Monroe L. Weber-Shir k S chool of Civil and Environmental Engi neering What’s New in Water Treatment? How well could filters remove Particles? Coagulants and Filter Aids Sticky Particles and Sticky Media
Monroe L. Weber-Shirk School of Civil and
Environmental Engineering
What’s New in Water Treatment?What’s New in Water Treatment?
How well could filters remove Particles?
Coagulants and Filter Aids
Sticky Particles and Sticky Media
Filter Performance ModelsFilter Performance Models
Iwasaki (1937) developed relationships describing the performance of deep bed filters.
0=dC
Cdz
C is the particle concentration [number/L3]0 is the initial filter coefficient [1/L]z is the media depth [L]
The particle’s chances of being caught are the same at all depths in the filter; pC* is proportional to depth
0=dC
dzC
0
0
0
=C z
C
dCdz
C 0
0
ln =C
zC
00
1log *
ln 10
CpC z
C
0
*C
CC
Filtration Performance: Dimensional Analysis
Filtration Performance: Dimensional Analysis
What is the parameter we are interested in measuring? _________________
How could we make performance dimensionless? ____________
What are the important forces?
Effluent concentration
C/C0 or pC*
Inertia London van der Waals Electrostatic
Viscous
Need to create dimensionless force ratios!
Gravitational Thermal
Only effective in the attachment phase
Choose viscosity as the common force that inhibits transport
Choose viscosity as the common force that inhibits transport
We will use viscosity as the repeating parameter and create a set of dimensionless force ratios
GravitationalViscous
ThermalViscous
But these forces are functions of …
GravityGravity
2
g0
( )=
18p w pgd
NV
2
g
( )=
18p w pgd
v
vpore
g0
= gvN
VGravity only helps when the streamline has a _________ component.horizontal
2fu
V
l
fg gr=
g = gfN
f
g02
=
p
gN
V
d
2
g0
( )= p w pgd
NV
velocities forces
Use this equation
Diffusion (Brownian Motion)Diffusion (Brownian Motion)
kB=1.38 x 10-23 J/°KT = absolute temperature
vpore
Br0 0
=3
B
c p c
k TDN
V d d V d
3B
p
k TD
d
2L
T
0Br
c
DN
V d
dc
Dv
d
dc is diameter of the collector
Diffusion velocity is high when the particle diameter is ________.small
Geometric ParametersGeometric Parameters
What are the length scales that are related to particle capture by a filter?______________________________________________________Porosity (void volume/filter volume) ()
Create dimensionless groupsChoose the repeating length ________
Filter depth (z)
Collector diameter (media size) (dc)
Particle diameter (dp)
pR
c
dN
d z
c
zN
d
(dc)
Number of collectors!
Write the functional relationshipWrite the functional relationship
, ,g Br* , , ,R zpC f N N N N
Length ratios
Force ratios
,g Br* , ,z RpC N f N N N
If we double depth of filter (or ) what does pC* do? ___________doubles
How do we get more detail on this functional relationship?
Empirical measurements
Numerical models
attachments
per contact
Total removal (SSF conditions)Total removal (SSF conditions)
20 cm/hr 0.2 mm sand 1 m deep Particle
density of 1040 kg/m3
0.1 1 101
10
100
particle diameter
Par
ticle
rem
oval
as
pC*
pCPe dp pCR dp pCg dp pC dp
dp
m
Plots based on numerical models
How deep must a filter (SSF) be to remove 99% of bacteria?
Assume is 1 and dc is 0.2 mm, V0 = 20 cm/hr
For 1 m of sand pC*=____ Depth for pC* of ____ is
_____ What does this mean?
10 cm
20
If the attachment efficiency were 1, then we could get great particle capture in a 1 m deep filter!
2
0.1 1 1010
100
particle diameter
Par
ticle
rem
oval
as
pC*
pC dp
dp
m
0.1 1 100.1
1
10
particle diameter
Par
ticle
rem
oval
as
pC*
pCPe dp pCR dp pCg dp pC dp
dp
m
Total removal (RSF conditions)Total removal (RSF conditions)
dc=0.5 mmApproach
velocity is 5 m/hr
1 m deepParticle
density of 1040 kg/m3
How deep a Rapid Sand Filter will remove 90% of cryptosporidium?
