The Role of Landfill Gas Emission Rate Calculation Methods in Solid Waste Landfill Risk Assessments

1
The Role of Landfill Gas Emission Rate
Calculation Methods in Solid Waste Landfill Risk
Assessments

• Sarah A. Foster
• Paul C. Chrostowski, Ph.D.
• CPF Associates, Inc.
• Steve Wilsey
• Conestoga Rovers Associates, Inc.
• Society for Risk Analysis
• 2006 Annual Meeting
• December 2006
• Baltimore, Maryland

2
Introduction

• MSW landfills
• Dominant disposal method in many countries
• Roughly 55 of US MSW
• Common community concerns
• Health risks from landfill gas
• Cancer and noncancer health effects
• Odors

• Risk assessments
• Can be used to help evaluate community health
  concerns

3
Presentation Outline

• What is landfill gas?
• Methods for calculating landfill gas emission
  rates for use in risk assessments
• Case studies
• Conclusions

4
What is Landfill Gas (LFG)?

• By-product of biodegradation of municipal solid
  waste
• 45-60 methane
• 40-60 carbon dioxide
• lt1 trace compounds including volatile organic
  compounds and sulfur-containing compounds

5
Overview of Landfill Gas Emission Sources
Total gas generated
Uncollected gas
Gas collection system
Landfill surface (cover, fissures)
Treatment devices (flare, gas-to-energy
6
Methods for Calculating Landfill Gas Emission
Rates

• Modeling
• Total gas generation rate
• Gas system collection efficiency
• LFG composition
• Calibration using site-specific data
• Measured gas collection rates
• Measured LFG composition
• Measured versus modeled downwind ambient air
  levels
• Measurement
• Surface flux boxes
• Optical remote sensing methods (emerging, limited)

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Case Studies LFG Emission Sources
8
Landfill 1 LFG Emissions and Dispersion

• Modeled emission rates
• EPA LANDGEM model
• Default inputs
• Used 70-year average gas generation rate
• Assumed gas collection efficiency (75)

• LFG concentrations
• Site-specific gt Waste Industry Air Coalition
  averages gt EPA AP-42
• SCREEN3 dispersion model
• Modeled as one area source
• Evaluated assumed residential locations
  100 m - 2,500 m from landfill

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Landfill 2 LFG Emissions and Dispersion

• Measured emission rates
• EPA surface flux box sampling method
• 8 flux box samples
• Analyzed for VOCs and sulfur compounds
• ISCST3 dispersion model
• Modeled as four area sources (active cover,
  interim cover, final cover, 1 fissure)
• Evaluated 15 nearby residential locations

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Landfill 3 LFG Emissions

• Modeled emission rates
• EPA LANDGEM model
• Calibrated to site-specific gas collection rates
• Assumed gas collection efficiency (90)
• Site-specific LFG concentrations
• Calibrated H2S emission rate using
  surface air measurements
• Jerome H2S analyzer
• 94 surface measurements

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Landfill 3 H2S Surface Emission Rates

• Measured 0.004 ppm
• Modeled 0.7 ppm (simple box model)
• Modeled concentrations 175 times gt measured
  concentrations
• Reduced H2S emission rate by scaling factor of
  100

4
12
Landfill 3 LFG Dispersion Modeling

• ISCST3 Dispersion model
• Modeled as 3 area sources

• Calculated average concentrations in 3
  surrounding areas

13
Landfill 4 LFG Emissions

• Measured emission rates
• EPA surface flux box sampling method
• 18 flux box samples
• Analyzed for VOCs and sulfur compounds

14
Landfill 4 LFG Dispersion Modeling

• ISCST3 Dispersion model
• Modeled as 4 area sources

• Calculated average
  concentrations in 2 nearby residential areas

15
Landfill 5 LFG Emissions and Dispersion

• Vinyl chloride only
• Modeled emission rates
• EPA LANDGEM model
• Initial Default inputs
• Refinement
• Site-specific calibration compared measured
  versus modeled ambient air concentrations at
  downwind sampling locations
• Reduced all emission rates by factor of 2.5
• FDM dispersion model
• Modeled as 4 area sources
• Evaluated residential locations within 1 km

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Landfill 5 - Model Calibration
Source RWDI 1999
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Case Study Risk Assessment Results

• Solid, non-hazardous waste landfills equipped
  with LFG collection and treatment systems
• Used variety of methods to calculate landfill gas
  emission rates
• Long-term and short-term risks associated with
  LFG compounds below regulatory and risk-based
  target levels
• Excess lifetime cancer risks 3x10-9 1x10-6
• Noncancer hazard index values 0.0005 0.1
• Surface area emissions dominated inhalation risk
  results

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Modeling LFG Emission Rates - Uncertainties

• Landfill gas generation models can overestimate
  emissions
• Gas generation model does not take into account
  cover effects
• Model based on methane NMOC releases may not be
  correlated with methane production
• Defining LFG concentrations
• Gas collection efficiency

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Conclusions and Future Research

• Tiered approach - tailor to project scope
• Hierarchy
• Measurement (flux box)
• Modeling with site-specific calibration
• Modeling with some site-specific data
• Research areas
• Attenuation effects of landfill gas covers
• Validation of landfill gas generation and
  emission models
• Validation of modeled off-site concentrations
  using measured ambient air data

Increasing uncertainty