into the atmosphere or the lateral and vertical movement through the surrounding soil. Recovered landfill gas can be used to produce energy or can be flared under controlled conditions to eliminate the discharge of harmful constituents to the atmosphere Composition and Characteristics of Landfill Gas andfill gas is composed of a number of gases that are present in large amounts(the principal gases)and a number of gases that are present in very small amounts(the trace gases). The principal gases are produced from the decomposition of the organic fraction of MSw. Some of the trace gases, although present in small quantities, can be toxic and could present risks to public health Principal Landfill Gas Constituents. Gases found in landfills include ammonia(NH3 ) carbon dioxide (CO2), carbon monoxide(Co), hydrogen(H2), hydrogen sulfide(H2S), methane(CHa), nitrogen(N3 and oxygen(O2). Data that can be used to determine the solubility of these gases in water(leachate are presented in Appendix F Methane and carbon dioxide are the principal gases produced from the anaerobic decomposition of the biodegradable organic waste components in MSw. When methane is present in the air in concentrations between 5 and 15 percent, it is explosive. Because only limited amounts of oxygen arc present in a landfill when methane concentrations reach this critical level, there is little danger that the landfill will explode. However, methane mixtures in the explosive range can form if landfill gas migrates off-site and mixes with air. The concentration of these gases that may be expected in the leachate will depend on their concentration in the gas phase in contact with the leachate. Because carbon dioxide will affect the ph of the leachate, carbonate equilibrium data can be used to estimate the pH of the leachate Trace Landfill Gas Constituents. The California Integrated Waste Management Board has performed an extensive landfill gas sampling program as part of its landfill gas characterization study. Summary data on the concentrations of trace compounds found in landfill gas samples from 66 landfills are reported in Table 15-1. In another study conducted in England, gas samples were collected from three different landfills and analyzed for 154 compounds. a total of 116 organic compounds were found in landfill gas. Many of the compounds found would be classified as volatile organic compounds (VOCs) The data presented in Table 15-1 are representative of the trace compounds found at most MSw landfills The presence of these gases in the leachate that is removed from the landfill will depend on their oncentrations in the landfill gas in contact with the leachate. Expected concentrations of these constituents in the leachate can be estimated using Henry's law as outlined in Appendix F. Note that the occurrence of significant concentrations of VOCs in landfill gas is associated with older landfills that accepted industrial and commercial wastes containing VOCs. In newer landfills. ' in which the disposal of hazardous waste has been banned, the concentrations of VOCs in the landfill gas have been extremely Generation of landfill gases The generation of the principal landfill gases, the variation in their rate of generation with time, and the sources of trace gases in landfills is considered in the following discussion Generation of the Principal Landfill Gases. The generation of the principal landfill gases is thought to occur in five more or less sequential phases. Each of these phases is described below Phase Iinitial adjustment. Phase I is the initial adjustment phase, in which the organic biodegradable omponents in MSW undergo microbial decomposition as they are placed in a landfill and soon after. In Phase I, biological decomposition occurs under aerobic conditions, because a certain amount of air is trapped within the landfill. The principal source of both the aerobic and the anaerobic organisms responsible for waste decomposition is the soil material that is used as a daily and final cover. Digested wastewater treatment plant sludge, disposed of in many MSw landfills, and recycled leachate are other sources of organisms Phase 11-transition phase. In Phase Il, identified as the transition phase, oxygen is depleted and anaerobic conditions begin to develop. As the landfill becomes anaerobic, nitrate and sulfate, which can serve as electron acceptors in biological conversion reactions, are often reduced to nitrogen gas and hydrogen sulfide. The onset of anaerobic conditions can be itored the oxidation/reduction potential of thewaste. Reducing conditions sufficient to bring about the reduction of nitrate and sulfate occur at about -50 to-100 millivolts. The production of methane occurs when the oxidation/reduction potential values are in the range from -150 to-300 millivolts. As the oxidation/reduction potential continues to decrease, members of the microbial community responsible for the conversion of the organic material in MSw to methane and carbon dioxide begin the three-step5 into the atmosphere or