34 Landfill Gas Management
Dr. Yogalakshmi K. N
Objectives:
1. To know the composition and characteristics of landfill gas
2. To understand the mechanism of landfill gas generation
3. To know the stages of bacterial decomposition that leads to gas generation
4. To understand the conditions favoring or affecting landfill gas generation and migration
5. To understand landfill gas migration and factors influencing them
6. To identify various landfill gas collection and treatment systems
1.0 Introduction
Solid wastes, especially Municipal Solid Waste (MSW) is ultimately disposed off and land disposal happens to be traditional means of disposing solid waste. However during older times, waste was simply collected at one place on the ground and piled in heap. Such site was referred to as dumping site. Open dumps cause number of undesirable consequences which adversely impacts human health and environment. It attracts disease causing vectors such as flies, mosquitoes; rodents etc. and emit foul odours. They pollute surface water as well as ground water and catch spontaneous fires regularly. In addition to these problems, open dump also pollute ground water and soil hence this practice is prohibited in some countries.
Sanitary landfills happen to be improved and secure method for the disposal of solid waste. It was first referred in 1930s for compacting solid waste in particular area. The solid waste is placed in a secure area and compacted mechanically. Each day the waste is layered and compacted. The designing of landfills is carefully carried out according to prescribed method in such a way that it does not cause any harm to human health and the surrounding.
The solid waste, once disposed in landfill undergo physical, chemical and biological reactions. The organic substances present in the landfills undergo biological degradation which leads to the generation and release of leachate and gases. The biological reactions occurring in the landfills depend on moisture content, waste composition, temperature, oxygen availability, microflora, compaction rate and many more.
2.0 Landfill gas
The waste that is contained in the landfill undergoes a wide range of reactions and release a mixture of different gases. Landfill gas is the product of methanogenesis and mainly consists of methane and carbon dioxide with minor concentration of nitrogen, carbon monoxide, oxygen, ammonia, sulfides, hydrogen, and non-methane organic compounds (NMOCs) like trichloroethylene, benzene, and vinyl chloride. The process of biological degradation of organic matter which release landfill gas in the landfill is affected by number of factors such as temperature, moisture, waste composition etc.
3.0 Generation of landfill gas
Landfill gas is produced chiefly by three processes which are mentioned below:
- Microbial decomposition: Out of total landfill gas production, major part is attributed to microbial degradation of organic waste mediated by microbes present in the waste material naturally or in the soil which is used as cover/ liner material. The biological degradation of the organic waste material occurs in different stages and the composition of gas changes in every stage.
- Volatilization: Apart from microbial decomposition, the processes of volatilization (i.e. solid changes to liquid or gas) which occur in landfill also produce landfill gases. Volatilization of some compounds in the landfills releases non-methane organic compounds.
- Chemical reactions: Chemicals present in the landfills react and generates gases. For example, ammonia and chlorine react to generate a harmful gas.
Table 1 summaries the composition of landfill gas generated by different biodegradable components present in the waste.
Table 2 Landfill gas generated by various biodegradable components of waste
4.0 Stages of bacterial decomposition in waste landfill:
The waste in the landfill undergoes bacterial decomposition in five stages which influences or determines the composition of landfill gas and leachate. Landfills are designed to contain the waste for nearly 20 to 30 years. As the waste gets older it undergoes a number of changes. The waste disposed at different time intervals might be in different stage of decomposition. The five stages of bacterial decomposition of solid waste are mentioned below:
Stage I: Hydrolysis or Aerobic degradation
As the name suggests, hydrolysis or aerobic degradation occurs in the presence of oxygen. This step occurs during waste emplacement and a period thereafter and depends on the availability of oxygen that is trapped within the waste. Aerobic micro-organisms consume organic waste in the presence of the available oxygen to generate simpler hydrocarbons, water, carbon dioxide and heat. These reactions can raise the temperature of the waste in the landfill up to 70-90 ⁰C. The wastes which are more compact receive lesser oxygen and hence attain comparatively lower temperature. Depending on the availability of oxygen, extent of compaction and the frequency with which the waste is covered, this stage can last for few days to weeks.
Stage II: Hydrolysis and Fermentation
Almost all the available oxygen is consumed in stage 1 and results in the predominance of anaerobic or facultative bacteria due to anaerobic condition. In this stage, proteins, carbohydrates and lipids are hydrolyzed into sugars which are further decomposed to organic acids, carbon dioxide, ammonia and hydrogen. The production of the gases in the waste may rise up to 80% of carbon dioxide and 20% hydrogen. The temperature drops to 30℃ to 50℃.
