Landfill Gas Generation Control and Recovery and Utilization

Gases found in landfills include ammonia (NH3), carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), hydrogen sulfide (H2S), methane (CH4), nitrogen (N2), and oxygen (O2). The typical percentage distribution of the gases found in the landfill is reported in Table 3.22. As shown in Table 3.22, methane and carbon dioxide are the principal gases produced from the anaerobic decomposition of the biodegradable organic waste components in MSW. In addition, a number of trace gases will also be found in landfill gas. The type and concentration of the trace gases will depend to a large extent on the past history of the landfill. Issues related to the generation, control of migration, and utilization of landfill gas are considered in the following discussion.

Generation of the Principal Landfill Gases The generation of principal landfill gases is thought to occur in five more or less sequential phases, as illustrated in Figure 3.31. Each of these phases is described briefly here. Additional details may be found in Ref. 15.

Phase I: Initial adjustment. Phase I is the initial adjustment phase, in which the organic biodegradable components in municipal solid waste begin to undergo bacterial decomposition soon after they are placed in a landfill. In phase I, biological decomposition occurs under aerobic conditions because a certain amount of air is trapped within the landfill.

Landfill Gas Formation

FIGURE 3.31 Generalized phases in generation of landfill gases (I—initial adjustment, II—transition phase; III—acid phase; IV—methane fermentation; V—maturation phase) (Source: G. Tchobanoglous, H. Theisen, and S. Vigil, Integrated Solid Waste Management: Engineering Principles and Management Issues, McGraw-Hill, New York, 1993.)

FIGURE 3.31 Generalized phases in generation of landfill gases (I—initial adjustment, II—transition phase; III—acid phase; IV—methane fermentation; V—maturation phase) (Source: G. Tchobanoglous, H. Theisen, and S. Vigil, Integrated Solid Waste Management: Engineering Principles and Management Issues, McGraw-Hill, New York, 1993.)

Phase II: Transition phase. In phase II, identified as the transition phase, oxygen is depleted and anaerobic conditions begin to develop.

Phase III: Acid phase. In phase III, the bacterial activity initiated in phase II is accelerated 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, organic polymers, and proteins) into compounds suitable for use by microorganisms as a source of energy and cell carbon. The second step in the process (acidogenesis) involves the bacterial conversion of the compounds resulting from the first step into lower molecular weight 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.

Phase IV: Methane fermentation phase. In phase IV, a second group of microorganisms that convert the acetic acid and hydrogen gas formed by the acid formers in the acid phase to methane (CH4) and CO2 becomes more predominant. Because the acids and the hydrogen gas produced by the acid formers have been converted 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.

Phase V: Maturation phase. Phase V 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.

Control of Landfill Gas Migration When methane is present in the air in concentrations between 5 and 15 percent, it is explosive. Because only limited amounts of oxygen are 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 be formed if landfill gas migrates off site and is mixed with air. The lateral migration of methane and other gases can be controlled by impermeable cutoff walls or barriers (see Figure 3.32) or by the provision of a ventilation system such as gravel-filled trenches around the perimeter of the landfill (see Figure 3.33). Gravel-packed perforated pipe wells or collectors may also be used to collect and diffuse the gas to the atmosphere, if not recovered. To be effective, the system must be carefully designed, constructed, and maintained.

Cutoff walls or barriers should extend from the ground surface down to a gas-impermeable layer such as clay, rock, or groundwater. Clay soils must be water saturated to be effective. Perforated pipes have been shown to be of limited effectiveness and are not recommended for the reduction of gas pressure when used alone. Gravel-filled trenches may permit migration of gases across the trench, especially when covered by snow or ice; vertical perforated pipes reduce somewhat the effect of snow or ice. Gravel-filled trenches require removal of leachate or water from the trench bottom and are susceptible to plugging by biomass buildup. Gravel-filled trenches in combination with an impermeable barrier provide good protection against gas migration when keyed to a gas-impermeable strata below the landfill. Induced exhaust wells or trenches with perforated pipes and pump or blower are reported to be very effective. Where enclosed structures are constructed over or in close proximity to a landfill, it is necessary to have these places continuously monitored. A combustible gas detection system to provide early warning (light and alarm) can alert personnel. The monitors comprising the detection system can also activate ventilation fans at present low methane levels. Soil and cement bentonite trenches or cutoff walls have also been used to prevent lateral gas migration.

Methane Recovery and Utilization Methane is produced in a landfill when anaerobic methane-producing bacteria are active. The condition shown

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  • Veijo
    How to control the generation of gas at land fill?
    2 years ago

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