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
process, with conversion of the complex organic material to organic acids and other intermediate products as described in Phase Ill. In Phase Il, the pH of the leachate, if any is formed, starts to drop due to the presence of organic acids and the effect of the elevated concentrations of COz within the landfill Phase Ill- acid phase. In Phase Ill, the acid phase, the microbial activity initiated in Phase ll accelerates with the production of significant amounts of organic acids and lesser amounts of hydrogen gas. The first step in the three-step process involves the enzyme-mediated transformation(hydrolysis)of higher-molecular mass compounds (e. g, lipids, polysaccharides, proteins, and nucleic acids) into compounds suitable for use by microorganisms as a source of energy and cell carbon. The second step in the process(acidogenesis) involves the microbial conversion of the compounds resulting from the first step into lower-molecular mass intermediate compounds as typified by acetic acid(CH3 COOH)and small concentrations of fulvic and other more Typical concentrations of trace compounds found complex organic acids. in landfil in landfill gas at 66 California sw landfills Carbon dioxide(CO2) is the principal gas compound Concentration, ppbve generated during Phase Median IIL Smaller amounts of hydrogen gas(H2) willChiorobenzene also be produced. The 1.1 chloroform 2,801 microorganIsms 1.15 nvolved in this Diethylene ethene conversio collectively 2,3- Dichloropr。pan nonmethanogenic consist of facultative and obligate anaerobic Methyl ethyl ketone These microorganisms Trichloroethylene bacteria ,1,2 14.500 are often identified 8,125 engineering 11. 2.2-Tetrachloroethane 6 literature as acidogens or acid formers inyl acetate The pH of the leachate, Xylenes 38.000 if formed. will often Ac od fron Rof. 5 drop to a value of 5 o pbv- parts per billion by volume. lower because of the presence of the organic acids and the elevated concentrations of COz within the landfill. The biochemical oxygen demand(BODs), the chemical oxygen demand (COD), and th conductivity of the leachate will increase significantly during Phase Ill due to the dissolution of the organic acids in the leachate. Also, because of the low pH values in the leachate, a number of inorganic constituents, principally heavy metals, will be solubilized during Phase Ill. Many essential nutrients are also removed in the leachate in Phase Ill. If leachate is not recycled, the essential nutrients will be lost from the system. It is important to note that if leachate is not formed, the conversion products produced during Phase Ill will remain within the landfill as sorbed constituents and in the water held by the waste as defined by the field capacity Phase IV-methane fermentation phase. In Phase IV, the methane fermentation phase, a second group of microorganisms, which convert the acetic acid and hydrogen gas formed by the acid formers in the acid phase to CHa and CO, becomes more predominant. In some cases, these organisms will begin to develop toward the end of Phase Ill. The microorganisms responsible for this conversion arc strict anaerobes and are called methanogenic. Collectively, they are identified in the literature as methanogens or methane formers In Phase Iv, both methane and acid formation proceed simultaneously, although the rate of acid formation is considerably reduced Because the acids and the hydrogen gas produced by the acid formers have been convened to Cha and CO2 in Phase IV, the ph within the landfill will rise to more neutral values in the range of 6.8 to 8. In turn, the ph of the leachate, if formed, will rise, and the concentration of BODs and COD and the conductivity value of the leachate will be reduced. With higher pH values, fewer inorganic constituents
6 process, with conversion of the complex organic material to organic acids and other intermediate products as described in Phase III. In Phase II, the pH of the leachate, if any is formed, starts to drop due to the presence of organic acids and the effect of the elevated concentrations of CO2 within the landfill . Phase Ill－acid phase. In Phase III, the acid phase, the microbial activity initiated in Phase II accelerates with the production of significant amounts of organic acids and lesser amounts of hydrogen gas. The first step in the three-step process involves the enzyme-mediated transformation (hydrolysis) of higher-molecular mass compounds (e.g., lipids, polysaccharides, proteins, and nucleic acids) into compounds suitable for use by microorganisms as a source of energy and cell carbon. The second step in the process (acidogenesis) involves the microbial conversion of the compounds resulting from the first step into lower-molecular mass intermediate compounds as typified by acetic acid (CH3COOH) and small concentrations of fulvic and other more complex organic acids. Carbon dioxide (CO2) is the