Stage III: Acetogenesis
The organic acids produced in stage 2 are converted into acetic acid by acetogens. Carbon dioxide and hydrogen are also produced in this stage under anaerobic conditions. In the presence of carbon dioxide and hydrogen, carbohydrates are converted into acetic acid which results in the reduction of carbon dioxide and hydrogen. The drop in pH causes dissolution of metal ions which increases their concentration in the leachate. During this stage, the sulphur reducing bacteria reduces sulphate compounds present in the waste into hydrogen sulphide.
Stage IV: Methanogenesis
The major portion of landfill gas is generated in this stage (approximately 60% methane and 40% carbon dioxide). The reactions in methanogenesis occur at very slow rate. The methanogens are induced to produce methane and CO2 from the organic acids because of low levels of hydrogen. Also microbes directly convert carbon dioxide and hydrogen into CH4 and water. During this stage, mesophillic as well as thermophillic bacteria are activated at the temperature range of 30 to 35 ⁰C and 45 to 65⁰C, respectively. In this stage the organic acids are degraded which leads to rise in pH of the landfill waste (pH 7 to 8). As methanogenesis stage lasts for longer duration, the landfill gas persists to generate for 15 to 30 years.
Stage V: Oxidation
Oxidation is the final stage of waste decomposition. In this stage, anaerobic microbes are replaced by new aerobic micro-organisms which convert the residual methane into carbon dioxide and water. If the waste contains high amount of sulphates, there is formation of some amounts of hydrogen sulfide.
5.0 Conditions affecting landfill gas production
The factors affecting landfill gas generation is as follows:
Waste composition: As the amount of organic waste increases, the quantity of landfill gas also increases accordingly. In case of increased amount of chemicals in the waste, the generation of non-methane organic compounds are accelerated through the process of volatilization or chemical reactions.
Age of refuse: The waste that are more than 10 years old produce less amount of landfill gas compared to those which are more newly buried. Maximum amount of landfill gas is generated between 5 to 7 years.
Presence of oxygen in the landfill: Oxygen inhibits the methane production. Methane production generally occurs under strict anaerobic condition in the landfill.
Moisture content: The presence of moisture promotes certain chemical reactions which help in gas production. Microbial degradation is enhanced in the presence of moisture leading to increased landfill gas production.
Temperature: Microbial activity is accelerated with the increase in the temperature of the landfill. Higher temperature also leads to the process of volatilization and other chemical reactions which generates more amount of landfill gas.
6.0 Characteristics of landfill gas
Landfill gas consists of combination of different gases such as methane, carbon dioxide, and non-methane organic compounds. Different characteristics of landfill gas are as follows.
Density and Viscosity: The quantity of individual gases determines the concentration of landfill gas. For example, a mixture of 90% carbon dioxide and 10% hydrogen, produced in the first stage of anaerobic process, will be heavier compared to air, while a mixture of 40% carbon dioxide and 60% methane, produced during methanogenic stage will be slightly lighter compared to air.
Heat Value Content: In ideal condition, the heating value of landfill gas is 500 Btu/ft3 during methanogenic phase, which is half of that of natural landfill gas. The actual heating value of the landfill gas from a landfill is dependent on the age of the waste and type of landfill cover.
Water Vapor: The landfill gas generated during the decomposition of waste generally contain 4% to 7% by volume water vapor. The quantity of water vapor in landfill gas is function of temperature and pressure inside the landfill. Temperatures are typically elevated over ambient during biological decomposition, increasing the evaporation of water into the LFG.
Others: Hydrogen is produced during waste decomposition, particularly during the initial anaerobic conversion of mixed organic acids to acetic acid. Significant amounts of hydrogen are later consumed in the formation of methane. Hydrogen is flammable between 4% and 74% by volume in air. The presence of carbon dioxide affects these ranges although little significant change occurs near the lower limit of the range.
