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?