compounds. Half-lives varying from a fraction of a year to over a thousand years have been reported for various compounds Management of landfill gas Typically, landfill gases that have been recovered from an active landfill are either flared or used for the recovery of energy in the form of electricity, or both. More recently, the separation of the carbon dioxide from the methane in landfill gas has been suggested as an alternative to the production of heat and electricit Flaring of Landfill Gases. A common method of treatment for landfill gases is thermal destruction; that methane and any other trace gases(including VOCs)are combusted in the presence of oxygen (contained in air) to carbon dioxide(co2), sulfur dioxide(So2), oxides of nitrogen, and other related gases. The thermal destruction of landfill gases is usually accomplished in a specially designed flaring operating specifications to ensure effective destruction of VOCs and other similar compounds that lip facility. Because of concerns over air pollution, modem flaring facilities are designed to meet rigor be present in the landfill gas. For example, a typical requirement might be a minimum combustion temperature of 1500.F and a residence time of 0.3 to 0.5 s, along with a variety of controls and instrumentation in the flaring station Landfill Gas Energy Recovery Systems. Landfill gas is usually converted to electricity In smaller installations(up to 5 MW), it is common to use dual fuel internal combustion piston engines or gas turbines. In larger installations, the use of steam turbines is common Where piston-type engines are used, the landfill gas must be processed to remove as much moisture as possible so as to limit damage to the cylinder heads. If the gas contains H2S, the combustion temperature must be controlled carefully to avoid corrosion problems. Alternatively, the landfill gas can be passed through a scrubber containing iron shavings, or through other proprietary scrubbing devices, to remove the H2s before the gas is combusted Combustion temperatures will also be critical where the landfill gas contains VOCs released from wastes placed m the landfill before the disposal of hazardous waste al landfills was banned. The typical service cycle for dual fuel engines running on landfill gas varies from 3000 to 10,000 hours before the engine must be overhauled. The typical service cycle for gas turbines running on landfill gas Gas Purification and Recovery. Where there is a potential use for the COz contained in the landfill gas the CH4 and COz in landfill gas can be separated. The separation of the COz from the CH4 can be accomplished by physical adsorption, chemical adsorption, and by membrane separation In physical and hemical adsorpdon, one component is adsorbed preferentially using a suitable solvent. Membrane eparation involves the use of a semipermeable membrane to remove the CO2 from the Ch Semipermeable membranes have been developed that allow COz, H2S, and H2o to pass while CHa is retained. Membranes are available as flat sheets or as hollow fibers. To increase efficiency of separation, the flat sheets are spiral wound on a support medium while the hollow fibers are grouped together in bundles 15-3 Composition, formation and control of leachate in landfills Leachate may be defined as liquid that has percolated through solid waste and has extracted dissolved or suspended materials. In most landfills leachate is composed of the liquid that has entered the landfill from external sources, such as surface drainage, rainfall, groundwater, and water from underground springs and the liquid produced from the decomposition of the wastes, if any. The composition formation, movement, and control of leachate arc considered in this section Composition of leachate When water percolates through solid wastes that are undergoing decomposition, both biologica materials and chemical constituents are leached into solution. Representative data on the characteristics of leachate are reported in Table 15-2 for both new and mature landfills. Because the range of the observed concentration values for the various constituents reported in Table 15-2 is rather large, especially for new landfills, great care should be exercised in using the typical values that are given8 compounds. Half-lives varying from a fraction of a year to over a thousand years have been reported for various compounds. Management of Landfill Gas Typically, landfill gases that have been recovered from an active landfill are either flared or used for the recovery of energy in the form of electricity, or both. More recently, the separation of the carbon dioxide from the methane in landfill gas has been suggested as an alternative to the production of heat and electricity. Flaring of Landfill Gases. A common method of treatment for landfill gases is thermal destruction; that is, methane and any other trace gases (including VOCs) are combusted in the presence of oxygen (contained in air) to carbon dioxide (CO2), sulfur dioxide (SO2), oxides of nitrogen, and other related gases. The thermal destruction of landfill gases is usually accomplished in a specially designed flaring facility. Because of concerns over air pollution, modem flaring facilities are designed to meet rigorous operating specifications to ensure effective destruction of VOCs and other similar compounds that may be present in the landfill gas. For example, a typical requirement might be a minimum combustion temperature of 1500°F and a residence time of 0.3 to 0.5 s, along with a variety of controls and instrumentation in the flaring station. Landfill Gas Energy Recovery Systems. Landfill gas is usually converted to electricity. In smaller installations (up to 5 MW), it is common to use dual fuel internal combustion piston engines or gas turbines. In larger installations, the use of steam turbines is common. Where piston-type engines are used, the landfill gas must be processed to remove as much moisture as possible so as to limit damage to the cylinder heads. If the gas contains H2S, the combustion temperature must be controlled carefully to avoid corrosion problems. Alternatively, the landfill gas can be passed through a scrubber containing iron shavings, or through other proprietary scrubbing devices, to remove the H2S before the gas is combusted. Combustion temperatures will also be critical where the landfill gas contains VOCs released from wastes placed m the landfill before the disposal of hazardous waste in municipal landfills was banned. The typical service cycle for dual fuel engines running on landfill gas varies from 3000 to 10,000 hours before the engine must be overhauled. The typical service cycle for gas turbines running on landfill gas is approximately 10,000 hours. Gas Purification and Recovery. Where there is a potential use for the CO2 contained in the landfill gas, the CH4 and CO2 in landfill gas can be separated. The separation of the CO2 from the CH4 can be accomplished by physical adsorption, chemical adsorption, and by membrane separation. In physical and chemical adsorpdon, one component is adsorbed preferentially using a suitable solvent. Membrane separation involves the use of a semipermeable membrane to remove the CO2 from the CH4. Semipermeable membranes have been developed that allow CO2, H2S, and H2O to pass while CH4 is retained. Membranes are available as flat sheets or as hollow fibers. To increase efficiency of separation, the flat sheets are spiral wound on a support medium while the hollow fibers are grouped together in bundles. 15-3 Composition, formation and control of leachate in landfills Leachate may be defined as liquid that has percolated through solid waste and has extracted dissolved or suspended materials. In most landfills leachate is composed of the liquid that has entered the landfill from external sources, such as surface drainage, rainfall, groundwater, and water from underground springs and the liquid produced from the decomposition of the wastes, if any. The composition, formation, movement, and control of leachate arc considered in this section. Composition of Leachate When water percolates through solid wastes that are undergoing decomposition, both biological materials and chemical constituents are leached into solution. Representative data on the characteristics of leachate are reported in Table 15-2 for both new and mature landfills. Because the range of the observed concentration values for the various constituents reported in Table 15-2 is rather large, especially for new landfills, great care should be exercised in using the typical values that are given