maintained and repaired to en-hance drainage, that gas and leachate control systems be maintained and operated, and that the pollution detection system be monitored Reactions Occurring in Landfills. Solid wastes placed in a sanitary landfill undergo a number of simul taneous and interrelated biological chemical, and physical changes, which are introduced in this section. The various reactions are considered in greater detail in subsequent sections of this chapter Biological reactions. The most important biological reactions occurring in landfills are those involving the organic material in MSw that lead to the evolution of landfill gases and, eventually, liquids. The biological decomposition process usually proceeds aerob for some short period diately afte sitIon of the waste until the oxvgen initially present is depleted. During aerobic decomposition CO2 is the principal gas produced. Once the available oxvgen has been consumed, the decomposition becomes anaerobic and the organic matter is converted to co. CHa and trace amounts of ammonia and hydrogen sulfide. Many other chemical reactions are biologically mediated as well. Because of the number of interrelated influences, it is difficult to define the conditions that will exist in any landfill or ortion of a landfill at any stated time Chemical reactions. Important chemical reactions that o ithin the landfill include dissolution and suspension of landfill materials and biological conversion products in the liquid percolating through the waste, evaporation and vaporization of chemical compounds and water into the evolving landfill gas. sorption of volatile and semivolatile organic compounds into the landfilled material, dehalogenation and The dissolution of biological conversion products and other compounds, particularly of organic compounds, into the leachate is of special importance because these materials can be transported out of the landfill with the leachate. These organic compounds can subsequently be released into the atmosphere either through the soil (where leachate has move away from an unlined landfill) or from uncovered leachate treatment facilities. Other important chemical reactions include those between certain organic compounds and clay liners, which may alter the structure and permeability of the liner material. The interrelationships of these chemical reactions within a landfill are not well understood Physical reactions. Among the more important physical changes in landfills are the lateral diffusion of gases in the landfill and emission of landfill gases to the surrounding environment. movement o underlying soils. and settlement caused by consolidation and decomposition of landfilled material. Landfill gas movement and emissions are particularly impor considerations in landfill management. As gas is evolved within a landfill, internal pressure may build, ausing the landfill cover to crack and leak. Water entering the landfill through the leaking cover may enhance the gas production rate, causing still more cracking. Escaping landfill gas may carry trace carcinogenic and teratogenic compounds into the surrounding environment. Because landfill gas usually has a high methane content, there may be a combustion and/or explosion hazard. Leachate migration is another concern. As leachate migrates downward in the landfill, it may transfer compounds and materials to new locations where they may react more readily. Leachate occupies pore spaces in the landfill and in doing so may interfere with the migration of landfill gas Concerns with the landfilling of solid wastes Concerns with the landfilling of solid waste are related to(1) the uncontrolled release of landfill gases that might migrate off-site and cause odor and other potentially dangerous conditions.(2)the impact of the uncontrolled discharge of landfill gases on the greenhouse effect in the atmosphere, (3)the trolled reles to underlying groundwater or to surfa 4) the breeding and harboring of disease vectors in improperly managed landfills. and(5) the he environmental impacts associated with the release of the trace gases arising from the hazardous materials that were often placed in landfills in the past. The goal for the design and operation of a modern landfill is to eliminate or minimize the impacts associated with these concerns 15-2 Composition and Characteristics, Generation and Control of Landfill Gases A solid waste landfill can be conceptualized as a biochemical reactor, with solid waste and water as the major inputs, and with landfill gas and leachate as the principal outputs. Material stored in the landfill includes partially biodegraded organic material and the other inorganic waste materials originally place in the landfill. Landfill gas control systems are employed to prevent unwanted movement of landfill gas4 maintained and repaired to en- hance drainage, that gas and leachate control systems be maintained and operated, and that the pollution detection system be monitored. Reactions Occurring in Landfills. Solid wastes placed in a sanitary landfill undergo a number of simultaneous and interrelated biological, chemical, and physical changes, which are introduced in this section. The various reactions are considered in greater detail in subsequent sections of this chapter. Biological reactions. The most important biological reactions occurring in landfills are those involving the organic material in MSW that lead to the evolution of landfill gases and, eventually, liquids. The biological decomposition process usually proceeds aerobically for some short period immediately after deposition of the waste until the oxygen initially present is depleted. During aerobic decomposition CO2 is the principal gas produced. Once the available oxygen has been consumed, the decomposition becomes anaerobic and the organic matter is converted to CO2, CH4, and trace amounts of ammonia and hydrogen sulfide. Many other chemical reactions are biologically mediated as well. Because of the number of interrelated influences, it is difficult to define the conditions that will exist in any landfill or portion of a landfill at any stated time. Chemical reactions. Important chemical reactions that occur within the landfill include dissolution and suspension of landfill materials and biological conversion products in the liquid percolating through the waste, evaporation and vaporization of chemical compounds and water into the evolving landfill gas, sorption of volatile and semivolatile organic compounds into the landfilled material, dehalogenation and decomposition of organic compounds, and oxidation-reduction reactions affecting metals and the solubility of metal salts. The dissolution of biological conversion products and other compounds, particularly of organic compounds, into the leachate is of special importance because these materials can be transported out of the landfill with the leachate. These organic compounds can subsequently be released into the atmosphere either through the soil (where leachate has move away from an unlined landfill) or from uncovered leachate treatment facilities. Other important chemical reactions include those between certain organic compounds and clay liners, which may alter the structure and permeability of the liner material. The interrelationships of these chemical reactions within a landfill are not well understood. Physical reactions. Among the more important physical changes in landfills are the lateral diffusion of gases in the landfill and emission of landfill gases to the surrounding environment, movement of leachate within the landfill and into underlying soils, and settlement caused by consolidation and decomposition of landfilled material. Landfill gas movement and emissions are particularly important considerations in landfill management. As gas is evolved within a landfill, internal pressure may build, causing the landfill cover to crack and leak. Water entering the landfill through the leaking cover may enhance the gas production rate, causing still more cracking. Escaping landfill gas may carry trace carcinogenic and teratogenic compounds into the surrounding environment. Because landfill gas usually has a high methane content, there may be a combustion and/or explosion hazard. Leachate migration is another concern. As leachate migrates downward in the landfill, it may transfer compounds and materials to new locations where they may react more readily. Leachate occupies pore spaces in the landfill and in doing so may interfere with the migration of landfill gas. Concerns with the Landfilling of Solid Wastes Concerns with the landfilling of solid waste are related to (1) the uncontrolled release of landfill gases that might migrate off-site and cause odor and other potentially dangerous conditions, (2) the impact of the uncontrolled discharge of landfill gases on the greenhouse effect in the atmosphere, (3) the uncontrolled release of leachate that might migrate down to underlying groundwater or to surface waters, (4) the breeding and harboring of disease vectors in improperly managed landfills, and (5) the health and environmental impacts associated with the release of the trace gases arising from the hazardous materials that were often placed in landfills in the past. The goal for the design and operation of a modern landfill is to eliminate or minimize the impacts associated with these concerns 15-2 Composition and Characteristics, Generation and Control of Landfill Gases A solid waste landfill can be conceptualized as a biochemical reactor, with solid waste and water as the major inputs, and with landfill gas and leachate as the principal outputs. Material stored in the landfill includes partially biodegraded organic material and the other inorganic waste materials originally placed in the landfill. Landfill gas control systems are employed to prevent unwanted movement of landfill gas