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7/14/2016
1
Thickening and Dewatering OptimizationMatt Van Horne, PE – Hazen and Sawyer
July 14, 2016
2
Agenda
• Equipment Specific Modifications
• Chemicals
• Feeding
• Cleaning and Maintenance
• Pretreatment
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Optimization
Approach
4
Dewatering and Thickening Optimization is a…
Balancing Act
Solids Capture
Throughput
Polymer Dosage
Cake Solids
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3
5
What is the Goal of Optimization?
• Key Performance Indicators
• Solids content output
• Solids capture
• Polymer dose
• Energy consumption
6
What Can We Do To Achieve These Goals?
• Vary different inputs
• Equipment parameters
• Hydraulic and solids loading rates
• Polymer characteristics
• Washwater
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Equipment Specific
Modifications
8
Belt Presses and Gravity Thickeners
• Belt speed
• Belt tension
• Chicane configuration
• Washwater pressure
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9
Belt Press and Gravity Thickener – Belt
Speed
• Faster speed
• More throughput
• Shorter retention time in gravity and pressure zones
10
Belt Press – Belt Tension
• Higher tension
• More force applied in pressure zone
• More force applied to belt
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11
Belt Filter Press and Gravity Thickener –
Chicane Configuration
• Balance turning of sludge with water drainage
time
12
Centrifuges
• Differential speed
• Torque
• Pond levels
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13
Centrifuge – Differential Speed
• Conveyor moving faster than bowl
• Increased differential speed
• Solids move through quicker
• Reduced cake solids
• Increased capture
14
Centrifuge - Torque
• Use torque measurement to estimate cake
dryness
Source: Andritz
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15
Centrifuge – Pond Levels
• Increased pond level
• More room for liquid clarification
• Likely reduced cake solids
• Possibility for increased solids capture
Shallow Levels
Deep Levels
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Gravity Thickener
• Elutriation water
• Solids removal rate
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17
Gravity Thickener – Elutriation Water
• More water = lower hydraulic retention time
• Lower retention time = less chance for septicity
• Temperatures increase → fermentation
increases → gas is produced → sludge floats
• Reduce residence time by increased hydraulic
load to reduce gas production potential
18
Gravity Thickener – Solids Removal Rate
• Pumping rates
faster than sludge
“refill” rates
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Solids Pumping
• Clarifier blanket level controls
• TSS meters
Chemicals
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What Types of Chemicals
• Iron salts
• Ferric chloride
• Coagulates colloids
• Lime
• Combine with iron salts for filter press applications
• Increase resistance to filtration
• Polymers
• Organic flocculants
• Fix destabilized particles on the long monomer chains
22
Most Applications Use Polymers (Flocculants)
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23
Typical Polymer Doses for Dewatering
0
5
10
15
20
25
30
Ce
ntr
ifu
ge
Be
lt P
ress
Scre
w P
ress
Ce
ntr
ifu
ge
Be
lt P
ress
Scre
w P
ress
Ce
ntr
ifu
ge
Be
lt P
ress
Scre
w P
ress
Waste Activated Aerobic Digested Anaerobic Digested
Po
lym
er
Do
sage
, lb
/to
n
Equipment / Sludge Type
Polymer Dosage, lb/ton
EPA 832-F-00-053, September 2000
24
Polymer Types
• Polymer is used for sludge conditioning and to
enhance settling, thickening, and dewatering
• Electronic charge
• Charge density
• Molecular weight
• Molecular structure
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Electronic Charge
• Anionic (negative) to attract mineral particles
• Cationic (positive) to attract organic particles
• Best determined via lab/bench testing
Source: SNF/Floerger
26
Charge Density
• More biological sludge requires more positive
charge required
Source: SNF/Floerger
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27
Molecular Weight
• Molecular weight = length of chain
• Longer chain is more able to handle high shear
forces (i.e. centrifuge)
• Low to medium weight promotes good gravity
drainage (i.e. GBT, RDT, BFP)
28
Molecular Structure
• Linear
• Low shear resistance
• Branched
• Cross linked
• High shear resistance
• Blocked charges
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Emulsion Polymer
• Milky/cloudy liquid totes
• Higher concentration of active
polymer
• Shorter self life than dry
polymer
• Usually 25% to 60% active
polymer
30
From Chemical Unloading
Polymer Storage
Tank
Dilution Water
Polymer
Mixing
Unit
Direct Feed Polymer System
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From Chemical Unloading
Polymer
Mixing UnitPolymer Storage
Tank
Mix/Age Tank
M
Dilution Water
Mix-Age Polymer Feed System
32
Dry Polymer
• Pellet or flake provided in large
bulk bags
• Lower concentration of active
polymer
• Longer shelf life than emulsion
polymer
