Buffer Zones Based on Air Dispersion Modeling - An Effective Approach to Odor Mitigation

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Buffer Zones Based on Air Dispersion Modeling
-An Effective Approach to Odor Mitigation

• Bill Tomerlin,
• Cliff Hoelscher, and
• Ray Kemp

Paul Bonnici and Perry Holland
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Project Description

• Study to define odor impact of City of Fort
  Collins Drake Water Reclamation Facility (DWRF)
  and Mulberry Water Reclamation Facility (MWRF) on
  surrounding community
• Model DWRF odors using an air dispersion model to
  justify a 1000 foot buffer zone
• Develop odor mitigation strategy based on model
  and treatment alternatives

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DWRF Description

• Capacity 23 mgd
• Treatment includes primary treatment and
  two-stage biological treatment
• Solids treatment facilities treat solids from
  both the DWRF and MWRF

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DWRF Schematic
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Drake Water Reclamation Facility
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Presentation Overview

• Buffer Zones and Odor Mitigation
• Air Dispersion Modeling
• Sampling/Testing for Odors at the DWRF
• Air Dispersion Model Output
• Conclusions

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Buffer Zones

• Zones intended to provide buffer between plant
  odor sources and developed areas
• Buffer Zones prevent development immediately
  adjacent to wastewater treatment facilities
• Buffer reduced the overall required odor
  mitigation measures at each facility

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Buffer Zone Regulations
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Buffer Zone Example

• www.srcsd.com/buffer.html

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Air Dispersion Modeling

• Uses topographical and meteorological data to
  estimate pollutant concentrations at downwind
  locations
• Air Dispersion Model Software - ISCST3

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Basic Steps to Develop an Air Dispersion Model

• Obtain meteorological data
• Data came from NCDC and private weather station
  1 mile from the DWRF site
• Looked at 5 years of data and chose the worst year

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Basic Steps to Develop an Air Dispersion Model

• Obtain odor source and building parameters
• Odor source coordinates, release height, areas,
  exit velocity, air flow factor, temp., line or
  area source designation
• Building parameters includes physical dimensions
  of plant structures
• Also included future anticipated structures and
  processes

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Basic Steps to Develop an Air Dispersion Model

• Obtain land use and terrain data
• Land use must be classified rural or urban
• Terrain data was incorporated into the model
  using USGS DEM (digital elevation model) data

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Basic Steps to Develop an Air Dispersion Model

• Build a receptor grid
• DWRF receptor grid consists of 100-meter receptor
  spacing
• The grid does not include receptors inside the
  fenceline but does include fenceline receptors

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Basic Steps to Develop an Air Dispersion Model

• Build receptor grid

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Basic Steps to Develop an Air Dispersion Model

• Develop an odor sampling plan
• Continuous H2S
• Sensory Analysis
• Gas Chromatography Analysis
• Liquid Sulfide Analysis
• Ammonia Draeger Tubes

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Basic Steps to Develop an Air Dispersion Model
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Basic Steps to Develop an Air Dispersion Model

• Continuous H2S Monitoring

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Basic Steps to Develop an Air Dispersion Model

• Sampling equipment

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Basic Steps to Develop an Air Dispersion Model

• Sample testing - Winter

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Basic Steps to Develop an Air Dispersion Model

• Sample testing - Summer

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Basic Steps to Develop an Air Dispersion Model

• Develop emissions estimates for each odor source
  based on measured DT values
• 1) Area and Line Sources
• Emission Rate ((DT/s)/m2) Air Flow Factor
  (cfm/m2) / Conversion Factor (0.58) DT
• 2) Point Sources
• Emission Rate (DT/s) Air Flow Rate From Stack
  (m3/s) DT

• Where
• Air Flow Factor 3 cfm/m2 (turbulent surfaces),
  1.37 cfm/m2 (quiescent surfaces)
• Conversion Factor 35.3 ft3/m3 / 60 s/min 0.58
• DT maximum sampling result

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Air Dispersion Model Output -Odor Impacts Extents
by Year
Legend 1993 1994 1995 1996 1997
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Air Dispersion Model Output -Cumulative Summary

• CDPHE regulations
• Hog Farms 2 DT
• Other Sources 7 DT
• Odor impact is typically defined as 2 DT

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Air Dispersion Model Output -Culpability Summary
Legend South Plant Aeration Basins North Plant
Aeration Basins Master Plan Aeration
Basins Primary Clarifier Weir Zone Thickening
Building Fan Primary Clarifier Settling
Zone Digested Sludge Odor Control Centrate
Holding Tank __ Buffer Zone
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Air Dispersion Model Output Frequency of Odor
Excursions Over 2 DT in hrs./yr.
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DWRF Odor Source Magnitude and Frequency
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DWRF Odor Source Rankings
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Conclusions

• Air Dispersion Model results show significant
  odors outside 1,000 foot buffer zone therefore,
  additional on-site odor mitigation will be
  required
• Buffer Zone will help reduce the amount of
  on-site mitigation required
• The air dispersion model will be used to model
  various odor mitigation technologies and their
  effectiveness