Since the development of the first landfill in 1937, landfilling has been the dominant waste management strategy nationally and internationally. As shown in Figure 1, in 2018, a total of 292.4 million tons of municipal solid waste were generated in the United States, and 50% of the generated waste was landfilled (EPA 2018). Compared to the 96% of waste landfilled in 1960, the overall percentage of waste placed in landfills has declined; however the quantity of the landfilled waste, in fact, increased from 82.5 million tons to 146.1 million tons (EPA 2018). With a tremendous amount of waste being placed in landfills annually, it is important to recognize the energy recovery potential of the waste stored inside the landfills and promote active utilization of waste as a valuable resource, specifically through landfill gas collection. In this paper, a thorough overview of the current understanding of landfill gas generation and collection is provided. The basic chemical decomposition of waste is explained as well as the factors that influence such reactions and the composition of gases produced as a result. Then, the current available collection methods are explored, as well as how the efficiency of gas collection may change with various parameters. Finally, the general utilization pathways of landfill gas are discussed to emphasize the potential to limit greenhouse gas emissions while simultaneously turning landfill gas into a useful output.
The organic portion of landfill waste undergoes anaerobic digestion carried out by various microbes to produce a wide range of landfill gases, mainly CH4 and CO2. These two compounds, along with a small volume of other compounds grouped into the term “trace gases”, make up landfill gas (LFG). This decomposition of wastes and subsequent production of the majority of LFG consists of five phases: hydrolysis, acidogenesis, acetogenesis, methanogenesis, and maturation. These are described below in Figure 2 and in the following section.
Fermentation continues in acetogenesis. More organic fatty acids are converted to acetic acids, H2, and CO2, and accumulation of acids generated through the two fermentation phases causes a decrease in pH, which could potentially inhibit the speed of degradation if excessive (Khalil et al. 2014).
Finally, in the maturation phase, most organic waste is already degraded and has been converted into landfill gases, and the rate of gas generation drastically declines as the landfill stabilizes. It is reported that the half-life of landfill gas production is about three to four years (Rettenberger 2018); however, slow but steady gas production continues to occur for 20 to 30 years or more (Khalil et al. 2014).
Due to the high variability in the settings of biodegradation, waste composition, and individual site characteristics, the rate of degradation and the volume of landfill gas produced per unit of waste can vary greatly, and accurately predicting how these parameters impact the overall gas production can be challenging. Many studies have been conducted to investigate such relationships, and the main factors influencing gas production are discussed in this section.
Moisture content of waste is one of the most critical components in successful decomposition of organic matter and gas generation. Typical moisture content of landfill waste lies between 20-30 percent, though some variance exists depending on the site conditions. Minimal changes in the moisture content do not incur drastic improvement in degradation; however, when raised to 60 to 80 percent, enhancement of waste biodegradation has been observed along with maximized CH4 generation (Wreford 1996). Such increase in gas production is mostly due to the improved nutrient and microorganism population distribution as a result of leachate flowing through the wider portion of the waste matrix. Despite the addition of moisture, the total population of methanogen remains relatively unchanged (Barlaz et al. 1992); thus, the benefit of high moisture content can be owed largely to the improved mixing of leachate, which increases nutrient availability while also removing compounds inhibiting regular microbial activity (Wreford 1996). Elevated moisture content can yield positive results, but it should be noted that uncontrolled addition of water, such as excessive infiltration of precipitation into a landfill, may produce adverse effects like interrupted CH4 production and increased leachate volume (Wreford 1996). The leachate generated from landfills must be collected and treated per regulations.
Adequate availability of nutrients such as carbon, nitrogen, oxygen, hydrogen, and phosphorus are crucial in ensuring proper digestion of organic materials (Gardner and Probert 1993). The ratio of 30 biodegradable carbon to nitrogen ratio was found to be ideal for maximizing CH4 production (Gardner and Probert 1993). On the other hand, a surplus of sulphate may prevent gas generation. Figure 3 shows that H2 gas is used both in sulfate reduction and methane generation; thus an increased increased activity of sulfate-reducing bacteria could cause exhaustion of the H2 gas needed during methanogenesis (Wreford 1996). Furthermore as shown in Figure 4, oxygen gas is lethal to methanogens, and an influx of O2 due to active gas extraction can inhibit CH4 production by reducing the number of bacteria (Rettenberger 2018).
