AREA A/B ENGINEERING REPORT - Waste Management

Geosyntec Consultants
various practitioners in the LFG industry in the 1980s and 1990s, as opposed to extensive field
test data. Similarly, the current AP-42 document also states that: “…average oxidation of
methane (on a volumetric basis) in some laboratory and case studies on landfill covers have indicated
ranges from 10 percent to over 25 percent with the lower portion of the range being found in clay
soils and higher in topsoils.” The USEPA thus recommended a conservative default factor of ten
percent methane oxidation for uncollected landfill gas that escapes through the cover. The
current AP-42 document makes no allowance for carbon sequestration in landfills.
The implications of the USEPA’s AP-42 default assumptions for gas collection efficiency, methane
oxidation, and carbon sequestration are profound because, in many cases, landfill
owner/operators and regulatory agencies calculate the collection system efficiency from the
actual volume of gas collected relative to the volume projected by LFG models for generation, for
example using the USEPA’s Landfill Gas Emissions Model, or LandGEM (USEPA, 2005). However,
LandGEM and other first-order decay models predict LFG generation using a variety of
conservative default inputs to the model, mainly because of difficulties in determining site-specific
values. As a result, LandGEM commonly overestimates LFG generation rates and, therefore, net
GHG emissions. This is examined in more detail in the remainder of Section B4.
B4.2.4 Gas Collection Efficiency
In more recent studies (cit. in SWICS, 2009), the collection efficiency of a gas collection system has
been demonstrated to be more proficient at reducing GHG emissions, where efficiencies are
dependent on the type of cover being used during the operation of the landfill. In evaluating
collection efficiency, it is important to recognize that the efficiency of a gas extraction system will
vary continuously while a landfill is in operation. While waste is received daily, gas extraction
systems can only be extended into newly filled sections of a landfill on a much less frequently
basis. Although an increasing number of landfills use horizontal trenches and other means of gas
extraction from active areas that allow for a reduced period between refuse placement and gas
recovery, the most practicable schedule expansion of a gas system is generally annually or even
biannually. Thus, at any time during the life of a landfill, there may be sections with only daily
cover and no gas extraction system, sections with intermediate soil cover and only limited gas
extraction, and areas with final cover and optimal placement of gas extraction wells. To address
this, SWICS (2009) provides collection efficiency values obtained from a comprehensive fieldtesting
program under different cover system types. Collection efficiencies are reported in the
range of 90-99 percent, with a mid-range default of 95 percent, for landfills that contain a final
soil and/or geomembrane cover system with an active LFG collection system. In other words, a
default collection efficiency of 95 percent should be assumed for landfills in the post-closure
phase with Subtitle D-compliant final cover systems.
In summary of the above, the USEPA’s default landfill gas collection system efficiency of only 75
percent in their AP-42 document is not supported by recent studies and field tests results provided
by SWICS (2009) which demonstrate that active gas collection systems at modern managed
landfill typically have a much higher collection efficiency that ranges from 90 to 99% for landfills
that contain a final soil and/or geomembrane cover system. These high gas collection efficiencies
MD10186.doc 134 29 March 2009