Filtration Part I Filtration Fundamentals

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Filtration - Part IFiltration Fundamentals

• KY Water and Wastewater Operators Conference
• March 2003
• Tim Wolfe
• Vice President
• MWH Americas

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Items to be Discussed - Part I

• Brief History of Water Filtration
• Todays Filters
• Filter Operating Parameters
• Particle Removal in Filters
• Headloss Development in Filters
• Unit Filter-Run Volume
• Backwashing
• Tomorrows Filter Challenges

3
Items to be Discussed - Part II

• Filter Coring Method
• Physical inspection
• Core sampling, for Solids-retention profiles
  (before and after backwashing)
• Backwash-turbidity profile
• Backwash-expansion measurement
• Core sampling, for Sieve analysis
• Filter-effluent turbidity profile
• Solids-retention profiles
• Filter Coring Analyzing Results

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Brief Historyof Water Filtration

• 1829 - Simpson builds slow sand filter at Chelsea
  Water Co. in London (2 to 4 MGAD and run length
  of 6 to 8 months)
• 1850 - Broad Street well epidemic
  (i.e., Filters assume the primary role for the
  removal of pathogenic microbes)
• 1872 - Poughkeepsie, NY builds first US slow sand
  filter

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Brief Historyof Water Filtration (Cont.)

• 1885 - Somerville, NJ builds first rapid sand
  filter
• 1890s
• Fuller establishes filtration rate of 2 gpm/sf
• Hyatt patents Alum as a coagulant
• 1900 to 1915 - Technology of disinfection
  developed (i.e., Chlorination assumes the primary
  role for inactivating pathogenic microorganisms)

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Brief Historyof Water Filtration (Cont.)

• 1914 - First USPHS standards set
• 1937 - John Baylis introduces activated silica as
  a coagulant aid
• 1940 to 1950
• Filtration rate increased to 2.5 - 4 gpm/sf due
  to baby boom
• Criteria established to terminate filter runs
• Organic polymers introduced as filter aids

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Brief Historyof Water Filtration (Cont.)

• Filtered-Water Turbidity Standards
• Prior to 1962 - 10.0 NTU
• 1962 to 1976 - 5.0 NTU
• 1977 to 1993 - 1.0 NTU
• 1993 to Recently - 0.5 NTU
• Interim LT-1 ESWTRs - 0.3 NTU
• AWWA/Partnership Goal - 0.1 NTU
• Bottom Line
• Filters are once again a primary barrier for
  removal of pathogenic microorganisms

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Todays Filters

• Typical media

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Typical Filter Media Used in Todays Filters
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Filter-OperatingParameters
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Conventional Filtration is ...

• The separation of particles from water by
  passing pre-treated water through a bed of porous
  media.
• The particles present in the filter influent are
  those that have escaped the clarification
  process
• Particles originally present in the source water
• Particles created during coagulation/flocculation

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Most WTPs Treat Surface Water Prior to
Filtration, by

• Adding a coagulant in the rapid-mix step to start
  creating floc particles,
• Forming larger floc particles in the flocculation
  step, and
• Clarifying the water during the
  sedimentation/clarification process.

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This Portion of Part I Discusses Key,
Filter-Operating Parameters

• - Particle storage/removal in filters,
• - Head-loss development in filters,
• - Unit, filter-run volumes, and
• - Backwashing.

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Filter-OperatingParameters (cont.)

• A - Particle Storage/Removal
• in Filters

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It Must be Noted that ...

• Particles are temporarily stored in a filter
  bed during filtration.
• These particles are only truly removed from the
  water when the filter is backwashed.

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Particles are Temporarily Stored in a Filter Bed
by

• Straining (i.e., the large particles are too big
  to fit through the porous volume between
  filter-media grains), and
• Sticking, or Attachment (i.e., the smaller
  particles come in contact with, and stick to, the
  surface of the filter-media grains).

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This Distinction between Particle Storage and
Removal is Important, because ...

• The smaller particles are stored in the filter
  bed by sticking to the surface of the filter
  media.
• Under certain conditions these smaller attached
  particles can detach from the media resulting in
  turbidity breakthrough in the filter effluent.

