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

2. Filter Performance Models
Iwasaki (1937) developed relationships describing the performance of deep bed filters.
Iwasaki, T. (1937). "Some Notes on Sand Filtration." Journal American Water Works Association 29: 1591.
C is the particle concentration [number/L3]
l0 is the initial filter coefficient [1/L]
z is the media depth [L]
The particle’s chances of being caught are the same at all depths in the filter; pC* is proportional to depth

3. Filtration Performance: Dimensional Analysis
What is the parameter we are interested in measuring? _________________
How could we make performance dimensionless? ____________
What are the important forces?
Effluent concentration
C/C0 or pC*
Inertia
London van der Waals
Electrostatic
Viscous
Gravitational
Thermal
Need to create dimensionless force ratios!

4. Choose viscosity!
In Fluid Mechanics inertia is a significant “force” for most problems
In porous media filtration viscosity is more important that inertia.
We will use viscosity as the repeating parameter and get a different set of dimensionless force ratios
Inertia
Gravitational
Viscous
Thermal
Viscous

5. Gravity
velocities
forces v
pore
Gravity only helps when the streamline has a _________ component.
horizontal
Use this equation

6. Diffusion (Brownian Motion)v
pore
Diffusion velocity is high when the particle diameter is ________.
kB=1.38 x J/°K
T = absolute temperature
small
dc is diameter of the collector
The exponent was obtained from an analytical model

7. Geometric Parameters
What are the length scales that are related to particle capture by a filter?
______________
__________________________
Porosity (void volume/filter volume) (e)
Create dimensionless groups
Choose the repeating length ________
Filter depth (z)
Collector diameter (media size) (dc)
Particle diameter (dp)
(dc)
Number of collectors!

8. Write the functional relationship
Length ratios
Force ratios
If we double depth of filter what does pC* do? ___________
doubles
How do we get more detail on this functional relationship?
Empirical measurements
Numerical models

9. Total removal (SSF conditions)
Number of contact opportunities
contacts per attachment
Transport velocity/advection
Tufenkji, N. and M. Elimelech (2004). "Correlation equation for predicting single-collector efficiency in physicochemical filtration in saturated porous media." Environmental-Science-and-Technology 38(2):
With my corrections to eliminate London dependency

10. How deep must a filter (SSF) be for diffusion to remove 99% of bacteria?
Assume a is 1 and dc is 0.2 mm, V0 = 10 cm/hr
NR is ____
pC* is ____
z is _____
What does this mean?
1/200 2
5.4 cm
If the attachment efficiency were 1, then we could get great particle capture in a 1 m deep filter!

11. Total removal (RSF conditions)
dc=0.5 mm
Approach velocity is 5 m/hr

12. How deep a Rapid Sand Filter) will remove 90% of cryptosporidium?
dp is 4 mm
Assume a is 1 and dc is 0.5 mm, V0 = 5 m/hr
NR is ____
pC* is ____
z is _____
4/500 1
0.18 m
If the attachment efficiency were 1, then we could get great particle capture in a 1 m deep filter!

13. Slow Sand Filtration First filters to be used on a widespread basis
Fine sand with an effective size of 0.2 mm
Low flow rates ( cm/hr)
Schmutzdecke (_____ ____) forms on top of the filter
causes high head loss
must be removed periodically
Used without coagulation/flocculation!
filter cake

14. 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 the filter only receives distilled water

15. 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?

16. 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 glass beads

17. 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
Physical/chemical
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?

18. Biological Poison Biofilms? Abiotic? predator
0.08
0.1 1 2 3 4 5 6
Time—h
Control
Sodium azide pulse
Sodium chloride pulse
Biofilms?
Abiotic? q
Fraction of influent E. coli remaining in the effluent
predator
Conclusion? _________ is removing bacteria
predator

19. 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)

20. 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

21. Role of Natural Particles 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!

22. 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 __________
Remember the diffusion surprise?
attachment

23. 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?

24. 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!

25. Attachment Mediating Polymer (AMP)
Concentrate particles from Cayuga Lake
Acidify with 1 N HCl
Centrifuge
Centrate contains polymer
Neutralize to form flocs

26. AMP Characterization
Alum!
Did I discover alum?

27. Which part of AMP is the important actor?
What causes the particle removal?
Alum
Iron
the organic matter (the volatile solids) or a
Combination of Al and organic matter

28. The dilution delay the organic matter was significant
Students compared filters treated with AMP, aluminum, and iron
They used the amount of aluminum and iron that was in the AMP
Found that AMP was far superior
We concluded _______________________________
4 years later we discovered that they had made a dilution error and hadn’t actually applied nearly as much aluminum and iron as was present in the AMP
Further experimentation revealed that alum improves filter performance just like AMP
the organic matter was significant

29. E. coli Removal as a Function of Time and Al Application Rate
No E. coli detected
Log remaining is proportional to accumulated mass of Al in filter

30. Head Loss Produced by Al

31. Aluminum feed methods
Alum must be dissolved until it is blended with the main filter feed above the filter column
Alum flocs are ineffective at enhancing filter performance
The diffusion dilemma (alum will diffuse efficiently and be removed at the top of the filter)

32. Performance Deterioration after Al feed stops?
Hypotheses
Decays with time
Sites are used up
Washes out of filter
Research results
Deterioration rate increases with higher particle loads

33. 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

34. Future Work
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 filter aid?

35. 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 alum in filter
attachment
Predation
Naturally occurring aluminum

36. Polymer in a void between glass beads

37. Polymer in a void between glass beads

38. Polymer on and bridging between glass beads

39. Polymer Bridge between Glass Beads

40. 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?