principal gas generated during Phase III. Smaller amounts of hydrogen gas (H2) will also be produced. The microorganisms involved in this conversion, described collectively as nonmethanogenic, consist of facultative and obligate anaerobic bacteria. These microorganisms are often identified in the engineering literature as acidogens or acid formers. The pH of the leachate, if formed, will often drop to a value of 5 or lower because of the presence of the organic acids and the elevated concentrations of CO2 within the landfill. The biochemical oxygen demand (BOD5), the chemical oxygen demand (COD), and the conductivity of the leachate will increase significantly during Phase III due to the dissolution of the organic acids in the leachate. Also, because of the low pH values in the leachate, a number of inorganic constituents, principally heavy metals, will be solubilized during Phase III. Many essential nutrients are also removed in the leachate in Phase III. If leachate is not recycled, the essential nutrients will be lost from the system. It is important to note that if leachate is not formed, the conversion products produced during Phase III will remain within the landfill as sorbed constituents and in the water held by the waste as defined by the field capacity. Phase IV—methane fermentation phase. In Phase IV, the methane fermentation phase, a second group of microorganisms, which convert the acetic acid and hydrogen gas formed by the acid formers in the acid phase to CH4 and CO2, becomes more predominant. In some cases, these organisms will begin to develop toward the end of Phase III. The microorganisms responsible for this conversion arc strict anaerobes and are called methanogenic. Collectively, they are identified in the literature as methanogens or methane formers. In Phase IV, both methane and acid formation proceed simultaneously, although the rate of acid formation is considerably reduced. Because the acids and the hydrogen gas produced by the acid formers have been convened to CH4 and CO2 in Phase IV, the pH within the landfill will rise to more neutral values in the range of 6.8 to 8. In turn, the pH of the leachate, if formed, will rise, and the concentration of BOD5 and COD and the conductivity value of the leachate will be reduced. With higher pH values, fewer inorganic constituents 15-1
can remain in solution; as a result, the concentration of heavy metals present in the leachate will also be reduced Phase V-maturation phase. Phase V, the maturation phase, occurs after the readily available biodegradable organic material has been converted to CH4 and CO2 in Phase IV. As moisture continues to migrate through the waste, portions of the biodegradable material that were previously unavailable, will be converted. The rate of landfill gas generation diminishes significantly in Phase V, because most of the available nutrients have been removed with the leachate during the previous phases and the substrates that remain in the landfill are slowly biodegradable. The principal landfill gases evolved in Phase V are CH4 and CO2 Depending on the landfill closure measures, small amounts of nitrogen and oxygen may also be found in the landfill gas. During maturation phase, the leachate will often contain humic and fulvic acids, which are difficult to process further biologically Duration of phases. The duration of the individual phases in the production of landfill gas will var depending on the distribution of the organic components in landfill, the availability of nutrients, the moisture content of waste, moisture routing through the fill, and the degree of initial compaction. For balpanple, if several loads of brush are compacted together the carbon/nitrogen ratio and the nutrient balance may not be favorable for the production of landfill gas. Likewise, the generation of landfill gas will be retarded if sufficient moisture is not available. Increasing the density of the material placed in the landfill will decrease the possibility of moisture reaching all parts of the waste and, thus, reduce the rate of bioconversion and gas production Variation in Gas Production with Time. Under normal conditions, the rate of decomposition, as measured by gas production, reaches a peak within the first two years and then slowly tapers off, continuing in many cases for periods up to 25 years or more. If moisture is not added to the wastes in a well-compacted landfill, it is not uncommon to find materials in their original form years after they were The variation in the rate of gas production from the anaerobic decomposition of the rapidly(five years or less-some highly biodegradable wastes are decomposed within days of being placed in a landfill) and slowly (5 to 50 years) biodegradable organic materials in MSW can be modeled. Gas production model in which the peak rate of gas production occurs one and five years, respectively, after gas production starts. Gas production is assumed to start