LandGEM: EPA’s Landfill Gas Emissions Model – Part 2

The 60% of MSW that is organic does not begin generating LFG all at once. Instead, LFG production occurs through four distinct stages over the lifetime of the landfill. These stages are described as follows:

Stage I: Aerobic Decomposition. After initial disposal and in-waste compaction, the air voids within the landfill’s waste mass are nearly identical to the atmosphere, with large amounts of oxygen. This oxygen supply allows for the first stage, which is driven by aerobic (oxygen-using) bacteria. This is a relatively short duration stage, and it begins almost immediately after waste disposal. The aerobic bacteria subject the organic portion of the waste to both hydrolysis (chemical reactions with moisture and water present in the waste mass that result in the breakdown of such complex organic molecules as carbohydrates into simpler ones, such as sugar) and aerobic degradation. This process generates heat, raising the waste’s temperature to as high as 160°F (70°C), producing both carbon dioxide and water vapor as the available oxygen in the waste void spaces is consumed. Once the oxygen has been almost completely removed, an anaerobic (non-oxygen) condition forms, and the next stage begins.

Stage II: Acidogenesis. Anaerobic bacteria are poisoned by oxygen, and with the oxygen consumed they quickly displace the previous aerobic microbes. The continuing hydrolysis by anaerobic bacteria is actually a form of fermentation that produces organic acids, hydrogen, carbon dioxide, water vapor, ammonia, and nitrogen. The hydrogen and carbon dioxide are produced as byproducts of the fermentation of the simpler organic material previously produced by the aerobic bacteria, creating volatile fatty acids. Concurrent with this stage, sulfur-reducing bacteria produce hydrogen sulfide (giving LFG its “rotten egg” smell). Anaerobic decomposition is a process that requires the addition of heat energy and the temperature of the waste usually falls. Like the first stage, this is also a relatively short-lived stage, compared with the main methane-production stage.

Stage III: Acetogenesis. This last, preparatory stage involves conversion of the volatile fatty acids produced by the previous stage’s activities into acetic acid, carbon dioxide, and hydrogen. This continues under anaerobic conditions, requiring additional heat. So, by this stage, the waste’s temperature has typically fallen to less than 100°F from its peak in the first stage. With this stage stable LFG production can now commence.

The 60% of MSW that is organic does not begin generating LFG all at once. Instead, LFG production occurs through four distinct stages over the lifetime of the landfill. These stages are described as follows: Stage I: Aerobic Decomposition. After initial disposal and in-waste compaction, the air voids within the landfill’s waste mass are nearly identical to the atmosphere, with large amounts of oxygen. This oxygen supply allows for the first stage, which is driven by aerobic (oxygen-using) bacteria. This is a relatively short duration stage, and it begins almost immediately after waste disposal. The aerobic bacteria subject the organic portion of the waste to both hydrolysis (chemical reactions with moisture and water present in the waste mass that result in the breakdown of such complex organic molecules as carbohydrates into simpler ones, such as sugar) and aerobic degradation. This process generates heat, raising the waste’s temperature to as high as 160°F (70°C), producing both carbon dioxide and water vapor as the available oxygen in the waste void spaces is consumed. Once the oxygen has been almost completely removed, an anaerobic (non-oxygen) condition forms, and the next stage begins. Stage II: Acidogenesis. Anaerobic bacteria are poisoned by oxygen, and with the oxygen consumed they quickly displace the previous aerobic microbes. The continuing hydrolysis by anaerobic bacteria is actually a form of fermentation that produces organic acids, hydrogen, carbon dioxide, water vapor, ammonia, and nitrogen. The hydrogen and carbon dioxide are produced as byproducts of the fermentation of the simpler organic material previously produced by the aerobic bacteria, creating volatile fatty acids. Concurrent with this stage, sulfur-reducing bacteria produce hydrogen sulfide (giving LFG its “rotten egg” smell). Anaerobic decomposition is a process that requires the addition of heat energy and the temperature of the waste usually falls. Like the first stage, this is also a relatively short-lived stage, compared with the main methane-production stage. Stage III: Acetogenesis. This last, preparatory stage involves conversion of the volatile fatty acids produced by the previous stage’s activities into acetic acid, carbon dioxide, and hydrogen. This continues under anaerobic conditions, requiring additional heat. So, by this stage, the waste’s temperature has typically fallen to less than 100°F from its peak in the first stage. With this stage stable LFG production can now commence. [text_ad] Stage IV: Methanogenesis. This fourth and longest stage converts available acetate to methane and carbon dioxide will consuming the last of the hydrogen in a process that also involve carbon-dioxide reduction by free hydrogen molecules. This phase is the longest duration, lasting for the bulk of the landfill’s operational lifetime and post closure care period, and beyond (longer than all the other phases combined). While earlier phases last for several years, this fourth stage lasts for decades, often extending even beyond the site’s post closure care period. Settlement of the waste because of decomposition also achieves maximum volume reduction at this time. However, once all of the available acetate is converted into methane, the landfill can theoretically revert back to its initial aerobic stage. However, most modern final cap-and-cover systems utilize impermeable high-density polyethylene (HDPE) geomembranes, which effectively preclude atmospheric intrusion into the landfill. As a practical matter, this possibility can be discounted for planning purposes, since methane production usually extends beyond the regulatory mandated post-closure care and planning period, and most Subtitle D landfills have not existed long enough for this stage to begin much less be fully played out. The major components of the landfill gas produced during the methanogenesis stage includes

