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Grit HappensYou Dont Know What Youre Missing
Indiana Water Environment FederationIndianapolis, IN
November 16, 2010
Pat HerrickHydro International
Wastewater Division
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Presentation Objectives
Why target fine grit
Designing the optimum Grit Removal System
Why efficiency in all three steps is key
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What is Grit?
High Density, Inert Solids
Non-putrescible
Sand, gravel, minerals, cinders, other heavysolid materials
Settling velocity greater than organicputrescible solids
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Takes up volume in process tanks Primaries, aeration, digesters,
incinerators
Manual labor to remove and dispose(time & cost)
Accelerates wear on equipment
Sludge transfer pumps
Digester mixing components
Sludge dewatering feed pumps
Centrifuges
Collectors and screws
Maintenance operator time and parts(time & cost)
Impact of Poor Grit Mgmt
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Conventional Process Design
Remove heavy grit (> 210 micron, 2.65 SG)
Pass organics (Reduce odors & simplify disposal)
Problem: Typically capture
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Conventional Results
Digesters
Aeration Basins
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Understanding Grit
Grit is a common and serious problem formany Wastewater Treatment Plants
Grit is often assumed to behaves like clean sand in
clean water and fine material is often overlooked.
However, municipal grit DOES NOT behave (settle) likeclean sand and many plants have a significantvolume of fine material.
Improving our understanding of grit will serve toimprove the success of our designs.
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Understanding Grit
Assumptions Reality
All silica sand Variety of material types
2.65 specific gravity Range of specific gravity
Perfect spheres Variety of shapes
Quiescent basin Basins not quiescent
Clean sand in clean water Fats, oils, greases, soap, scum incollection system
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Distribution of Grit at WWTPsCompiled Particle Size Distribution from Treatment Plants
0
10
20
30
40
50
60
70
80
90
100
10 100 1000 10000
Particle Size (micron)
PercentFinerThan(%)
Chicago (10/27/2004) Chicago (10/28/2004) Chicago (10/29/2004) Florida (Ormand Beach)
Florida (Iron Bridge WRF) Florida (Eastern WRF) Florida (Largo WWTP) Florida (St. Petersberg SW)
Florida (Three Oaks WWTP) New England (Hartford, CT) Various (Atlanta) Various (Baltimore)
Calumet City WRP (7/15/2003) Calumet City WRP (7/18/2003) Clearwater , FL (Northeast Plant) Clearwater , FL (Marshall St. Plant)
Clearw ater, FL (East Plant) Green Bay, WI Tampa, FL
Wastewater Management Grit Products
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Specific Gravity
Specific Gravity of Various Materials
Quartz Sand 1.2 Earth 1.4
Limestone 1.55 Granite 1.65Clay 1.8 Red Brick 1.9
Sand, wet 1.92 Gravel 2.0
Asphalt 2.2 Concrete 2.4
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Deer Island Treatment Plant, Boston, MASG: 1.22
East Bay Municipal Utility District WWTP, CASG: 1.24 1.61, Average 1.35
Green Bay WWTP, WISG: 1.53
Particle Size(microns)
AggregateClass
Time Required toSettle 1
SG = 2.65
Time Required toSettle 1
SG = 1.35
100 Very Fine Sand 38 Seconds 2 min. 48 sec.
Specific Gravity
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Settling Velocity
CleanSand
ParticleSettlingVelocity(Fast)
Same SizeParticles
Grit
ParticleSettlingVelocity(Slow)
SandParticle
WithSame
SettlingVelocity
Sand Equivalent Size (SES) Of Grit
CLEANSAND
GRIT
Grit settling velocity affected by:
Size/ShapeSpecific GravityFats, Oils & Grease
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Average Median SizeDistribution of Grit
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Collection System Basics
Grit remains in the collectionsystem until transported to theplant Low flow moves only small and
light grit Increased flow/velocity suspendsgrit First flush significantly increases
grit load (10
40X avg.)
