filtration and backwashing
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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
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FILTRATION: THE GREAT BARRIER TO PARTICLES,
PARASITES, AND ORGANICS
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Particle Removal
Improve taste, appearanceImprove taste, appearance
Sorbed metals and pesticidesSorbed metals and pesticides
Pathogens: bacteria, viruses, protozoaPathogens: bacteria, viruses, protozoa
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Organic Removal in Biofiltration
Prevent biofouling of distribution systemPrevent biofouling of distribution system
Remove DBP precursorsRemove DBP precursors
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Multiple-Barrier Concept
watershed protection
chemical addition
screenwaste sludge
coagulation flocculation
sedimentation filtration
backwash recycle
waste sludge
disinfection
distribution system
direct filtration
raw water
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Fundamental and Microscopic View
1.1. Filtration:Filtration:AttachmentAttachmentDetachmentDetachment
2.2. Backwashing:Backwashing:DetachmentDetachment
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Mechanisms of Filtration
transporttransport
attachmentattachment
detachmentdetachment
fluid streamlinefluid streamline
collector, dcollector, dcc
particle, dparticle, dpp
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History of Filtration Theory(1)
Phenomenological - Macroscopic ViewPhenomenological - Macroscopic View
Basic Equations:Basic Equations:
Ives:Ives:
(2) . . . .
(1) . . . . 0
czc
tzcu
c
u
b
o
a
oo
111
1
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Trajectory Theory
dcdcdc
dp
Diffusion
dp < 1 m
Sedimentation
dp > 1 m
Interception
Viruses0.01 -0.025 m
Bacteria0.2 - 1 m
Cryptosporidium3 - 5 m
Giardia6 - 10 m
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History of Filtration Theory (2)
Trajectory Analysis - Microscopic ViewTrajectory Analysis - Microscopic View
dcdz d
cc
D G I
1 5
1.
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Detachment - Macroscopic View
Mintz:Mintz:
Ginn et al.:Ginn et al.: c zc
uac
zc
da
o
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Particle Size Distribution Function
1.0E+01.0E+11.0E+21.0E+31.0E+41.0E+51.0E+61.0E+71.0E+8
0.1 1 10 100dp (m)
n (#
/mL m
)
Ao
n (d ) = A (d )p 0 p-
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Variation inAcross a Water Treatment Plant
2
2.5
3
3.5
4
Raw water Coagulatedwater
Filtered water
va
lue
from
pow
er la
w fu
nctio
n
n = 11
n = 22
n = 11
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Time
Filter Ripening
Efflu
ent T
urbi
dity
Clean back-wash
Backwash remnants
Function
of influent
Filter breakthrough
TU TM TB TR
TB
TM
TU
Outlet
Media
Strainer
Filter Effluent Quality
above
media
in
media
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Zeta
Pot
entia
l
L
og (A
l) -
mol
/LA
lum - m
g/L
as Al
2 (SO4 )3 •14.3H
2 O
Alum Coagulation Diagram
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Alum Coagulation Diagram
pH of Mixed Solution
Alum -
mg/L
Log [Al
] - mol/L
ChargeNeutralization
4TOTAL
-8.5
-7.5
Al
5 6
2+Al(OH)
-5.5
-6.5
-4.5
-3.5
Restabilization Zone(boundaries vary withdifferent waters)
7 8
Al(OH)
9
0.3
1
SweepCoagulation
10
3
30
100
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Conceptual Model of FiltrationFi
lter c
oeffi
cient
()
(-
) D
etac
hmen
tAt
tach
men
t (+
)
0
Time
Filter Ripening
Effective Filtration
Turbidity Breakthrough
Wormhole Flow
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QuestionQuestion::
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?
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Sphere - Flat Plate Interactions (1)
1
za
z6AFv - =
F = - 64 akTZe
Ze4kT
Ze4kT
ze1 2
2tanh tanh exp
aa
zz
Van der Waals Force:Van der Waals Force:
Electrostatic Double Layer Force:Electrostatic Double Layer Force:
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Sphere - Flat Plate Interactions (2)
Born Repulsion: F = -Aa
180zb
6
8
Structural Forces
Hydration Force: F = - 2 aKh exp -zhh
Hydrophobic Force: F = aC exp -zDH
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Detachment During Backwashing
Hydrodynamic Forces > Adhesive ForcesHydrodynamic Forces > Adhesive Forces
1.1. Spherical Particles - pH and Ionic Spherical Particles - pH and Ionic StrengthStrength
2.2. Non-spherical Particles - Ionic StrengthNon-spherical Particles - Ionic Strength Kaolinite PlateletsKaolinite Platelets
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Backwashing Filters
Weakness of fluidization backwashWeakness of fluidization backwash
Improvement due to surface washImprovement due to surface wash
Collapse-pulsing air scourCollapse-pulsing air scourThe best for cleaningThe best for cleaning
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bV
V%aQmf
2a
Theory for Collapse-Pulsing
a, b = coefficients for a given mediaa, b = coefficients for a given mediaQQaa = air flow rate = air flow rate
Percentage of minimum fluidization Percentage of minimum fluidization water flowwater flow
mfVV%
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Equations Describing Collapse-Pulsing for all Filter Beds
Filter Media Equation Applicable range of QaSand 0.8 Qa
2 + %(V/Vmf) = 43.5 1.8 to 4.6 scfm/sq ft
Anthracite 1.7 Qa2 + %(V/Vmf) = 43.0 1.5 to 4.2 scfm/sq ft
Dual Media 1.7 Qa2 + %(V/Vmf) = 39.5 0.8 to 2.4 scfm/sq ft
GAC 3.3 Qa2 + %(V/Vmf) = 26.6 Qa < 2.7 scfm/sq ft
GAC-Sand 3.0 Qa2 + %(V/Vmf) = 27.2 Qa < 2.0 scfm/sq ft
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.
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