15 Disposal of solid Wastes and Residual matter le safe and reliable long-term disposal of solid waste residues is an important component of integrated waste management. Solid waste residues are waste components that are not recycled, that remain after processing at a materials recovery facility, or that remain after the recovery of conversion products and/or energy. Historically, solid waste has been placed in the soil in the earth's surface or deposited in the oceans. Although ocean dumping of municipal solid waste was officially abandoned in the United States in 1933, it is now argued that many of the wastes now placed in landfills or on land could be used as fertilizers to increase productivity of the ocean or the land It is also argued that the placement of wastes in ocean trenches where tectonic folding is occurring is an effective method of waste disposal Nevertheless, landfilling or land disposal is today the most commonly used method for waste disposal by far. Disposal of solid waste residues in landfills is the primary subject of this chapter The planning, design, and operation of modem landfills involve the application of a variety of scientific, engineering, and economic principles. The major topics covered in this chapter include: (1)a description of the landfill method of solid waste disposal. including environmental concerns and regulatory requirements:(2) a description of types of landfills and landfilling methods:(3) landfill siting considerations:(4) landfill gas management; (5) landfill leachate control:(6) surface water control: (7 landfill structural characteristics and settlement:( 8)environmental quality monitoring:(9) the layout and preliminary design of landfills: (10) development of landfill operation plan:(11 landfill closure and st-closure care: and(12)landfill design computations 15-1 The Landfill Method of Solid Waste Disposal Historically, landfills have been the most economical and environmentally acceptable method for the disposal of solid wastes, both in the United States and throughout the world. Even with implementation of waste reduction, recycling, and transformation technologies, disposal of residual solid waste in landfills still remains an important component of an integrated solid waste management strategy Landfill management incorporates the planning, design, operation, closure, and postclosure control of landfills. The purposes of this section are(I)to introduce the reader to the landfilling process,(2)to review the principal reactions occurring in landfills, (3) to identify environmental concerns associated with landfills, and(4)to review briefly some federal and state regulations governing the disposal of solid waste in landfills The Landfilling process Definition of Terms. Landfills are the physical facilities used for the disposal of residual solid wastes in the surface soils of the earth. In the past, the term sanitary landfill was used to denote a landfill in which the waste placed in the landfill was covered at the end of each day's operation. Today. sani refers to an engineered facility for the disposal of msw designed and operated to minimize nealth and environmental impacts. Landfills for the disposal of hazardous wastes are called secure landfills. A sanitary landfill is also sometimes identified as a solid waste management unit. Landfilling is the process by which residual solid waste is placed in a landfill. Landfilling includes monitoring of the incoming waste stream, placement and compaction of the waste, and installation of landfill environmental monitoring and control facilities. The term cell is used to describe the volume of material placed in a ndfill during one operating period. usually one day. a cell includes the solid waste deposited and the daily cover material surrounding it. Daily cover usually consists of 6 to 12 in of native soil or alternative ing period. The purposes of daily cover are to control the blowing of waste materials, to prevent rats, flies, and other disease vectors from entering or exiting the landfill; and to control the entry of wate nto the landfill during operation
1 15 Disposal of Solid Wastes and Residual Matter The safe and reliable long-term disposal of solid waste residues is an important component of integrated waste management. Solid waste residues are waste components that are not recycled, that remain after processing at a materials recovery facility, or that remain after the recovery of conversion products and/or energy. Historically, solid waste has been placed in the soil in the earth's surface or deposited in the oceans. Although ocean dumping of municipal solid waste was officially abandoned in the United States in 1933, it is now argued that many of the wastes now placed in landfills or on land could be used as fertilizers to increase productivity of the ocean or the land. It is also argued that the placement of wastes in ocean trenches where tectonic folding is occurring is an effective method of waste disposal. Nevertheless, landfilling or land disposal is today the most commonly used method for waste disposal by far. Disposal of solid waste residues in landfills is the primary subject of this chapter. The planning, design, and operation of modem landfills involve the application of a variety of scientific, engineering, and economic principles. The major topics covered in this chapter include: (1) a description of the landfill method of solid waste disposal, including environmental concerns and regulatory requirements; (2) a description of types of landfills and landfilling methods; (3) landfill siting considerations; (4) landfill gas management; (5) landfill leachate control; (6) surface water control; (7) landfill structural characteristics and settlement; (8) environmental quality monitoring; (9) the layout and preliminary design of landfills; (10) development of landfill operation plan; (11) landfill closure and post-closure care; and (12) landfill design computations. 