Jae K. (Jim) Park Dept. of Civil and Environmental Engineering University of Wisconsin-Madison 1

 Gas Migration Pathways: pressure gradients and diffusion processes  Methane gas > 5~15% by vol.: explosive  Create vegetation stress and odor problems  Toxicity: H 2 S > 20 ppm; benzene and toluene  Corrosion: acidic condition Refuse Thick cover will impede gas release and increase gas pressure Barrier: pavement, frost, saturated soil Barrier: clay strata Permeable strata Frost cover Gravel lens Trenches and conduits Water table generally acts as a barrier 2

 Resource Conservation Recovery Act (RCRA) (1980)  The combustible gas concentration shall not exceed 5% at or beyond the property boundary.  Natural barriers: moist fine-grained soils and saturated coarse-grained soils  Constructed barriers such as trenches, membranes, wells, and vents:  Passive systems: barriers and vents or refuse and adjacent media with large differences in conductivity  Active systems: induce a vacuum to control the flow of gas; preferred when the refuse age is 10 m, and development to be protected is < 1.5 km away  Selection: site specific - economics, degree of protection required, and system reliability 3

4 Approximate zone of influence Condensate trap Gas monitoring probes Landfill boundary Gas header Blower station Gas headerGas well Landfill gas extraction well

5 Perforated gas collection pipe Gas extraction trench Gas monitoring probes Gas collection pipe Condensate trap Blower station Backfilled trench Synthetic membrane liner Landfill boundary

 LFG: saturated with water vapor  Temperature in the gas collection pipe system drops 6 LFG containing water vapor Droplets formed Temp. Condensate Pressure buildup Blocking of flow Needs condensate trap

7 Environmental regulations prohibit the return of condensate to the landfill. Condensate

8 Recommended

 Based on hazardous characteristics tests for ignitability and TCLP toxicity, LFG condensate and particularly the organic phase may be a hazardous waste.  Some priority pollutants were detected in the organic phase that exceed the proposed regulatory limits. 9 Organic phase (float) (< 1~5% by vol.) Aqueous phase Hydrocarbons, other priority organics, trace moisture Water and trace organics EPA 1988:

UnpublishedDomesticTypical Parameterdata EPA (1988)sewageleachate mg/Lmg/Lmg/Lmg/L BOD1,000~31,2504,000~17,500100~5001,050~32,400 COD476~14,7201,042~30,500250~1,000800~50,700 TOC575~4,90094~23,500100~300700~68, EPA 1988: Comparison of BOD, COD, and TOC values for condensate, domestic sewage, and landfill leachate

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12 Landfill Impermeable liner system Leachate collection system Gravel packed well Blower station Impermeable cover system Landfill gas flared or converted to energy

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 Moisture content: moisture content   permeability and diffusion coefficient  K g = f(  ) and K w = f(  ) where:n =  +  = total porosity; K g = effective K for the gas mixture (cm/sec); and K w = effective K for the soil moisture (cm/sec).  Absolute viscosity: temp.   viscosity  for low density gases Temp. adjustment: Soil condition adj.: D d o = unobstructed diffusion coefficient (cm 2 /sec); and D d = effective diffusion coefficient (cm 2 /sec). (Millington and Quirk, 1961) (Farmer et al., 1980) 14

 Methane: colorless, odorless gas, less dense than air, combustible when > 5% by vol.  CO 2 : denser than air, not combustible, acidic conditions  release of alkaline earth metals from soil materials and production of a hardness halo downgradient from the landfill site CH 4 CO 2 Landfill gasAir Viscosity (  pa  s) Mass density,  (kg/m 3 ) Molecular mass, M (g) Diffusion coefficient in air, D’ (m 2 /sec)1.57  Note: Landfill gas defined as 50% CH 4 and 50% CO 2 ; all properties reported at 1 atm and 0  C. 15

 Affect gas migration pathways as well as gas velocity and spreading  Grain size, stratification, and spatial variations  Crack or fissure formation, soil pore water content, and intrusion such as an excavation  Hydraulic conductivity of the gas, K g (m/sec) where  = weight density of the gas (N/m 3 ) =  g;  = absolute viscosity (Pa  sec); k s = intrinsic permeability (m 2 ); and g = gravitational constant (m/sec 2 ).  k s : vary by between clay and gravel - needs accurate assessment of soil conditions  Pressure: < 12 in. of water (3 kPa) - incompressible, Darcy’s law to calculate gas flow 16

 400~1600 gal. of condensable liquids per million ft 3 of gas  water and various hydrocarbons (dichloromethane, dichloroethane, trichloroethylene, toluene, acetone, xylenes, vinyl chloride, and benzene)  Young and Parker (1983) - no significant hazard due to the trace gases from domestic wastes  Odors are worst during the first year after deposition, and organosulfurs and esters play particularly important roles.  Collection and ultimate flaring of landfill gas - min. combustion temp. of 1,500  F and a residence time of 0.3 to 0.5 sec. 17

 Very large quantities of air are introduced into the landfill in a localized area, either through natural occurrence or overly aggressive operation of the LFG system  a poorly supported subsurface combustion  CO detected and landfill temp. increase to 400~500°F.  Result from short-circuiting air intrusion into:  Landfill/cover soil interface  Cracks, breaks or imperfections in the cover/cap  Breaks in buried collection piping and extraction wells  Backfill surrounding collection system components (e.g., from the filter or gravel pack of an extraction well or the gravel backfill around a sump)  Subsurface fires: difficult to control or extinguish once initiated 18

 Monitor carbon monoxide and temperature for testing potential presence of a landfill fire.  CO: byproduct of incomplete combustion - good indicator  Measurement Diffusion tubes, gas analyzers, gas chromatographs, or aerial thermal imaging  Temperatures within the landfill should be stabilized at or below 140°F and maintained as low as possible for as long as possible. 19

 Starve the fire of oxygen - seal all sources of air intrusion - reduce the rate of extraction of LFG or shut down the LFG control system in and adjacent to the affected mass.  Carefully use heavy equipment in areas affected by subsurface fires to avoid a very dangerous hazard that may result from sudden, rapid subsidence (cave-in).  In extreme cases, supply CO 2 to lower temperatures in the area of the fire and thus arresting the chemical pyrolysis that sustains the fire. Applicable for only small fires.  Applying water to subsurface landfill fires is not recommended. Ineffective against CH 4 gas fires. Use along with a surfactant or ‘wet water’ (fire fighting trade). Never attempt to fight an above ground methane fire with water.  Consider landfill fire as a special emergency situation 20