AREA A/B ENGINEERING REPORT - Waste Management

A4. LONG-TERM TRENDS IN LANDFILL GAS GENERATION AND COMPOSITION
A4.1 Technical Synopsis
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• The primary factors affecting LFG generation are similar to those affecting waste
degradation;
• Methane generation from MSW landfills decreases after closure, typically peaking
about one year after cessation of waste placement before tapering off in exponential
form; and
• Concentrations of non-methane organic compounds (NMOC) in LFG at closed landfills
decrease at rates that are similar to, or in advance of, the rate of decline in methane
concentration.
Seminal Supporting References: Farquhar & Rovers (1973); Pohland & Harper, 1986;
Barlaz, et al, 1990 and 2004; SWANA, 2002; Sullivan & Michels, 2000; Soltani-Ahmadi,
2002; Sullivan, et al, 2004.
A4.2 Summary of Supporting Body of Knowledge
The primary factors affecting LFG generation are similar to those affecting waste degradation as
described in Section A1.1 (i.e., mass of waste, percentage of total organic material in the waste
when placed, as-placed moisture content of waste, and temperature). It is well documented (e.g.,
Pohland & Harper, 1986; Barlaz, et al, 1990) that methane generation from MSW landfills
decreases with waste age (i.e., after closure). Under normal conditions, LFG generation rates
typically reach a peak about one year after cessation of waste placement before tapering off in
exponential form. Although the total quantity of LFG that can be generated is fixed by the mass
and nature of the MSW in place, it is well known that the rate of LFG production can be
enhanced by liquid injection into a landfill (van Zanten & Sheepers, 1995; Sullivan & Stege,
2000; SWANA, 2002). In addition, a number of researchers (e.g., Hsin-Mei & Kuo, 2000; Green,
et al, 2000; Sullivan, et al, 2004) have demonstrated declining concentration trends for nonmethane
organic compounds (NMOC) in LFG at closed landfills under a wide range of conditions,
operational practices, and timeframes, and at rates that are similar to, or in advance of, the rate
of decline in methane concentration. This body of knowledge forms an important technical basis
for using the trends in LFG quantity and quality as an indicator that evaluation of LFG generation
(and associated pressure buildup) as the principal cause of uncontrolled surface emissions or
subsurface LFG migration may be based upon a first-order decay curve for LFG production after
closure.
The data used by USEPA to develop default values for NMOC production were collected mainly
during the 1980s (i.e., prior to RCRA’s hazardous waste exclusion rules) and have not been
MD10186.doc 115 29 March 2009