AREA A/B ENGINEERING REPORT - Waste Management

LONG-TERM BEHAVIOR OF MANAGED LANDFILLS
A1. OVERVIEW
A1.1 Degradation of Waste in Landfills
Appendix A
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The contents of MSW landfills have physical, chemical, and biochemical properties that change
over time as they degrade. Of these, anaerobic biochemical transformation processes are
typically of most significance in landfills (Kjeldsen, et al., 2003). A number of factors affect the
rate of anaerobic waste decomposition in landfills, and hence the rate and quality of landfill gas
(LFG) and leachate production. These factors include waste composition and biodegradability,
environmental factors (e.g., moisture content and distribution, pH and alkalinity, availability of
nutrients, and the presence of inhibitors to microbial activity), and operational and process-based
factors such as the physical state of the waste, addition of degradation enhancing additives, and
practices of liquid addition (Christensen & Kjeldsen, 1989; Barlaz, et al., 1990; Tchobanoglous, et
al., 1993).
The release of constituents from a solid into solution involves a number of interrelated transport
mechanisms that are either predominantly controlled by diffusion, or by percolation and kinetics.
Understanding the interrelations between waste degradation, the mechanisms by which waste
constituents are released into leachate or LFG, and the factors affecting them enables
identification of the various stages of waste decomposition. Based primarily on data from
laboratory lysimeters and test cell studies, biodegradation of MSW in landfills has traditionally
been considered to occur in five more or less sequential and predictable phases in which
biochemical transformation processes occur as described by Farquhar & Rovers (1973), Rees
(1980), Pohland & Harper (1986), Christensen & Kjeldsen (1989), Christensen, et al. (1992), and
others. The initial phases include Phase I (aerobic), Phase II (acid), Phase III (initial methanogenic)
and Phase IV (stable methanogenic). However, more recent research findings, including data
from field-based studies and full-scale landfills (e.g., Calmano, et al., 1993, Bozkurt, et al., 1999
and 2000; Revans, et al., 1999; Kjeldsen, et al., 2003), propose that the fifth (i.e., long-term)
phase be sub-divided into three separate phases – Phase V (methane oxidation), Phase VI (air
intrusion), and Phase VII (carbon dioxide or humic) – to better describe landfill behavior over the
very long-term. Figure A-1 provides a qualitative depiction of expected leachate and LFG
composition over the seven phases of waste decomposition.
MD10186.doc 105 29 March 2009