distinguishing elevated temperature landfill
DISTINGUISHING ELEVATED TEMPERATURE LANDFILL CHARACTERISTICS FROM LANDFILL FIRES AND SUBSURFACE OXIDATION EVENTS B. STALEY1, M. BARLAZ2, C. BENSON3, M. CASTALDI4, S. LEUTTICH5 1
Environmental Research & Education Foundation, Raleigh NC USA North Carolina State University, Raleigh NC USA 3 University of Virginia, Charlottesville VA USA 4 City University of New York, New York City NY USA 5 Geosyntec Consultants, Augusta ME USA 2
SUMMARY: Recently, a small number of landfills across the U.S. have experienced conditions where temperatures of the landfill gas or within the waste mass have persistently exceeded 100 C (212 F) over a substantial area of a landfill cell. The phenomenon has exhibited traits that contrast with symptoms typically observed with landfill fires or subsurface oxidation. Affected landfills exhibit combined traits that have not been observed for other types of elevated temperature events, including: increased hydrogen concentrations, odor problems,inhbition or cessation of methane production, increased leachate production, substantially higher leachate strength and increased settlement of the waste mass over short periods of time.
1. INTRODUCTION Recently, a small number of landfills across the U.S. have experienced conditions where temperatures of the landfill gas or within the waste mass have persistently exceeded 100 C (212 F) over a substantial area of a landfill cell (Figure 1). The phenomenon has exhibited traits that are in contrast to symptoms typically observed with landfill fires or subsurface oxidation. Landfills with such traits have been designated as elevated temperature landfills (ETLFs). While some ETLFs received reactive wastes that are a source of excessive heat, others have not. Moreover, there is considerable uncertainty as to the mechanisms controlling heat accumulation in landfills. A research project funded by the Environmental Research and Education Foundation (EREF) aims to better understand the mechanisms behind why some landfills are experiencing elevated temperatures. The objectives of this paper are to: (1) provide an overview of characteristics and traits that make ETLFs distinct from landfill fires or subsurface oxidation events, and (2) describe ongoing research by the EREF-funded researchers to understand the factors and mechanisms that contribute to ETLFs.
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 1. Temperature profile in a landfill experiencing elevated temperatures. ETLFs pose a number of potential problems including temperature damage to gas and leachate collection system infrastructure, rapid settlement with subsequent implications for the integrity of the landfill cover and slope stability, elevated leachate volume and strength, reduced methane content that may impact landfill gas treatment and energy generation processes, odorous gases, and challenges with regulatory compliance. In cases where the elevated temperature extends to the bottom of the landfill, the geomembrane liner may also be affected. Consequently, ETLFs often require increased monitoring and management.
2. DIFFERENCES BETWEEN LANDFILL FIRES AND SUBSURFACE REACTIONS Landfill fires occur at the landfill surface or near the surface where waste is burning or smouldering. These combustion reactions require a source of oxygen. While large landfill fires are rare, smaller fires are known to occur and are caused by phenomena such as hot loads from trucks, hot equipment, lightening strikes and via air intrusion near the landfill surface at shallow depths via the landfill cover or over pulling on gas collection wells. In contrast, a subsurface reaction (SSR) that creates an ETLF appears to generally occur relatively deep in the landfill waste mass where conditions are anaerobic and in many cases very wet. While not universal, these conditions have been largely observed in larger, deeper landfills where the highest temperatures are observed far below the landfill surface and in saturated or near saturated waste materials.