at the end of the first full year of landfill operation The area under the triangle is equal to one half the base times the altitude, therefore, the total amount of gas produced from the waste placed the first year of operation is equal to Total gas produced, ft/lb 1/2(base, yr)x(altitude, peak rate of gas production, ft/lb. yr) (15-1) Using a triangular gas production model, the total rate of gas production from a landfill in which wastes were placed for a period of five years is obtained graphically by summing the gas produced from the rapidly and slowly biodegradable portions of the MSw deposited each year. The total amount of gas roduced corresponds to the area under the rate curve As noted previously, in many landfills the available moisture is insufficient to allow for the complete conversion of the biodegradable organic constituents in the MS w. The optimum moisture content for the conversion of the biodegradable organic matter in MSw is on the order of 50 to 60 percent. Also in many landfills, the moisture that is present is not uniformly distributed. When the moisture content of the landfill is limited, the gas production curve is more flattened out and is extended over a greater period of Sources of Trace Gases. Trace constituents in landfill gases have two basic sources. They may be brought to the landfill with the incoming waste or they may be produced by biotic and abiotic reactions ccurring within the landfill. Of the trace compounds found in landfill gases, many are mixed into the incoming waste in liquid form, but tend to volatilize. The tendency to volatilize can be shown to be approximately proportional to the vapor pressure of the liquid, and inversely proportional to the surface area of a sphere of the volatile liquid within the landfill. In newer landfills where the disposal of hazardous waste has been banned, the concentrations of VOCs in the landfill gas have been reduced Complex biochemical pathways can exist for the production or consumption of any of the trace constituents. For example, vinyl chloride is a byproduct of the degradation of di- and trichloroethene Because of the organic nature of these gases they can be sorbed by waste constituents in the landfill. At present, very little can be stated definitively about the rates of biochemical transformation of the trace
7 can remain in solution; as a result, the concentration of heavy metals present in the leachate will also be reduced. Phase V—maturation phase. Phase V, the maturation phase, occurs after the readily available biodegradable organic material has been converted to CH4 and CO2 in Phase IV. As moisture continues to migrate through the waste, portions of the biodegradable material that were previously unavailable, will be converted. The rate of landfill gas generation diminishes significantly in Phase V, because most of the available nutrients have been removed with the leachate during the previous phases and the substrates that remain in the landfill are slowly biodegradable. The principal landfill gases evolved in Phase V are CH4 and CO2 Depending on the landfill closure measures, small amounts of nitrogen and oxygen may also be found in the landfill gas. During maturation phase, the leachate will often contain humic and fulvic acids, which are difficult to process further biologically. Duration of phases. The duration of the individual phases in the production of landfill gas will vary depending on the distribution of the organic components in landfill, the availability of nutrients, the moisture content of waste, moisture routing through the fill, and the degree of initial compaction. For example, if several loads of brush are compacted together the carbon/nitrogen ratio and the nutrient balance may not be favorable for the production of landfill gas. Likewise, the generation of landfill gas will be retarded if sufficient moisture is not available. Increasing the density of the material placed in the landfill will decrease the possibility of moisture reaching all parts of the waste and, thus, reduce the rate of bioconversion and gas production. Variation in Gas Production with Time. Under normal conditions, the rate of decomposition, as measured by gas production, reaches a peak within the first two years and then slowly tapers off, continuing in many cases for periods up to 25 years or more. If moisture is not added to the wastes in a well-compacted landfill, it is not uncommon to find materials in their original form years after they were buried. The variation in the rate of gas production from the anaerobic decomposition of the rapidly (five years or less－some highly biodegradable wastes are decomposed within days of being placed in a landfill) and slowly (5 to 50 years) biodegradable organic materials in MSW can be modeled. Gas production model in which the peak rate of gas production occurs one and five years, respectively, after gas production starts. Gas production is assumed to