Filtration Part II Filter Coring

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Filtration - Part IIFilter Coring

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

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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

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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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Filter CoringMethod
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1. Physical Inspection

• Media Surface
• Surface variation, Cracks, Media boils
• Mudballs
• Bed depth, and Gravel migration
• Distance from top of trough to bed
• Distance between surface-wash nozzles and bed
• Backwash
• Media Loss
• Air release during backwash
• Level of backwash troughs

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Measuring Bed Depth,and Gravel Migration
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2. Core Sampling
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Array of Core-Sampling Tools
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Core-Sampling Method

• Sample from three (3) bore holes for first sample
  (0 - 2 in.)
• Sample from two (2) bore holes for second sample
  (2 - 6 in.)
• Sample from one (1) bore hole for remaining
  samples (6 - 12 in., etc.)
• Place samples from each depth in a baggy

Core
10
Collecting Core Samples, andPlacing Samples in a
Baggy
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Filter-Media Depth Profile
0 - 2
2 - 6
6 - 12
Anthracite - 24
12 - 18
36 total
18 - 24
Anthracite / Sand 21 - 27
24 - 30
Sand - 12
30 - 36
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Core-Sampling Pipe
Approximately 6 ft
Lightweight Pipe (e.g., Electrical conduit) PVC
pipe is not suitable
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Filter-Media Samples for Solids-Retention
Profiles(Before and After Backwash)
Sample No.
Sample Depth
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Dual-Media Interface Potential for Significant
Solids Storage
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3. Backwash-Turbidity Profile

• Standard backwash procedure using seasonal
  backwash rate
• Surface wash
• Surface wash, with low-rate backwash
• High-rate backwash, without surface wash
• Collect first sample of backwash water when
  filter troughs begin to overflow
• 100-ml sample volume at 30 second intervals

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Backwash-Rate Relationship

• BW rate is a function of terminal velocity of
  media grains
• media grain size (effective size d10)
• uniformity coefficient (d60/d10)
• specific gravity
• media sphericity
• Scouring action provides effective BW of media at
  10 settling velocity

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Backwash Rate Independent of Media Depth
Media scouring vs. terminal velocity of media
grains
When the expanded-bed porosity is around 0.7,
media grains are scratching against each other to
unstick the stuck solids.
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Appropriate Backwash Rate at 20c
30
25
Sand S. G. 2.65
20
15
Backwash Rate (gpm/sf)
10
Anthracite Coal S. G. 1.65
5
0
0
1
2
60 Weight grain size (ESUC)
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Temperature-Correction Factor for Backwash Rate
Multiplier for Appropriate Backwash Rate
Water Temperature (C)
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4. Backwash-Expansion Measurement
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Backwash-Expansion Measurement (Cont.)
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5. Core Sampling for Sieve Analysis

• Collect full-depth, filter-media core samples for
  both the anthracite and the sand after backwash
• Send to lab for sieve analysis of both the
  anthracite and the sand samples
• Effective grain size (d10), uniformity
  coefficient (d60/d10) and specific gravity
• ASTM methods C136 and C128

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Example of an Anthracite Core Sample for Sieve
Analysis
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6. Filter-EffluentTurbidity Profile

• Perform a mini-wash of the filter following the
  filter coring conducted after backwash
• Bring filter back on-line
• Measure filter-effluent turbidity every 5 min for
  30 minutes
• Measure filter-effluent particle counts every 5
  min for 30 minutes
• Continue measuring if effluent turbidity is not
  lt 0.1 NTU at 30 minutes

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7. Solids-Retention Profiles
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Solids-Retention Lab Method
a. Remove and weigh a 50-gram sample of filter
media from a baggy b. Mix 100 ml of tap water
with media shaking for 30 sec at uniform
intensity c. Decant supernatant, and shaking
repeated with additional 100 ml water d. Final
volume of supernatant 500 ml e. Turbidity
measurement converted to NTU/100 grams of
filter media f. Develop solids-retention
profile for each sample, before and after BW
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Solids-Retention Method (cont.)
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Solids-Retention Method (cont.)
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Solids-Retention ProfileGuidelines for
Backwashed Filter

• lt 30 NTU/100 grams

• Unconditioned Media

• 60 -120 NTU/100 grams

• Well Matured Filter Bed

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Filter CoringAnalyzing Results
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1. Physical Inspection Results

• Media Surface
• Evidence of Media Boiling, Cracks in Bed
• Presence of Mudballs
• Sloping of Filter Surface Bed Contours
• Backwash
• Air Boiling
• Vigorous Backwash on One Side
• Unequal flow of backwash in troughs - troughs may
  be uneven

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2. Core Samples for Solids-Retention Profile
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Lab Instruments for Developing Solids-Retention
Profiles
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3. Backwash-Turbidity Samples
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Backwash-Turbidity Profile Dual-media Filters
Turbidity (NTU)
Time of Backwash (minutes)
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BW-Turbidity Profile Comparison Rapid-sand Filters
Backwash Duration (minutes)
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BW-Turbidity Profile Results Rapid-sand Filters

• Sharp Initial Backwash-Turbidity Peak
• Second Turbidity Peak after Start of Surface Wash
• Final Backwash Turbidity over Target of 20 NTU

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4. 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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5. Sieve-Analysis Results

• Allows us to define, for both the anthracite and
  the sand
• d10 - 10 of the media is finer (smaller) than
  that size (diameter), on a weight basis -
  referred to as the effective size of the media
• d60 - 60 of the media is finer (smaller) than
  that size (diameter), on a weight basis
• Uniformity coefficient (UC) - Ratio of d60 size
  to the d10 size
• Specific gravity ( i.e., weight of media / weight
  of water )

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Sieve-Analysis Results (Cont.)

• Also allows us to define, for the anthracite
• d90 - 90 of the anthracite is finer (smaller)
  than that size (diameter), on a weight basis
• Important because we want a ratio for the d90
  coal size to the d10 sand size of approx. 3
  (i.e., 3 - 6 in. of intermixing at interface)
• Ratio significantly bigger than 3 ( i.e., too
  much intermixing at interface )
• Ratio significantly smaller than 3 ( i.e., no
  intermixing at interface )

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6. Filter-Effluent Turbidity Profile Results
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7. Solids-Retention Profiles
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Ideal FilterSolids-Retention Profile
0
5
10
15
Media depth (in)
Before Backwash
20
After Backwash
25
30
Turbidity (NTU/100 g of Media)
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Poorly Backwashed Filter(No Surface Wash)
Media Depth (inches)
Before Backwash
After Backwash
Turbidity (NTU/100 g of Media)
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Poorly Backwashed Filter(With Surface Wash)
Before Backwash
Media Depth (in)
After Backwash
Turbidity (NTU/100 g of Media)
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Poorly Matched Media (Sand is not Fluidized)
0
5 0
100
150
200
250
300
0
5
Before Wash
Anthracite Coal
10
Dual Media Filter
Depth From Top of Bed (in)
15
Sand
20
Coal Mixed
25
Sand
After Wash
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Floc Deposition on Filter Media
Turbidity (NTU/100 g of Media)
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Core-Sampling Outcomes

• Actual depth of filter media L/d ratios
• Backwash-turbidity profile
• Effectiveness of backwash
• Grain-size distribution media size and
  uniformity coefficient
• Filter-effluent turbidity profile
• Solids-retention profiles

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Filter-Coring Conclusions

• Effective Tool for Diagnosis of Filter-Media
• Condition, and
• Backwash Effectiveness
• Provides Specific Recommendations
• for Change in
• Filter Operation

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Items 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