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Filtration Rate Filtration Velocity multiplied
by theArea Available for Flow
Q V x A
Q Filtration Rate, is constant V Filtration
Velocity A Area Available for Flow
between Media Grains
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As Particles are Stored in a Filter Bed, A
becomes Smaller
Q V x A
Q Filtration Rate, is constant V Filtration
Velocity A Area Available for Flow
between Media Grains
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As A becomes SmallerV becomes Larger
Q V x A
Q Filtration Rate, is constant V Filtration
Velocity A Area Available for Flow
between Media Grains
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And, if Vbecomes too Large ...

• Particles stored in a filter bed by
• straining can be driven deeper into the filter
  bed,
• sticking can be detached from the surface of the
  filter media,
• And, both types of particles can exit the bottom
  of the filter ...
• Resulting in particle or turbidity
  break-through

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Filter-OperatingParameters (cont.)

• B - Head-loss Development
• in Filters

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The Driving Force Available for Pushing Water
through a Filter
Filter
Total Available Head is the Driving Force that is
available for pushing water through the filter
Water Surface
Filter Media
Water Surface
Rate-of-Flow Control Valve
Clearwell
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Head-loss Developmentin Conventional Filters
ROF Control Valve
14
Nearly Closed
Nearly Open
12
ROF Control Valve Head loss
10
Total Available Head
Head loss ( ft )
8
6
Solids-Storage Head loss
4
2
Clean-bed Head loss
Filter Run Time ( hrs )
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Head-loss Developmentin Conventional Filters
(cont.)

• The floc particles (e.g., aluminum hydroxide,
  clay, silt, microorganisms, etc.) that are
  temporarily stored in the filter bed are
  compressible solids.
• The filter-media grains (e.g., anthracite and
  sand) are incompressible solids.

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Head-loss Developmentin Conventional Filters
(cont.)

• Therefore, head-loss development depends on the
  particle-storage mechanism
• Surface removal of compressible solids
  (Straining)
• Depth removal of compressible solids (Sticking)
• Surface/Depth removal of compressible solids
  (i.e., a combination of Staining and Sticking).

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Straining - Surface Storageof Compressible Solids
Filter-run Length
Head loss
Q filtration rate constant
Q x t, volume of filtrate

• Straining is the primary solids-storage
  mechanism.
• Head loss builds up fairly quickly because the
  compressible
• solids form an impermeable mat on top of the
  filter media.

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Sticking - Depth Storageof Compressible Solids
Q 6 gpm/sf
Q 4 gpm/sf
Q 2 gpm/sf
Head loss
Q constant
Clean-Bed Head loss
Typical of most dual-media filters
Q x t, volume of filtrate

• Sticking and squatting are the primary
  solids-storage mechanisms.
• Head loss builds up fairly slowly because the
  sticky, compressible solids are
• stuck to (and inter-mixed with) the
  non-compressible, filter-media grains.

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Combination of Surface Storage andDepth Storage
of Compressible Solids
2
3
4
6
5 gpm/sf
Q
Head loss
Terminal Head loss
Optimum Rate
Q x t, volume of filtrate

• As the filtration rate increases more solids are
  driven down into
• the filter bed for storage by sticking to the
  filter-media grains.
• Surface removal by straining decreases as
  filtration rate increases.

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Filter-OperatingParameters (cont.)

• C - Unit, Filter-Run Volume

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The Goal for a Unit,Filter-Run Volume (UFRV) is

• Sand Media - 5,000 gallons of water filtered
  through each square foot of media surface between
  backwashes (i.e., during a filter run).
• Dual Media - 10,000 gal/sf-run

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Why Not a Longer UFRV ?

• Particles are temporarily stored in a filter bed
  during filtration ...

These particles are only truly removed from the
water when the filter is backwashed.
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Example Calculations ofUnit, Filter-Run Volumes

• 3 gals 60 min 36 hrs 6,480 gals
• X X
• min-sf hr 1 sf
• 5 gals 60 min 36 hrs 10,800 gals
• X X
• min-sf hr 1 sf

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Filter Runs are Terminated at Most WTPs when the

• Goal for the filtered-water, total particle count
  has been exceeded
• Goal (0.1 - 0.2 NTU) for the filtered-water,
  turbidity value has been exceeded
• Total available head has been exceeded by the
  head loss developed across the bed or
• Pre-determined, filter-run length (36-48 hrs) has
  been exceeded.