Assume is 1 and dc is 0.5 mm, V0 = 5 m/hr
dp is 4 m pC* is ____ for 1 m deep
filter z is ________________1 m/1.8=0.55 m
1.8
We need flocculation to produce larger and more dense particles to get good removal in RSF
0.1 1 100.1
1
10
particle diameterP
artic
le r
emov
al a
s pC
*
pCPe dp pCR dp pCg dp pC dp
dp
m
Slow Sand FiltrationSlow Sand Filtration
First filters to be used on a widespread basisFine sand with an effective size of 0.2 mmLow flow rates (10 - 40 cm/hr)Schmutzdecke (_____ ____) forms on top
of the filtercauses high head lossmust be removed periodically
Used without coagulation/flocculation!
filter cake
Typical Performance of SSF Fed Cayuga Lake Water
Typical Performance of SSF Fed Cayuga Lake Water
0.05
0.1
1
0 1 2 3 4 5Time (days)
Frac
tion
of
infl
uent
E. c
oli
rem
aini
ng in
the
effl
uent
Filter performance doesn’t improve if the filter only receives distilled water
(Daily samples)
How do Slow Sand Filters Remove Particles?
How do slow sand filters remove particles including bacteria, Giardia cysts, and Cryptosporidium oocysts from water?
Why does filter performance improve with time? Why don’t SSF always remove Cryptosporidium
oocysts? Is it a biological or a physical/chemical mechanism? Would it be possible to improve the performance of
slow sand filters if we understood the mechanism?
Slow Sand Filtration Research Apparatus
Sampling tubeLower to collect sample
Manifold/valve blockPeristaltic pumps
Manometer/surge tubeCayuga Lake water(99% or 99.5% of the flow)
Auxiliary feeds(each 0.5% of the flow)
1 liter E. coli feed
1 liter sodium azide
To waste
Filter cell with18 cm of glass beads
Sampling Chamber
Biological and Physical/Chemical Filter Ripening
0.05
Quiescent Cayuga Lake water
0.1
1
0 2 4 6 8 10Time (days)
Control
Sodium azide (3 mM)
Continuously mixed Cayuga Lake water
0.05
0.1
1
0 1 2 3 4 5Time (days)
Frac
tion
of
infl
uent
E. c
oli
rem
aini
ng in
the
effl
uent
What would happen with a short pulse of poison?
Gradual growth of _______ or ________biofilm predator
Physical/chemical
___________________________ are removing bacteria
Biological Poison Fr
acti
on o
f in
flue
nt E
. col
i re
mai
ning
in th
e ef
flue
nt
predators
Biofilms?Abiotic?
0.08
0.1
1
0 1 2 3 4 5 6Time—h
Control
Sodium azide pulse
Sodium chloride pulse
Grazers or suspension feeders?
Suspension feeding predators
Chrysophyte
long flagellum used for locomotion and to provide feeding current
short flagellum
stalk used to attach to substrate (not actually seen in present study)
1 µm
Particle Removal by Size
0.001
0.01
0.1
1
0.8 1 10Particle diameter (µm)
control
3 mM azide
Fra
ctio
n of
infl
uent
par
ticl
es
rem
aini
ng in
the
effl
uent
Effect of the Chrysophyte
What is the physical-chemical mechanism?
Recall quiescent vs. mixed?
Role of Natural Particles in SSF
Could be removal by strainingBut SSF are removing particles 1 m in
diameter!To remove such small particles by straining
the pores would have to be close to 1 m and the head loss would be excessive
Removal must be by attachment to the sticky particles!
Particle Capture Efficiency
Sand filters are inefficient capturers of particles
Particles come into contact with filter media surfaces many times, yet it is common for filters to only remove 90% - 99% of the particles.
