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. 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 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 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 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 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 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 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. 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 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 be adsorbed per cubic foot is equal to 60 g. If the concentration of heavy metals in the leachate was 100 mg/L, the heavy metals could be removed from about 600 L of leachate. If the permeability of the clay is equal to 1 × 10~7 cm/s, then 2.83 L would pass through 1 ft2 each year. At this rate of percolation, it would take 212 years to saturate the original ft3 of clay. If the amount of leachate allowed to percolate through the liner were limited to one tenth of that value by designing the leachate collection system properly, then the time required lo saturate the ft3 of clay would be approximately 2000 years. Even with ail of the simplifying assumptions that went into the above analysis, it can be concluded that with a properly designed landfill cover and clay liner, heavy metals should not pose a problem. Trace Organics. Adsorption is the most common way in which the organic constituents in leachate are removed as it moves through a porous medium. If hydrodynamic dispersion is neglected, the materials balance for a contaminant subject to adsorption m a groundwater aquifer is given by the following modified Form of Eq. (15-2): The mass of material sorbed per unit mass of dry soil is related to the concentration of the contaminant in the liquid phase and the soil distribution coefficient, as described m the following equation: S = KSD×C (15-3) where Ksd = soil distribution coefficient, m3/g Retardation of the organic constituents found in leachate is important because the retained material can be subjected to biological and chemical conversion reactions, in some cases rendering the retained material hannless. Control of Leachate in Landfills As leachate percolates through the underlying strata, many of the chemical and biological constituents originally contained in it will be removed by the filtering and adsorptive action of the material composing the strata. In general, the extent of this action depends on the characteristics of the soil, especially the clay content. Because of the potential risk involved in allowing leachate to percolate to the groundwater, best practice calls for its elimination or containment. Landfill liners are now commonly used to limit or eliminate the movement of leachate and landfill gases from the landfill site. To date (1992), the use of clay as a liner material has been the favored method of reducing or eliminating the seepage (percolation) of leachate from landfills. Clay is favored for its ability to adsorb and retain many of the chemical constituents found in leachate and for its resistance to the flow of leachate. However, the use of combination composite geomembrane and clay liners is gaining in popularity, especially because of the resistance afforded by geomembranes to the movement of both leachate and landfill gases. The characteristics, advantages, and disadvantages of the geomembrane liners (also known as flexible membrane liners, FMLs) that have been used for MSW landfills are summarized in Table 15-3. Typical specifications for geomembrane liners are given in Table 15-4. Liner Systems for MSW. The objective in the design of landfill liners is to minimize the infiltration of leachate into the subsurface soils below the landfill thus eliminating the potential for groundwater contamination. A number of liner designs have been developed to minimize the movement of leachate into the subsurface below the landfill. The sand or gravel layer serves as a collection and drainage layer for any leachate that may be generated within the landfill. The geotextile layer is used to minimize the intermixing of the soil and sand or gravel layers. The final soil layer is used to protect the drainage and barrier layers. A modification of the liner design involves the installation of leachate collection pipes in the leachate collection layer. Composite liner designs employing a geomembrane and clay layer provide more protection and are hydraulically more effective than either type of liner alone. A specifically designed open weave plastic mesh (geonet) and geotextile filter cloth are placed over the geomembrane which, in turn, is placed over compacted clay layer. A protective soil layer is placed above the geotextile. The geonet and the geotextile function together as the drainage layer to convey leachate to the leachate collection system. The permeability of the liner system that is composed of a drainage layer and a filter layer is equivalent to that of coarse sand. Because of the potential for the geotextile filter cloth to clog, many designers favor the use of a sand or gravel layer as the drainage layer. In the liner system shown, two composite liners, commonly identified as the primary and secondary composite liners, are used. The primary composite liner is used for the collection of leachate, whereas the secondary composite liner serves as a teak-detection system and a backup for the primary composite liner. A modification of the liner system, involves replacing the sand drainage layer with a geonet drainage system. A manufactured product, the GCL is made from a high-quality bentonite clay (from Wyoming) and an appropriate binding material. The bentonite clay is essentially a sodium montmorillonite mineral that has the capacity to absorb as much as 10 times its weight in water. As the clay absorbs water, it becomes putty-like and very resistant to the movement of water. Permeabilities aslow