Slow Sand FiltrationSlow Sand Filtration

The Slow Sand Filter Mystery Major Events in Slow Sand Filtration

History Research at Cornell

Particle Removal Mechanisms Search for the Mystery Compound

SSF research by CEE 453

The Slow Sand Filter Mystery Major Events in Slow Sand Filtration

History Research at Cornell

Particle Removal Mechanisms Search for the Mystery Compound

SSF research by CEE 453

Slow Sand Filtration

An old technology that is poorly understood Particle removal improves with time! Until recently no one knew how particles

were removed by slow sand filters The mystery is not yet solved Potential for new useful knowledge

Slow Sand Filter Schematic

A. Valve for raw water inlet and regulation of filtration rate

B. Valve for draining unfiltered water

C. Valve for back-filling the filter bed with clean water

D. Valve for draining filter bed and outlet chamber

E. Valve for delivering treated water to waste

F. Valve for delivering treated water to the clear-water reservoir

A

B

C

D E

F

Filter Cake

Sand

GravelUnderdrains

Slow Sand Filtration:A Brief History

1790 - SSF used in Lancashire, England to provide clean water for textile industry

1829 - SSF used to filter municipal water (London) 1850: John Snow established the link between drinking

water (from a contaminated well) and Cholera 1885- SSF shown to remove bacteria 1892 - Cholera outbreak in Hamburg, Altoona saved by

slow sand filters 1980s - Giardia shown to be removed by SSF 1990s - Cryptosporidium not always removed by SSF

Bioengineering in the 1800's

In 1885 Percy F. Frankland used the recently devised 'gelatin process' of Robert Koch to enumerate bacteria in raw and filtered water

Frankland showed that filtration reduced the average bacteria concentration from Thames water 97.9%“It is most remarkable, perhaps, that these hygienically satisfactory results have been obtained without any knowledge on the part of those who construct these filters, as to the conditions necessary for the attainment of such results.” (Percy F. Frankland)

1892 Cholera outbreak in Hamburg, Germany

Large outbreak of Cholera in Hamburg 17,000 cases; 8,600 deaths Very few cases in neighborhoods served by

Altoona's filtered water supply Hamburg's sewers were upstream from Altoona's

intake!

Hamburg'swater intake

Altoona'swater intakeand filter beds Hamburg's sewer

outfalls

HamburgAltoona

Elbe River

The Challenge of the 1990's: Cryptosporidiosis

Milwaukee (March 1 to April 10 1993): an estimated 370,000 city residents suffered from diarrhea, nausea, and stomach cramps caused by Cryptosporidiosis

Evidence suggests that a substantial proportion of non-outbreak-related diarrheal illness may be associated with consumption of water that meets all current water quality standards

Slow sand filters shown to remove less than 50% of Cryptosporidium oocysts at an operating plant in British Columbia

In Search of the Secret in the 1990's

How do slow sand filters remove particles including bacteria, Giardia cysts, and Cryptosporidium oocysts from water?

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?

Particle Removal Mechanisms

SuspensionSuspensionfeedersfeeders

GrazersGrazers

Attachment toAttachment tobiofilmsbiofilms

Capture byCapture bypredatorspredators

to mediumto medium

to previouslyto previouslyremovedremovedparticlesparticles

by mediumby medium

bybypreviouslypreviouslyremovedremovedparticlesparticles

StrainingStraining(fluid and(fluid and

gravitationalgravitationalforces)forces)

AttachmentAttachment(electrochemical(electrochemical

forces)forces)

Physical-ChemicalPhysical-Chemical

BiologicalBiological

ParticleParticleRemovalRemoval

MechanismsMechanisms

Slow Sand Filtration Research Apparatus

Sampling tubeLower to collect sample

Manifold/valve block

Peristaltic pumps

Manometer/surge tube

Cayuga 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

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

Effluent Mystery ParticlesEffluent Mystery Particles

0

1

2

3

4

5

6

7

8

9

1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9 1.95 2Particle diameter (µm)

1.962

3.007

3.986

4.965

5.958

Eff

luen

t par

ticl

e co

unt

(nu

mbe

r/µ

l/p

arti

cle

diam

eter

)

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

Chrysophyte CultureChrysophyte Culture

1 1.5 2 2.5 3 3.50

500

1000

1500

2000

2500

3000

3500

4000

Particle diameter (µm)

Chrysophyte InoculumChrysophyte Inoculum

0.001

0.01

0.1

1

0 1 2 3 4Time (days)

Control

Chrysophyte inoculum

Mechanisms

Particle Removal by SizeParticle Removal by Size

0.001

0.01

0.1

1

0.8 1 10Particle diameter (µm)

control

3 mM azide

Biological MechanismsBiological Mechanisms

The biological activity of microorganisms being removed in the filter column was not significant

The biological activity of the filter biopopulation was only significant for removal of particles smaller than 2 µm.

