What s new in water treatment
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What’s New in Water Treatment?. How well could filters remove Particles? Coagulants and Filter Aids Sticky Particles and Sticky Media. Filter Performance Models. Iwasaki (1937) developed relationships describing the performance of deep bed filters.

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What’s New in Water Treatment?

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What s new in water treatment

What’s New in Water Treatment?

How well could filters remove Particles?

Coagulants and Filter Aids

Sticky Particles and Sticky Media

Filter performance models

Filter Performance Models

  • Iwasaki (1937) developed relationships describing the performance of deep bed filters.

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

Filtration performance dimensional analysis

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*

Only effective in the attachment phase


London van der Waals





Need to create dimensionless force ratios!

Choose viscosity as the common force that inhibits transport





Choose viscosity as the common force that inhibits transport

  • We will use viscosity as the repeating parameter and create a set of dimensionless force ratios

But these forces are functions of …







Gravity only helps when the streamline has a _________ component.


Use this equation

Diffusion brownian motion

Diffusion (Brownian Motion)



Diffusion velocity is high when the particle diameter is ________.

kB=1.38 x 10-23 J/°K

T = absolute temperature


dc is diameter of the collector

Geometric parameters

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)


Number of collectors!

Write the functional relationship

Write the functional relationship

attachments per contact

Length ratios

Force ratios


If we double depth of filter (or a) what does pC* do? ___________

How do we get more detail on this functional relationship?

Empirical measurements

Numerical models

Total removal ssf conditions

20 cm/hr

0.2 mm sand

1 m deep

Particle density of 1040 kg/m3

Total removal (SSF conditions)

Plots based on numerical models

How deep must a filter ssf be to remove 99 of bacteria

How deep must a filter (SSF) be to remove 99% of bacteria?

  • Assume a is 1 and dc is 0.2 mm, V0 = 20 cm/hr

  • For 1 m of sand pC*=____

  • Depth for pC* of ____ is _____

  • What does this mean?



10 cm

If the attachment efficiency were 1, then we could get great particle capture in a 1 m deep filter!

Total removal rsf conditions

Total removal (RSF conditions)

  • dc=0.5 mm

  • Approach velocity is 5 m/hr

  • 1 m deep

  • Particle density of 1040 kg/m3

How deep a rapid sand filter will remove 90 of cryptosporidium

How deep a Rapid Sand Filter will remove 90% of cryptosporidium?

  • Assume a is 1 and dc is 0.5 mm, V0 = 5 m/hr

  • dp is 4 mm

  • pC* is ____ for 1 m deep filter

  • z is ________________


1 m/1.8=0.55 m

We need flocculation to produce larger and more dense particles to get good removal in RSF

Slow sand filtration

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 (10 - 40 cm/hr)

  • Schmutzdecke (_____ ____) forms on top of the filter

    • causes high head loss

    • must be removed periodically

  • Used without coagulation/flocculation!

filter cake

Typical performance of ssf fed cayuga lake water

Typical Performance of SSF Fed Cayuga Lake Water


Fraction of influent E. coli remaining in the effluent









Time (days)

(Daily samples)

Filter performance doesn’t improve if the filter only receives distilled water

How do slow sand filters remove particles

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

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

Biological and physical chemical filter ripening

Quiescent Cayuga Lake water


Sodium azide

(3 mM)










Time (days)

Biological and Physical/Chemical Filter Ripening

Continuously mixed Cayuga Lake water



Fraction of influent E. coli remaining in the effluent

Gradual growth of _______ or ________











Time (days)

What would happen with a short pulse of poison?

Biological poison



Sodium azide pulse

Sodium chloride pulse











Biological Poison




Fraction of influent E. coli remaining in the effluent


Grazers or suspension feeders?

___________________________ are removing bacteria

Suspension feeding predators



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

Particle Removal by Size



3 mM azide


Recall quiescent vs. mixed?

Fraction of influent particles remaining in the effluent

Effect of the Chrysophyte


What is the physical-chemical mechanism?





Particle diameter (µm)

Role of natural particles in ssf

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!

Particle capture efficiency

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?


Techniques to increase particle attachment efficiency

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

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!

Attachment mediating polymer amp

Attachment Mediating Polymer (AMP)

  • Concentrate particles from Cayuga Lake

  • Acidify with 1 N HCl

  • Centrifuge

  • Centrate contains polymer

  • Neutralize to form flocs

Amp characterization

AMP Characterization


Did I discover alum?

Which part of amp is the important actor

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

The dilution delay

The dilution delay

  • 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

E coli removal as a function of time and al application rate

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

Head loss produced by al

Head Loss Produced by Al

Aluminum feed methods

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 microflocs will diffuse efficiently and be removed at the top of the filter)

Performance deterioration after al feed stops

Performance Deterioration after Al feed stops?

  • Hypotheses

    • Decays with time

    • Sites are used up

    • Washes out of filter

  • Research results

    • Not yet clear which mechanism is responsible – further testing required

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

Sticky Media vs. Sticky Particles

Future work

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 aluminum as a filter aid in SSF and in RSF?



  • 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



Naturally occurring aluminum

Polymer in a void between glass beads

Polymer in a void between glass beads

Polymer in a void between glass beads1

Polymer in a void between glass beads

Polymer on and bridging between glass beads

Polymer on and bridging between glass beads

Polymer bridge between glass beads

Polymer Bridge between Glass Beads

How can we make filter media sticky why do slow sand filters work

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?

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