as l0-10 cm/s have been observed. Available in large sheets (12 to 14 by 100 ft), GCLs are overlapped in the construction of a liner system.. In the two-layer composite layer landfill systems through f, leak-detection sensors are usually placed between the two liners. Liner Systems for Monofills. Liner systems for monofills usually comprise two geomembranes, each provided with a drainage layer and a leachate collection system. A leachate detection system is placed between the first and second liners as well as below the lower liner. In many installations, a thick (3 to 5 ft) clay layer is used below the two geomembranes for added protection. Construction of Clay Liners. Great care must be exercised in the construction of the clay layer. Perhaps the most serious problem with the use of clay is its tendency to form cracks due to desiccation. It is critical that the clay not be allowed to dry out as it is being placed. To insure that the clay liner performs as designed, the clay liner should be laid in 15- to 6-in layers with adequate compaction between the placement of succeeding layers. Laying the clay in thin layers avoids the possibility of leaks due to the alignment of clods that could occur if the clay layer is applied in a single pass. Another problem that has been encountered when clays of different types have been used is cracking due to differential swelling. To avoid differential swelling only one type of clay must be used in the construction of the liner. Leachate Collection Systems The design of a leachate collection system involves (1) the selection of the type of liner system to be used, (2) the development of the grading plan including the placement of the leachate collection and drainage channels and pipelines for the removal of leachate, and (3) the layout and design of the leachate removal, collection, and holding facilities. Selection of Liner System. The type of liner system selected will depend to a large extent on the local geology and environmental requircments of the landfill site. For example, in locations where there is no groundwater, a single compacted lay liner may be sufficient in locations where both leachate and gas migration must be controlled, a combined liner comprising a clay liner and a geomembrane liner with an appropriate drainage and soil protection layer will be necessary. Design of Leachate Collection Facilities. A variety of liner designs have been used for the removal of leachate from landfills. The sloped terrace and piped bottom designs are discussed below. Sloped terraces. To avoid the accumulation of leachate in the bottom of a landfill, the bottom area is graded into a series of sloped terraces. The terraces are shaped so that the leachate that accumulates on the surface of the terraces will drain to leachate collection channels. Perforated pipe placed in each leachate collection channel is used to convey the collected leachate to a central location, from which it is removed for treatment or reapplication to the surface of the landfill. The cross-slope of the terraces is usually 1 to 5 percent, and the slope of the drainage channels is 0.5 to 1.0 percent. The slope and maximum length of the drainage channel is selected based on the capacity of the drainage facilities. The flow rate capacity of the drainage facilities is estimated using Manning's equation. The design objective is not to allow the leachate to pond in the bottom of the landfill so as to create a significant hydraulic head on the landfill liner (less than 1 ft at the highest point as specified in the new federal Subtitle D landfill regulations). The depth of flow in the perforated drainage pipe increases continually from the upper reaches of the drainage channel to the lower reaches. In very large landfills, the drainage channels will be connected to a larger cross-collection system. Piped bottom. The bottom area is then divided into a series of rectangular strips by clay barners placed at appropriate distances. The barrier's spacing corresponds to the width of a landfill cell. Leachate collection pipes are then placed lengthwise directly on the geomembrane. The 15-in leachate collection pipes have laser-cut perforations, similar to a welt screen, over one-half of the circumference. The laser-cuts are spaced 0.25 in apart and the size of the laser cut is 0.0001 in, corresponding to the smallest sand size. To promote effective drainage, the bottom is sloped from 1.2 to 1.8 percent. The leachate collection pipes, spaced every 20 ft, are covered with a two-foot layer of sand before landfilling commences. The use of a multiple-pipe leachate collection system will ensure the rapid removal of leachate from the bottom of the landfill. Further, the use of a 2-ft sand layer serves to filter the leachate before it is collected for treatment. The first 3-ft layer of solid waste, placed directly on the sand layer, is not compacted. A unique featurc of the design is the method used to remove the stormwater from the unused portion of the landfill. The method is detailed. In theunused portion ofthe landfill, stormwater is collected in the lines that will ultimately be used for the collection of leachate. When the next landfill cell is to be placed in service, the leachate piping is reconnected to the leachate collection system, and the leachate collection pipe which extends into the next diked strip is capped. Leachate Removal, Collection, and Holding Facilities. Two methods have been used for the removal of leachate that accumulates within a landfill. The leachate collection pipe is passed through the side of the landfill. Where this method is used, great care must be taken to ensure that the seal where the pipe penetrates the landfill liner is sound. An altemative method used for the removal of leachate from landfills involves the use of an inclined collection pipe located within the landfill. Leachate collection facilities are used where the leachate is