• CO₂—45% to 55%
• CH₄—45% to 55%
• Trace chemicals—usually less than 1%

The trace chemicals and non-methane organic compounds (NMOCs) that make up the less than 1% fraction include

• H₂S
• Benzene
• Ethyl Benzene
• Toluene
• Vinyl Chloride
• Dichloromethane
• Trichloroethylene
• 1, 2, -cis-Dichloroethylene
• Tetrachloroethylene

NMOCs are usually measured in terms of the representative molecular weight of hexane (C₆H₁₄ as parts per million). However, as mentioned above, the amount and constituents of LFG can vary considerably from landfill to landfill, within a single landfill, over its operational lifetime, or even from season to season.

Model Assumptions and Computational Methodology It should be clear by know that projecting LFG production rates is a tricky business due to the wide variety of facility, material, and situational characteristics that impact the amount of LFG produced per amount of waste in a given time frame. The best anyone can hope to do is come up with a range of values for production estimates with a standard value based on typical characteristics for the purpose of planning. LandGEM is a first-order decay model that mimics the actual first-order decay rate of landfill gas production that occurs after its overall production peaks. Reactions whose rate depends only on the concentration of one reactant (known as first-order reactions) consequently follow exponential (i.e., ever-increasing) decay. LandGEM relies on several model parameters with assumed values to estimate landfill emissions. These parameters include the projected methane generation rate (K), the potential methane generation capacity (L), assumed NMOC (non-methane organic compound) concentrations, and the assumed methane content of the overall LFG emissions. For the purposes of modeling, methane is typically assumed to be 50% of the total LFG emissions by volume (with the other 50% being carbon dioxide). NMOC concentrations are usually assumed to be insignificant (4,000 parts per million as hexane being a typical value). The first two values, K and L, have the greatest impact on projected annual LFG production rates. The value of K can be a wide range of values that are derived from four factors:

• Moisture content of the waste mass (which ranges from near zero in arid landfills to nearly saturated in wet bioreactors)
• Availability of the organic and carbon nutrients in the waste for microorganisms to break down and generate methane and carbon dioxide
• pH of the waste mass, which depends on part on its moisture content and organics percentage
• Temperature of the waste mass, which in turn depends on the temperatures achieved by the previous exothermic and endothermic reactions prior to methanogenesis

Based on this model, Table 1 summarizes the default values of K. Increases in K result in increased LFG generation rates, and wet conditions (either natural or man-made) result in higher values of K. Wet conditions enhance and accelerate waste stabilization, which results in increased gas production. Lo, however, depends only on the amount of cellulose contained in the deposited waste mass. The greater concentration of cellulose, the higher the value of Lo. The default Lo values employed by the model are typical of average MSW wastestreams. Lo is measured in cubic meters of gas generated per megagram of waste. The default Lo values used by the model are provided in Table 2. The actual equation used by the model to calculate projected methane production for each of the landfill’s operational and post-closure year is as follows: where: ^QCH₄ = annual methane generation in the year of the calculation (cubic meters / year) i = summation using 1-year time increments n = (year of the calculation) – (initial year of waste acceptance) j = additional summation using 0.1-year time increments, not months k = methane generation rate (1/year) Lo = potential methane generation capacity (cubic meters/megagram, or metric ton of waste) Mi = mass of waste accepted in the “ith” year (megagrams, or metric tons, equal to 1,000 kilograms or 2,204.622 pounds) tij = age of the “jth” section of waste mass Mi accepted in the “ith” year (decimal years, not months) The projected methane generation rate (K) determines the rate of methane production for each sub-mass of waste in the landfill. K is a constant that determines the rate of LFG generation. The value of K is a function of waste moisture content, the abundance of nutrients for the anaerobic microbes, the pH value of the waste and the temperature of the waste. The higher the value of K the faster the methane rate increases and then decreases over time. The model assumes that the value of K is the same before and after peak production of methane occurs (a point which coincides with the last receipt of waste and close out of the landfill). A standard value of K used by the LandGEM model is K = 0.05 per year. However, field observations indicate a wide range of potential values of K, from 0.003 per year in arid climates to 0.70 per year for wet bioreactor landfills.  

Stage IV: Methanogenesis. This fourth and longest stage converts available acetate to methane and carbon dioxide will consuming the last of the hydrogen in a process that also involve carbon-dioxide reduction by free hydrogen molecules. This phase is the longest duration, lasting for the bulk of the landfill’s operational lifetime and post closure care period, and beyond (longer than all the other phases combined). While earlier phases last for several years, this fourth stage lasts for decades, often extending even beyond the site’s post closure care period. Settlement of the waste because of decomposition also achieves maximum volume reduction at this time.

