The U.S. Environmental Protection Agency (EPA) has released the 17th annual U.S. greenhouse gas inventory in which it revealed that landfills accounted for approximately 16.2% of total U.S. anthropogenic methane (CH4) emissions in 2010.
The EPA said that it prepares the annual report in collaboration with experts from multiple federal agencies and after gathering comments from stakeholders across the country.
The inventory tracks annual greenhouse gas emissions at the national level and presents historical emissions from 1990 to 2010. The inventory also calculates carbon dioxide emissions that are removed from the atmosphere by "sinks," e.g., through the uptake of carbon by forests, vegetation and soils.
According to the report, in 2010, landfill CH4 emissions were approximately 5.1 million tones of CH4 - or 107.8 million tones of CO2 equivalent - representing the third largest source of CH4 emissions in the U.S. behind natural gas systems and enteric fermentation (methane emissions from animals).
Emissions from municipal solid waste (MSW) landfills, which received about 69% of the total solid waste generated in the U.S. accounted for about 94% of total landfill emissions, while industrial landfills accounted for the remainder.
The report found that approximately 1900 operational landfills exist in the country, with the largest landfills receiving most of the waste and generating the majority of the CH4.
According to the EPA, while the number of landfills has decreased significantly over the past 20 years, from 6326 in 1990 to 1908 in 2009, the average landfill size has increased.
The report added that CH4 production in these landfills continues for between 10 and 60 years after the waste has been deposited at the site.
The estimated annual quantity of waste placed in MSW landfills increased from about 206 million tonnes in 1990 to 254 million tonnes in 2010 - an increase of 23%.
However, despite increased waste disposal, the amount of decomposable materials such as paper, food scraps, and yard trimmings deposited at MSW landfills has decreased by approximately 21% from 1990 to 2009. In addition, the amount of landfill gas collected and combusted has increased.
In 1990, for example, approximately 960,000 tonnes of CH4 were recovered and combusted - either for energy recovery or flared - from landfills, while in 2010, 7.6 million tonnes of CH4 was reported to have been combusted - a 5% increase from 2009 levels.
The report said that an estimated 54 new landfill gas-to-energy projects and 46 new flares began operation.
Over the next several years, the EPA said that the total amount of municipal solid waste generated is expected to increase as the U.S. population continues to grow.
However, according to the report the%age of waste landfilled, may decline due to increased recycling and composting practices.
In addition, the EPA said that it expected the quantity of CH4 that is recovered and either flared or used for energy purposes to continue to increase as a result of 1996 federal regulations that require large municipal solid waste landfills to collect and combust landfill gas as well as voluntary program such as the Landfill Methane Outreach Program (LMOP).
Waste to Energy Emissions
Meanwhile the incineration of waste accounted for some 12.1 million metric tonnes of C02 emissions - compared to 1745 million tonnes emitted by transportation. According to the report incineration is used to treat around 26.5 million tonnes of waste in the U.S. - or between 7% and 19% of the total solid waste generated in the country.
The report said that emissions of CO2 from the incineration of waste include CO2 generated by the incineration of plastics, synthetic fibres, and synthetic rubber, as well as the incineration of synthetic rubber and carbon black in tires.
These emissions were estimated by multiplying the amount of each material incinerated by the carbon content of the material and the fraction oxidised (98%). Various other assumptions were also applied to approximate a range of values for different wastes being incinerated.
Statistical analysis tools such as a Tier 2 Monte Carlo formula were also applied to the date to determine the level of uncertainty in the figures. The final calculation showed that the actual quantity of CO2 emitted by waste incineration was in the range of 9.6 to 14.9 million tonnes.
In addition to landfill and incineration, the report also showed that wastewater treatment and composting of organic waste accounted for approximately 2.5% and less than 1% of U.S. methane emissions, respectively.
Meanwhile, the final report shows overall emissions in 2010 increased by 3.2% from the previous year.
According to the EPA the trend is attributed to an increase in energy consumption across all economic sectors, due to increasing energy demand associated with an expanding economy, and increased demand for electricity for air conditioning due to warmer summer weather during 2010.
Total emissions of the six main greenhouse gases in 2010 were equivalent to 6.82 billion metric tonnes of carbon dioxide.
These gases include carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride. The report indicates that overall emissions have grown by over 10% from 1990 to 2010.
The Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2010 is the latest annual report that the U.S. has submitted to the Secretariat of the United Nations Framework Convention on Climate Change - which sets an overall framework for intergovernmental efforts to tackle the challenge posed by climate change.
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