to be recycled from or treated at a central location. The capacity of the holding tank will depend on the type of treatment facilities that are available and the maximum allowable discharge rate to the treatment facility. Typically, leachate holding tanks are designed to hold from 1 to 3 days of leachate production during the peak leachate production period. Both double- and single-walled tanks have been used, but the double walled tanks are preferred over single-walled tanks because of the added safety afforded. Although both plastic and metallic tanks have been used, plastic tanb are more corrosion resistant. Leachate Management Options The management of leachate, when and if it forms, is key to the elimination of the potential for a landfill to pollute underground aquifers. A number of altematives have been used to manage the leachate collected from landfills including: (1) leachate recycling, (2) leachate evaporation, (3) treatment followed by disposal, and (4) discharge to municipal wastewater collection systems. Theseopdons are discussed briefly below. Leachate Recycling. An effective method for the treatment of leachate is to collect，and recirculate the leachate through the landfill. During the early stages of landfill operadon the leachate will contain significant amounts of TDS, BOD5, COD, nutrients, and heavy metals. When the leachate is recirculated, the constituents are, attenuated by the biological activity and by other chemical and physical reactions occurring within the landfill. For example, the simple organic acids present in the leachate will be converted to CH4 and CO2. Because of the rise in pH within the landfill when CH4 is produced, metals wiil be precipitated and retained within the landfill. An additional benefit of leachate recycling is the recovery of landfill gas that contains CH4. Typically, the rate of gas production is greater in leachate recirculatnon systems. To avoid the uncontrolled release of landfill gases when leachate is recycled for treatment, the landfill should be equipped with a gas recovery system. Ultimately, it will be necessary to collect, treat, and dispose of the residual leachate. In large landfills it may be necessary to provide leachate storage facilities. Leachate Evaporation. One of the simplest leachate management systems involves the use of lined leachate evaporation ponds. Leachate that is not evaporated is sprayed on the completed portions ofthe landfill In locations with high rainfall, the lined leachate storage facility is covered with a geomembrane during the winter season to exclude rainfall. The accumulated leachate is disposed of by evaporation during the warm summer months, by uncovering the storage facility, and by spraying the leachate on the surface of the operating and completed landfill. Odorous gases that may accumulate under the surface cover are vented to a compost or soil filter. Soil beds are typically 2 to 3 ft deep, with organic loading rates of about 0.1 to 0.25 lb/ft3 of soil. During the summer when the pond is uncovered, surface aeration may be requnred to control odors. If the storage pond is not large it can be teft covered year round. Another example involves treatment of the leachate (usually biologically) with winter storage and spray disposal of the treated effluent on nearby lands during the summer. If enough land is available, spraying of effluent can be carried out on a continuous basis, even when it is raining. Leachate Treatment. Where leachate recycling and evaporation is not used, and the direct disposal of leachate to a treatment facility is not possible, some fonn of pretreatment or complete treatment will be required. Because the characteristics of the collected leachate can vary so widely, a number of options have been used for the treatment of leachate. The principal biological and physical/chemical treatment operations and processes used for the treatment of leachate are summarized in Table 15-5. The treatment process or processes selected will depend to a large extent on the contaminant(s) to be reinoved. Selection of treatment facilities. The type of treatment facilities used will depend primarily on the characteristics of the leachate and secondarily on the geographic and physical location of the landfill. Leachate characteristics of concem include TDS, COD, SO42-, heavy metals, and nonspecific toxic constituents. Leachate containing extremely high TDS concentrations (e.g., > 50,000 mg/L) may be difficult to treat biologically. High COD values favor anaerobic treatment processes because aerobic treatment is expensive. High sulfate concentrations may limit the use of anaerobic treatment processes because of the production of odors from the biological reduction of sulfate sulfide. Heavy metal toxicity is also a problem with many biological treatment processes. Another important question is how large should the treatment facilities be? The capacity of the treatment facilities will depend on the size of the landfill and the expected useful life. The presence of nonspecific toxic constituents is often a problem with older landfills that received a variety of wastes, before environment regulations governing the operation of landfills were enacted. Integrated leachate management system. Liquid (leachate) that moves down through the solid waste is first filtered as it passes the sand layer m the landfill. The collected leachate is transported to a treatment lagoon where septage is also added. The liquid in the lagoon is aerated to reduce the organic content and to control odors. Liquid from the lagoon is then applied to shredded MSW that is to be composted and used for intermediate cover material in the landfill. Recyclable materials and metals arc removed before the MSW is shredded. Application of the leachate to the shredded MSW provides the moisture needed for optimum composting and reduces the volume of leachate through evaporation. The excess leachate is