FILTRATION AND BACKWASHING

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FILTRATION AND BACKWASHING

• A. Amirtharajah
• School of Civil and Environmental Engineering
• Georgia Institute of Technology
• Atlanta, GA 30332

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FILTRATION THE GREAT BARRIER TO PARTICLES,
PARASITES, AND ORGANICS
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Particle Removal

• Improve taste, appearance
• Sorbed metals and pesticides
• Pathogens bacteria, viruses, protozoa

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Organic Removal in Biofiltration

• Prevent biofouling of distribution system
• Remove DBP precursors

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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
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Fundamental and Microscopic View

• 1. Filtration
• Attachment
• Detachment
• 2. Backwashing
• Detachment

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Mechanisms of Filtration
particle, dp
transport
fluid streamline
attachment
collector, dc
detachment
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History of Filtration Theory(1)

• Phenomenological - Macroscopic View
• Basic Equations
• Ives

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Trajectory Theory
dp
Viruses 0.01 -0.025 mm Bacteria 0.2 - 1
mm Cryptosporidium 3 - 5 mm Giardia 6 - 10 mm
dc
dc
dc
Diffusion dp lt 1 mm
Sedimentation dp gt 1 mm
Interception
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History of Filtration Theory (2)

• Trajectory Analysis - Microscopic View

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Detachment - Macroscopic View

• Mintz
• Ginn et al.

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Particle Size Distribution Function
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Variation in???Across a Water Treatment Plant
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Filter Effluent Quality
Filter Ripening
Outlet
Backwash remnants
TB
above media
in media
TM
Effluent Turbidity
Function of influent
Clean back-wash
Media
Strainer
TU
Filter breakthrough
TU
TM
TB
TR
Time
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Alum Coagulation Diagram
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Alum Coagulation Diagram
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Conceptual Model of Filtration
Attachment () ?
Filter coefficient (?)
0
?(-) Detachment
Filter Ripening
Effective Filtration
Turbidity Breakthrough
Wormhole Flow
Time
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• Question
• Why is it easier to remove alum or clay particles
  in contrast to polymer coated particles or
  micro-organisms during backwash?

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Sphere - Flat Plate Interactions (1)
Van der Waals Force
a
z
Electrostatic Double Layer Force
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Sphere - Flat Plate Interactions (2)
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Detachment During Backwashing

• Hydrodynamic Forces gt Adhesive Forces
• 1. Spherical Particles - pH and Ionic Strength
• 2. Non-spherical Particles - Ionic Strength
• Kaolinite Platelets

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Backwashing Filters

• Weakness of fluidization backwash
• Improvement due to surface wash
• Collapse-pulsing air scour
• The best for cleaning

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Theory for Collapse-Pulsing

• a, b coefficients for a given media
• Qa air flow rate

Percentage of minimum fluidization water flow
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Equations Describing Collapse-Pulsing for all
Filter Beds
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Total Interaction Force Hydrophilic Clay Vs
Hydrophobic Bacteria
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Biofiltration

• Ozonation
• Microbial counts in effluent
• Head loss
• Effect of biocides
• Particle removal

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Biological Filtration and Backwashing

• Precursor Removal
• Minimize DBPs
• Effect of Hydrophobicity

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Bacterial Adhesion
Energy barrier
Repulsion
Potential Energy of Interaction
Distance
Secondary
Attraction
minimum
Release of
extracellular
polymeric substances at
secondary minimum
Primary minimum
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Turbidity and Bacterial Removal During Backwashing
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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

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Pathogenic Protozoa

• Low infective doses
• Resistant to chlorine disinfection
• Analytical techniques

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Outbreaks of Cryptosporidiosis

• Surface and groundwater sources
• Runoff
• Sewage spills
• Coagulation
• Filtration
• rate changes
• Backwash recycle
• Contaminated distribution system

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Particle Counts

• Continuous on-line monitoring
• Low operating costs
• High sensitivity
• Detachment of aggregates

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Cyst Removal vs Particle Removal
Nieminski and Ongerth (1995)
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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 lt 0.1 NTU

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Concluding Statement

• In the multiple-barrier concept, filtration is
  the great barrier to particles, parasites and
  organics.

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Summary and Conclusions

• Importance of particle destabilization
• Micromechanical force model
• Biofiltration for organics removal
• Effectiveness of collapse-pulsing air scour
• Multiple-barrier concept

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References

• Amirtharajah, A., Some Theoretical and
  Conceptual Views of Filtration, JAWWA, Vol. 80,
  No. 12, 36-46, Dec. 1988.
• Amirtharajah, A., Optimum Backwashing of Filters
  with Air Scour - A Review, Water Sci. and Tech.,
  Vol. 27, No. 10, 195-211, 1993.
• Ahmad, R. et al., Effects of Backwashing on
  Biological Filters, JAWWA, Vol. 90, No. 12,
  62-73, Dec. 1998.

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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.