However, once all of the available acetate is converted into methane, the landfill can theoretically revert back to its initial aerobic stage. However, most modern final cap-and-cover systems utilize impermeable high-density polyethylene (HDPE) geomembranes, which effectively preclude atmospheric intrusion into the landfill. As a practical matter, this possibility can be discounted for planning purposes, since methane production usually extends beyond the regulatory mandated post-closure care and planning period, and most Subtitle D landfills have not existed long enough for this stage to begin much less be fully played out.

The major components of the landfill gas produced during the methanogenesis stage includes

• CO₂—45% to 55%
• CH₄—45% to 55%
• Trace chemicals—usually less than 1%

The trace chemicals and non-methane organic compounds (NMOCs) that make up the less than 1% fraction include

• H₂S
• Benzene
• Ethyl Benzene
• Toluene
• Vinyl Chloride
• Dichloromethane
• Trichloroethylene
• 1, 2, -cis-Dichloroethylene
• Tetrachloroethylene

NMOCs are usually measured in terms of the representative molecular weight of hexane (C₆H₁₄ as parts per million). However, as mentioned above, the amount and constituents of LFG can vary considerably from landfill to landfill, within a single landfill, over its operational lifetime, or even from season to season.

Model Assumptions and Computational Methodology
It should be clear by know that projecting LFG production rates is a tricky business due to the wide variety of facility, material, and situational characteristics that impact the amount of LFG produced per amount of waste in a given time frame. The best anyone can hope to do is come up with a range of values for production estimates with a standard value based on typical characteristics for the purpose of planning.

LandGEM is a first-order decay model that mimics the actual first-order decay rate of landfill gas production that occurs after its overall production peaks. Reactions whose rate depends only on the concentration of one reactant (known as first-order reactions) consequently follow exponential (i.e., ever-increasing) decay.

LandGEM relies on several model parameters with assumed values to estimate landfill emissions. These parameters include the projected methane generation rate (K), the potential methane generation capacity (L), assumed NMOC (non-methane organic compound) concentrations, and the assumed methane content of the overall LFG emissions. For the purposes of modeling, methane is typically assumed to be 50% of the total LFG emissions by volume (with the other 50% being carbon dioxide). NMOC concentrations are usually assumed to be insignificant (4,000 parts per million as hexane being a typical value). The first two values, K and L, have the greatest impact on projected annual LFG production rates.

The value of K can be a wide range of values that are derived from four factors:

• Moisture content of the waste mass (which ranges from near zero in arid landfills to nearly saturated in wet bioreactors)
• Availability of the organic and carbon nutrients in the waste for microorganisms to break down and generate methane and carbon dioxide
• pH of the waste mass, which depends on part on its moisture content and organics percentage
• Temperature of the waste mass, which in turn depends on the temperatures achieved by the previous exothermic and endothermic reactions prior to methanogenesis

Based on this model, Table 1 summarizes the default values of K.

Increases in K result in increased LFG generation rates, and wet conditions (either natural or man-made) result in higher values of K. Wet conditions enhance and accelerate waste stabilization, which results in increased gas production.

Lo, however, depends only on the amount of cellulose contained in the deposited waste mass. The greater concentration of cellulose, the higher the value of Lo. The default Lo values employed by the model are typical of average MSW wastestreams. Lo is measured in cubic meters of gas generated per megagram of waste. The default Lo values used by the model are provided in Table 2.

The actual equation used by the model to calculate projected methane production for each of the landfill’s operational and post-closure year is as follows:

where:

^QCH₄ = annual methane generation in the year of the calculation (cubic meters / year)

i = summation using 1-year time increments

n = (year of the calculation) – (initial year of waste acceptance)

j = additional summation using 0.1-year time increments, not months

k = methane generation rate (1/year)

Lo = potential methane generation capacity (cubic meters/megagram, or metric ton of waste)

Mi = mass of waste accepted in the “ith” year (megagrams, or metric tons, equal to 1,000 kilograms or 2,204.622 pounds)

tij = age of the “jth” section of waste mass Mi accepted in the “ith” year (decimal years, not months)

The projected methane generation rate (K) determines the rate of methane production for each sub-mass of waste in the landfill. K is a constant that determines the rate of LFG generation. The value of K is a function of waste moisture content, the abundance of nutrients for the anaerobic microbes, the pH value of the waste and the temperature of the waste. The higher the value of K the faster the methane rate increases and then decreases over time. The model assumes that the value of K is the same before and after peak production of methane occurs (a point which coincides with the last receipt of waste and close out of the landfill). A standard value of K used by the LandGEM model is K = 0.05 per year. However, field observations indicate a wide range of potential values of K, from 0.003 per year in arid climates to 0.70 per year for wet bioreactor landfills.