filtered as it passes through the shredded waste and the sand filter underdrain system. The collected leachate is piped to a series of constructed wetlands. The wetlands are used to remove organic material, nutrients, heavy metals, and other trace organics. The effluent from the constructed wetlands is passed through a slow sand filter and then used for spray irrigation on the grass-covered landscape at the landfill. Discharge to Wastewater Treatment Plant. In those locations where a landfill is located near a wastewater collection system or where a pressure sewer can be used to connect the landfill leachate collection system to a wastewater collection system, leachate is often discharged to the wastewater collection system. In many cases pretreatment, using one or more of the metliods reported in Table 11-18, may be required to reduce the organic content before the leachate can be discharged to the sewer. In locations where sewers are not available, and evaporation and spray disposal are not feasible, complete treatment followed by surface discharge may be required. 15-4 Environmental Quality Monitoring at Landfills Environmental monitoring is conducted at sanitary landfills to ensure that no contaminants that may affect public health and the surrounding environment are released from the landfill. The monitoring required may be divided into three general categories: (1) vadose zone monitoring for gases and liquids, (2) groundwater monitoring, and (3) air quality monitoring. Environmental monitoring involves the use of both sampling and nonsampling methods. Sampling methods involve the collection of a sample for analysis, usually at an offsite laboratory. Nonsampling methods are used to detect chemical and physical changes in the environment as a function of an indirect measurement such as a change in e?ectrical current. Representative devices that have been used to monitor landfill sites are listed in Table 15-6. Vadose Zone Monitoring. The vadose zone is defined as that zone from the ground surface to where the permanent groundwater is found. An important characteristic of the vadose zone is that the pore spaces are not filled with water, and that the small amounts of water that are present coexist with air. Vadose zone monitoring at landfills involves both liquids and gases. Liquid Monitoring in the Vadose Zone. Monitoring for liquids in the vadose zone is necessary to detect any leakage of leachate from the bottom of a landfill. In the vadose zone, moisture held in the interstices of the soil particles or within porous rock is always held at pressures below atmospheric pressure. To remove the moisture it is necessary to develop a negative pressure or vacuum to pull the moisture away from the soil particles. Because suction must be applied to draw moisture out of the soil in the vadose zone, conventional wells or other open cavities cannot be used to collect samples in this zone. The sampling devices used for sample extraction in the unsaturated zone are called suction lysimeters. Three commonly used classes of lysimeters are (1) the ceramic cup, (2) the hollow fiber, and (3) the membrane filter. The most commonly used device for obtaining samples of moisture in the vadose zone is the ceramic cup sampler, which consists of a porous cup or ring made of ceramic material that is attached to a short section of nonporous tubing (e.g., PVC). When placed in the soil, because of its pores it becomes an extension of the pore space of the soil. Soil moisture is drawn in through the porous ceramic element by the application of a vacuum. When a sufficient amount of water has collected in the sampler, the collected sample is pulled to the surface through a narrow tube by the application of a vacuum or is pushed up by air pressure. Gas Monitoring in the Vadose Zone. Monitoring for gases in the vadose zone is necessary to detect the lateral movement of any landfill gases. In many monitoring systems, gas samples are collected from multiple depths in the vadose zone. Groundwater Monitoring Monitoring of the groundwater is necessary to detect changes in water quality that may be caused by the escape of leachate and landfill gases. Both down- and up gradient wells are required to detect any contamination of the underground aquifer by leachate from the landfill. To obtain a representative sample, the liquid in permanent sample collection tubing, where used, must be purged before the sample is collected. Landfill Air Quality Monitoring Air quality monitoring at landfills involves (1) the monitoring of ambient air quality at and around the landfill site, (2) the monitoring of landfill gases extracted from the landfill, and (3) the monitoring of the off gases from any gas processing or treatment facilities. Monitoring Ambient Air Quality. Ambient air quality is monitored at landfill sites to detect the possible movement of gaseous contaminants from the boundaries of the landfill site. Gas sampling devices can be divided into three categories: (1) passive, (2) grab, and (3) active. Passive sampling involves the collection of a gas sample by passing a stream of gas through a collection device in which the contaminants contained in the gas stream are removed for subsequent analysis. Commonly used in the past, passive sampling is seldom used today. Grab samples are collected using an evacuated flask, gas syringe, or an air collection bag made of a synthetic material. An active sampler involves the collection and analysis of a continuous stream of gas. Monitoring Extracted Landfill Gas. Landfill gas is monitored to assess the composition of the gas, and to determine the presence of trace constituents that may pose a health or environmental risk. Monitoring Off-Gases. Monitoring off-gases from treatment and energy recovery facilities is done to determine compliance with local air pollution control requirements. Both grab and continuous sampling have been used for this purpose.