Landfill Biodegradation

In 2010, landfill CH4 emissions were approximately 107.8 Tg CO2 Eq. (5,135 Gg of CH4), representing the third largest source of CH4 emissions in the United States, behind natural gas systems and enteric fermentation. Emissions from municipal solid waste (MSW) landfills, which received about 69 percent of the total solid waste generated in the United States, accounted for about 94 percent of total landfill emissions, while industrial landfills
accounted for the remainder. Approximately 1,900 operational landfills exist in the United States, with the largest landfills receiving most of the waste and generating the majority of the CH4 (EPA 2010; BioCycle 2008, adjusted to include missing data from five states). While the number of landfills has decreased significantly over the past 20 years, from 6,326 in 1990 to 1,908 in 2009), the average landfill size has increased (EPA 2010).
After being placed in a landfill, waste (such as paper, food scraps, and yard trimmings) is initially decomposed by aerobic bacteria. After the oxygen has been depleted, the remaining waste is available for consumption by anaerobic bacteria, which break down organic matter into substances such as cellulose, amino acids, and sugars. These
substances are further broken down through fermentation into gases and short-chain organic compounds that form the substrates for the growth of methanogenic bacteria. These CH4-producing anaerobic bacteria convert the
fermentation products into stabilized organic materials and biogas consisting of approximately 50 percent biogenic carbon dioxide (CO2) and 50 percent CH4, by volume. Significant CH4 production typically begins one or two years
after waste disposal in a landfill and continues for 10 to 60 years or longer.
Methane emissions from landfills are a function of several factors, including: (1) the total amount of waste in MSW landfills, which is related to total waste landfilled annually; (2) the characteristics of landfills receiving waste (i.e., composition of waste-in-place, size, climate); (3) the amount of CH4 that is recovered and either flared or used for energy purposes; and (4) the amount of CH4 oxidized in landfills instead of being released into the atmosphere.
From 1990 to 2010, net CH4 emissions from landfills decreased by approximately 27 percent . This net CH4 emissions decrease can be attributed to many factors, including changes in waste composition, an increase in the amount of landfill gas collected and combusted, a higher frequency of composting,
and increased rates of recovery for degradable materials (e.g., paper and paperboard).
The estimated annual quantity of waste placed in MSW landfills increased from about 206 Tg in 1990 to 254 Tg in 2010, an increase of 23 percent (see Annex 3.14). Despite increased waste disposal, the amount of decomposable materials (i.e., paper and paperboard, food scraps, and yard trimmings) discarded in MSW landfills have decreased by approximately 21 percent from 1990 to 2009 (EPA 2010). In addition, the amount of landfill gas collected and combusted has increased. In 1990, for example, approximately 960 Gg of CH4 were recovered and combusted (i.e., used for energy or flared) from landfills, while in 2010, 7,627 Gg CH4 was combusted, which represents a 5 percent increase in the quantity of CH4 recovered and combusted from 2009 levels (see Annex 3.14). In 2010, an estimated 54 new landfill gas-to-energy (LFGTE) projects and 46 new flares began operation (EPA 2011).

Over the past 9 years, however, the net CH4 emissions have fluctuated from year to year, but a slowly increasing trend has been observed. While the amount of landfill gas collected and combusted continues to increase every year, the rate of increase in collection and combustion no longer exceeds the rate of additional CH4 generation from
the amount of organic MSW landfilled as the U.S. population grows. Over the next several years, the total amount of municipal solid waste generated is expected to increase as the U.S. population continues to grow. The percentage of waste landfilled, however, may decline due to increased recycling and composting practices. In addition, the quantity of CH4 that is recovered and either flared or used for energy purposes is expected to continue to increase as a result of 1996 federal regulations that require large municipal solid waste landfills to collect and combust landfill gas (see 40 CFR Part 60, Subpart Cc 2005 and 40 CFR Part 60, Subpart WWW 2005), voluntary programs that encourage CH4 recovery and use such as EPA’s Landfill Methane Outreach Program (LMOP), and federal and state incentives that promote renewable energy (e.g., tax credits, low interest loans, and Renewable Portfolio Standards).

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