filtration and backwashing

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


FILTRATION: THE GREAT BARRIER TO PARTICLES,

PARASITES, AND ORGANICS


Particle Removal

Improve taste, appearanceImprove taste, appearance

Sorbed metals and pesticidesSorbed metals and pesticides

Pathogens: bacteria, viruses, protozoaPathogens: bacteria, viruses, protozoa


Organic Removal in Biofiltration

Prevent biofouling of distribution systemPrevent biofouling of distribution system

Remove DBP precursorsRemove DBP precursors


Multiple-Barrier Concept

watershed protection

chemical addition

screenwaste sludge

coagulation flocculation

sedimentation filtration

backwash recycle

waste sludge

disinfection

distribution system

direct filtration

raw water


Fundamental and Microscopic View

1.1. Filtration:Filtration:AttachmentAttachmentDetachmentDetachment

2.2. Backwashing:Backwashing:DetachmentDetachment


Mechanisms of Filtration

transporttransport

attachmentattachment

detachmentdetachment

fluid streamlinefluid streamline

collector, dcollector, dcc

particle, dparticle, dpp


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


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


History of Filtration Theory (2)

Trajectory Analysis - Microscopic ViewTrajectory Analysis - Microscopic View

dcdz d

cc

D G I

1 5

1.


Detachment - Macroscopic View

Mintz:Mintz:

Ginn et al.:Ginn et al.: c zc

uac

zc

da

o


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-


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


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

CEE - Georgia TechCEE - Georgia TechpH of Mixed Solution

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


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


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


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?


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:


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


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


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


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%


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.


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


Biofiltration

OzonationOzonation Microbial counts in effluentMicrobial counts in effluent Head lossHead loss Effect of biocidesEffect of biocides Particle removalParticle removal


Biological Filtration and Backwashing

Precursor RemovalPrecursor Removal

Minimize DBP’sMinimize DBP’s

Effect of HydrophobicityEffect of Hydrophobicity


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


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


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


Pathogenic Protozoa

Low infective dosesLow infective doses Resistant to chlorine disinfectionResistant to chlorine disinfection Analytical techniquesAnalytical techniques


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


Particle Counts

Continuous on-line monitoringContinuous on-line monitoring Low operating costsLow operating costs High sensitivityHigh sensitivity Detachment of aggregatesDetachment of aggregates


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)


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


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.


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


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.


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.

filtration and backwashing

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