Smaller anthropogenic sources of methane from the industrial, agricultural, and waste sectors are not covered in this study. The two largest sources excluded are rice cultivation and wastewater treatment. Methane emissions from land use, land use change, and forestry activities are also not covered.
Methane from wetland (paddy) rice cultivation accounts for about 2 percent of anthropogenic methane emissions in the United States (EPA, 2017b), and production is confined to a few regions of the country. Globally, however, rice cultivation is responsible for approximately 10 percent of total emissions.1
Methane emissions from rice cultivation are the result of anaerobic decomposition of organic material (i.e., methanogensis), which escapes to the atmosphere primarily by diffusive transport through the rice plants during the growing season, when soils are waterlogged. In the United States, a combination of Tier 1 and 3 methodologies (IPCC, 2006) is used to estimate methane emissions from rice cultivation (EPA, 2014). The activity data are represented by the rice paddy annual harvested area taken from the National Resources Inventory (USDA-NRCS, 2015). Most of the estimates use a Tier 3 approach, where a process-based model (DAYCENT) is used to simulate rice cultivation and associated methane emissions. In instances where DAYCENT cannot be used because of lack of representation of production systems (e.g., when rice is grown in rotation with crops that are not included in DAYCENT, such as vegetables and perennial/horticultural crops, and for organic soils), an Intergovernmental Panel on Climate Change (IPCC) Tier 1 emission factor is employed using a default base estimation rate. This estimation rate assumes a continuously flooded field with no organic amendments and scaling factors to represent water management regimes that differ from this base case, as outlined by IPCC (2006). According to EPA (2017b), extensive improvements have been planned to update time-series management data, which is expected to improve methane emission estimates by more accurately reflecting fertil-
izer rates, updated tillage and water management practices, organic amendments, and planting/harvesting dates. Uncertainties related to estimating emissions from rice cultivation (±28 percent in the GHGI) are largely due to their high spatial heterogeneity and data scarcity on field irrigation, soil properties, rice variety, and rice production systems. Improving data availability for the variables used in process-based models could significantly reduce uncertainties in emission estimates.
Methane from wastewater is currently estimated to contribute about 2 percent of anthropogenic methane emissions in the United States (EPA, 2017b). Wastewater channeled to septic systems and via sewers to centralized wastewater treatment facilities can produce methane as a result of methanogenesis when anaerobic conditions develop during transport and treatment steps. Methane can also be emitted due to fugitive emissions from anaerobic digesters or from anaerobic conditions developing in residual biosolids following land disposal. The principal factors used in current IPCC (2006) emission estimates are population, the estimated quantity of degradable organic carbon, assumed temperature, and the type of treatment system. For septic systems, the U.S. population is multiplied by the estimated fraction of wastewater treated in septic systems (about 19 percent) and an emission factor. For centralized systems, the organic carbon fraction of the wastewater is estimated using biochemical oxygen demand (an estimate of the amount of oxygen needed to degrade the waste aerobically, thereby avoiding methane emissions) and chemical oxygen demand (amount of material available for chemical oxidation). Then, 81 percent of the U.S. population is multiplied by factors indicating the amount of waste entering the system, the relative use of aerobic versus anaerobic treatment systems, and system-specific emission factors. In general, systematic accounting of degradable carbon to anaerobic pathways through the various process steps for individual treatment facilities is lacking, as is process-specific modeling for fugitive methane generation and emissions, leading to very high uncertainties regarding methane emissions from wastewater treatment. A limited body of recent research currently exists on methane production and emissions from wastewater systems in the United States. Bellucci (2010) concluded from a field and laboratory study of three wastewater plants in the Chicago area that IPCC (2006) methodologies yielded reasonable values when compared to field observations. However, many researchers point to very high uncertainties for methane emissions from these systems (−26 to +22 percent in the GHGI), as well as the paucity of field studies
of U.S. systems. Complications include fossil carbon, which is unavailable to anaerobic microbial pathways (Tseng et al., 2016), and/or sequestered carbon through wastewater treatment steps (Rosso and Stenstrom, 2008). In addition to direct emissions from sewers and during treatment steps, there can also be methane emissions from residual biosolids (Tian et al., 2009).
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