【正文】 ions as well as the characteristics of the refuse, the land?ll surface, and underlying soil. The quality of land?ll leachate is highly dependent upon the stage of fermentation in the land?ll, waste position, operational procedures, and codisposal of industrial wastes (Hoeks and Harmsen, 1980。 Balba, 1987). These steps are highly interdependent and include hydrolysis, acidogenesis, acetogenesis, and methanogenesis(Figure 1). Generally, the breakdown of organic matter in anaerobic ecosystems proceeds sequentially from the plex to the simple starting with the hydrolysis of plex particulate matter to simpler polymers like proteins, carbohydrates and lipids which are further hydrolyzed to yield biomonomers like amino acids, sugars, and high molecular fatty acids. Amino acids and sugars are converted into either intermediate byproducts (. propionic, butyric and other volatile acids) or directly fermented to acetic acid. High molecular fatty acids are oxidized to intermediate byproducts and hydrogen. Methane and carbon dioxide generation occurs primarily through acetate cleavage. Methane is also produced through carbon dioxide reduction with a land?ll environment, methane generation from the latter route is often limited by the lack of hydrogen which is consumed by sulfate reducers (Kasali, 1986). Qualitatively, land?ll gas is highly dependent on the deposition stage within the land?ll (Rovers and Farquhar, 1973。 McCarty 1981。 Nozhevnikova et al., 1992。 Environmental Impacts of Solid Waste Landfilling Abstract:Inevitable consequences of the practice of solid waste disposal in landfills are gas and leachate generation due primarily to microbial deposition, climatic conditions, refuse characteristics and landfilling operations. The migration of gas and leachate away from the landfill boundaries and their release into the surrounding environment present serious environmental concerns at both existing and new facilities. Besides potential health hazards, these concerns include, and are not limited to, fires and explosions, vegetation damage, unpleasant odors, landfill settlement, ground water pollution, air pollution and global warming. This paper presents an overview of gas and leachate formation mechanisms in landfills and their adverse environmental impacts, and describes control methods to eliminate or minimize these impacts. Keywords: landfill, solid waste disposal, biodegradation, gas and leachate generation, environmental impacts, control methods Solid waste disposal in landfills remains the most economic form of disposal in the vast majority of cases (Thompson and Zandi, 1975。 Cossu, 1989。 Wolfe, 1979。 Archer and Robertson, 1986。 Barlaz et al.,1989c). Under a stabilized methanogenic condition which is the stage of interest from a bene?cial recovery perspective, methane and carbon dioxide are by far the two principal ponents of land?ll gas and form more than 90% of the total gas and oxygen are normally present in small quantities primarily as a result of air entrapment during waste deposition, atmospheric air di?usion through the land?ll cover especially in the near surface layers, or air intrusion from negative land?ll pressure when land?ll gas is extracted. 3. Leachate Formation Mechanisms Leachate formation is the result of the removal of soluble pounds by the nonuniform and intermittent percolation of water through the refuse mass. Soluble pounds aregenerally encountered in the refuse at emplacement or are formed in chemical and biological processes. The sources of percolating water are primarily the precipitation,irrigation, and run o? which cause in?ltration through the land?ll cover。 Pohland et al., 1983). Many chemicals (. metals, aliphatics, acyclics, terpenes, and aromatics) have been detected in land?ll leachate from domestic, mercial, industrial, and codisposal sites. 4. Environmental Impacts Historically, land?lls were initiated largely as a result of a need to protect the environment and society from adverse impacts of alternative methods of refuse disposal such as openair burning, openpit dumping, and ocean dumping (Senior, 1990). Although land?lls eliminated some impacts of old practices, new ones arose, primarily due to gas and leachate formation. Besides potential health hazards, these concerns include ?res and explosions, vegetation damage, unpleasant odors, land?ll settlement, ground water pollution, air pollution and global warming. Fire and Explosion Hazards Although land?ll gas rich in methane provides an energy recovery opportunity, it has often been considered to be a liability because of its ?ammability, its ability to form explosive mixtures with air, and its tendency to migrate away from the land?ll boundaries by di?usion and advection. Di?usion is the physical process that causes a gas to seek a uniform concentration