1. On removing little particles with big particles
Filtration Theory
On removing little particles with big particles
BY-
Prakash Sundram

2. Topics to be Covered Filtration
How Filters Work:  Filtration Mechanisms
Filter Materials
Types of Filter
Principles of Slow Sand Filtration
Slow Sand Filters vs. Rapid Sand Filters

3. 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.

4. Particle Removal Improve taste, appearance Removes odour, turbidity
Removes Pathogens: bacteria, viruses, protozoa

5. How Filters Work: Filtration Mechanisms
SEDIMENTATION
INTERCEPTION
BROWNIAN DIFFUSION
INERTIA

6. Sedimentation
The mechanism of sedimentation is due to force of gravity and the associate settling velocity of the particle, which causes it to cross the streamlines and reach the collector.

7. Interception
Interception of particles is common for large particles. If a large enough particle follows the streamline, that lies very close to the media surface it will hit the media grain and be captured.

8. Brownian Diffusion
Diffusion towards media granules occurs for very small particles, such as viruses. Particles move randomly about within the fluid, due to thermal gradients. This mechanism is only important for particles with diameters < 1 micron

9. Inertia
Attachment by inertia occurs when larger particles move fast enough to travel off their streamlines and bump into media grains.

10. Contact Points

11. Filter Materials Sand Effective size(D10)
Uniformity coefficient(D60/D10)
Gravel
Other material

12. Filter Media- Fine Sand Filter Bed
Criteria for top layer of filter media 
Size of particles:
~ mm
Particles must be
uniform
Free of organic matter,
loam, and clay
Depth greater than
0.6 m (every 0.3
meters of extra sand will help the filter media to last an extra 3-4 years)
Different sized sand particles

13. Filter Media – Gravel Layer
Construction: Separate into 3 grades
Top
Finest
2-8 mm
Middle
Size 8-16 mm
Bottom
Coarsest
Size mm
Purpose
Supports the sand 
Contributes to uniform
filtration
Coarse gravel

14. Other Materials
Instead of using sand, sometimes, Anthrafilt is used as filter media. Anthrafilt is made from anthracite, which is a type of coal-stone that burns without smoke or flames. It is cheaper and has been able to give a high rate of filtration.

15. Types of Filter
Slow sand filter
Rapid-sand filter
Multimedia filters

16. The Slow Sand Filter Large scale community water treatment system
Water filters through a layer of sand with gravel base
Gravity is the driving force
Collective pipes
No chemicals added
Small pore space traps
and removes particles
Formation of biofilm
above the sand
contributes to the
reduction of bacteria,
viruses, protozoa and colloidal particles
Slow sand filter

17. Advantages long design life Can use local materials and labor
Inexpensive and easy to construct
Minimal sludge handling problems
Close operator supervision is not necessary
Filters enough water for a community (up to 15,000 gal/day)
Reduces bacteria, cloudiness, and organic levels
Minimal power and chemical requirements

18. How does the filter remove contaminants?
The schmutzdecke consumes and adsorbs/absorbs organic contaminants
Layers of sand strain out particulate contaminants due to the small pores created by fine sand particles
A layer of Schmutzde

19. Typical Slow Sand Filter Schematic

20. Filter Maintenance To address head loss or slow flow rate
Scrape off the schmutzdecke and top layer of sand
Clean sand or add new, clean sand to the top of filter
The length of time between cleaning can range from several weeks to a year depending on the raw water quality
Filtered water quality will be poor for a few days after cleaning (water should not be used until the biological layer builds up again)

21. Biological Layer: The Schmutzdecke
Due to the organic and biological matter in the water to be filtered, a bio-layer grows on top of the sand layer
The bio-layer contributes to water treatment by consuming organic contaminants including bacteria and viruses
A four week period of saturation before drinking is suggested so the layer fully develops 
The majority of biological activity occurs in the top 20 cm of the filter
Biofilm Development
Layer of Schmutzdecke

22. What will make the filter suitable for rural communities?
Simple filtration technology
Low construction and operation cost  
Excellent removal of pathogenic organisms
Good removal of turbidity
Low energy consumption
Minimal sludge handling 
Does not need constant operator attention

23. Filtration-Influencing Factors
Treatment conditions
Influent microorganism density
Biological condition of sand bed
Established biota on ripened bed
More effective removal
Better turbidity control
Water temperature
Cold water inhibits biological processes
Filtration rate
Increased removal with lower rate
Sand size
Increased removal with smaller size
Influent nutrient levels
Promote biological growth

24. Rapid Sand Filters Typically closed, pressurized units
Handle high flow rates: 20 gpm/ft2
Downside: very high head loss (30-90 ft)
Only really good for low solids process streams with some sort of pre-trt
Backwashing can be made automatic

25. Rapid Sand Filter (Conventional US Treatment)
Specific
Gravity
1.6
2.65
Depth
(cm) 30 45
Size
(mm)
0.70
5 - 60
Anthracite
Influent
Sand
Gravel
Drain
Effluent
Wash water

26. 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.

27. Backwashing Wash water is treated water! WHY?
Anthracite
Only clean water should ever be on bottom of filter!
Sand
Influent
Gravel
Drain
Effluent
Wash water

28. Multimedia filters
They consist of two or more layers of different granular materials, with different densities. Usually, anthracite coal, 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, even after backwashing.

29. Sand Filters vs. Rapid Sand Filters
Base material:
3 to 65 mm in size and 30 to 75 cm in depth
3 to 40 mm in size and 60 to 90 cm in depth
Filter sand:
E.Z between 0.2 to 0.4 and U.C between 1.2 to 3.0
E.Z between and U.C between 1.2 to 1.8
Rate of filtration:
l/h/sq.m
l/h/sq.m
Flexibility:
No flexibility according to demand
Quite flexible according to demand
Post treatment:
Almost pure water obtained
Disinfection required
Method of cleaning:
Scrapping and removal of dirty skin
Sand is agitated and backwashed with or without compressed air
Loss of head:
Initial limit 10 cm and final limit is 0.8 to 1.2 m
Initial loss is 0.3m and final limit is 2.5 to 3.5m

30. Sand Filters and Raw water Tank

31. New Rapid Gravity Filters

32. Sludge Storage

33. Adapted from the following:
-->good case st

34. Image Sources http://www.marlerblog.com/E_coli_lge.jpg
Precast beds : from manufacturers sucs as Milton Precast
Source: Vigneswaran, S. and C. Visvanathan. 1995
prometheus.mse.uiuc.edu/glossary/biofilms/
biosandfilter.org