Life-Cycle Analysis of Energy and Greenhouse Gas Emissions from Anaerobic Biodegradation of Municipal Solid Waste

Abstract

Energy requirements and greenhouse gas (GHG) emissions for current landfilling of municipal solid waste (MSW) was compared to potential biodegradation of MSW in anaerobic digesters (AD) throughout the United States. A hybrid life-cycle analysis was completed to assess the potential for anaerobic biodegradation of MSW to methane, a valuable energy source. Conversion of MSW to methane in AD would generate a form of renewable energy, reduce GHG emissions, and save landfill space for nonbiodegradable materials. Based on laboratory- and pilot-scale studies conducted in the United States, full-scale data from facilities in Europe, and economic input-output life-cycle analysis, the annual 127 million t of MSW landfilled in the United States could be biologically converted to $5.9\text{billion}{\text{m}}^3$ of methane. Net methane production would have an estimated value of $1.5 billion/year when converted to an equivalent amount of electricity at an assumed value of $0.1/kWh. The 15 billion kWh/year of renewable energy released through the biodegradation process is estimated to satisfy the annual consumption of 1.3 million United States households. The analysis also suggests that diversion of MSW from landfills to AD systems would result in GHG emissions reductions of 146 million t ${\text{CO}}_2\text{e}$ per year, due to decreased landfill activity and use of biogenic methane instead of fossil fuel for electricity production. This represents a reduction in total emissions of 1.9% compared to U.S. GHG emissions in 2006. Nationwide AD systems are projected to reduce cumulative energy consumption by nearly 15 million TJ and reduce GHG emissions by 7.2 billion t ${\text{CO}}_2\text{e}$ , over a 50-year period. Logistics and capital costs of developing a nationwide reactor-based system for MSW management are considerable. Development of appropriate national policy and incentives would be needed to stimulate such a transition from the current landfill-based system that currently exists. It is estimated that a carbon emissions credit on the order of $30 to $60/t ${\text{CO}}_2\text{e}$ would facilitate break-even economics for nationwide implementation of AD systems. Alternatively, renewable energy credits would enhance the value of electricity produced from AD biogas. Carbon emissions taxes on landfills would further improve the economics of AD systems.