AREA A/B ENGINEERING REPORT - Waste Management

A2.1 Summary of Supporting Body of Knowledge
A2.1.1 Long-Term MSW Leachate Generation
Geosyntec Consultants
Leachate is produced when the field moisture-holding capacity of the waste contained in the
landfill is exceeded. This occurs when the waste moisture deficit (the difference between the
waste moisture content at placement and field capacity) is exceeded. Leachate generation rates
are most greatly affected by the type and condition of the in-place engineered cover system at
the landfill. Four other factors affecting leachate production at a landfill (Rees, 1980) include: (i)
the water content of the waste when placed; (ii) the volume of leachate recirculation or other
liquids addition; (iii) the volume of liquids or sludges co-disposed with the waste; and (iv) waste
compaction and density.
From the above, it is clear that good landfill cover design is the most important limiting factor
controlling the amount of leachate generated at a site. For example, in a broad study cited by
Bonaparte, et al (2002a) that included 11 MSW and 26 hazardous waste landfill cells that had
a leachate collection system and cover systems including a geomembrane layer, it was found that
the rate of leachate generation decreased by approximately three orders of magnitude less than
ten years after closure. The above finding, coupled with the expected longevity of Subtitle D
liner systems (Othman, et al, 2002) and demonstrated improvement in leachate quality over time
(as liner performance gradually declines), therefore means that closed landfills relying on Subtitle
D compliant cover and liner systems to limit generation and emission of leachate can expect to
continue to be protective of HHE over the very long term (Pivato & Morris, 2005).
It should also be noted that new approaches to landfill operations and management have been
promulgated to promote long-term threat reduction through enhanced waste degradation (i.e.,
enhanced organic stability) rather than reduced infiltration and leachate generation in the PCC
period (see Wisconsin NR 514.07.9). This regulation uses methane and carbon dioxide
generation rates as a surrogate for waste decomposition (i.e., there is an expected direct
correlation between improving leachate quality and reduction in methane/carbon dioxide
generation rates with time, which is clearly a function of waste decay as illustrated in Figure A-1).
However, in most cases achieving the required waste degradation cannot realistically be attained
without consistently adding liquid to the refuse during operations and the post-closure period.
Therefore, implementing a waste degradation approach to managing the long-term impact
potential will require proactive landfill operations (e.g., leachate recirculation, bioreactor
operations, and/or alternative all-soil covers) to optimize the moisture content necessary for
enhanced waste degradation (ITRC, 2003 and 2006a). This performance-based PCC approach
will require maintaining optimum moisture contents in the waste mass while effectively managing
leachate and LFG generation until the landfill becomes stable.
A2.1.2 Characterization of MSW Leachate
MSW landfill leachate contains organic compounds (typically represented by BOD and COD),
inorganic ions and nutrients, and relatively low concentrations of heavy metals and volatile
organic compounds (VOCs). A large number of reviews of leachate composition from multiple
MD10186.doc 109 29 March 2009