15-1 The Landfill Method of Solid Waste Disposal Historically, landfills have been the most economical and environmentally acceptable method for the disposal of solid wastes, both in the United States and throughout the world. Even with implementation of waste reduction, recycling, and transformation technologies, disposal of residual solid waste in landfills still remains an important component of an integrated solid waste management strategy. Landfill management incorporates the planning, design, operation, closure, and postclosure control of landfills. The purposes of this section are (1) to introduce the reader to the landfilling process, (2) to review the principal reactions occurring in landfills, (3) to identify environmental concerns associated with landfills, and (4) to review briefly some federal and state regulations governing the disposal of solid waste in landfills. The Landfilling Process Definition of Terms. Landfills are the physical facilities used for the disposal of residual solid wastes in the surface soils of the earth. In the past, the term sanitary landfill was used to denote a landfill in which the waste placed in the landfill was covered at the end of each day's operation. Today, sanitary landfill refers to an engineered facility for the disposal of MSW designed and operated to minimize public health and environmental impacts. Landfills for the disposal of hazardous wastes are called secure landfills. A sanitary landfill is also sometimes identified as a solid waste management unit. Landfilling is the process by which residual solid waste is placed in a landfill. Landfilling includes monitoring of the incoming waste stream, placement and compaction of the waste, and installation of landfill environmental monitoring and control facilities. The term cell is used to describe the volume of material placed in a landfill during one operating period, usually one day. A cell includes the solid waste deposited and the daily cover material surrounding it. Daily cover usually consists of 6 to 12 in of native soil or alternative materials such as compost that are applied to the working faces of the landfill at the end of each operating period. The purposes of daily cover are to control the blowing of waste materials; to prevent rats, flies, and other disease vectors from entering or exiting the landfill; and to control the entry of water into the landfill during operation
Final cover system Bench(terrace) as required Final cel Fina 3: 1 typical slope 6 in intermediate variable andfill liner system Sectional view through a sanitary landfill a lifi is a complete laver of cells over the active area of the landfill(see Fig 15-1) ally. landfills are comprised of a series of lifts. A bench (or terrace) is co will exceed 50 to 75 ft. Benches are used to maintain the slope stability of the landfill. for of surface water drainage channels, and for the location of landfill gas recovery piping. The final lift includes the cover laver. The final cover laver is applied to the entire landfill surface after all landfilling materials designed to enhance surface drainage, intercept percolating water. and support surface The liquid that collects at the bottom of a landfill is known as leachate. In deep landfills, leachate is often collected at intermediate points. In general, leachate is a result of the percolation of precipitation. uncontrolled runoff. and irrigation water into the landfill leachate can also include water initially ell as infiltrating groundwater. Leachate contains a variety of chemical constituents derived from the solubilization of the materials deposited in the landfill and from the products of the chemical and biochemical reactions occurring within the landfill Landfill gas is the mixture of gases found within a landfill. The bulk of landfill gas consists of methane (CHa)and carbon dioxide (COz), the principal products of the anaerobic decomposition of the biodegradable organic fraction of the msw in the landfill. Other components of landfill gas include atmospheric nitrogen and oxygen, ammonia, and trace organic compounds Landfill liners are materials(both natural and manufactured) that are used to line the bottom area and below-grade sides of a landfill. Liners usually consist of layers of compacted clay and/or geomembrane material designed to prevent migration of landfill leachate and landfill gas. Landfill control facilities include liners, landfill leachate collection and extraction systems, landfill gas collection and extraction systems, and daily and final cover layers Environmental monitoring involves the activities, associated with collection and analysis of water and r samples, that are used to monitor the movement of landfill gases and leachate at the landfill site Landfill closure is the term used to describe the steps that must be taken to close and secure a landfill site once the filling operation has been completed. Postclosure care refers to the activities associated with the long-term monitoring and maintenance of the completed landfill (typically 30 to 50 years) Overview of Landfill Planning, Design, and Operation. The principal elements that must be considered in The planning design, and operation of landfills include(1) landfill layout and design; (2) landfill operations and management;(3) the reactions occurring in landfills; (4)the management of landfill gases; (5)the management of leachate;(6)environmental monitoring; and (7 landfill clo and postclosure care Preparation of the site for landfilling The first step in the process involves the preparation of the site for landfill construction Existing site drainage must be modified to route any runoff away from the intended landfill area. rerouting of
2 A lift is a complete layer of cells over the active area of the landfill (see Fig. 15-1). Typically, landfills are comprised of a series of lifts. A bench (or terrace) is commonly used where the height of the landfill will exceed 50 to 75 ft. Benches are used to maintain the slope stability of the landfill, for the placement of surface water drainage channels, and for the location of landfill gas recovery piping. The final lift includes the cover layer. The final cover layer is applied to the entire landfill surface after all landfilling operations are complete. The final cover usually consists of multiple layers of soil and/or geomembrane materials designed to enhance surface drainage, intercept percolating water, and support surface vegetation. The liquid that collects at the bottom of a landfill is known as leachate. In deep landfills, leachate is often collected at intermediate points. In general, leachate is a result of the percolation of precipitation, uncontrolled runoff, and irrigation water into the landfill. Leachate can also include water initially contained in the waste as well as infiltrating groundwater. Leachate contains a variety of