Effective grit removal systems mustfunction at peak flow and peak gritload simultaneously
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Designing a Grit Removal
System That Works
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Design Guideline
Define Design Requirements:
Grit Particle Size Analysis
Settleability (SES)
Required System Efficiency
Screening Requirements Evaluate Equipment
Removal Efficiency/Performance
Equipment Design/Features
Space
Headloss
Cost: Capital, Installation, Operational
Maintenance Requirements
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Types of Grit Removal
Gravity Sedimentation
Detritor Tanks
Velocity Channels
Aerated Grit Chambers
Vortex Grit Systems
Open Vortex or Free Vortex
Forced Vortex (Mechanical or Hydraulic)
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Forced Vortex
Wall velocity is greater thanat the center Increasing performance asflows decrease (lower SLR) Low headloss < 12
2D
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Forced Vortex Systems
Vortex Type Headloss RemovalEfficiency
Cut PointParticle
Hydraulic Forced Vortex Medium 6-12
95% 75 + micron
Mechanical Forced Vortex Low < 6 95% 300 + micron
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Mechanically Induced Vortex
Performance:
95% removal of 300micron particle
85% removal of 212micron particle
65% removal of 150micron particle
Low headloss
Mechanical vortex
Paddle maintains vortex
Power required
Inlet / Outlet limitations
270& 360
Hundreds of installations
Wastewater Management Grit Products
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Stacked Tray Vortex
Performance: 95% removal down to 75 micron
particle
Large surface area in a smallfootprint
Short settling distance
Handles wide flow ranges
Sized for peak flow and grit load
No moving parts (in-basin)
Ideal for retrofitting into existingbasins
Proven Track Record > 100 US installations
Wastewater Management Grit Products
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Unit Processes of Grit Separation
Washing
Dewatering
Collection
ORGANICS
WATER
Slurry
Grit
Influent Effluent
Slurry
Wastewater Management Grit Products
System is only as strong as its weakest link
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Conventional Washing/Dewatering
Liquid CycloneBorrowed from the mining industry
Centrifugal separator/concentrator Capable of removing grit 150 m
Apex valve size minimized to reduceoutput volume to classifier (not adjustable)
Requires constant pressure drop toachieve separationCannot handle grit concentration spikes
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Screw ClassifierSettling tank with screw Typically sized based on
screw conveying capacity Clarification capacity
typically not a sizing criteria Hydraulic load washes fine
grit out of the unit causingrecirculation issues
Conventional Washing/Dewatering
Fox Lake IL
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Fox Lake, ILSystem Comparison
PerformanceAerated Grit Basin
with Cyclone/Screw
HeadCell/SlurryCup/Grit Snail
Separator Capture 58% 95%
Washing/Dewatering Capture 17% 93%
Overall System Capture 10% 88%
Grit Quality 5 lb FS/ft3 65 lb FS/ft3
Aerated Grit Basin 225 micronHeadCell System 100 micron
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Wastewater Management Grit Products
High Efficiency Washing & Dewatering
Grit SnailSlurryCup
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Wastewater Management Grit Products
Eutek SlurryCup
Guaranteed performance:
95% removal down to 50micron particle
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Boundary LayerSeparation by Size
Grit Particle Stays inBL: Swept to Center
Wastewater Management Grit Products
Organic ParticleWashed out of BL:
Not Swept to Center
Washed GritTo Grit Snail
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Quiescent DewateringEscalator
Large clarifier area
3.2 GPM/FT2
Slow moving belt (1-2 FPM)
Guaranteed 95% removal of50 micron grit
Guaranteed 60%TS dry grit
Durable construction longproduct life
Eutek Grit Snail
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Overview Base design on native grit behavior
Target fine grit
Define design requirements
Consider efficiency of each step
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Copyright Hydro International 2008
Pat HerrickHydro International
Wastewater [email protected]
(503) 679-0273
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