3. KEY CHARACTERISTICS OF ELEVATED TEMPERATURE LANDFILLS Data collected from ETLFs suggest that there are a number of traits that may suggest elevated temperatures are becoming problematic. Many landfills periodically have gas wells that exhibit elevated temperatures (i.e., greater than US limit of 55 C). A primary difference between a hot gas well and an
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
ETLF is the presence of a self-propagating reaction in an ETLF. In addition to increased temperatures, there are a number of traits that collectively serve as indicators matic of an ETLF. 3.1 Increased Hydrogen and Odors in the Landfill Gas Increased hydrogen has been routinely observed at ETLFs. Anaerobic decomposition is a multistep process facilitated by specific groups of microorganisms. The first steps involve hydrolysis and fermentation of organic materials by Bacteria that are converted to volatile fatty acids (e.g. acetate), carbon dioxide, and hydrogen (Eqn. 1). Eqn. 1 (Fermentation): C6H12O6 (glucose) + 2H2O à 2C2H4O2 (acetate) + 2CO2 + 4H2 The second step involves methanogenic Archaea consuming acetate and hydrogen and converting these compounds to methane (e.g., Eqn. 2). Eqn 2 (Methanogenesis): 4H2 + CO2 à CH4 + 2H2O In a healthy landfill, this process results in a biogas composition that is approximately 50% carbon dioxide and 50% methane. However, in an ETLF, conditions develop that inhibit methanogenesis. When this occurs, methane content drops substantially as the hydrogen produced from the fermentation step is not consumed, thus resulting in increased hydrogen concentrations in the landfill gas. Volatile fatty acid concentrations may also increase, resulting in substantially increased odors. Temperatures can get hot enough in ETLFs to damage gas collection systems, reducing gas collection capabilities and causing more landfill gas to escape to the atmosphere. The low methane content noted above decreases the effectiveness of combustion at the flare. Despite this, higher gas output and pressures can occur. Collectively, these factors create a situation where odor is far more of an issue compared to typical landfills. 3.2 Rapid and Widespread Settlement Under typical conditions, anaerobic biodegradation of MSW in a landfill results in waste settlement at a rate of less than a meter every few years. It is hypothesized that the increased temperatures and self-propagating attributes of an ETLF result in a breakdown of MSW at a substantially accelerated pace. While the exact mechanisms causing an ETLF are still under investigation, landfills experiencing this condition have observed settlement of a meter or more over a time period of several months and, in some cases, a few weeks. 3.3 Increased Leachate Volume and Strength As solids are converted to gas, the waste compresses and the field capacity of the waste mass decreases. This phenomenon is normal in a landfill and occurs over many years. In ETLFs, the process occurs over much shorter time frames, which is manifested as more rapid settlement and larger volumes of leachate to manage over shorter periods of time. This can introduce complexity in managing leachate since existing treatment processes and storage capacity may not be sufficient. The excessive liquid can also flood gas collection wells, making collection of landfill gas more difficult to collect.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Leachate from a normally operating landfill has biochemical oxygen demand (BOD) concentrations ranging from 1000 to 5000 mg/L. In contrast, ETLFs can have BOD concentrations in excess of 100,000 mg/L. The increased leachate volume coupled with substantially higher strength can make leachate from ETLFs very challenging to manage. 3.4 Low Methane to Carbon Dioxide Ratio in the Landfill Gas Temperature increases in ETLFs inhibit methanogenesis, resulting in a significant drop in methane concentration. As noted in Section 3.1, methane and carbon dioxide are stoichiometrically formed at a 1:1 ratio. Thus, when methane production is inhibited, one can expect the primary gas ratio (methane to carbon dioxide, CH4/CO2) ratio to be less than one. Operators of ETLFs report that a decreasing trend in the CH4/CO2 ratio is a leading indicator of temperature increases to a level at which biological methane generation is interrupted. Because CH4/CO2 is a leading indicator, decreases may be observed before the gas temperature increases. Figure 2 shows how this ratio may decrease abruptly, emphasizing the importance of regular monitoring and data review.
Figure 2. Primary gas (CH4/CO2) ratio over time at an ETLF where the ratio decreased rapidly.
In many cases, a decrease in the CH4/CO2 ratio has been observed to spread across a landfill over a period of months to even years before significant increases in temperature have been observed. Figure 3 depicts a gas well field at a landfill suspected of elevated temperatures over a four year period. As can be seen, even though nearly half of the wells exhibited an inverted CH4/CO2 ratio by year 2, no increased temperatures were observed. Yet by Year 4, nearly all gas wells had a primary gas ratio less
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
than 1 and nearly 40% of wells exhibited temperatures in excess of 80 °C. In this example, there was a lag time of nearly 3 years before increased temperatures were observed in 10% of the gas wells, even though at this point over 80% of wells had a primary gas ratio less than 1.
Figure 3. Percent of gas wells with CH4/CO2 ratio < 1 and gas well temperatures > 80 oC over time demonstrating the ratio as a leading indicator to elevated temperatures.
4. OPERATIONAL CONSIDERATIONS AND ONGOING WORK ETLF conditions tend to develop deep in the waste mass and in saturated or near saturated waste and can cause significant challenges in landfill operation, particularly with settlement, landfill gas and leachate management, and in some cases odors. Understanding the difference between a landfill fire (oxidation in the presence of oxygen) and a subsurface reaction that occurs anaerobically in wet conditions is important, as the mechanisms differ and different management strategies are required. Turning off the gas collection system is common practice during a landfill fire to starve the fire of oxygen. For an ETLF, this practice traps heat and volatile reactants, which may exacerbate or accelerate heat propagation. In an ETLF situation, the pace of the reaction can be slowed by increasing removal of heat and gaseous compounds. Thus, keeping the gas collection system on is important when managing ETLFs. Ongoing research conducted by N.C. State University, the University of Virginia, City University of New York, and Geosyntec Consultants under EREF’s sponsorship has focused on causal mechanisms and triggers of high temperatures such as anaerobic metal corrosion (e.g. aluminium, iron) and ash hydration/carbonation as well as the strategies to manage ETLFs. The working hypothesis of this research team is that these conditions lead to self-propagating pyrolytic reactions. If proven, the research will lead to the development of best management practices to avoid and/or mitigate elevated temperature landfill conditions.