Biofilms were expected to increase removal of particles larger than 2 µm as well by increasing the attachment efficiency. The lack of biologically enhanced removal of particles larger than 2 µm suggested that “sticky” biofilms did not contribute significantly to particle removal.

The biological activity of microorganisms being removed in the filter column was not significant

The biological activity of the filter biopopulation was only significant for removal of particles smaller than 2 µm.

Biofilms were expected to increase removal of particles larger than 2 µm as well by increasing the attachment efficiency. The lack of biologically enhanced removal of particles larger than 2 µm suggested that “sticky” biofilms did not contribute significantly to particle removal.

Biological MechanismsBiological Mechanisms

The immediate and reversible response of slow sand filters to sodium azide was consistent with bacterivory and inconsistent with particle removal by biofilms.

Biologically mediated mechanisms together with physical-chemical mechanisms accounted for removal of particles smaller than about 2 µm in diameter. In this research bacterivory was the only significant biologically mediated particle removal mechanism.

The immediate and reversible response of slow sand filters to sodium azide was consistent with bacterivory and inconsistent with particle removal by biofilms.

Biologically mediated mechanisms together with physical-chemical mechanisms accounted for removal of particles smaller than about 2 µm in diameter. In this research bacterivory was the only significant biologically mediated particle removal mechanism.

Mechanisms

Filter with Few Particles in Influent

Filter with Few Particles in Influent

0.01

0.1

1

10

0.8 1 10Particle diameter (µm)

Low particle with azide

Low particle controlDay 5

Filters with Many Particles in Influent

Filters with Many Particles in Influent

0.001

0.01

0.1

1

0.8 1 10Particle diameter (µm)

High particle with azide

High particle control

Day 5

Physical-Chemical Particle Removal Mechanisms

Physical-Chemical Particle Removal Mechanisms

Physical-chemical particle removal mechanisms are significant in slow sand filters.

Physical-chemical particle removal efficiency was greatest when particles previously had been retained by the filter (within the bed or in the filter cake) and was considered to be caused by attachment of particles to retained particles.

Further work is necessary to determine what types of particles are most effective for filter ripening.

Physical-chemical particle removal mechanisms are significant in slow sand filters.

Physical-chemical particle removal efficiency was greatest when particles previously had been retained by the filter (within the bed or in the filter cake) and was considered to be caused by attachment of particles to retained particles.

Further work is necessary to determine what types of particles are most effective for filter ripening.

Mechanisms

Sludge from Bolton PointEureka! CEE 453 1997

Sludge from Bolton PointEureka! CEE 453 1997

0.001

0.010

0.100

1.000

0 20 40 60

Time (min)

frac

tion

rem

aini

ng Completely Mixed

2 cm layer

Top Layer

Control

Sludge from Bolton Point = Alum(oops) CEE 453 1998

Sludge from Bolton Point = Alum(oops) CEE 453 1998

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70

slurryAlumdistilled controltap water controlC/Co

Time (minutes)

?

Research project 2000Research project 2000

Successfully extracted a coagulant from Cayuga Lake Seston using 1.0 N HCl

The CLSE fed filters removed up to 99.9999% of the influent coliforms.

Analysis of the CLSE Nonvolatile solids was 44% of the TSS Volatile solids was 56% of the TSS Aluminum was dominant metal

Successfully extracted a coagulant from Cayuga Lake Seston using 1.0 N HCl

The CLSE fed filters removed up to 99.9999% of the influent coliforms.

Analysis of the CLSE Nonvolatile solids was 44% of the TSS Volatile solids was 56% of the TSS Aluminum was dominant metal

CLSE Experiment 2001CLSE Experiment 2001

Groups of 4 Assemble filter apparatus

Measure head loss, flow rate, turbidity Coat filter with CLSE

Observe _______________ Challenge filter with kaolin

Observe ________and _______ Control?

Groups of 4 Assemble filter apparatus

Measure head loss, flow rate, turbidity Coat filter with CLSE

Observe _______________ Challenge filter with kaolin

Observe ________and _______ Control?

increased head loss

turbidity head loss

ApparatusApparatus

Raw Water