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Ideal Filter Run
Filter Ripening Period (Turbidity lt 0.1 NTU in
15 min)
Terminal Head loss
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Filter-OperatingParameters (cont.)

• D - Backwashing

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Optimal Backwash Rates for Commonly-Used Filter
Media
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Appropriate Backwash Rate at 20c
30
Anthracite Coal S. G. 1.65
25
Sand S. G. 2.65
20
15
Backwash Rate (gpm/sf)
10
5
0
0
1
2
60 Weight grain size (ES x UC d60)
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Temperature-Correction Factor for Backwash Rate
Multiplier for Appropriate Backwash Rate
Water Temperature (C)
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Determining OptimumBackwash Duration
Peak Turbidity Value Typically Between 250 - 400
NTU
Turbidity Typically Drops Below 10 NTU in 6 to 8
minutes
10 NTU
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Typical BackwashSequence - Water Only

• Surface wash prior to water wash
• Break up the layer of solids stored, by
  straining, as a mat on top of the filter-media
  bed
• Surface wash continued while water wash is being
  ramped up
• Begin to detach the solids stored, by sticking,
  in the upper layer of the filter media

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Typical BackwashSequence (cont.)

• High-rate water wash
• Expand the media so the expanded-bed porosity is
  roughly 0.7 for the upper layer of the media
• At an expanded-bed porosity of 0.7 the media
  grains are barely touching each other, and the
  scouring action between the surfaces of adjacent
  media grains continues the detachment of solids
  that have been stored by sticking
• Carry the solids out of the filter bed and into
  the backwash trough, for removal

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Backwashing - with Water Only

Surface Wash (minutes)
Concurrent
wash
High-Rate Water Wash (minutes)
Notes
1. Draw down water level in filter box to
approx. 6 inches above the media surface
before starting surface wash.

2. Continue the surface wash while the high-rate
water wash is being ramped up (i.e.,
concurrent wash).
3. Longer concurrent wash will result in higher
media loss.
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Why We Expand the MediaDuring the Backwash
(cont.)
When the Expanded-bed porosity is roughly 0.7,
media grains are scratching against each other to
un-stick the stuck solids.
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Backwash ExpansionDual-Media Filter
Anthracite
( 46 in. - 36 in. ) / 36 in. x 100 28
Bed Expansion
Anthracite
29 in.
46 in.
24 in.
36 in.
Sand
Sand
17 in.
12 in.
Fixed, Filter Bed
Expanded, Filter Bed
Want the same expansion all year long - change
BW rate.
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d90 (coal) / d10 (sand) 3
Top of Conc. Filter Wall
Water Surface
Trough
Top of Media to Bottom of Trough gt 18
24 (anthracite) 12 (sand) 36
Anthracite - 24
d90
d10
Sand - 12
Under-drain
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Backwashing - with Air and Water
Air-purge washing if needed
2
Air (cfm/sq ft )
Backwash in rinsing stage
Backwash
with air
Air-scouring
Washing duration (minutes)
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Optimizing FilterPerformance

• Adopt a low filtered-water, turbidity goal (e.g.,
  0.1 - 0.2 NTU)
• Develop a well-defined chemical control strategy
  for various seasons of the year
• Regular jar testing in the lab
• zeta potential measurements
• particle count data

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Optimizing FilterPerformance (Cont.)

• Off-line monitoring of full-scale treatment
• streaming current detectors
• particle counters / monitors
• pilot filters
• Chemical pretreatment ahead of filtration is
  important
• Consider coagulant aids (e.g., cationic polymers)
• Consider filter aids (e.g., nonionic polymers)

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Optimizing FilterPerformance (Cont.)

• Chemical pretreatment (cont.)
• Pre-ozonation promotes micro-flocculation
• Rapid mixing is more critical than flocculation
• Paddle flocculators are superior to turbine or
  pitched-blade flocculators, but require more
  maintenance and use more energy
• Increase flocculator speed when water
  temperature is low

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Optimizing FilterPerformance (Cont.)

• Chemical pretreatment (cont.)
• Where applicable, use a ported baffle wall
  between flocculation and clarification to avoid
  floc break-up
• Filterability of solids in clarified
• water is more important than
• the turbidity

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Optimizing FilterPerformance (Cont.)