Temperature can have a great impact on landfill gas production, especially for shallower landfills that are vulnerable to seasonal conditions such as heavy rain or snowfall. Methanogens can carry out normal functions in the range 59℉ to 113℉, and the optimal temperature for CH4 production was found to be between 90℉ to 95℉. Below 50℉, a drastic reduction of degradation rate was observed (Khalil et al. 2014). While higher temperature does improve volatilization and stimulate chemical reactions (Khalil et al. 2014), injecting heat into a landfill might be challenging and costly, and thus the focus should be to seal the heat generated through reactions in the landfill by carefully designing thermal insulating soil covers.
For maximum CH4 production, the landfill waste should maintain a neutral pH level around 7 (Wreford 1996), and methanogenic activity could be severely inhibited if the pH drops below 6 or rises above 8 (Gardner and Probert 1993). Due to the acids produced during the anaerobic reaction, the pH may decrease to an undesirable level for gas production. Such phenomenon can be exacerbated with the addition of digested sludge or old refuse, which accelerates and stimulates more microbial activity but also produces more acids that will inhibit methanogenesis (Khalil et al. 2014). To mitigate this, a layer of dry buffer material can be placed along with landfill waste (Wreford 1996), or the waste can be pretreated before burial to control the acid release in the landfill (Khalil et al. 2014).
Figure 5 shows a typical waste composition in the United States; however there is a great variance. Depending on the population and mean standard of living near each landfill, landfill waste composition can differ significantly, meaning the fraction of organic waste, which dominates landfill gas production, will also fluctuate greatly (Daskalopoulos and Probert 1998). Wastes like food, paper, and garden cuttings are easily biodegradable, thus contributing mostly to earlier gas production, whereas materials like plastic, wood, and rubber are harder to biodegrade and will biodegrade much more slowly (Khalil et al. 2014). For an accelerated gas production and more rapid stabilization of the landfill, waste can be shredded prior to placement such that the surface area to mass ratio is increased, resulting in a greater chance of interaction between microbes and waste (Khalil et al. 2014). However with reduction in waste particle size, surplus of acid release is of serious concern, which could slow down methanogenic activities (Wreford 1996).
The level of waste compaction can also influence the landfill gas production. With higher compaction, wastes are physically closer to nutrients and microorganisms, promoting more degradation and increased CH4 production (Wreford 1996). Yet high compaction may not be ideal for some landfills, such as a landfill with high moisture content or a bioreactor landfill with leachate circulation, as the compacted waste could be difficult for leachate to flow through and therefore result in a buildup of pore pressure. Thus, a looser compaction level might be more suitable for waste with high moisture content to ensure an easier circulation of leachate (Khalil et al. 2014). Furthermore, the timing of compaction is also important since compaction in the beginning of methane production with only a small population of microorganisms could inhibit methane production, but compaction during the active methane production phase could in fact enhance the decomposition rate (Ko et al. 2016). Figure 6 shows the difference in methane generation rate depending on the timing of compaction, and the increase of the generation was recorded when the compaction was applied at the peak of methanogenesis.
As described in Figure 4, CH4 and CO2 constitute almost the entirety of landfill gas, with 50-60 percent of landfill gas being CH4 and 40-50 percent being CO2 (Khalil et al. 2014). During the fermentation phases of acidogenesis and acetogenesis, a large volume of CO2 yield occurs, while CH4 generation mainly happens during methanogenesis (Wreford 1996).
Trace gases constitute around 1 percent of total landfill gas generated (Duan et al. 2021). Among many trace gas that can be observed, as shown in Figure 8, seven main trace gases are discussed in the following sections. While the emission amounts are minimal, trace gases can pose threats to air quality and public health and should be monitored with caution.
