Lesson 12: MSW Landfills

• Biological reactions (microbially mediated)
  • Mediate reduction
  • Decompose organic waste
• Chemical oxidation/reduction
  • Loss or gain of electrons/oxygen
• Gas production, movement, and diffusion
  • Bio activity and volatilization of waste constituents produce gas
  • Stoichiometric equation for breakdown of organic wastes:
    • 
    • Degradation rates, slow vs. long
  • Significance
    • Potential energy recovery of methane
    • Methane is a potent greenhouse gas
    • Explosive danger - migration to nearby structures
    • Health hazards associated with trace gases
    • Odor nuisance
  • Composition
    • Typically 40-60% methane
    • Balance primarily carbondioxide
    • Higher percetages of methane can result from CO₂ absorption by leachate
    • Trace gases - volatilization, daughter products,
• Liquid production , movement, and migration - hydraulics
• Leaching and Dissolution of organic & inorganics to the liquid phase
• Contaminant transport (advection, dispersion, diffusion)
• Settlement
  • Biological activity
  • Height of landfill
  • Initial compaction and specific weight
  • Characteristics of SW and cover materials
  • Age of LF
  • Decomposition of SW components
  • 90% of settlement occurs in first 5 yrs

Microbial Metabolism - anaerobic fermentation,

• Steps
  • Hydrolysis; substrate converted to a usable form, enzyme mediated
  • Conversion of step one products to identifiable intermediates
  • Intermediates converted to CO₂ and CH₄
• Organisms -
  • Group which hydrolyzes organic polymers and lipids to building blocks, fatty acids, amino acids, monosaccharides, etc
  • Acidogens or acid formers - ferment hydrolysis products to organic acids, acetic acid is most common.
    • Non-methanogenic, facultative and obligate anaerobic bacteria
  • Methanogen or methane formers -convert hydrogen and acetic acid produced by acidogens to CH₄ and CO₂, methane is insoluble
    • Can only use certain substrates,
  • Synotrophic relationship between methanogens and acidogens
    • Acidogens produce hydrogen, formate, acetate
    • Methanogens consume hydrogen, hydrogenase, low H2 partial pressures
    • Shifts equilibrium to right by removing end products
    • Interspecies hydrogen transfer
    • Methanogenic bacteria remove compounds that inhibit acidogen growth
    • Acid sticking conditions
    • Leachate recirculation, buffer addition

• Initial Adjustment - associated with placement
  • Aerobic decomposition until trapped O2 used up
  • Accumulation of moisture
  • Lag time until favorable stabilization conditions develop
• Transition Phase
  • Oxygen depleted and anaerobic conditions develop
  • ORP drops to negative values, <300 mV for methanogens
  • pH drops due to formation of organic acids
• Acid Phase
  • Acidogens kick in, continuous hydrolysis or liquification with fermentation of SW
  • Formation of organic acids and smaller amount of H2
  • Heavy metals mobilized due to decrease in pH
  • Possible removal of nutrients with leachate stream
• Methane Fermentation Phase
  • Methanogens become more predominant
  • Methane and acid production procedd simultaneously, acid production rate is reduced
  • pH rises to a more neutral value, carbonate system
  • Metals are complexed/preciptated
  • Gas production peaks
  • BOD, COD, and conductivity of leachate will decrease
• Maturation Phase
  • Nutrients limit growth
  • Conditions shift from active degradation to relative dormancy
  • Gas production drops (doesn’t stop)
  • O2 and oxidized species may reappear depending on capping technique
  • Humic like substance produced from continuing degradation

• Convective and diffusive forces drive movement
• May be present in sub-surface outside un-lined landfills
• Control concerns
  • Prevent migration
  • Relieve pressure build up on LF cap and liner
  • Prevent groundwater contamination
  • Prevent vegetative damage on surface of LF (methane)
  • Energy recovery
  • Minimize odor
  • Minimize air pollution
• Control techniques
  • Passive extraction systems
    • Driven by natural pressure gradient within LF
  • Active system
    • Design a vacuum to direct the flow and control gas flow

Gas Collection System (GCS)

• Extraction wells
• Vacuum Pumps
• Flare
   

  Landfill Gas Flare

Review: Text, Example Problem 4-4 (pp. 129-130)
Review: Text, Example Problem 4-6 (pp. 146-147)
Review: Text, Example Problem 11-9