1. FILTRATION AND BACKWASHING
A. Amirtharajah
School of Civil and Environmental Engineering
Georgia Institute of Technology
Atlanta, GA

2. FILTRATION: THE GREAT BARRIER TO PARTICLES, PARASITES, AND ORGANICS

3. Particle Removal Improve taste, appearance
Sorbed metals and pesticides
Pathogens: bacteria, viruses, protozoa

4. Organic Removal in Biofiltration
Prevent biofouling of distribution system
Remove DBP precursors

5. Multiple-Barrier Concept
chemical addition
direct filtration
filtration
watershed protection
sedimentation
disinfection
raw water
distribution system
screen
coagulation
flocculation
waste sludge
backwash recycle
waste sludge

6. Fundamental and Microscopic View
1. Filtration:
Attachment
Detachment
2. Backwashing:

7. Mechanisms of Filtration
particle, dp
transport
fluid streamline
attachment
collector, dc
detachment

8. History of Filtration Theory(1)
Phenomenological - Macroscopic View
Basic Equations:
Ives:

9. Trajectory Theory Viruses 0.01 -0.025 mm Bacteria 0.2 - 1 mmdp
Viruses mm
Bacteria mm
Cryptosporidium
3 - 5 mm
Giardia mm dc dc dc
Diffusion
dp < 1 mm
Sedimentation
dp > 1 mm
Interception

10. History of Filtration Theory (2)
Trajectory Analysis - Microscopic View

11. Detachment - Macroscopic View
Mintz:
Ginn et al.:

12. Particle Size Distribution Function

13. Variation inAcross a Water Treatment Plant

14. Filter Effluent Quality
Filter Ripening
Backwash remnants
Outlet TB
above
media in
media TM
Effluent Turbidity
Function
of influent
Clean back-wash
Media
Strainer TU
Filter breakthrough TU TM TB
Time TR

15. Alum Coagulation Diagram

16. Alum Coagulation Diagram

17. Conceptual Model of Filtration
Attachment (+) 
Filter coefficient ()
(-) Detachment
Filter
Ripening
Effective
Filtration
Turbidity
Breakthrough
Wormhole
Flow
Time

18. Question:
Why is it easier to remove alum or clay particles in contrast to polymer coated particles or micro-organisms during backwash?

19. Sphere - Flat Plate Interactions (1)
Van der Waals Force: a z
Electrostatic Double Layer Force:

20. Sphere - Flat Plate Interactions (2)

21. Detachment During Backwashing
Hydrodynamic Forces > Adhesive Forces
1. Spherical Particles - pH and Ionic Strength
2. Non-spherical Particles - Ionic Strength
Kaolinite Platelets

22. Backwashing Filters Weakness of fluidization backwash
Improvement due to surface wash
Collapse-pulsing air scour
The best for cleaning

23. Theory for Collapse-Pulsing
a, b = coefficients for a given media
Qa = air flow rate
Percentage of minimum fluidization
water flow

24. Equations Describing Collapse-Pulsing for all Filter Beds

25. Total Interaction Force: Hydrophilic Clay Vs Hydrophobic Bacteria

26. Biofiltration Ozonation Microbial counts in effluent Head loss
Effect of biocides
Particle removal

27. Biological Filtration and Backwashing
Precursor Removal
Minimize DBP’s
Effect of Hydrophobicity

28. Bacterial Adhesion Energy barrier Repulsion
Potential Energy of Interaction
Distance
Secondary
Attraction
minimum
Release of
extracellular
polymeric substances at
secondary minimum
Primary minimum

29. Turbidity and Bacterial Removal During Backwashing

30. Backwashing Biofilters
Collapse-pulsing air scour
Cleans better
No deleterious effect
Chlorinated backwash reduces TOC removal over time
Chloraminated backwash less than 2.0 mg/L may be used

31. Pathogenic Protozoa Low infective doses
Resistant to chlorine disinfection
Analytical techniques

32. Outbreaks of Cryptosporidiosis
Surface and groundwater sources
Runoff
Sewage spills
Coagulation
Filtration
rate changes
Backwash recycle
Contaminated distribution system

33. Particle Counts Continuous on-line monitoring Low operating costs
High sensitivity
Detachment of aggregates

34. Cyst Removal vs Particle Removal
Nieminski and Ongerth (1995)

35. Minimizing Risk of Outbreaks
Optimal destabilization of particles
Filter-to-waste
Coagulants in backwash
Slow-start filtration
Minimizing flow rate changes in dirty filters
Treatment of backwash water
Filter effluent turbidity < 0.1 NTU

36. Concluding Statement
In the multiple-barrier concept, filtration is the “great” barrier to particles, parasites and organics.

37. Summary and Conclusions
Importance of particle destabilization
Micromechanical force model
Biofiltration for organics removal
Effectiveness of collapse-pulsing air scour
Multiple-barrier concept

38. References
Amirtharajah, A., “Some Theoretical and Conceptual Views of Filtration,” JAWWA, Vol. 80, No. 12, 36-46, Dec
Amirtharajah, A., “Optimum Backwashing of Filters with Air Scour - A Review,” Water Sci. and Tech., Vol. 27, No. 10, , 1993.
Ahmad, R. et al., “Effects of Backwashing on Biological Filters,” JAWWA, Vol. 90, No. 12, 62-73, Dec

39. Acknowledgments
This paper includes the work of several former students at Georgia Tech:
M.S. students T.M. Ginn, L. Zeng and X. Wang and Ph.D students, Drs. P. Raveendran, R. Ahmad, K.E. Dennett and T. Mahmood.
They were not only students but teachers too! Their work is acknowledged with gratitude.