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Dry Polymer
34
Polymer Conclusions
• Polymer aging is important for dewatering
• Polymer dosing location can be important for
thickening
• SCADA can provide real time polymer
performance tracking
• Feed rates
• Solids concentrations
• Close mass balance
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Feeding
36
Sludge is Inherently Variable
• But thickening and dewatering processes like
consistent characteristics…
• Influent changes
• Biological process variations
• Digestion process variations
• Seasonal fluctuations
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37
Control Feeding Equipment
• Manage pumping times and intervals
• Monitor solids content – ratholing
• Improve control over upstream processes (if
possible)
• Sludge storage and blending
38
Consistent Feeding Has Multiple Benefits
• Predictability of thickening/dewatering
performance
• Consistent polymer dosing
• Reduces equipment adjustments
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39
Feed Distribution Can Impact Performance
Credit: BDP
Cleaning and
Maintenance
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41
Poor Cleaning Can Reduce Capture
Credit: BDP
42
Uneven Belt Stretching
Credit: BDP
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43
Clogged Spray Nozzle
Credit: BDP
Pretreatment
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45
An Emerging Approach
• Likely are many options similar to those used for
digestion pretreatment
• One manufacturer recently working in the North
American market
46
Orege SLG Approach
• Compressed air injection into sludge
• Short retention time
• Allows release of free water from sludge
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47
Orege SLG
48
Initial Installation – Allentown, PA
• Goals
• Improve cake solids – 15.3% baseline
• Increase BFP throughput – 100 gpm baseline
• Decrease polymer dose – 60 lbs/dry ton baseline
• Initial Results
• Cake solids – 18.6%
• BFP throughout – 150 gpm
• Polymer dose – 45 lbs/dry ton
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Case Study
RDT Optimization
F. Wayne Hill WRF, Gwinnett County, GA
50
PS and WAS Holding Tank
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Rotary Drum Thickeners
52
Rotary Drum Thickener
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Thickened Sludge Hopper & Pump
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Optimization Approach
• Understand performance
• Long term historical
• Variability on different time scales
• Field sampling
• Full scale parameter adjustment
• Tracking key performance indicators
• Develop “real life” guidance and approaches
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55
RDT Analysis – Historical Data
• Average solids and hydraulic loading rates
• Sludge feed to digesters
• Field sampling
• Conclusions
56
WAS Load
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WAS + PS Load
58
Average RDT Solids Loading Rate
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Average RDT Hydraulic Loading Rate
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Calculated Thickened Sludge Loading to Digesters
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61
Thickened Sludge Flow to Digesters
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Digester SRT
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Feb 1-14, 2014
Individual RDT Thickened Sludge Flow
64
Oct 14-21, 2014
Individual RDT Thickened Sludge Flow
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Oct 14 – 21, 2014
Thickened Sludge flow to digesters
66
Oct 2014 – PS and WAS Loading
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RDT Data Update Conclusions
• Periods of:
• Good Thickening and High Digester SRTs
• Sporadic Thickening and Lower Digester SRTs
• Variable Individual RDT Performance
68
RDT Field Sampling
• Measured TSL concentration of operating RDTs
• Including new drum of RDT 4 (same day as install)
• Varied polymer dose on one RDT
• Measured WAS, PSL, and TSL concentrations
• Field Observations:
• RDTs had different polymer doses & different TSL
concentrations
• TSL concentration can be optimized by varying polymer
dose
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Field Sampling Results
70
Field Sampling Results
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Diurnal Sampling Results
72
RDT Field Sampling
• Still have variable RDT performance
• Use Thickened Sludge Flow as quick
assessment of individual thickening performance
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RDT Analysis – Key Conclusions
• Target thickened sludge concentration of 6.5%
(Max 8%)
• Necessary for maintaining digester capacity
• Variable performance across different RDTs
• Likely due to varying polymer doses
• Target uniform polymer dose
• Average % capture >99.5%
• Average filtrate TSS range = 100-200 mg/L TSS
Wrap Up
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75
What Did We Learn?
• Equipment adjustments
• Polymer optimization
• Feeding controls
• Cleaning and maintenance
• Pretreatment potential
76
Questions and Discussion
Matt Van Horne, P.E.
703-267-2738