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“GRIT HAPPENS: KNOW HOW TO DEAL WITH IT” A CASE STUDY ON SELECTION OF GRIT REMOVAL SYSTEMS FOR WWTPs Murali Erat, P.E. November 18, 2014
“GRIT HAPPENS: KNOW HOW TO DEAL
WITH IT”
A CASE STUDY ON SELECTION OF GRIT REMOVAL SYSTEMS FOR WWTPs
Murali Erat, P.E.
November 18, 2014
Outline § What is Grit? § Characteristics of Grit § Grit Removal Technologies § Case Study:
§ Grit Removal System Evaluation for City of Houston’s Sims Bayou South WWTP
§ Conclusion § Key Factors for Design
EPA’s Wastewater Technology Fact Sheet deFines grit as:
– Particles > 210-‐micron (.0084 in or 65 mesh) – SpeciFic gravity > 2.65
What is Grit?
Grit Characteristics SpeciFic Gravity (SG)
Specific Gravity of Various Materials in WW
Quartz Sand 1.20 Sand, wet 1.92
Limestone 1.55 Gravel 2.00
Granite 1.65 Asphalt 2.20
Clay 1.80 Concrete 2.40
Red Brick 1.90 Grit 2.65
ParNcle Size (microns)
Aggregate Class Time Required to SeTle 1’ SG = 2.65
Time Required to seTle 1’ SG = 1.35
100 Fine Sand 38 Seconds 2 min. 48 sec.
Settling Velocity
Grit Characteristics • Modeling “Grit” as “Sand” (Se6ling Characteris:cs):
Clean Sand
Particle Settling Velocity (Fast)
Same Size Particles
Grit Particle Settling Velocity (Slow)
Sand Particle
With Same
Settling Velocity
Sand Equivalent Size (SES)
CLEAN SAND GRIT
Design Criteria Target grit cut point: 150-‐105μ SpeciHic gravity: 1.2-‐2.7
Types of Grit Removal System Aerated Grit Chambers
Mechanical Vortex
Gravity Vortex (HeadCellTM)
Gravity Vortex (HeadCellTM)
Degritted Effluent
Grit to dewatering
Screened Influent
Case Study – Sims Bayou South WWTP (City of Houston)
Sims Bayou South WWTP
Sims Bayou South WWTP
Headworks
InHluent Lift Station
Aeration Basins
ClariHiers
Chlorine Contact Basins
Sims Bayou South WWTP
§ Plant built in 1987
§ Current Permit: § Annual Average Daily Flow (AADF) – 36 MGD
§ 2-‐hour Peak Flow – 90 MGD
§ Future Ultimate 2-‐hour Peak Flow – 190 MGD
§ Actual Flow Conditions: § Current average Flow – 16 MGD
§ Max. peak Flows during wet weather -‐ 160 MGD
Existing Aerated Grit Chambers
48’
30’
Total Capacity of Aerated Grit Chambers -‐ 190 MGD
Out of Service for ~ 10 years
Existing Aerated Grit Chambers
Air Li] Pumps
Existing Grit Dewatering System: Vibra-‐Screens
Existing Aerated Grit Chambers DeHiciencies
§ Vibra-‐Screen dewatering system failed – was hard to operate and maintain
§ Air lift pumps failed to remove settled grit § Air lift pumps were manually operated § Existing Basin Geometry:
§ Width to Depth ratio – 2:1 (Recommended 0.8-‐0.9:1) § Length to Width – 1.6:1 (Recommended 3-‐8:1) § Floor slope – 22 deg (Recommended 30 deg)
Refurbish Exis:ng Aerated Grit Chambers 1
Retrofit with Mechanical Vortex System 2
Retrofit with HeadCellTM 3
Grit Removal System Options
Refurbish Exis:ng Aerated Grit Chambers 1
Replace air lift pumps with self-‐
priming grit pumps
Replace vibra-‐screen unit with traditional grit washer/classiHier
100 MGD
100 MGD
Entrance Channel: 39’
Retrofit with Mechanical Vortex System 2
31.7 MGD
31.7 MGD
31.7 MGD
18
Three (3) 12’ dia tray units, Capacity per basin – 95 MGD
Retrofit with HeadCellTM 3
31.7 MGD
31.7 MGD
31.7 MGD
Add partition walls
Three (3) 12’ dia tray units, Capacity per basin – 95 MGD
Relocate efHluent troughs
Add isolation gates
Retrofit Exis:ng Aerated Grit Chambers with HeadCellTM 3
EfHluent Trough
Existing Aerated Grit Chamber
BafHle Wall
Option 3: RetroHit with HeadCellTM
HeadCell
Grit Pump
Option 3: RetroHit with HeadCellTM
Basin #1 Basin #2
Grit Washing/Dewatering Station #2
Grit Washing/Dewatering Station #1
Grit Removal EfHiciency
50% Aerated Grit Chambers
85% HeadCell/Vortex
Refurbish Existing Aerated Grit Chambers 1 Mechanical Vortex System 2 RetroFit with HeadCellTM 3
Grit Removal System Options -‐ Cost
Capital Annual O&M
Life Cycle Cost (25 years)
$1.44 M $114 K $12.6 M
$4.5 M $45 K $7.5 M
$4.0 M $64 K $7.3 M
NO GRIT REMOVAL
Grit Removal System Options -‐ Cost
More frequent cleaning of
aeration basins
Increase in Blower HP
Damage to diffusers during grit cleanout
Wear & tear of sludge piping
Wear & tear of mechanical equip.
$$$ $$$
$$
$$
$
Refurbish Existing Aerated Grit Chambers 1 Mechanical Vortex System 2 RetroFit with HeadCellTM 3
Grit Removal System Options -‐ Cost Capital Annual
O&M Life Cycle Cost (25 years)
$211 K $90 K $12.4 M
$1.44 M $114 K $12.6 M
$4.5 M $45 K $7.5 M
$4.0 M $64 K $7.3 M
Status Quo – No Grit Removal 0
Option 3: HeadCell
• Easily retroFitted in existing grit basins
• Flexible operation • No mechanical parts within the basin
• Very efFicient in removing Fine grit
• Pilot HeadCell system for use at other plants
• SOLE SOURCED!!
Option 2: Mechanical Vortex
• Not a sole-‐sourced technology
• EfFicient in removing Fine grit
• Cannot be retroFitted into existing grit basins
• Larger footprint • No turn down
Grit Removal Options -‐ Recommendation
Retrofit Exis:ng Aerated Grit Chambers with HeadCellTM 3
Includes: • Six (6) HeadCell Units • Six (6) Tea-‐Cup girt washes and four (4) Grit Snail grit dewatering equipment
• Grit pumps • Structural repair to grit basins • New isolation gates • Contingency and overhead factors
Capital Cost Annual O&M Life Cycle Cost (25 years)
$4.0 M $64 k $7.3 M
Grit Removal System Design – Key Points
Grit Separa:on
Grit Washing
Grit Dewatering
Aerated grit chambers Mechanical Vortex Gravity Vortex -‐HeadCell
Cyclone Type (Tea-‐Cup), Conical Type (Pista Turbo, Huber etc.)
Inclined Screw Type (Pista Turbo) Escalator Type (Grit-‐Snail)
70%
80%
90%
Overall EfHiciency =50%
Grit Removal System Design – Key Points
• Know your grit!
• Fine grit? There are treatment options!
• Keep grit dewatering system close
• Grit Slurry Pipe
ü Velocity 4-‐7 ft/sec
ü Avoid 90 deg bends
ü Provide cleanouts
GRIT HAPPENS
DEAL WITH IT