• Filter Maintenance
• Check the condition of existing media
• effective size (E.S., or d10 value)
• uniformity coefficient (U.C., or ratio of d60 /
  d10)
• depth
• presence of mudballs
• consider performing a
• filter-coring

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Optimizing FilterPerformance (Cont.)

• Filter Maintenance (cont.)
• Minimize the ripening period of a freshly
  backwashed filter
• rewash (i.e., filter-to-waste)
• increase rate slowly (e.g., keep one backwashed
  filter off-line)
• polymer, etc. added to the influent as a filter
  aid
• polymer, etc. added to the backwash water during
  the last stage of the backwash as a media
  conditioner

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Optimizing FilterPerformance (Cont.)

• Filter Maintenance (cont.)
• Dual-media filters
• choose depths of each carefully (i.e., maximize
  L/d ratio within constraints of existing system)
• consider paying more to obtain media with a lower
  U.C.
• pick an appropriate ratio for d90 coal / d10 sand
• Clean the filter effectively
• make sure underdrain system provides uniform
  dispersion of the backwash water

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Optimizing FilterPerformance (Cont.)

• Clean the filter effectively (cont.)
• use appropriate backwash rates
• expanded bed porosity of 0.7 for top layer (i.e.,
  d10 size) of each medium
• higher rates when water temperature is high
• use appropriate backwash duration
• consider surface wash before and during backwash
• consider air backwash prior to water backwash
• make sure washwater troughs are high enough above
  the media surface to allow for proper expansion

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Optimizing FilterPerformance (Cont.)

• Filter Maintenance (cont.)
• Keep track of unit filter run volumes (UFRVs)
• UFRV (filtration rate, gpm/sf) X (filter run
  length, min) gal/sf
• 10,000 gal/sf - for conventional filtration
• 5,000 gal/sf - for direct filtration
• UFRV -
  UBWV
• Production efficiency ------------------- X 100
  
• 
  UFRV
• Where UBWV unit backwash volume, gal/sf

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Tomorrows Filter Challenges

• Based on Raw-Water Densities Measured
• Current, 3.0-log Total-Reduction requirement for
  Giardia could become as high as 6.0 log
• Current, 2.0-log Total-Reduction requirement for
  Crypto could become as high as 6.0 log
• Remember
• Removal Inactivation Total Reduction

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Tomorrows Filter Challenges (Cont.)

• SWTR assumption is a conventional water treatment
  plant that produces a combined, filtered-water
  turbidity lt 0.5 NTU achieves 2.5-log removal of
  Giardia
• Interim LT-1 ESWTRs assumption is a
  conventional water treatment plant that produces
  a combined, filtered-water turbidity lt 0.3 NTU
  achieves 2.0-log removal of Crypto

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Tomorrows Filter ChallengesReductions being
Considered

• Raw Water Density
• lt l cyst/oocyst per 100 L
• 1 - 9 cysts/oocysts per 100 L
• 10 - 99 cysts/oocysts per 100 L
• gt 99 cysts/oocysts per 100 L

• Total Reduction
• 0 to 3-log reduction
• 2 to 4-log reduction
• 3 to 5-log reduction
• 4 to 6-log reduction

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Tomorrows Filter Challenges (Cont.)

• Therefore, Based on Raw-Water Densities gt 1
  cyst/oocyst per 100 L
• Current, 0.5-log Inactivation of Giardia could
  become 1.5 to 3.5-log Inactivation
  ( based on assumed Removal of 2.5 logs )
• Current, no log-Inactivation for Crypto could
  become 2.0 to 4.0-log Inactivation
  ( based on assumed Removal of 2.0 logs )
• Inactivation of Crypto offers a challenge

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Items Discussed - Part I

• Brief History of Water Filtration
• Todays Filters
• Filter Operating Parameters
• Particle Removal in Filters
• Headloss Development in Filters
• Unit Filter-Run Volume
• Backwashing
• Tomorrows Filter Challenges

62
Items to be Discussed - Part II

• Filter Coring Method
• Physical inspection
• Core sampling, for Solids-retention profiles
  (before and after backwashing)
• Backwash-turbidity profile
• Backwash-expansion measurement
• Core sampling, for Sieve analysis
• Filter-effluent turbidity profile
• Solids-retention profiles
• Filter Coring Analyzing Results