chemical constituents derived from the solubilization of the materials deposited in the landfill and from the products of the chemical and biochemical reactions occurring within the landfill. Landfill gas is the mixture of gases found within a landfill. The bulk of landfill gas consists of methane (CH4) and carbon dioxide (CO2), the principal products of the anaerobic decomposition of the biodegradable organic fraction of the MSW in the landfill. Other components of landfill gas include atmospheric nitrogen and oxygen, ammonia, and trace organic compounds. Landfill liners are materials (both natural and manufactured) that are used to line the bottom area and below-grade sides of a landfill. Liners usually consist of layers of compacted clay and/or geomembrane material designed to prevent migration of landfill leachate and landfill gas. Landfill control facilities include liners, landfill leachate collection and extraction systems, landfill gas collection and extraction systems, and daily and final cover layers. Environmental monitoring involves the activities, associated with collection and analysis of water and air samples, that are used to monitor the movement of landfill gases and leachate at the landfill site. Landfill closure is the term used to describe the steps that must be taken to close and secure a landfill site once the filling operation has been completed. Postclosure care refers to the activities associated with the long-term monitoring and maintenance of the completed landfill (typically 30 to 50 years). Overview of Landfill Planning, Design, and Operation. The principal elements that must be considered in The planning, design, and operation of landfills include (1) landfill layout and design; (2) landfill operations and management; (3) the reactions occurring in landfills; (4) the management of landfill gases; (5) the management of leachate; (6) environmental monitoring; and (7) landfill closure and postclosure care. Preparation of the site for landfilling. The first step in the process involves the preparation of the site for landfill construction. Existing site drainage must be modified to route any runoff away from the intended landfill area. Rerouting of Fig. 15-1 Sectional view through a sanitary landfill. Fig 15-1
drainage is particularly important for ravine landfills where a significant watershed may drain through the site. In addition, drainage of the landfill area itself must be modified to route water away from the initial fill area. Other site preparation tasks include construction of access roads and weighing facilities, and installation of fenc The next step in the development of a landfill is the excavation and preparation of the landfill bottom only a small part of the unprotected landfill surface to be exposed to precipitation at any time. In addition, excavations are carried out over time, rather than preparing the entire landfill bottom at once. Excavated material can be stockpiled on unexcavated soil near the active area and the problem of precipitation collecting in the excavation is minimized. Where the entire bottom of the landfill is lined at once, provision must be made to remove storm-water runoff from the portion of the landfill that is not To minimize costs, it is desirable to obtain cover materials from the landfill site whenever possible. The initial working area of the landfill is excavated to the design depth, and the excavated material stockpiled or later use. Vadose zone(zone between ground surface and permanent groundwater) and groundwater monitoring equipment is installed before the landfill liner is laid down. The landfill bottom is shaped to provide drainage of leachate, and a low-permeability liner is installed. Leachate collection and extraction facilities are placed within or on top of the liner. Typically, the liner extends up the excavated walls of the landfill. Horizontal gas recovery trenches may be installed at the bottom of the landfill, particularly if emissions of volatile organic compounds(vOCs) from the newly placed waste is expected to be a problem. To minimize the release of VOCs. a vacuum is applied and air is drawn through the completed portions of the landfill. The gas that is removed must be burned under controlled conditions to destroy the VOCs. Before the fill operation begins, a soil berm is constructed at the downwind side of the planned fill area. The berm serves as a windbreak to control blowing materials and as a face agains which the waste can be compacted. For excavated landfills, the wall of the excavation usually serves as the initial compaction face The placement of wastes. Once the landfill site has been prepared, the next step in the process involves the actual placement of waste material. The waste is placed in cells beginning along the compaction face. continuing outward and upward from the face. The waste deposited in each operating period, usually one day, forms an individual cell. Wastes deposited by the collection and transfer vehicles are spread out in 18-to 215-in layers and compacted. Typical cell heights vary from 8 to 12 ft. The length of the working face varies with the site conditions and the size of the operation. The working face is the area of a landfill where solid waste is being unloaded, placed and compacted during a given operating period. The width of a cell varies from 10 to 30 ft, again depending on the design and capacity of the landfill. All exposed faces of the cell are covered with a thin layer of soil (6 to 12 in) or other suitable material at the end of each operating period After one or more lifts have been placed, horizontal gas recovery trenches can be excavated in the completed surface. The excavated trenches are filled with gravel, and perforated plastic pipes are installed in the trenches. Landfill gas is extracted through the pipes as the gas is produced. Successive lifts are placed on top of one another until the final design grade is reached. Depending on the depth of the landfill, additional leachate collection facilities may be placed in successive lifts. A cover layer is applied to the completed landfill section. The final cover is designed to minimize infiltration of precipitation and to route drainage away from the active section of the landfill. The cover is landscaped to control erosion. Vertical gas extraction wells may be installed at this time through the completed landfill surface. The gas extraction system is tied together and the extracted gas may be flared or routed to energy recovery facilities as appropriate Additional sections of the landfill are constructed outward from the co leted sections, repeating the construction steps outlined above. As organic materials deposited the landfill decompose completed sections may settle. Landfill construction activities must include refilling and repairing of settled landfill surfaces to maintain the desired final grade and drainage. The gas and leachate control systems al so must be extended and maintained. Upon completion of all fill activities, the landfill surface is repaired and upgraded with the installation of a final cover. The site is landscaped appropriately and prepared for other uses Postclosure management. Monitoring and maintenance of the completed landfill must continue by law for some time after closure(30 to 50 vears). It is particularly important that the landfill surface be