Environmental Research & Education Foundation, Raleigh NC USA North Carolina State University, Raleigh NC USA 3 University of Virginia, Charlottesville VA USA 4 City University of New York, New York City NY USA 5 Geosyntec Consultants, Augusta ME USA 2
SUMMARY: Recently, a small number of landfills across the U.S. have experienced conditions where temperatures of the landfill gas or within the waste mass have persistently exceeded 100 C (212 F) over a substantial area of a landfill cell. The phenomenon has exhibited traits that contrast with symptoms typically observed with landfill fires or subsurface oxidation. Affected landfills exhibit combined traits that have not been observed for other types of elevated temperature events, including: increased hydrogen concentrations, odor problems,inhbition or cessation of methane production, increased leachate production, substantially higher leachate strength and increased settlement of the waste mass over short periods of time.
1. INTRODUCTION Recently, a small number of landfills across the U.S. have experienced conditions where temperatures of the landfill gas or within the waste mass have persistently exceeded 100 C (212 F) over a substantial area of a landfill cell (Figure 1). The phenomenon has exhibited traits that are in contrast to symptoms typically observed with landfill fires or subsurface oxidation. Landfills with such traits have been designated as elevated temperature landfills (ETLFs). While some ETLFs received reactive wastes that are a source of excessive heat, others have not. Moreover, there is considerable uncertainty as to the mechanisms controlling heat accumulation in landfills. A research project funded by the Environmental Research and Education Foundation (EREF) aims to better understand the mechanisms behind why some landfills are experiencing elevated temperatures. The objectives of this paper are to: (1) provide an overview of characteristics and traits that make ETLFs distinct from landfill fires or subsurface oxidation events, and (2) describe ongoing research by the EREF-funded researchers to understand the factors and mechanisms that contribute to ETLFs.
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 1. Temperature profile in a landfill experiencing elevated temperatures. ETLFs pose a number of potential problems including temperature damage to gas and leachate collection system infrastructure, rapid settlement with subsequent implications for the integrity of the landfill cover and slope stability, elevated leachate volume and strength, reduced methane content that may impact landfill gas treatment and energy generation processes, odorous gases, and challenges with regulatory compliance. In cases where the elevated temperature extends to the bottom of the landfill, the geomembrane liner may also be affected. Consequently, ETLFs often require increased monitoring and management.
2. DIFFERENCES BETWEEN LANDFILL FIRES AND SUBSURFACE REACTIONS Landfill fires occur at the landfill surface or near the surface where waste is burning or smouldering. These combustion reactions require a source of oxygen. While large landfill fires are rare, smaller fires are known to occur and are caused by phenomena such as hot loads from trucks, hot equipment, lightening strikes and via air intrusion near the landfill surface at shallow depths via the landfill cover or over pulling on gas collection wells. In contrast, a subsurface reaction (SSR) that creates an ETLF appears to generally occur relatively deep in the landfill waste mass where conditions are anaerobic and in many cases very wet. While not universal, these conditions have been largely observed in larger, deeper landfills where the highest temperatures are observed far below the landfill surface and in saturated or near saturated waste materials.
3. KEY CHARACTERISTICS OF ELEVATED TEMPERATURE LANDFILLS Data collected from ETLFs suggest that there are a number of traits that may suggest elevated temperatures are becoming problematic. Many landfills periodically have gas wells that exhibit elevated temperatures (i.e., greater than US limit of 55 C). A primary difference between a hot gas well and an
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
ETLF is the presence of a self-propagating reaction in an ETLF. In addition to increased temperatures, there are a number of traits that collectively serve as indicators matic of an ETLF. 3.1 Increased Hydrogen and Odors in the Landfill Gas Increased hydrogen has been routinely observed at ETLFs. Anaerobic decomposition is a multistep process facilitated by specific groups of microorganisms. The first steps involve hydrolysis and fermentation of organic materials by Bacteria that are converted to volatile fatty acids (e.g. acetate), carbon dioxide, and hydrogen (Eqn. 1). Eqn. 1 (Fermentation): C6H12O6 (glucose) + 2H2O à 2C2H4O2 (acetate) + 2CO2 + 4H2 The second step involves methanogenic Archaea consuming acetate and hydrogen and converting these compounds to methane (e.g., Eqn. 2). Eqn 2 (Methanogenesis): 4H2 + CO2 à CH4 + 2H2O In a healthy landfill, this process results in a biogas composition that is approximately 50% carbon dioxide and 50% methane. However, in an ETLF, conditions develop that inhibit methanogenesis. When this occurs, methane content drops substantially as the hydrogen produced from the fermentation step is not consumed, thus resulting in increased hydrogen concentrations in the landfill gas. Volatile fatty acid concentrations may also increase, resulting in substantially increased odors. Temperatures can get hot enough in ETLFs to damage gas collection systems, reducing gas collection capabilities and causing more landfill gas to escape to the atmosphere. The low methane content noted above decreases the effectiveness of combustion at the flare. Despite this, higher gas output and pressures can occur. Collectively, these factors create a situation where odor is far more of an issue compared to typical landfills. 