A Methodology To Design and/or Assess Baffles for Floatables Control Thomas L. Newman II, P.E. HydroQual, Inc.

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

HydroQual, Inc. Objective Develop an Improved Method to Assess the Floatables-Removal Efficiency of Baffles

HydroQual, Inc. Application of Baffles For Floatables Control Typical Regulator (Without Baffle) Dry Weather: 100% capture of –Flow –Floatables Section View Plan View

HydroQual, Inc. Application of Baffles For Floatables Control Typical Regulator (Without Baffle) Wet Weather: CSO Discharge of –Flow –Floatables Section View Plan View (continued)

HydroQual, Inc. Application of Baffles For Floatables Control (continued) Typical Regulator With Baffle Installed Wet Weather: CSO Discharge of –Flow –Fewer Floatables Section View Plan View Baffle

HydroQual, Inc. Application of Baffles For Floatables Control (continued) Typical Regulator With Baffle Installed Wet Weather: CSO Discharge of –Flow –Fewer Floatables Section View Plan View Baffle

HydroQual, Inc. - Laminar streamlines - Neutrally buoyant items follow streamlines, V x Previous Analytical Approaches Non-turbulent-Flow Case Channel Baffle - Floatables: rise velocity, V z XoXo - Capture if trajectory intercepts baffle ZoZo - Minimum V z for capture (from given release point): V z,min = Z o V x / X o (Dalkir, 1996; Cigana, 1998, 1999)

HydroQual, Inc. Turbulent-Flow Case –Mixing between streamlines –reduces effective V z by the RMS velocity component of the vertical turbulence, V* = V x (n g R h 1/3 ) 1/2 Previous Analytical Approaches Channel Baffle Drawdown Zone - Minimum V z must also compensate for downward turb. component V z,min = Z o V x / X o + C V * (C factor ) (Dalkir, 1996; Cigana, 1998, 1999) - Minimum V z (compensating for extra required rise, Z d ) V z,min = (Z o + Z d ) V x / X o + C V * (C = ) (Dalkir, 1996) (continued) ZdZd

HydroQual, Inc. Previous Analytical Approaches 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 % (continued)

HydroQual, Inc. Shortcomings of Previous Approach (and the solutions!) 1Requires multiple calculations: –for overall performance (each release point over the depth) –for each change in baffle position, flow rate, water level, etc.

HydroQual, Inc. Shortcomings of Previous Approach (and the solutions!) Solution: Spreadsheet Model inputs standardized automatic integration (gives overall efficiency) easy for sensitivity runs compare results using different approaches (Continued)

HydroQual, Inc. Shortcomings of Previous Approach (and the solutions!) 2Does 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! (Continued) Section View

HydroQual, Inc. Shortcomings of Previous Approach (and the solutions!) Solution: Assume A Simple Flow Path accounts for effect of baffle position and regulator geometry on flowstream Example... (Continued) Section View

HydroQual, Inc. Shortcomings of Previous Approach (and the solutions!) Example: – Item in top streamline must rise a small distance. – Item in bottom streamline must rise full distance (Z s +Z d ) before traveling the distance S: Therefore: V z,min = (Z s +Z d )V s / S ( + C V* ) where V s is speed along streamline (Continued) S ZsZs Section View ZdZd

HydroQual, Inc. Shortcomings of Previous Approach (and the solutions!) 3Does 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 (Continued) Section View

HydroQual, Inc. Shortcomings of Previous Approach (and the solutions!) 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. (continued) Section View

HydroQual, Inc. Shortcomings of Previous Approach (and the solutions!) 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... (continued)

HydroQual, Inc. Comparison / Verification of Results 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 Percent Capture

HydroQual, Inc. 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

HydroQual, Inc. For More Information Tom Newman HydroQual, Inc. (201)