Monroe L. Weber-Shirk School of Civil and

Environmental Engineering

What’s New in Water Treatment?What’s New in Water Treatment?

How do Slow Sand Filters remove Particles?

Coagulants and Filter Aids

Sticky Particles and Sticky Media

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 it 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 with 18 cm of medium

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

Biological Poison Biological Poison

0.08

0.1

1

0 1 2 3 4 5 6Time—h

Control

Sodium azide pulse

Sodium chloride pulse

Fra

ctio

n of

infl

uent

E. c

oli

rem

aini

ng in

the

effl

uent

predator

predator

Biofilms?Abiotic?

Conclusion? _________ is removing bacteria

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 SizeParticle 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 Particulates in SSF

Role of Natural Particulates 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 EfficiencyParticle 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 attachment (not limited by transport) – Proof coming up!

Mechanisms of Particle Transport

Mechanisms of Particle Transport

Gravity

Diffusion

Interception

These are dimensionless groups

Transport by mechanism x nondimensionalized by dividing by advective transport

If term is 1 then transport toward a surface is as fast as transport through the filter

2/3

d = D

rv

2

g

( )=

18p w pgd

v

2

i

3=

2pd

d

3B

p

k TD

d

Model ParametersModel Parameters

viscosity 1.00E-03 Ns/m2

p Particle density 1040 kg/m3

w Fluid density 1000 kg/m3

dp Particle diameter 1.00E-06 m

g Acceleration due to gravity 9.8 m/s2

kB Boltzman constant 1.38E-23 J/°K

r Pore radius 3.0E-05 m

T Absolute temperature 293 °K

v Filter approach velocity 3.0E-05 m/s

z Depth of filter 1 m

Constant for diffusion transport 4.04

Estimate Dimensionless Transport for a Bacteria Cell by Diffusion

Estimate Dimensionless Transport for a Bacteria Cell by Diffusion

2/3

d = D

rv

3B

p

k TD

d

viscosity 1.00E-03 Ns/m2

d

p

Particle diameter 1.00E-06 m

k

B

Boltzman constant 1.38E-23 J/°K

r Pore radius 3.00E-05 m

T Absolute temperature 293 °K

v Filter approach velocity

3.0E-05 m/s

Constant for diffusion transport

4.04

232

13

3 62

1.38 10 2934.3 10

N s3 1 10 1 10

m

JK

mKD

sm

2/32

13

d5 5

4.3 10

= 4.043.0 10 3.0 10

ms

mm

s

d = 0.025

Advection is 40x greater than diffusion

Fraction RemainingFraction Remaining

2 T T

z

r

o

Cf

C e

T d g i

2/3

d = D

rv 3

B

p

k TD

d

2/3

23 =

BT

p

k T z

rv d rf e

How deep must filter be for diffusion to remove 99% of bacteria?

How deep must filter be for diffusion to remove 99% of bacteria?

Assume attachment efficiency is 1

T is ____

f is ____ z is _____ What does this mean?

2 T dz

rf e

53.0 10 ln 0.01

2 1 0.025

mz

ln

2 T d

r fz

3 mm

1

0.01

Techniques to Increase Particle Attachment Efficiency

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 AgentMystery 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!

Cayuga Lake Seston ExtractCayuga Lake Seston Extract

Concentrate particles from Cayuga LakeAcidify with 1 N HClCentrifugeCentrate contains polymerNeutralize to form flocs

CLSE CharacterizationCLSE Characterization

11%

13%

17%

56%

volatile solidsAlNaFePSSiCaother metalsother nonvolatile solids

How much CLSE should be added to a filter?

Hypothesis: The organic fraction is most important

Organic Carbon Accumulation in Filters Fed Cayuga Lake Water

Organic Carbon Accumulation in Filters Fed Cayuga Lake Water

0.0000001

0.000001

0.00001

0.0001

0.001

0.0001 0.0010 0.0100 0.1000 1.0000x (m)

G (g

carb

on/g

glas

s be

ads)

day 1

day 3

day 7

day 70

Filters fed Cayuga Lake Water

Organic Carbon Accumulation Rate

Organic Carbon Accumulation Rate

Approximately 100 ppb (g/L) in Cayuga Lake

230 mg TOC /m2/day accumulated in filters fed Cayuga Lake Water

620 mg to 15,000 mg CLSE as TSS /m2/day fed to filters

Total Suspended Solids

Total organic carbon

E. coli Removal as a Function of Time and CLSE Application Rate E. coli Removal as a Function of Time and CLSE Application Rate

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

0 2 4 6 8 10

time (days)

control

0.62

3.1

15

end azide

..

out

in

E coliE coli

2

gm day×

Horizontal bars indicate when CLSE feed was operational for each filter.

Log remaining is proportional to accumulated mass of polymer in filter

Head Loss Produced by CLSEHead Loss Produced by CLSE

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

time (days)

head

loss

(m)

control

0.62

3.1

15

end azide

2

gm day×

What do we know about this Polymer?

What do we know about this Polymer?

Soluble at very low (<1) and at very high (>13) pH

Forms flocs readily at neutral pHContains protein (amino acids)

In acid solution amino acids are protonated and exist as cations

In basic solution amino acids are deprotonated and exist as anions

Dipolar Structure of Amino Acids

Dipolar Structure of Amino Acids

H—N —CH—C—O—H

H

R O..

In acid solution In base solution

H—N —CH—C—O

H

R O..

H—N —CH—C—O—H

H

R O

H

+

Carboxyl group

Amino group

cation anion

Sticky Media vs. Sticky ParticlesSticky 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 WorkFuture Work

Characterize the polymer Develop a better source of the polymer (algae

culture, bacteria culture, or synthetic?) Develop application techniques to optimize filter

performanceHow 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 this new coagulant?

ConclusionsConclusions

Filters could remove particles more efficiently if the _________ efficiency increased

SSF remove particles by two mechanisms________________________

Log remaining is proportional to accumulated mass of polymer in filter

Predation

Sticky polymer

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?