start at the end of the first full year of landfill operation. The area under the triangle is equal to one half the base times the altitude, therefore, the total amount of gas produced from the waste placed the first year of operation is equal to Total gas produced, ft3 /lb = 1/2 (base, yr) × (altitude, peak rate of gas production, ft3 /1b • yr) (15-1) Using a triangular gas production model, the total rate of gas production from a landfill in which wastes were placed for a period of five years is obtained graphically by summing the gas produced from the rapidly and slowly biodegradable portions of the MSW deposited each year. The total amount of gas produced corresponds to the area under the rate curve. As noted previously, in many landfills the available moisture is insufficient to allow for the complete conversion of the biodegradable organic constituents in the MSW. The optimum moisture content for the conversion of the biodegradable organic matter in MSW is on the order of 50 to 60 percent. Also in many landfills, the moisture that is present is not uniformly distributed. When the moisture content of the landfill is limited, the gas production curve is more flattened out and is extended over a greater period of time. Sources of Trace Gases. Trace constituents in landfill gases have two basic sources. They may be brought to the landfill with the incoming waste or they may be produced by biotic and abiotic reactions occurring within the landfill. Of the trace compounds found in landfill gases, many are mixed into the incoming waste in liquid form, but tend to volatilize. The tendency to volatilize can be shown to be approximately proportional to the vapor pressure of the liquid, and inversely proportional to the surface area of a sphere of the volatile liquid within the landfill. In newer landfills where the disposal of hazardous waste has been banned, the concentrations of VOCs in the landfill gas have been reduced significantly. Complex biochemical pathways can exist for the production or consumption of any of the trace constituents. For example, vinyl chloride is a byproduct of the degradation of di- and trichloroethene. Because of the organic nature of these gases they can be sorbed by waste constituents in the landfill. At present, very little can be stated definitively about the rates of biochemical transformation of the trace
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 given
8 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
TABLE小2 Typical data on the composition of leachate from new and mature landfills Value, mg/L- New landfill (less than 2 years) Mature landfill (greater the BODs(5-day biochemical oxygen demand) 2,00030,000 10,000 TOC(total organic carbon) 150020,0o CoD(chemical oxygen demand) 3,00060,000 Total suspended solids 200-2,000 100-400 Organic nitrogen 10-800 Nitrate 5-40 Total phosphorus Ortho phosphorus 1,00010.000 3000 200-1,00 4.57.5 Total hardness as Caco3 300-10.000 Calcium 2003,000 m 100400 Magnesium 50-1.500 50200 Potassium 2001,000 300 50400 Sodium 2002500 00 100-200 2003,000 500 Sulfate 60-1,000 300 2050 Total iron 50-1,200 60 20200 Developed from Refs. 2, 8, 9, 11, 39, 46. bExcept pH, which has no units. ative range of values. Higher maximum values have been reporte pical values for new landfills will vary with the metabolic state of the landfil Variations in Leachate Composition. Note that the chemical composition of leachate will vary greatly depending on the age of landfill and the events preceding the time of sampling For example, if a leachate sample lected during the acid phase of decomposition, the ph value will be low and the concentrations of BODs, TOC, COD, nutrients, and heavy metals will be high If, on the other hand, a leachate sample is collected during the methane fermentation phase, the pH will be m the range from 6.5 to 7.5, and the BODs, TOC, COD, and nutrient concentration values will be significantly lower Similarly the concentrations of heavy metals will be lower because most metals are less soluble at neutral pH values. The pH of the leachate will depend not only on the concentration of the acids that are resent but also on the partial pressure of the COz in the landfill gas that is m contact with the leachate The biodegradabil ity of the leachate will vary with time. Changes in the biodegradability of the leachate n be monitored by checking the BOD/COD ratio. Initially, the ratios will be in the range of 0.5 or greater. Ratios in the range of 0. 