3 drainage is particularly important for ravine landfills where a significant watershed may drain through the site. In addition, drainage of the landfill area itself must be modified to route water away from the initial fill area. Other site preparation tasks include construction of access roads and weighing facilities, and installation of fences. The next step in the development of a landfill is the excavation and preparation of the landfill bottom and subsurface sides. Modern landfills typically are constructed in sections. Working by sections allows only a small part of the unprotected landfill surface to be exposed to precipitation at any time. In addition, excavations are carried out over time, rather than preparing the entire landfill bottom at once. Excavated material can be stockpiled on unexcavated soil near the active area and the problem of precipitation collecting in the excavation is minimized. Where the entire bottom of the landfill is lined at once, provision must be made to remove storm-water runoff from the portion of the landfill that is not being used. To minimize costs, it is desirable to obtain cover materials from the landfill site whenever possible. The initial working area of the landfill is excavated to the design depth, and the excavated material stockpiled for later use. Vadose zone (zone between ground surface and permanent groundwater) and groundwater monitoring equipment is installed before the landfill liner is laid down. The landfill bottom is shaped to provide drainage of leachate, and a low-permeability liner is installed. Leachate collection and extraction facilities are placed within or on top of the liner. Typically, the liner extends up the excavated walls of the landfill. Horizontal gas recovery trenches may be installed at the bottom of the landfill, particularly if emissions of volatile organic compounds (VOCs) from the newly placed waste is expected to be a problem. To minimize the release of VOCs. a vacuum is applied and air is drawn through the completed portions of the landfill. The gas that is removed must be burned under controlled conditions to destroy the VOCs. Before the fill operation begins, a soil berm is constructed at the downwind side of the planned fill area. The berm serves as a windbreak to control blowing materials and as a face against which the waste can be compacted. For excavated landfills, the wall of the excavation usually serves as the initial compaction face. The placement of wastes. Once the landfill site has been prepared, the next step in the process involves the actual placement of waste material. The waste is placed in cells beginning along the compaction face, continuing outward and upward from the face. The waste deposited in each operating period, usually one day, forms an individual cell. Wastes deposited by the collection and transfer vehicles are spread out in 18- to 215-in layers and compacted. Typical cell heights vary from 8 to 12 ft. The length of the working face varies with the site conditions and the size of the operation. The working face is the area of a landfill where solid waste is being unloaded, placed and compacted during a given operating period. The width of a cell varies from 10 to 30 ft, again depending on the design and capacity of the landfill. All exposed faces of the cell are covered with a thin layer of soil (6 to 12 in) or other suitable material at the end of each operating period. After one or more lifts have been placed, horizontal gas recovery trenches can be excavated in the completed surface. The excavated trenches are filled with gravel, and perforated plastic pipes are installed in the trenches. Landfill gas is extracted through the pipes as the gas is produced. Successive lifts are placed on top of one another until the final design grade is reached. Depending on the depth of the landfill, additional leachate collection facilities may be placed in successive lifts. A cover layer is applied to the completed landfill section. The final cover is designed to minimize infiltration of precipitation and to route drainage away from the active section of the landfill. The cover is landscaped to control erosion. Vertical gas extraction wells may be installed at this time through the completed landfill surface. The gas extraction system is tied together and the extracted gas may be flared or routed to energy recovery facilities as appropriate. Additional sections of the landfill are constructed outward from the completed sections, repeating the construction steps outlined above. As organic materials deposited within the landfill decompose, completed sections may settle. Landfill construction activities must include refilling and repairing of settled landfill surfaces to maintain the desired final grade and drainage. The gas and leachate control systems also must be extended and maintained. Upon completion of all fill activities, the landfill surface is repaired and upgraded with the installation of a final cover. The site is landscaped appropriately and prepared for other uses. Postclosure management. Monitoring and maintenance of the completed landfill must continue by law for some time after closure (30 to 50 years). It is particularly important that the landfill surface be
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 gas
4 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
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-step
5 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