CEE - Georgia Tech CEE - Georgia Tech FILTRATION AND BACKWASHING A. Amirtharajah A. Amirtharajah School of Civil and Environmental School of Civil and Environmental Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Atlanta, GA 30332 Atlanta, GA 30332
FILTRATION AND BACKWASHING. A. Amirtharajah School of Civil and Environmental Engineering Georgia Institute of Technology Atlanta, GA 30332. FILTRATION: THE GREAT BARRIER TO PARTICLES, PARASITES, AND ORGANICS. Particle Removal. Improve taste, appearance Sorbed metals and pesticides - PowerPoint PPT Presentation
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CEE - Georgia TechCEE - Georgia Tech
FILTRATION AND BACKWASHING
A. AmirtharajahA. AmirtharajahSchool of Civil and Environmental EngineeringSchool of Civil and Environmental Engineering
Georgia Institute of TechnologyGeorgia Institute of TechnologyAtlanta, GA 30332Atlanta, GA 30332
Why is it easier to remove alum or clay Why is it easier to remove alum or clay particles in contrast to polymer coated particles in contrast to polymer coated particles or micro-organisms during particles or micro-organisms during backwash?backwash?
Quarles WTP Dual Media 1.2 Qa2 + %(V/Vmf) = 49.1 1.4 to 4.0 scfm/sq ft
Vmf based on d90% size.
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Total Interaction Force: Hydrophilic Clay Vs Hydrophobic Bacteria
-60-50-40-30-20-10
0102030405060
0 1 2 3 4 5 6 7 8 9 10Separation distance (nm)
Tota
l for
ce (
nN)
ClayBacteria
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Biofiltration
OzonationOzonation Microbial counts in effluentMicrobial counts in effluent Head lossHead loss Effect of biocidesEffect of biocides Particle removalParticle removal
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Biological Filtration and Backwashing
Precursor RemovalPrecursor Removal
Minimize DBP’sMinimize DBP’s
Effect of HydrophobicityEffect of Hydrophobicity
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Bacterial Adhesion
Energy barrier
Secondaryminimum
Primary minimum
Distance
Release of extracellularpolymeric substances atsecondary minimum
Pote
ntia
l Ene
rgy
of In
tera
ctio
n
Rep
ulsi
onA
ttrac
tion
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Turbidity and Bacterial Removal During Backwashing
0 2 4 6 8Backwash time (min)
HPC
(cf
u/m
L)
0
10
20
30
40
50
60
70
Turb
idit
y (N
TU)
HPCTurbidity
105
103
106
104
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Backwashing Biofilters
Collapse-pulsing air scourCollapse-pulsing air scour Cleans betterCleans better No deleterious effectNo deleterious effect
Chlorinated backwash reduces TOC Chlorinated backwash reduces TOC removal over timeremoval over time
Chloraminated backwash less than 2.0 Chloraminated backwash less than 2.0 mg/L may be usedmg/L may be used
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Pathogenic Protozoa
Low infective dosesLow infective doses Resistant to chlorine disinfectionResistant to chlorine disinfection Analytical techniquesAnalytical techniques
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Outbreaks of Cryptosporidiosis
Surface and groundwater sourcesSurface and groundwater sources RunoffRunoff Sewage spillsSewage spills CoagulationCoagulation FiltrationFiltration
rate changesrate changes Backwash recycleBackwash recycle Contaminated distribution systemContaminated distribution system
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Particle Counts
Continuous on-line monitoringContinuous on-line monitoring Low operating costsLow operating costs High sensitivityHigh sensitivity Detachment of aggregatesDetachment of aggregates
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Cyst Removal vs Particle Removal
00.5
11.5
22.5
33.5
44.5
0 1 2 3 4 5
Log Removal of 4 - 7 m ParticlesLo
g Re
mov
al o
f Cryptosporidium
00.5
11.5
22.5
33.5
44.5
0 1 2 3 4
Log Removal of 7 - 11 m Particles
Log
Rem
oval
of Giardia
Nieminski and Ongerth (1995)
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Minimizing Risk of Outbreaks
Optimal destabilization of particlesOptimal destabilization of particles Filter-to-wasteFilter-to-waste Coagulants in backwashCoagulants in backwash Slow-start filtrationSlow-start filtration Minimizing flow rate changes in dirty filtersMinimizing flow rate changes in dirty filters Treatment of backwash waterTreatment of backwash water Filter effluent turbidity < 0.1 NTUFilter effluent turbidity < 0.1 NTU
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Concluding Statement
In the multiple-barrier concept, In the multiple-barrier concept, filtration is the “great” barrier to filtration is the “great” barrier to particles, parasites and organics.particles, parasites and organics.
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Summary and Conclusions
Importance of particle destabilizationImportance of particle destabilization Micromechanical force modelMicromechanical force model Biofiltration for organics removalBiofiltration for organics removal Effectiveness of collapse-pulsing air scourEffectiveness of collapse-pulsing air scour Multiple-barrier conceptMultiple-barrier concept
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References
Amirtharajah, A., “Some Theoretical and Conceptual Amirtharajah, A., “Some Theoretical and Conceptual Views of Filtration,” Views of Filtration,” JAWWAJAWWA, Vol. 80, No. 12, 36-46, , Vol. 80, No. 12, 36-46, Dec. 1988.Dec. 1988.
Amirtharajah, A., “Optimum Backwashing of Filters with Amirtharajah, A., “Optimum Backwashing of Filters with Air Scour - A Review,” Air Scour - A Review,” Water Sci. and Tech.Water Sci. and Tech., Vol. 27, No. , Vol. 27, No. 10, 195-211, 1993.10, 195-211, 1993.
Ahmad, R. et al., “Effects of Backwashing on Biological Ahmad, R. et al., “Effects of Backwashing on Biological Filters,” Filters,” JAWWAJAWWA, Vol. 90, No. 12, 62-73, Dec. 1998., Vol. 90, No. 12, 62-73, Dec. 1998.
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Acknowledgments
This paper includes the work of several former This paper includes the work of several former students at Georgia Tech: students at Georgia Tech:
M.S. students T.M. Ginn, L. Zeng and X. Wang M.S. students T.M. Ginn, L. Zeng and X. Wang and Ph.D students, Drs. P. Raveendran, R. and Ph.D students, Drs. P. Raveendran, R. Ahmad, K.E. Dennett and T. Mahmood.Ahmad, K.E. Dennett and T. Mahmood.
They were not only students but teachers too! They were not only students but teachers too! Their work is acknowledged with gratitude.Their work is acknowledged with gratitude.