4 to 0.6 are taken as an indication that the organic matter in the leachate is readily biodegradable. In mature landfills, the BODs/COd ratio is often in the range of 0.05 to 0.2 The ratio drops because leachate from mature landfills typically contains humic and fulvic acids, which are not readily biodegradable As a result of the variability in leachate characteristics, the design of leachate treatment systems is omplicated. For example, a treatment plant designed to treat a leachate with the characteristics reported for a new landfill would be quite different from one designed to treat the leachate from a mature landfill, The problem of Interpreting the analytical results is complicated further by the fact that the leachate that is being generated at any point in time is a mixture of leachate derived from solid waste of different ages Water balance and Leachate generation In landfills The potential for the formation of leachate can be assessed by preparing a water balance on the landfill The water balance involves summing the amounts of water entering the landfill and subtracting the amounts of water consumed in chemical reactions and the quantity leaving as water vapor. The potential leachate quantity is the quantity of water in excess of the moisture-holding capacity of the landfill material Description of Water Balance Components for a Landfill Cell The principal sources include the
9 Variations in Leachate Composition. Note that the chemical composition of leachate will vary greatly depending on the age of landfill and the events preceding the time of sampling. For example, if a leachate sample is collected during the acid phase of decomposition, the pH value will be low and the concentrations of BOD5, TOC, COD, nutrients, and heavy metals will be high. If, on the other hand, a leachate sample is collected during the methane fermentation phase, the pH will be m the range from 6.5 to 7.5, and the BOD5, TOC, COD, and nutrient concentration values will be significantly lower. Similarly the concentrations of heavy metals will be lower because most metals are less soluble at neutral pH values. The pH of the leachate will depend not only on the concentration of the acids that are present but also on the partial pressure of the CO2 in the landfill gas that is m contact with the leachate. The biodegradability of the leachate will vary with time. Changes in the biodegradability of the leachate can be monitored by checking the BOD5/COD ratio. Initially, the ratios will be in the range of 0.5 or greater. Ratios in the range of 0.4 to 0.6 are taken as an indication that the organic matter in the leachate is readily biodegradable. In mature landfills, the BOD5/COD ratio is often in the range of 0.05 to 0.2. The ratio drops because leachate from mature landfills typically contains humic and fulvic acids, which are not readily biodegradable. As a result of the variability in leachate characteristics, the design of leachate treatment systems is complicated. For example, a treatment plant designed to treat a leachate with the characteristics reported for a new landfill would be quite different from one designed to treat the leachate from a mature landfill, The problem of Interpreting the analytical results is complicated further by the fact that the leachate that is being generated at any point in time is a mixture of leachate derived from solid waste of different ages. Water Balance and Leachate Generation In Landfills The potential for the formation of leachate can be assessed by preparing a water balance on the landfill. The water balance involves summing the amounts of water entering the landfill and subtracting the amounts of water consumed in chemical reactions and the quantity leaving as water vapor. The potential leachate quantity is the quantity of water in excess of the moisture-holding capacity of the landfill material. Description of Water Balance Components for a Landfill Cell. The principal sources include the 15-2
water entering the landfill cell from above the moisture in the solid waste, the moisture in the cover material, and the moisture in the sludge, if the disposal of sludge is allowed. The principal sinks are the leaving the landfill as part of the landfill gas (i., water used in the formation of the gas), as s leachate. Each of these components 'is considered below. tlayers low the upper lav ct,y orresponds to the r e la m yer. Where a e dfill cover can b hat moistu ed from the atmosphere or fre em y climates, some of the inhe ons of the storage. The mois Ht n ent, as reported in Table 15-1. However, wet and dry seasons, it may be necessary to e year. The ma lepen moIs t e e field capac hat is gravity. Iypical values for soils range oottom of the first cell of the landfill is termed sponds ntermediate irectly over the r entering the of leachate e cases leachate The capa cec EC of th 37 1b/ft3(specific gravity equals 2.2), foot of liner material. Using a typical value of 20 mg/meq for the heavy metals, the amount of metal that could 10
