Transcript New Treatment Technologies - School of Civil & Environmental

What’s New in Water Treatment?
How do Slow Sand Filters remove Particles?
Coagulants and Filter Aids
Sticky Particles and Sticky Media
Monroe L. Weber-Shirk
School of Civil and
Environmental Engineering
Fraction of influent E. coli
remaining in the effluent
Typical Performance of SSF Fed
Cayuga Lake Water
1
0.1
0.05
0
1
2
3
Time (days)
4
5
(Daily samples)
Filter performance doesn’t improve if it only
receives distilled water
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
Cayuga Lake water
(99% or 99.5% of the
flow)
Manometer/surge tube
Peristaltic
pumps
Manifold/valve block
Sampling Chamber
Auxiliary feeds
(each 0.5% of
the flow)
Sampling tube
Lower to collect sample
To waste
1 liter E.
coli feed
1 liter
sodiu
m
Filter cell with
18 cm of medium
Biological and Physical/Chemical
Filter Ripening
Fraction of influent E. coli
remaining in the effluent
Continuously mixed
Cayuga Lake water
Quiescent Cayuga Lake
water
1
1
Sodium azide
(3 mM)
Control
0.1
0.05
Gradual growth of
biofilm or ________
predator
_______
0
1
2
3
Time (days)
4
0.1
0.05
5
0
2
4
6
Time (days)
8
What would happen with a short pulse of poison?
10
Biological Poison
1
Fraction of influent E. coli
remaining in the effluent
Control
Sodium azide pulse
Biofilms?
Abiotic?
Sodium chloride pulse
predator
0.1
0.08
0
1
2
3
Time—h
4
5
6
Conclusion? _________
predator is removing bacteria
Chrysophyte
long flagellum used for
locomotion and to provide
feeding current
short flagellum
1 µm
stalk used to attach to
substrate (not actually
seen in present study)
Particle Removal by Size
1
Fraction of influent particles
remaining in the effluent
control
3 mM azide
0.1
Effect of
the Chrysophyte
Recall quiescent
vs. mixed?
0.01
What is the physicalchemical mechanism?
0.001
0.8
1
Particle diameter (µm)
10
Role of Natural Particulates in
SSF
Could be removal by straining
But SSF are removing particles 1 mm in
diameter!
To remove such small particles by straining
the pores would have to be close to 1 mm
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 attachment (not limited by
transport) – Proof coming up!
Mechanisms of Particle Transport
 Gravity
 These are dimensionless
groups
(  p   w ) gd p2
Pg =
 Transport by mechanism
18m v
 Diffusion
x nondimensionalized by
2/3
dividing by advective
D
  D  k BT
Pd =   
3m d p transport
 rv 
 Interception
 If P term is 1 then
transport toward a
2
3 dp 
surface is as fast as
Pi =  
2 d 
transport through the
filter
Model Parameters
m
p
w
dp
g
kB
r
T
v
z

viscosity
Particle density
Fluid density
Particle diameter
Acceleration due to gravity
Boltzman constant
Pore radius
Absolute temperature
Filter approach velocity
Depth of filter
Constant for diffusion transport
1.00E-03
1040
1000
1.00E-06
9.8
1.38E-23
3.0E-05
293
3.0E-05
1
4.04
Ns/m2
kg/m3
kg/m3
m
m/s2
J/°K
m
°K
m/s
m
Estimate Dimensionless Transport
for a Bacteria Cell by Diffusion

23 J 
1.38

10

  293 K 
2
K 

13 m
D
 4.3 10
N s 
s

3 1103 2  1106 m 
m 

k BT
D
3m d p
D
Pd =   
 rv 
2/3
2



13 m 
4.3

10




s



P d = 4.04 


5
5 m  
3.0

10
m
3.0

10





s



P d = 0.025
m
viscosity
1.00E-03 Ns/m2
d
Particle diameter
1.00E-06 m
Boltzman constant
1.38E-23 J/°K
r
Pore radius
3.00E-05
T
Absolute temperature
v
Filter approach
velocity

Constant for diffusion
transport
p
k
2/3
B
Advection is 40x greater than diffusion
m
293 °K
3.0E-05
4.04
m/s
Fraction Remaining
C
f 

Co
e
2T PT
z
r
PT  P d  P g  P i
D
Pd =   
 rv 
2/3
k BT
D
3m d p
2/3
f =e
 k BT   z 
2T  
 rv 3m d p   r 


How deep must filter be for diffusion
to remove 99% of bacteria?
 Assume attachment
efficiency is 1
1
 T is ____
0.01
 f is ____
 z is _____
3 mm
 What does this mean?
f e
2T P d
z
r
 r ln f
z
2T P d
  3.0 10 m  ln  0.01
5
z
2 1 0.025 
Techniques to Increase Particle
Attachment Efficiency
Make the particles stickier
The technique used in conventional water
treatment plants
Control 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 acid
Maybe the sticky agent is acid soluble
Maybe the sticky agent will become sticky
again if the acid is neutralized
Eureka!
Cayuga Lake Seston Extract
Concentrate particles from Cayuga Lake
Acidify with 1 N HCl
Centrifuge
Centrate contains polymer
Neutralize to form flocs
CLSE Characterization
volatile solids
Al
13%
Na
Fe
11%
P
S
Si
17%
Ca
other metals
other nonvolatile solids
56%
Hypothesis:
The organic
fraction is
most
important
How much CLSE should be added to a filter?
G (gcarbon/gglass beads)
Organic Carbon Accumulation in
Filters Fed Cayuga Lake Water
day 1
0.001
day 3
0.0001
day 7
0.00001
day 70
0.000001
0.0000001
0.0001
0.0010
0.0100
x (m)
0.1000
Filters fed Cayuga Lake Water
1.0000
Organic Carbon Accumulation
Rate
Approximately 100 ppb (mg/L) in Cayuga
Lake
Total organic carbon
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
E. coli Removal as a Function of
Time and CLSE Application Rate
control
1
0.1
0.62
0.01
E. coliout
E. coliin
3.1
0.001
g
m 2 ×day
15
0.0001
end azide
0.00001
0.000001
0.0000001
0
2
4
6
8
10
Horizontal bars indicate
when CLSE feed was
operational for each filter.
time (days)
Log remaining is proportional to accumulated mass of polymer in filter
Head Loss Produced by CLSE
control
0.62
g
3.1 m 2 ×day
15
end azide
head loss (m)
1.2
1
0.8
0.6
0.4
0.2
0
0
2
4
6
time (days)
8
10
What do we know about this
Polymer?
Soluble at very low (<1) and at very high
(>13) pH
Forms flocs readily at neutral pH
Contains 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
R
O
Carboxyl group
..
H—N —CH—C—O—H
Amino group
H
In base solution
In acid solution
R
O
H
+
H—N —CH—C—O—H
H
cation
R
O
..
H—N —CH—C—O
H
anion
Sticky Media vs. Sticky Particles
 Sticky Media
 Potentially 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 Particles
 Easier to add coagulant
to water than to coat
the filter media
Future Work
 Characterize the polymer
 Develop a better source of the polymer (algae
culture, bacteria culture, or synthetic?)
 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 this new coagulant?
Conclusions
Filters could remove particles more
efficiently if the attachment
_________ efficiency
increased
SSF remove particles by two mechanisms
Predation
____________
____________
Sticky polymer
Log remaining is proportional to
accumulated mass of polymer in filter
Polymer in a void between glass
beads
Polymer in a void between glass
beads
Polymer on and bridging between
glass beads
Polymer Bridge between Glass
Beads
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?