slow sand filtration ä the slow sand filter mystery ä major events in slow sand filtration history...
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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