Landfill gas recovery The recent article on landfill gas recovery (“Is converting landfill gas to energy the best option?”, DOI 10.1021/es803266t) mixes several issues and, in so doing, does not do justice to a complex topic. It is indeed news that the Environmental Defense Fund (EDF) plans to sue EPA to tighten regulations governing the collection of landfill gas. However, by referring to landfills as dumps, the article creates a misimpression. Modern landfills cost several hundred thousand dollars an acre to construct. Today’s engineered landfills are anything but the dumps of decades past. The article attempts to address the question raised in the title, which is not a news item but a complex issue that demands detailed analysis. It is stated that the global warming potential of methane is 72 times that of CO2. This is correct for a 20-year time horizon, but a factor of 25, which reflects a 100-year time horizon, is typically used in analyses such as the one required to address the question in the title.
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Later, it is stated that the small amounts of methane generated during composting are oxidized. This is not consistent with published literature or the discussion during a recent workshop sponsored by the U.S. EPA on how to model both composting facilities and landfills. Methane (and N2O) emissions from composting will depend on facility design and operation. The article then indicates that methane collection efficiencies can be as low as 40-50%. This could be true for the period of time when a landfill has a permeable intermediate cover and limited gas well coverage. Equally true is that methane collection efficiencies can be as high as 90-99% for landfills with a final cover. It is more appropriate to consider lifetime weighted average gas collection efficiency. For a typical landfill, 75% is a reasonable estimate, recognizing that this value is uncertain and difficult to measure. It would have been more appropriate to consult with landfill owners to get their experience; consult the peer-reviewed literature for the limited available data; or state that the EPA, in its AP-42 database, assumes a value of 75%. To be sure, there are legitimate questions about the optimal man-
2009 American Chemical Society
Published on Web 04/29/2009
ner in which to manage food and yard waste (note that I have excluded paper, which is also an organic). The answers revolve around the relative emissions from composting facilities and landfills and the degree to which either finished compost or landfill gas is used beneficially. A complete life-cycle analysis, with transparent and defensible assumptions, is needed. The article concludes with the statement that landfill owners have a financial incentive to bring in more waste to increase revenue. Of course they do, but is not the same true for the composting industry? MORTON A. BARLAZ North Carolina State University
[email protected] The author notes that he receives financial support from the landfill industry. Environmental Science & Technology edits all Letters for length, punctuation, and clarification of references. Authors approve of changes prior to publication.
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May 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2995
