A Methodology To Design andor Assess Baffles for Floatables Control

1
A MethodologyTo Design and/or AssessBaffles for
Floatables Control

• Thomas L. Newman II, P.E.
• HydroQual, Inc.

2
Introduction

• Interest in Baffles
• EPA CSO Control Policy / 9 Minimum Controls
• Municipalities seek cost-effective alternatives
• Advantages of Baffles
• Low Cost (capital and maintenance)
• Simple Design
• Easy to Retrofit
• Usable with Other Technologies
• Disadvantages of Baffles
• Not much information available
• Limited analytical tools to assess performance

3
Objective

• Develop an Improved Method
• to Assess the
• Floatables-Removal Efficiency
• of Baffles

4
Application of Baffles For Floatables Control
Plan View

• Typical Regulator (Without Baffle)
• Dry Weather
• 100 capture of
• Flow
• Floatables

Section View
5
Application of Baffles For Floatables Control
Plan View
(continued)

• Typical Regulator (Without Baffle)
• Wet Weather
• CSO Discharge of
• Flow
• Floatables

Section View
6
Application of Baffles For Floatables Control
(continued)
Baffle
Plan View

• Typical Regulator With Baffle Installed
• Wet Weather
• CSO Discharge of
• Flow
• Fewer Floatables

Baffle
Section View
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Application of Baffles For Floatables Control
(continued)
Baffle
Plan View

• Typical Regulator With Baffle Installed
• Wet Weather
• CSO Discharge of
• Flow
• Fewer Floatables

Baffle
Section View
8
Previous Analytical Approaches

• Non-turbulent-Flow Case

- Laminar streamlines - Neutrally buoyant items
follow streamlines, Vx
- Floatables rise velocity, Vz
- Minimum Vz for capture (from given release
point) Vz,min Zo Vx / Xo (Dalkir, 1996
Cigana, 1998, 1999)
- Capture if trajectory intercepts baffle
Baffle
Zo
Xo
Channel
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Previous Analytical Approaches
(continued)

• Turbulent-Flow Case
• Mixing between streamlines
• reduces effective Vz by the RMS velocity
  component of the vertical turbulence, V Vx (n
  g Rh1/3 )1/2

- Minimum Vz must also compensate for downward
turb. component Vz,min Zo Vx / Xo C V (C
factor 0.4 - 1.6)
(Dalkir, 1996 Cigana, 1998, 1999)
- Minimum Vz (compensating for extra required
rise, Zd) Vz,min (Zo Zd) Vx / Xo C V (C
0.4 - 1.6)
(Dalkir, 1996)
Drawdown Zone
Zd
Channel
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Previous Analytical Approaches
(continued)

• Determine Removal Efficiency from Rise
  Velocity
• Use distribution curve
• Laboratory tests on 2,000 items from 2 Montreal
  CSOs
• Example
• Vz,min 10 cm/s
• Efficiency 20

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Shortcomings of Previous Approach (and the
solutions!)

• Requires multiple calculations
• for overall performance (each release point over
  the depth)
• for each change in baffle position, flow rate,
  water level, etc.

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Shortcomings of Previous Approach (and the
solutions!)
(Continued)

• Solution
• Spreadsheet Model
• inputs standardized
• automatic integration (gives overall efficiency)
• easy for sensitivity runs
• compare results using different approaches

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Shortcomings of Previous Approach (and the
solutions!)
(Continued)

• Does Not Account for Effect of Flow Path
• only release point and baffle position
  
• ignores downward velocity component of flow
• predicts 100 capture if baffle extends
  below inlet invert level
• overpredicts capture!

Section View
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Shortcomings of Previous Approach (and the
solutions!)
(Continued)

• Solution
• Assume A Simple Flow Path
• accounts for effect of baffle position and
  regulator geometry on flowstream
• Example...

Section View
15
Shortcomings of Previous Approach (and the
solutions!)
(Continued)

• Example
• Item in top streamline must rise a small
  distance.
• Item in bottom streamline must rise full distance
  (ZsZd) before traveling the distance S
• Therefore
• Vz,min (ZsZd)Vs / S ( C V )
• where Vs is speed along streamline

Zd
S
Zs
Section View
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Shortcomings of Previous Approach (and the
solutions!)
(Continued)

• Does Not Account for Underflow
  Capture
• some floatables captured in the underflow
• model not applicable to pre-baffle condition
• cannot determine Net Effectiveness
  of Baffle Installation

Section View
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Shortcomings of Previous Approach (and the
solutions!)
(continued)

• Solution
• Account for Escape Velocity
• Example
• Underflow 20 of Inflow,
• Bottom 20 of streamlines to underflow
• Floatables that can rise out of underflow
  streamlines escape but remaining are captured
• Add underflow capture to baffle capture for
  overall capture.

Section View
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Shortcomings of Previous Approach (and the
solutions!)
(continued)

• Efficiency based on 2 Montreal CSOs, but these
  appear to differ from NYC composition
• fewer on high and low end of spectrum
• cause under- or over-estimate of performance
• NYC tests coming...

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Comparison / Verification of Results
Percent Capture

• Previous Approaches Predict Higher Removal
  Efficiency Than New Model
• New Model Still Predicts Relatively High
  Performance
• Comparison to Lab Data is Favorable, but
• Not Apples to Apples

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Conclusions

• New, Improved Model to Assess the
  Floatables-Removal Efficiency of Baffles
• Fully Compatible with Previous Approaches
• Spreadsheet format
• Considers flow path
• Accounts for underflow capture
• Enables assessment of pre-baffle condition and
  the net effectiveness of the installation
• Awaiting experimental data to further verify model

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For More Information

• Tom Newman
• HydroQual, Inc.
• tnewman_at_hydroqual.com
• www.hydroqual.com
• [phone]