6.0 Landfill gas migration
The gas produced in the landfill does not remain idle, it keeps migrating and fills the void and pore space within the refuse and soil covering the landfill. Methane is lighter than air and hence travels vertically (i.e) upwards and reaches the surface of the landfill. However, the upward movement of the gas is hindered by the overlying compacted waste or the cover material. During such conditions, the air moves horizontally to find weak areas through which it can move upwards. Always gases use materials of least resistance to travel across namely sand and gravel. Carbon dioxide generally is collected in the subsurface due to density difference. Migration of gas (i.e) direction, speed, and distance of movement is governed by various factors such as
Diffusion: Landfill gas moves and spreads to the surrounding area through the process of diffusion (movement of gas from higher to lower concentration)
Pressure: Pressure plays an important role in migration of landfill gas. A high pressure is created in areas of landfill where gas accumulation or gas concentration is high. Accumulation generally occurs due to compaction of overlying waste. The variation in pressure results in movement of gas from high to low pressure areas. Increase in gas concentration increases the sub-surface pressure which tends to be higher than atmospheric pressure leading to release of gas to the atmosphere.
Temperature: When temperature increases gas diffusion and rate of migration also increases. Frozen soil over the landfill may act as a physical barrier for the upward movement of the landfill gas.
Landfill cover type and Permeability: Material used in landfill cover influences the migration of gas. Higher the permeability easier the migration of gases. Sand and gravel have more permeability than clay and silt.
Natural or cultural pathways: Landfill gas migration is also influenced by natural geology and manmade structures. Fractures, faults, fractured rocks and porous rocks are natural geological features that influence migration of gas. Likewise, drains, trenches, vents, pipelines and tunnels are man-made structures that act as channels for landfill gas movement.
Wind speed and direction: Wind carries the landfill gas and mixes the gas with air
Moisture: The gas accumulated within the pores are pushed out by rain water. Wet soil in the cover does not allow gas to escape from the surface.
Groundwater levels: Gas migration is influenced by variations in the groundwater table below the landfill. When the water table rises the gas moves upwards
7.0 Landfill gas collection
Landfill gas control systems include collection, conveyance, and treatment. An ideal collection system consists of a sequence of gas collection wells which are positioned throughout the landfill. The gas collection wells are designed based on the volume and density of waste, depth, and area of landfill. Collection wells in the landfill offer pathways for the passage of gas. The landfill gas is either discharged directly to the atmosphere or collected for the treatment depending upon the potential health, environmental risks and local regulatory criteria.
7.1 Passive Gas Collection Systems: Passive gas collection systems are generally established either during active operation or after the closure of a landfill. Collection wells or extraction wells are used in passive system for the gas collection. The collection wells are made of perforated plastic material which is mounted vertically throughout the landfill upto the depths of about 50% to 90% of the landfill waste thickness. If there is groundwater within the waste, the wells are terminated at the groundwater table. A passive collection system may also contain horizontal wells to facilitate passage of gas through the landfill. Horizontal wells are more suitable for landfills that need to recuperate gas rapidly. Gas can be controlled with the help of liners on the top, on the sides, and bottom of the landfill. An impermeable liner traps the landfill gas and is used to generate ideal gas passage pathways. The efficiency of a passive gas collection system depends on the way the gas is controlled and on the environmental conditions. When the pressure is not sufficient enough to drive the gas to the control/ venting device, the passive systems fail to eliminate landfill gas efficiently. In case of high pressure, outside air enters inside the landfill through passive vents which do not direct gas to control device. Therefore, passive collection systems are not consistent enough in areas with a high risk of gas migration.
7.2 Active Gas Collection: Active gas collection systems are considered the most effective means of landfill gas collection. It consists of both vertical and horizontal gas collection wells analogous to passive collection systems. Unlike passive gas collection system, the active system includes valves to control gas flow and to provide as a sampling port. Sampling facilitates measurement of gas generation, composition, and pressure. Active collection system includes vacuums or pumps which help to push gas out of the landfill. Vacuums or pumps create pressure in the gas collection wells and draw gas from the landfill. Based on the amount of gas generated the requirement of the size, type, and number of vacuums to draw the gas is decided. The gas migrates in the direction of low pressure. On the basis of landfill gas generation, composition, and pressure, a landfill operator can decide or modify the collection system.
8.0 Landfill gas treatment
The landfill gas generated is collected in controlled manner and subjected to treatment for reducing the environment and health hazard before discharging it into the atmosphere. But when the quantity of gas generated is relatively low and there is no human settlement in the nearby areas then the gas is directly released to the atmosphere. The treatment of landfill gas is generally carried out using the method of combustion, non-combustion technologies and odor control technologies.
