Comment by Prof. Nickolas J. Themelis, Earth Engineering Center, Columbia University to EPA Docket: EPA-HQ-OAR-2019-0424

We have studied extensively methane generation in landfills and we are in agreement with increasing the first order decay rate (k) for biodegradation. However, there is no reason to decrease the degradable organic carbon (DOC) content of MSW landfilled throughout the country. A 2010 study (https://ascelibrary.org/doi/abs/10.1061/%28ASCE%29EE.1943-7870.0000189?download=true) of the biogenic (i.e. biodegradable) carbon in the stack gas of 40 U.S. waste-to-energy power showed the DOC to be 0.2 (20%) across the U.S., i.e., the same as the current default value of DOC used by EPA. The U.S. MSW landfills receive the same MSW as the waste-to-energy plants and inert waste that should go to C&D landfills and not to MSW landfills.

 

The major problem in the EPA estimate of methane emission from U.S. landfills (114 million metric tons of CO2 equivalent; https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gasemissions- and-sinks) is that the tonnage of MSW landfilled in 2018 is grossly underestimated by EPA, at 146.1 million short tons (132.8 million metric tons; (www.epa.gov/facts-and-figuresabout- materials-waste-and-recycling). However, Themelis and Bourtsalas (Journal of Environmental Science and Engineering A 10, p.199-206; 2021) conducted a study of the 1,164 U.S. operating landfills, using the data provided by the landfill operators to the government. Analysis of the 2018 data showed that in fact 348 million short tons (316 million metric tons) were disposed in MSW landfills. This number is 138% higher than the EPA estimate of 146.1 million short tons and results in the underestimate of methane emissions of U.S. landfills.

 

Also, the Columbia University study of U.S. operating landfills ((Themelis and Bourtsalas, Journal of Environmental Science and Engineering A 10, p.199-206; 2021) showed that, after subtraction of methane captured by landfills in EPA’s Landfill Methane Outreach Program (LMOP: 5.1 million short tons of methane), the U.S. operating landfills in 2018 emitted 11.9 million short tons of methane. On a 100-year horizon, the MSW landfill emissions were equivalent to 270 million metric tons of carbon dioxide, i.e., 85% higher than the EPA estimate.

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