3.2 Rapid and Widespread Settlement Under typical conditions, anaerobic biodegradation of MSW in a landfill results in waste settlement at a rate of less than a meter every few years. It is hypothesized that the increased temperatures and self-propagating attributes of an ETLF result in a breakdown of MSW at a substantially accelerated pace. While the exact mechanisms causing an ETLF are still under investigation, landfills experiencing this condition have observed settlement of a meter or more over a time period of several months and, in some cases, a few weeks. 3.3 Increased Leachate Volume and Strength As solids are converted to gas, the waste compresses and the field capacity of the waste mass decreases. This phenomenon is normal in a landfill and occurs over many years. In ETLFs, the process occurs over much shorter time frames, which is manifested as more rapid settlement and larger volumes of leachate to manage over shorter periods of time. This can introduce complexity in managing leachate since existing treatment processes and storage capacity may not be sufficient. The excessive liquid can also flood gas collection wells, making collection of landfill gas more difficult to collect.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Leachate from a normally operating landfill has biochemical oxygen demand (BOD) concentrations ranging from 1000 to 5000 mg/L. In contrast, ETLFs can have BOD concentrations in excess of 100,000 mg/L. The increased leachate volume coupled with substantially higher strength can make leachate from ETLFs very challenging to manage. 3.4 Low Methane to Carbon Dioxide Ratio in the Landfill Gas Temperature increases in ETLFs inhibit methanogenesis, resulting in a significant drop in methane concentration. As noted in Section 3.1, methane and carbon dioxide are stoichiometrically formed at a 1:1 ratio. Thus, when methane production is inhibited, one can expect the primary gas ratio (methane to carbon dioxide, CH4/CO2) ratio to be less than one. Operators of ETLFs report that a decreasing trend in the CH4/CO2 ratio is a leading indicator of temperature increases to a level at which biological methane generation is interrupted. Because CH4/CO2 is a leading indicator, decreases may be observed before the gas temperature increases. Figure 2 shows how this ratio may decrease abruptly, emphasizing the importance of regular monitoring and data review.
Figure 2. Primary gas (CH4/CO2) ratio over time at an ETLF where the ratio decreased rapidly.
In many cases, a decrease in the CH4/CO2 ratio has been observed to spread across a landfill over a period of months to even years before significant increases in temperature have been observed. Figure 3 depicts a gas well field at a landfill suspected of elevated temperatures over a four year period. As can be seen, even though nearly half of the wells exhibited an inverted CH4/CO2 ratio by year 2, no increased temperatures were observed. Yet by Year 4, nearly all gas wells had a primary gas ratio less
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
than 1 and nearly 40% of wells exhibited temperatures in excess of 80 °C. In this example, there was a lag time of nearly 3 years before increased temperatures were observed in 10% of the gas wells, even though at this point over 80% of wells had a primary gas ratio less than 1.
Figure 3. Percent of gas wells with CH4/CO2 ratio < 1 and gas well temperatures > 80 oC over time demonstrating the ratio as a leading indicator to elevated temperatures.
4. OPERATIONAL CONSIDERATIONS AND ONGOING WORK ETLF conditions tend to develop deep in the waste mass and in saturated or near saturated waste and can cause significant challenges in landfill operation, particularly with settlement, landfill gas and leachate management, and in some cases odors. Understanding the difference between a landfill fire (oxidation in the presence of oxygen) and a subsurface reaction that occurs anaerobically in wet conditions is important, as the mechanisms differ and different management strategies are required. Turning off the gas collection system is common practice during a landfill fire to starve the fire of oxygen. For an ETLF, this practice traps heat and volatile reactants, which may exacerbate or accelerate heat propagation. In an ETLF situation, the pace of the reaction can be slowed by increasing removal of heat and gaseous compounds. Thus, keeping the gas collection system on is important when managing ETLFs. Ongoing research conducted by N.C. State University, the University of Virginia, City University of New York, and Geosyntec Consultants under EREF’s sponsorship has focused on causal mechanisms and triggers of high temperatures such as anaerobic metal corrosion (e.g. aluminium, iron) and ash hydration/carbonation as well as the strategies to manage ETLFs. The working hypothesis of this research team is that these conditions lead to self-propagating pyrolytic reactions. If proven, the research will lead to the development of best management practices to avoid and/or mitigate elevated temperature landfill conditions.
