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Taking Your Digester Gas on the Road: The Case to Upgrade Digester Gas for Vehicle Fuel
Author: Eron Jacobson, Jim Schettler, Christopher Muller and Jing Luo
Date: 4/112
Preprint, Residuals and Biosolids 2012, March 25-28, 2012, Raleigh, NC

Often digester gas generated by anaerobic digestion at WWTPs is used to produce heat and electricity through cogeneration, but under the right market conditions digester gas may provide better economic and environmental value as a vehicle fuel. Electricity is relatively inexpensive in some parts of the country and the payback for the installation of cogeneration is not economically attractive. While the use of digester gas for anaerobic digester process and space heating in a boiler is still often inexpensive, not all of the digester gas is used throughout the year—especially during the spring and summer months. This leaves some or all of the digester gas available for an alternative end use. Two case studies—the City of Tacoma, Washington, and Pima County (Tucson), Arizona—are presented where digester gas end-use alternatives were evaluated and resulted in vehicle fuel production being identified as the most cost-effective alternative, over other more traditional end uses. Both of these locations have relatively low electricity costs (less than about $0.06 per kilowatt-hour). A quantity of digester gas sufficient to make 100 kilowatt-hours of electricity can displace roughly 23 to 26 liters of diesel fuel or 26 to 30 liters of gasoline for vehicle fuel. This paper discusses the technologies, life-cycle costs, and carbon emissions reductions that demonstrate that for these utilities, converting their digester gas to vehicle fuel was the best economical and a good environmental use of their resource. As the role of wastewater utilities expands from treatment to resource recovery, understanding the best use of those recovered resources will serve only to improve the long-term sustainability of operations, fiscal and environmental. This paper discusses the technical aspects of vehicle fuel production and the likely impacts to long-term operating costs and capital budgets, along with the benefits to plant operators, design engineers, and utility managers.