1. What’s New in Water Treatment?
How do Slow Sand Filters remove Particles?
Coagulants and Filter Aids
Sticky Particles and Sticky Media

2. Typical Performance of SSF Fed Cayuga Lake Water1
Fraction of influent E. coli remaining in the effluent
0.1
0.05 1 2 3 4 5
Time (days)
(Daily samples)
Filter performance doesn’t improve if it only receives distilled water

3. 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?

4. Slow Sand Filtration Research Apparatus
Manometer/surge tube
Cayuga Lake water
(99% or 99.5% of the flow)
Manifold/valve block
Peristaltic pumps
Sampling Chamber
Auxiliary feeds
(each 0.5% of the flow)
Sampling tube
Lower to collect sample
To waste
1 liter sodium azide
1 liter E. coli feed
Filter cell with
18 cm of medium

5. Biological and Physical/Chemical Filter Ripening
Continuously mixed Cayuga Lake water
0.05
Quiescent Cayuga Lake water
0.1 1 2 4 6 8 10
Time (days)
Control
Sodium azide
(3 mM) 1
Fraction of influent E. coli remaining in the effluent
Gradual growth of _______ or ________
0.1
biofilm
predator
0.05 1 2 3 4 5
Time (days)
What would happen with a short pulse of poison?

6. Biological Poison Biofilms? Abiotic? predator
Fraction of influent E. coli remaining in the effluent
predator
Conclusion? _________ is removing bacteria
predator

7. 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)

8. Particle Removal by Size1
control
3 mM azide
0.1
Recall quiescent vs. mixed?
Fraction of influent particles remaining in the effluent
Effect of the Chrysophyte
0.01
What is the physical-chemical mechanism?
0.001
0.8 1
Particle diameter (µm) 10

9. 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!

10. 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!

11. Mechanisms of Particle Transport
Gravity
Diffusion
Interception
These are dimensionless groups
Transport by mechanism x nondimensionalized by dividing by advective transport
If P term is 1 then transport toward a surface is as fast as transport through the filter

12. Model Parameters m viscosity 1.00E-03 Ns/m2 rp Particle density 1040
kg/m3 rw
Fluid density
1000 dp
Particle diameter
1.00E-06 g
Acceleration due to gravity
9.8
m/s2 kB
Boltzman constant
1.38E-23
J/°K r
Pore radius
3.0E-05 T
Absolute temperature
293 °K v
Filter approach velocity
m/s z
Depth of filter 1 f
Constant for diffusion transport
4.04

13. Estimate Dimensionless Transport for a Bacteria Cell by Diffusion
viscosity
1.00E-03
Ns/m2 dp
Particle diameter
1.00E-06 kB
Boltzman constant
1.38E-23
J/°K r
Pore radius
3.00E-05 T
Absolute temperature
293 °K v
Filter approach velocity
3.0E-05
m/s f
Constant for diffusion transport
4.04
Advection is 40x greater than diffusion

14. Fraction Remaining

15. How deep must filter be for diffusion to remove 99% of bacteria?
Assume attachment efficiency is 1
aT is ____
f is ____
z is _____
What does this mean? 1
0.01
3 mm

16. 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?

17. 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!

18. Cayuga Lake Seston Extract
Concentrate particles from Cayuga Lake
Acidify with 1 N HCl
Centrifuge
Centrate contains polymer
Neutralize to form flocs

19. CLSE Characterization
Hypothesis: The organic fraction is most important
How much CLSE should be added to a filter?

20. Organic Carbon Accumulation in Filters Fed Cayuga Lake Water

21. Organic Carbon Accumulation Rate
Approximately 100 ppb (mg/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 organic carbon
Total Suspended Solids

22. E. coli Removal as a Function of Time and CLSE Application Rate
Horizontal bars indicate when CLSE feed was operational for each filter.
Log remaining is proportional to accumulated mass of polymer in filter

23. Head Loss Produced by CLSE

24. 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

25. Dipolar Structure of Amino Acids
H—N —CH—C—O—H H R O ..
Carboxyl group
Amino group
In acid solution
In base solution
H—N —CH—C—O—H H R O +
H—N —CH—C—O H R O ..
cation
anion

26. Sticky Media vs. Sticky Particles
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

27. 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?

28. 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 polymer in filter
attachment
Predation
Sticky polymer

29. Polymer in a void between glass beads

30. Polymer in a void between glass beads

31. Polymer on and bridging between glass beads

32. Polymer Bridge between Glass Beads

33. 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?