Boundaries matter: Greenhouse gas emission reductions from alternative waste treatment strategies for California's municipal solid waste

How waste is managed – whether as a nuisance to be disposed of, or as a resource to be reused – directly affects local and global environmental quality. This analysis explores the GHG benefits of five treatment options for residual municipal solid waste (MSW) in California: Business As Usual (landfilling), Anaerobic Digestion, Incineration, 40% Reduction, and MaxEnergy (both incineration and anaerobic digestion). Because recycling efforts in California are already strong, this analysis focuses on non-recyclables and asks what else can be done with the material fractions that are currently reaching landfills. Using two different waste LCA models, EASEWASTE (a Danish model) and WARM (a U.S. model), we find that improved biogenic waste management through anaerobic digestion and waste reduction can lead to life-cycle GHG savings when compared to Business As Usual. The magnitude of the benefits depends strongly on a number of model assumptions: the type of electricity displaced by waste-derived energy, how biogenic carbon is counted as a contributor to atmospheric carbon stocks, and the landfill gas collection rate. Assuming that natural gas is displaced by waste-derived energy, that 64% of landfill gas is collected, and that our system boundary begins when waste is thrown away and ends with disposal or conversion to air emissions, reducing California's residual waste by 40% can lead to a savings of 6Mt (million metric tonnes) of CO₂-e per year, and digesting California's biogenic waste could save 0.6Mt CO₂-e per year. Source reduction is the most robust means to mitigate GHG emissions from waste, though either increasing landfill gas capture rates within the current management plan or digesting biogenic waste (and designing landfills to maximize carbon sequestration) provide two other important means for greenhouse gas mitigation from waste management.