the lateral and vertical movement through the surrounding soil. Recovered landfill gas can be used to produce energy or can be flared under controlled conditions to eliminate the discharge of harmful constituents to the atmosphere. Composition and Characteristics of Landfill Gas Landfill gas is composed of a number of gases that are present in large amounts (the principal gases) and a number of gases that are present in very small amounts (the trace gases). The principal gases are produced from the decomposition of the organic fraction of MSW. Some of the trace gases, although present in small quantities, can be toxic and could present risks to public health. Principal Landfill Gas Constituents. Gases found in landfills include ammonia (NH3), carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), hydrogen sulfide (H2S), methane (CH4), nitrogen (N3), and oxygen (O2). Data that can be used to determine the solubility of these gases in water (leachate) are presented in Appendix F. Methane and carbon dioxide are the principal gases produced from the anaerobic decomposition of the biodegradable organic waste components in MSW. When methane is present in the air in concentrations between 5 and 15 percent, it is explosive. Because only limited amounts of oxygen arc present in a landfill when methane concentrations reach this critical level, there is little danger that the landfill will explode. However, methane mixtures in the explosive range can form if landfill gas migrates off-site and mixes with air. The concentration of these gases that may be expected in the leachate will depend on their concentration in the gas phase in contact with the leachate. Because carbon dioxide will affect the pH of the leachate, carbonate equilibrium data can be used to estimate the pH of the leachate . Trace Landfill Gas Constituents. The California Integrated Waste Management Board has performed an extensive landfill gas sampling program as part of its landfill gas characterization study. Summary data on the concentrations of trace compounds found in landfill gas samples from 66 landfills are reported in Table 15-1. In another study conducted in England, gas samples were collected from three different landfills and analyzed for 154 compounds. A total of 116 organic compounds were found in landfill gas. Many of the compounds found would be classified as volatile organic compounds (VOCs). The data presented in Table 15-1 are representative of the trace compounds found at most MSW landfills. The presence of these gases in the leachate that is removed from the landfill will depend on their concentrations in the landfill gas in contact with the leachate. Expected concentrations of these constituents in the leachate can be estimated using Henry's law as outlined in Appendix F. Note that the occurrence of significant concentrations of VOCs in landfill gas is associated with older landfills that accepted industrial and commercial wastes containing VOCs. In newer landfills.' in which the disposal of hazardous waste has been banned, the concentrations of VOCs in the landfill gas have been extremely low. Generation of Landfill Gases The generation of the principal landfill gases, the variation in their rate of generation with time, and the sources of trace gases in landfills is considered in the following discussion. Generation of the Principal Landfill Gases. The generation of the principal landfill gases is thought to occur in five more or less sequential phases. Each of these phases is described below. Phase I—initial adjustment. Phase I is the initial adjustment phase, in which the organic biodegradable components in MSW undergo microbial decomposition as they are placed in a landfill and soon after. In Phase I, biological decomposition occurs under aerobic conditions, because a certain amount of air is trapped within the landfill. The principal source of both the aerobic and the anaerobic organisms responsible for waste decomposition is the soil material that is used as a daily and final cover. Digested wastewater treatment plant sludge, disposed of in many MSW landfills, and recycled leachate are other sources of organisms. Phase 11—transition phase. In Phase II, identified as the transition phase, oxygen is depleted and anaerobic conditions begin to develop. As the landfill becomes anaerobic, nitrate and sulfate, which can serve as electron acceptors in biological conversion reactions, are often reduced to nitrogen gas and hydrogen sulfide. The onset of anaerobic conditions can be monitored by measuring the oxidation/reduction potential of thewaste. Reducing conditions sufficient to bring about the reduction of nitrate and sulfate occur at about -50 to -100 millivolts. The production of methane occurs when the oxidation/reduction potential values are in the range from -150 to -300 millivolts. As the oxidation/reduction potential continues to decrease, members of the microbial community responsible for the conversion of the organic material in MSW to methane and carbon dioxide begin the three-step