8.1 Combustion Technologies: The combustion technique is most commonly utilized for the management and treatment of landfill gas. Combustion technologies comprise of flares, incinerators,boilers, gas turbines, and internal combustion engines which thermally degrade the substances present in landfill gas. In combustion technique, methane is converted into carbon dioxide resulting in the reduction of the risk of a large amount of greenhouse gas emission. Combustion or flaring technique is considered to be most efficient if the landfill gas consists of minimum of 20% methane by volume because the landfill gas readily develops combustible mixture with air and therefore require less energy for the operation. When the concentration of methane is less than 20% by volume in the landfill, then a supplemental fuel such as natural gas is required for the operation of flares which makes it expensive to operate. There are two types of flares used in the combustion technologies, open or enclosed flares.
Open flame flares come with a pipe through which the gas is impelled. It consists of a pilot light for igniting the gas alongside the regulation of the gas flow pathway. This technology is easy to use because of the simplicity of the design and the operation. The disadvantage of this technique includes inefficient combustion, difficulties in monitoring and aesthetic problems. Open flame flares may be partly covered to bury the flame and for the improvement of the accuracy in monitoring.
Compared to open flame flares, enclosed flame flares are more expensive and complex. But it comes with several other advantages which are absent in open flame flares. Enclosed flame flares contain multiple burners contained by fire-resistant walls which enclose above the flame. The enclosed flame flare system is such that it is capable to control the amount of incoming gas contrasting to open flame flares which makes combustion more consistent and proficient.
Other enclosed combustion systems such as gas turbines, boilers, process heaters, and internal combustion engines are also used to efficiently eliminate the organic compounds in landfill gas. In addition to this, these techniques help in generating useful electricity.
8.2 Non-combustion Technologies: Non-combustion technologies are alternative to combustion technologies. The process of combustion produces substances which cause smog such as nitrogen oxides, carbon monoxide, sulfur oxides, and particulate matter. Energy recovery technologies and Gas-to-product conversion technologies are the two types of non-combustion technologies. Landfill gas is directly converted into energy in energy recovery technologies, whereas they are converted into commercial products, like Compressed Natural Gas (CNG), methanol, purified carbon dioxide and methane, or Liquefied Natural Gas (LNG) in gas to product conversion technologies. Landfill gas has to be pretreated to remove impurities such as NMOCs, water, and carbon dioxide before subjecting non-combustion technologies.
8.3 Odor Control Technologies: As the name suggests, odour control technology help in preventing odour causing gases to escape from the landfill. The top cover restricts the odour from newly deposited waste and release of the landfill gases produced during the biological decomposition to escape into the open environment. Extensive covers are used as landfill top cover to restrict moisture from leaching the waste and promoting bacterial growth and decomposition. Plantation of vegetation on the landfill cover can also decrease odors. The process also helps in eliminating the odour from the landfill gas by combusting the odor causing gases thermally. Venting of landfill gas with the help of filter can be utilized to lessen the odors.
9.0 Beneficial uses of collected landfill gas
Landfill gas happens to be one of the biggest sources of man-made emissions especially methane gas. Methane gas is an important greenhouse gas as its global warming potential is much higher than that of carbon dioxide. Because of huge concentration of methane in the composition of landfill gas, it can be widely applied for energy use. The collection of the landfill gas for the energy use decreases the hazard of explosions. It also helps in conserving other energy resources, provide economic benefits to the people, and greatly decrease the risk of global climate change.
The landfill gas released from the landfill can be collected and made use in various ways. The landfill gas can be used on-site by using a boiler or any other types of combustion system which provide heat. With the help of microturbines, steam turbines, or fuel cells, electricity can also be generated on site. The landfill gas can be collected and sold off-site through natural gas pipelines. The quantity of energy recovered from the waste depends on the efficiency of gas collection system in at landfills. The landfills that are closed and do not accept waste any longer are capable of collecting more gas than those of open landfills which continue to accept waste. Landfill gas is also used for the evaporation of leachate produced as a byproduct of the reactions occurring in the landfill.
Summary
To summarize, in this module we have studied about
- Landfill gas
- Composition of landfill gas
- Characteristics of landfill
- Factors influencing landfill gas generation
- Landfill gas migration
- Landfill gas collection and treatment
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Reference
- Waste Management Paper 27, Landfill Gas, Department of the Environment, HMSO, London,1994.
- Paul T William (2005) Waste Treatment and Disposal, John Wiley and sons, Second Edition.