Changes in Landfill Gas Quality as a Result of Controlled Air Injection JON POWELL, † PRADEEP JAIN, † HWIDONG KIM, † TIMOTHY TOWNSEND,* ,† AND DEBRA REINHART ‡ Department of Environmental Engineering Sciences, University of Florida, PO Box 116450 Gainesville, Florida 32611-6450, and Department of Civil and Environmental Engineering, University of Central Florida, PO Box 162450 Orlando, Florida 32816-2450 Air addition has been proposed as a technique for rapid stabilization of municipal solid waste (MSW) in landfills. The objective of this study was to observe the change in concentration of trace constituents of landfill gas in response to air addition. Air injection tests were conducted at a MSW landfill in Florida, and the concentrations of several gaseous constituents at adjacent wells within the waste were measured. The concentrations of methane, carbon dioxide, and oxygen, as well as several trace constituents, were measured both prior to and during air addition. The trace components investigated included a suite of volatile organic compounds (VOCs), nitrous oxide (N 2 O), carbon monoxide (CO), and hydrogen sulfide (H 2 S). A significant increase in CO was observed in 9 of 14 monitoring points; overall, CO concentrations were found to increase as the ratio of CH 4 to CO 2 decreased. A significant decrease in H 2 S was observed at 6 of 14 monitoring points. Air injection did not have a noticeable affect on VOC or N 2 O concentrations compared to initial levels. Introduction Gas emissions are a concern that must be addressed in the design and operation of landfills. In addition to the dominant gases that are emitted from municipal solid waste (MSW) landfills (methane (CH4) and carbon dioxide (CO2)), trace components, both organic (nonmethane organic compounds (NMOCs)) and inorganic (carbon monoxide (CO), hydrogen sulfide (H2S), and nitrous oxide (N2O)) are also a concern. NMOCs are a class of compounds that includes hazardous air pollutants (HAPs) and compounds that deplete strato- spheric ozone and contribute to the greenhouse effect (1). CO plays a role in the tropospheric formation of ozone, an important greenhouse gas (2), and can have a deleterious effect on human health (3). Additionally, the presence of CO in landfills may be an indication of subsurface fire (1). Like NMOCs, N2O contributes to the greenhouse effect, with a global warming potential approximately 300 times that of CO2 (4). H2S can cause odor problems as well as negative human health effects if inhaled (5). MSW landfills in the United States are traditionally designed and operated in a manner that minimizes liquid infiltration in order to reduce leachate production. Operation of a landfill as a bioreactor, on the other hand, promotes the stabilization of waste through the addition of moisture (anaerobic bioreactors) or air and moisture (aerobic biore- actors). The amount and composition of gas emissions expected from MSW landfills are well documented, and the U.S. Environmental Protection Agency (EPA) has developed regulations regarding management of these systems (6). Operating a landfill as an anaerobic bioreactor increases gas production relative to that of conventional MSW landfills (7, 8), and the EPA has recently promulgated regulations to address this concern (6). However, the emission of trace gas constituents from aerobic bioreactors has not been suf- ficiently explored. If aerobic bioreactors are to be widely implemented, the effect of aerobic decomposition on the concentration of gaseous compounds that pose a risk to human health and the environment must be investigated. The objective of this study was to evaluate the impact of air addition on the concentration of trace constituents present in landfill gas. The concentration of several volatile organic compounds (VOCs), N2O, H2S, and CO in landfill gas was measured prior to and during the addition of air to recently landfilled solid waste. The results provide insight to the potential changes in concentration of these compounds as a result of air addition to landfills; such data are needed to guide future regulations pertaining to aerobic bioreactor technology and to develop strategies for monitoring and control of these operations. Background Investigators have shown that faster reaction kinetics can be achieved with aerobic decomposition of MSW compared to those of anaerobic decomposition (9), which means the waste can reach a more stabilized state (i.e., less biologically active) more quickly. In addition to faster reaction kinetics, aerobic decomposition offers other potential benefits such as miti- gation of CH4 generation, with CO2 and water vapor as the primary end products (5, 10, 11). Currently, conventional MSW landfills with an actual or design capacity greater than 2.5 million Mg of waste (or 2.5 million m 3 ) are subject to New Source Performance Standards and Emission Guidelines (EG/ NSPS) for air emissions. These guidelines require the collection of landfill gas and 98% (wt %) destruction of NMOCs or reduction of outlet NMOCs to 20 ppm (as hexane by volume, dry basis) at less than or equal to 3% oxygen (1). New rules for the National Emission Standards for Hazardous Air Pollutants (NESHAP) concerning anaerobic bioreactors have been promulgated (6); these facilities must comply with gas collection requirements sooner than tra- ditional landfills. No similar rule specific to aerobic bioreactor landfills exists. The EPA addressed the subject of aerobic bioreactors in a Federal Register notice stating “an insufficient amount of aerobic landfill data is available to properly characterize HAP emissions from aerobic bioreactors” and that “portions of a landfill that are operated as aerobic bioreactors would continue to be subject to EG/NSPS and the final rule requirements for conventional landfills” (6). The bulk of studies investigating the aerobic landfill concept have focused on evaluating air addition and creating aerobic conditions within the waste (11, 12). Measurements of trace gas constituents such as benzene, toluene, and vinyl chloride from aerobic bioreactors are lacking. The source of trace gas constituents in MSW landfills is the volatilization of discarded chemicals or chemical-containing products as well as waste degradation (13). Several studies have reported the concentrations of VOCs in anaerobic MSW landfill gas (14-16). Additionally, researchers have examined the VOC * Corresponding author phone: (352)392-0846; fax: (352)392-3076; e-mail: ttown@ufl.edu. † University of Florida. ‡ University of Central Florida.