Currently, landfill gas (LFG) is required by the Clean Air Act to be collected from landfills that are above a specific size in order to minimize greenhouse gas emissions (EPA). According to Barlaz et al., landfills that require gas collection include those “with a design capacity of 2.5 million Mg… that emit greater than 50 Mg of non-methane organic carbon per year” (Barlaz et al. 2012). Collected landfill gas can be utilized for electricity generation, directly used as a fuel, or disposed of through a process called flaring. The use of LFG after collection will be discussed in Section 4. LFG can and should be collected both during the period in which the landfill is still accepting waste and during the post-closure period (Rettenburger 2018). The length of the post-closure period is determined by the landfill’s size and other factors, but it is typically about 30 years at minimum (EPA). Including this post-closure period, LFG can be collected from a single landfill for 60 to 80 years.
Landfill gas collection systems are separated into passive and active systems. A passive system collects gas simply by inserting vents into the landfill and allowing LFG to travel ambiently up through the vents (Meegoda et al.). The vents may be vertical or horizontal within the waste, and may extend lengths up to 75% of the depth of the landfill waste (Meegoda et al.). In order for a passive system to be effective, the landfill must have adequate internal pressure to push air upwards (Rettenburger 2018). Generally, a passive system releases the collected gas into the atmosphere; thus, passive systems are not frequently used in large landfills since many LFG components are greenhouse gases regulated by the Clean Air Act or other legislation. Rettenburger recommends that passive systems only be used in cases where methane generation is below 0.5 liters per square meter of waste volume, and where the gas released through the cover system of the landfill is negligible (Rettenburger 2018). Passive systems are often practical for lower volume landfills (less than 40,000 cubic meters, according to Meegoda et al.) or landfills that have been closed for a significant period of time and are no longer producing large volumes of gas (Meegoda et al., Rettenberger 2018).
Active systems of landfill gas collection generate a vacuum inside extraction wells in order to extract landfill gas from inside the system (Meegoda et al). The vacuum is created using blowers that are sized and distributed proportionally to the amount of gas that is being collected through the overall system (Meegoda et al.). The extraction wells all connect to a larger header pipe, which may be located just below the surface in a sand trench (Meegoda et al.). Extraction wells may extend vertically or horizontally within the landfill (Meegoda et al.). Calculations must be completed to ensure that the distribution of wells adequately extracts gas from all areas of the landfill (Meegoda et al.).
The efficiency of LFG collection is defined by Barlaz et al. as “the amount of LFG that is collected relative to the amount generated in the landfill”, and it depends on a number of factors. According to a study titled “Evaluation of Landfill Gas Emissions from Municipal Solid Waste Landfills for the Life Cycle Analysis of Waste-to-Energy Pathways” done by Lee et al., these factors mostly revolve around collection strategy and include collector operation, cover type, cover installation time, and others (Lee et al. 2017). Efficiency factors are explored below.
Lee et al. explains that because they often serve slightly different purposes, horizontal and vertical LFG collectors have different levels of efficiency (Lee et al. 2017). Horizontal extraction wells are more often used while the landfill is still active - in other words, while waste is still being added to the system and the top cover system is not yet in place (Lee et al. 2017). However, vertical collectors are usually installed after the cover system is placed and the landfill is closed to further waste; thus, a much stronger vacuum suction is generated because of the density of waste surrounding the well and the largely-impermeable boundaries of the system as a whole (Lee et al. 2017). Because of this difference, vertical systems tend to be more efficient than horizontal systems (Lee et al. 2017).
Lee et al. also elaborates on the effect of “waste decay speed” on LFG collection efficiency (Lee et al. 2017). Essentially, any waste decay that occurs before or after landfill gas collection systems are installed and in use will subtract from the overall collection efficiency because of the LFG that is generated and not collected (Lee et al. 2017). Thus, any waste decay that is extremely fast or slow will result in gases that are released outside of the time frame in which the collection system is in place and employed (Lee et al. 2017). The decay speed largely depends on the type of waste deposited in the landfill (Lee et al. 2017). As discussed previously, organic waste items such as food and paper will degrade much more quickly than non-organics such as plastics (Khalil et al. 2014).
