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Page 12 of 18 Shang et al. Carbon Footprints 2025, 4, 24 https://dx.doi.org/10.20517/cf.2025.28

nationwide implementation of the "waste-free cities" initiative starting in 2018, which significantly reduced
landfill volumes. Chen et al. reported that methane emissions from wastewater treatment plants in China
[43]
increased 2.8-fold, from 0.5 million tons in 2006 to 1.37 million tons in 2019 . Ma et al. found that
[44]
methane emissions from wastewater treatment reached 2.2 million tons in 2010 . Wang et al. estimated
emissions from municipal wastewater treatment plants (WWTPs) at 0.26 million tons in 2019 . Using a
[45]
bottom-up approach, Zhao et al. estimated 1.17 million tons of methane emissions from wastewater
treatment plants in 2014, lower than the 1.667 million tons calculated in this study for 2018, as their analysis
[26]
[46]
excluded methane from sludge anaerobic digestion . According to Xu et al. and Zhao et al. , methane
[47]
emissions from livestock and poultry manure in China range from 1.95 to 4 million tons annually. Lan et al.
and Du et al. estimated total livestock methane emissions (including enteric fermentation) between 10 and
12 million tons during 2010-2020 [48,49] . In comparison, this study estimates methane emissions from manure
alone at 2.044-2.263 million tons per year. Research on methane emissions from agricultural cultivation has
primarily focused on rice paddies. Fewer studies have incorporated emissions from agricultural off-field fuel
use into the waste methane system. Table 2 summarizes the differences between this study and previous
research in terms of parameters, system boundaries, and key findings.

Overall, the methane emission estimates from the four waste sub-sectors in this study fall within a
reasonable range compared to the existing literature. Differences in emission trends largely stem from
variations in policy implementation, data accuracy, and system boundary definitions. As public awareness
of waste-related methane emissions and their economic implications grows, significant reductions could be
[42]
achieved under low-carbon development and related policy scenarios, as noted by Cai et al. . Moreover,
methane emissions from waste are strongly correlated with urban population density and agricultural
activity intensity, with high-emission regions concentrated in densely populated urban centers and areas of
intensive agriculture, consistent with findings by Wang et al. and Lu et al. .
[51]
[50]
Uncertainty analysis
The uncertainty in the results of this study primarily arises from data collection methods, the costs of
methane capture technologies, and the extent of policy enforcement. The methane emission factors for the
four sub-sectors analyzed are largely derived from IPCC inventories and relevant literature. These factors
may deviate from actual measurements, potentially leading to variations in the results. Moreover,
substantial differences in emission factors may exist among neighboring provinces. For example, in Hunan
and Guizhou, variations in terrain and farming practices result in differing methane emission factors for
agricultural cultivation and livestock farming. Such regional discrepancies can affect the spatial distribution
patterns of emission reductions. Currently, methane capture technologies for landfills are relatively mature.
However, technologies for capturing methane from urban sewage and agricultural waste have not yet been
widely adopted. The high costs of these technologies may delay policy implementation, thereby introducing
considerable uncertainty into quantitative assessments of policy benefits.

Limitations
This study integrates methane emissions from waste systems into a unified framework, focusing on the
emission characteristics and trends across four sub-areas linked to urban and agricultural activities.
However, beyond these four sub-areas, methane-producing waste also includes organic matter from
industrial production, kitchen waste from households, and garden waste. The potential for methane
emissions from these sources depends on technological pathways, treatment processes, and treatment
capacities, which may vary regionally. For instance, processes such as direct incineration of garden waste or
converting kitchen waste into feedstock through drying and oil refining do not produce methane. Due to
the complexity of these scenarios, they were not included in this study. In addition, this study accounts for
methane emissions from agricultural straw only when used as fuel, while excluding its use as a soil