10 water entering the landfill cell from above, the moisture in the solid waste, the moisture in the cover material, and the moisture in the sludge, if the disposal of sludge is allowed. The principal sinks are the water leaving the landfill as part of the landfill gas (i.e., water used in the formation of the gas), as saturated water vapor in the landfill gas, and as leachate. Each of these components 'is considered below. Water entering from above. For the upper layer of the landfill, the water from above corresponds to the precipitation that has percolated through the cover material. For the layers below the upper layer, water from above corresponds to the water that has percolated through the solid waste above the layer in question. One of the most critical aspects in the preparation of a water balance for a landfill is to determine the amount of the rainfall that actually percolates through the landfill cover layer. Where a geomembrane is not used, the amount of rainfall that percolates through the landfill cover can be determined using the Hydrologic Evaluation of Landfill Performance (HELP) model. Water entering in solid waste. Water entering the landfill with the waste materials is that moisture inherent in the waste material as well as moisture that has been absorbed from the atmosphere or from rainfall (where the storage containers are not sealed properly). In dry climates, some of the inherent moisture contained in the waste can be lost, depending on the conditions of the storage. The moisture content of residential and commercial MSW is about 20 percent, as reported in Table 15-1. However, because of the variability of the moisture content during the wet and dry seasons, it may be necessary to conduct a series of tests during the wet and dry periods. Water entering in cover material. The amount of water entering with the cover material will depend on the type and source of the cover material and the season of the year. The maximum amount of moisture that can be contained in the cover material is defined by the field capacity (FC) of the material, that is, the liquid which remains in the pore space subject to the pull of gravity. Typical values for soils range from 6-12 percent for sand to 23-31 percent for clay loams. Water leaving from below. Water leaving from the bottom of the first cell of the landfill is termed leachate. As noted previously, water leaving the bottom of the second and subsequent cells corresponds to the water entering from above for the cell below the cell in question. In landfills where intermediate leachate collection systems are used, water leaving from the bottom of the cell placed directly over the intermediate leachate collection system is also termed leachate. In general, the quantity of leachate is a direct function of the amount of external water entering the landfill. In fact, if a landfill is constructed properly the production of measurable quantities of leachate can be eliminated. When wastewater treatment plant sludge is added to solid wastes to increase the amount of methane produced, leachate control facilities must be provided. In some cases leachate treatment facilities may also be required. Fate of Constituents in Leachate in Subsurface Migration The major concern with the movement of leachate into the subsurface aquifer below unlined and lined landfills is the fate of the constituents found in leachate. Mechanisms that are operative in the attenuation of the constituents found in leachate as the leachate migrates through the subsurface soil include mechanical filtration, precipitation and coprecipitation, sorption (including ion exchange), gaseous exchange, dilution and dispersion, and microbial activity. The fate of heavy metals and trace organics, the two constituents of greatest interest, is considered in the following discussion. Heavy Metals. In general, heavy metals are removed by ion exchange reactions as leachate travels through the soil while trace organics are removed primarily by adsorption. The ability of a soil to retain the heavy metals found in leachate is a function of the cation exchange capacity (CEC) of the soil. The uptake and release of positively charged ions by a soil is referred to as cation, or base, exchange. The total CEC of a soil is defined as the number of milliequivalents (meq) of cations that 100 grams of soil will adsorb. The CEC of a soil depends on the amount of mineral and organic colloidal matter present in the soil matrix. Typical CEC values, at a pH value of 7, are 100 to 200 meq/100 g for organic colloids, 40 to 80 meq/100 g for 2:1 clays (montmorillonite minerals), and 5 to 20 meq/100 g for 1:1 clays (kaolinite minerals). The reported CEC values are affected by the pH of the solution; they drop to about 10 percent of the given values at a pH value of 4. As noted previously, the presence of carbon dioxide in the bottom of a landfill will tend to lower the pH of the leachate. The capacity of a clay landfill liner to take up heavy metals can be estimated as follows. Assume the CEC of the liner material is 100 meq/100 g. If the density of the clay material used in the liner is 137 1b/ft3 (specific gravity equals 2.2), then about 3000 meq of cations can be adsorbed per cubic foot of liner material. Using a typical value of 20 mg/meq for the heavy metals, the amount of metal that could