Barlaz et al. 2012 describes the influence of daily, intermediate, and final cover systems on the efficiency of LFG collection. Both daily and intermediate covers are not regulated in terms of hydraulic conductivity, so the material chosen for each of these covers can greatly affect the LFG collection efficiency (Barlaz et al. 2012). Some materials often used for daily covers include wood chips, foams, or plastics, which can vary greatly in hydraulic conductivity (Barlaz et al. 2012). The site’s native soils are usually used as an intermediate cover, so a sandy site may have lower efficiency due to sand’s high hydraulic conductivity (Barlaz et al. 2012). The final cover is regulated much more specifically, but Barlaz et al. emphasizes that different cells within a landfill will have different cover types at any given time (Barlaz et al. 2012). Thus, the construction sequence - how many cells are covered with each cover type at any given point - of a landfill will greatly influence its overall LFG collection efficiency (Barlaz et al. 2012).
Lastly, the layout and design decisions of the collection system will affect the overall efficiency of LFG collection. As mentioned previously, calculations are performed to determine design parameters of an LFG collection system. These calculations determine the spacing of extraction wells, the size of blowers in the case of an active system, and other specifications. Barlaz et al. points out that specific landfills are designed for varying purposes and according to various regulations: some are preventing the migration of LFG in the subsurface, others are minimizing methane above the top cover of the landfill, and still others are maximizing methane concentration in collected gas in order to generate energy (Barlaz et al. 2012). Obviously, these changes in the desired landfill gas controls will affect the overall collection efficiency (Barlaz et al. 2012).
As stated previously, landfill gas that is not released to the atmosphere can be flared or used for energy through a variety of processes.
Flaring describes the combustion of landfill gas that is collected (Environment Agency 2002). This combustion can happen in open air or in an enclosed container, but both systems include the same process of burning landfill gas to convert methane to carbon dioxide (Environment Agency 2002). Morgan et al. describes open flares as having “less efficient combustion, [possibly resulting] in aesthetic complaints, and [being] difficult to test for emissions [content]” (Morgan et al. 2001). Alternatively, enclosed flares are more costly, but generally have better efficiency and may require less repairs or maintenance (Morgan et al. 2001). Flaring does not result in utilization of landfill gas or production of energy, but rather is considered as a method of “disposal” for LFG, and largely is used to reduce the greenhouse gas contribution of landfills (Morgan et al. 2001).
Morgan et al. describes direct, local use as the “simplest and most cost-effective way to exploit recovered gas” (Morgan et al. 2001). This gas is sold as a medium-Btu substitute or supplement for natural gas, and requires the presence of a pipeline for transport (Morgan et al. 2001). Thus, the purchase of this gas is generally only feasible for industry clients with the resources to install a pipeline for transport (Morgan et al. 2001). However, when possible, direct use is an excellent way to utilize landfill gas.
Electricity generation is the most common usage of collected landfill gas, making up approximately 70% of landfill energy projects in the U.S. (Morgan et al. 2001; EPA). There are multiple technologies available for the conversion of LFG to electric power via combustion, gas turbines, or other methods (Morgan et al. 2001). Internal combustion engines are the most common (Morgan et al. 2001). Currently, the U.S. Environmental Protection Agency’s Landfill Methane Outreach Program (LMOP) reports 386 operational landfills using LFG for electricity generation.
The emissions and energy production associated with a given landfill are highly associated with the active landfill conditions, including moisture content, primary waste types, compaction, pH, and temperature. In addition, once landfill gas is produced, the subsequent capture of the gas greatly affects the emissions and energy potential as well. The gas collection is influenced by the collection system and its design, cover types, waste type, and regulations associated with the particular landfill based on size, location, and type of waste it accepts. The collected gas can then be converted to a less potent greenhouse gas (CO2) through flaring, used as a form of natural gas, or used for electricity generation. Overall, landfill gas generation, collection, and use are all highly affected by the landfill’s type and management, and there is extensive potential to expand the utilization of landfill gas as a sustainable energy source with the knowledge of these factors.
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