Failure to capture more particles is due to ineffective __________
Remember the diffusion surprise?attachment
0.1 1 101
10
100
particle diameter
Par
ticle
rem
oval
as
pC*
pCPe dp pCR dp pCg dp pC dp
dp
m
Techniques to Increase Particle Attachment Efficiency
Make the particles stickierThe technique used in conventional water
treatment plantsControl coagulant dose and other coagulant aids
(cationic polymers)Make the filter media stickier
Potato starch in rapid sand filters?Biofilms in slow sand filters?Mystery sticky agent imported into slow sand
filters?
Mystery Sticky Agent
Serendipity!Head loss through a clogged filter decreases
if you add acidMaybe the sticky agent is acid solubleMaybe the sticky agent will become sticky
again if the acid is neutralizedEureka!
Attachment Mediating Polymer (AMP)
Concentrate particles from Cayuga LakeAcidify with 1 N HClCentrifugeCentrate contains polymerNeutralize to form flocs
AMP Characterization
11%
13%
17%
56%
volatile solidsAlNaFePSSiCaother metalsother nonvolatile solids
Alum!
Did I discover alum?
Which part of AMP is the important actor?
What causes the particle removal?AlumIronthe organic matter (the volatile solids) or aCombination of Al and organic matter
The dilution delay
Students compared filters treated with AMP, aluminum, and iron
They used the amount of aluminum and iron that was in the AMP
Found that AMP was far superior We concluded _______________________________ 4 years later we discovered that they had made a dilution
error and hadn’t actually applied nearly as much aluminum and iron as was present in the AMP
Further experimentation revealed that alum improves filter performance just like AMP
the organic matter was significant
0
1
2
3
4
5
6
7
0 2 4 6 8 10
time (days)
E. co
li re
main
ing
(pC*
)
control
4
20
100
end azideHorizontal bars indicate when polymer feed was operational for each filter.
E. coli Removal as a Function of Time and Al Application Rate
..
out
in
E coliE coli
2
mmol Al
m day
Log remaining is proportional to accumulated mass of Al in filter
No E. coli detected
Head Loss Produced by Al
0
0.2
0.4
0.6
0.8
1
0 50 100 150
Total Al applied
head
loss
(m)
3.9
202
mmol Al
m day
Aluminum feed methods
Alum must be dissolved until it is blended with the main filter feed above the filter column
Alum flocs are ineffective at enhancing filter performance
The diffusion dilemma (alum microflocs will diffuse efficiently and be removed at the top of the filter)
0.1 1 101
10
100
particle diameter
Par
ticle
rem
oval
as
pC*
pCPe dp pCR dp pCg dp pC dp
dp
m
Performance Deterioration after Al feed stops?
HypothesesDecays with timeSites are used upWashes out of filter
Research resultsNot yet clear which
mechanism is responsible – further testing required
0
1
2
3
4
5
6
7
0 2 4 6 8 10
time (days)E
. col
i r
emai
ning
(pC
*)
control
4
20
100
end azideHorizontal bars indicate when polymer feed was operational for each filter.
Sticky Media vs. Sticky Particles
Sticky MediaPotentially treat filter
media at the beginning of each filter run
No need to add coagulants to water for low turbidity waters
Filter will capture particles much more efficiently
Sticky ParticlesEasier to add coagulant
to water than to coat the filter media
Future Work
Develop application techniques to optimize filter performance
How can we coat all of the media?Will the media remain sticky through a
backwash?Will it be possible to remove particles from
the media with a normal backwash?What are the best ways to use aluminum as a
filter aid in SSF and in RSF?
Conclusions
Filters could remove particles more efficiently if the _________ efficiency increased
SSF remove particles by two mechanisms___________________________________
Log remaining is proportional to accumulated mass of alum in filter
Predation
Naturally occurring aluminum
attachment
Polymer in a void between glass beads
Polymer in a void between glass beads
Polymer in a void between glass beads
Polymer in a void between glass beads
Polymer on and bridging between glass beads
Polymer on and bridging between glass beads
Polymer Bridge between Glass Beads
Polymer Bridge between Glass Beads
How can we make filter media sticky?Why do slow sand filters work?
How can we make filter media sticky?Why do slow sand filters work?
Slow sand filters don’t use any coagulants, yet their performance improves with time
Their improved performance is due to natural particulate matter that is captured by the filter
What is it about this particulate matter that makes the filters work better?