rapid sand filter
Filtration –
– Retained alum- flocs is held back on sand bed
• Forms a slimy layer [comparable to the zoological layer in
slow sand filter]
• adsorbs bacteria from water
– Oxidation of ammonia also takes place during the
passage of water

19. Rapid sand filter
• Cleaning & Maintenance
–When the loss of head reaches 7-8 ft, it needs to be
cleaned
–cleaned by Backwashing.
•Backwashing is done by reversing the flow of
water through the sand bed.
dislodges the impurities and cleans up sand bed.
•Compressed air can be used in back washing

20. Rapid sand filter
• Advantages –
deal with raw water
directly
Occupies less space
Filtration is rapid
Washing of filter is easy
More flexibility in
operation
• Disadvantages
× Relatively high
skill operation
× Costlier
× Removes bacteria
by 98-99%

21. Slow sand filter
Space
Rate filtration
Rapid sand filter
Occupies large area
Small area
0.1-0.4 m3/m2/ hour
5-15 m3 / m2/ hour
Effective size of sand
0.2-0.3 mm
0.4-0.7 mm
Preliminary treatment
Plain sedimentation
Chemical coagulation &
sedimentation
Washing
By scraping the sand
bed
Backwashing
Operation
Less skilled
Highly skilled
Loss of head allowed
4 feet
6-8 feet
Removal of turbidity
Good
Good
Removal of colour
fair
Good
Removal of bacteria
99.9-99.99 %
98-99%

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