Concerns are growing in the solid waste industry as researchers attempt to pinpoint the cause of elevated temperatures at some landfills.
Dr. Morton Barlaz, distinguished university professor and head of the department of civil, construction and environmental engineering at North Carolina State University in Raleigh, N.C., says there is a lot of speculation about why the temperatures are elevated, however no definitive explanation has been established.
“At some landfills, it is believed that the disposal of reactive industrial wastes caused the temperature to become elevated," he says. "At other landfills, the cause is not known. The presence elevated temperatures is often detected by the presence of elevated temperatures in landfill gas—the landfill gas temperature is monitored routinely.”
Henry Kerfoot, principal for Civil & Environmental Consultants Inc. (CEC), in Phoenix, Ariz., says the current elevated temperature issue is due to heat-generating reactions that are neither combustion nor fire.
“We should recognize that there have been elevated temperatures in landfills in the past from smoldering, or flameless, combustion, often due to oxygen from air intrusion into the waste. These can produce temperatures above 700 to 1,000 degrees Fahrenheit, along with ash and smoke,” he says. “However, current concerns regarding elevated temperatures are for a different type of reaction. Temperatures below 375 degrees Fahrenheit, well below the smoldering combustion range, are observed in waste that is saturated or nearly saturated with water. Combustion does not occur in these conditions.”
Kerfoot adds that it is likely that a combination of many reactions is involved in the heat-generating process.
“We have noted evidence in the leachate, gas and residual solids for an aqueous pyrolysis process that occurs in water at temperatures lower than combustion," he says. "Because of lower temperatures and the presence of water, aqueous pyrolysis is known to incompletely destroy organic matter.” he says. “Smoldering combustion or fire, on the other hand, completely destroys organic matter, forming ash, which is not observed at these sites. Because it occurs in water, aqueous pyrolysis also affects the leachate chemical composition, while combustion does not, and major changes in leachate composition are observed at these sites.”
Navid Jafari, assistant professor in the department of civil and environmental engineering at Louisiana State University based in Baton Rouge, La., says that several factors can lead to elevated landfill temperatures, including air intrusion, partially extinguished surface fires, and waste placed at elevated temperatures.
“The common mechanism causing elevated temperatures is the introduction of ambient air into a landfill during gas collection and control operations and/or poor cover maintenance,” he says. “When landfill operators increase the vacuum to enhance methane recovery for energy production, oxygen can enter the landfill … Aerobic decomposition can start from these and other actions that allow oxygen to enter the waste. Landfill temperatures typically range from around 75 to 115 degrees Fahrenheit, but waste temperatures in aerobic conditions can increase to 175 degrees Fahrenheit and higher if smoldering combustion develops. As a result, it is paramount to limit air intrusion into the landfill.”
According to Jafari, landfills can also experience elevated temperatures from reactive industrial wastes, like aluminum production waste, incinerator ash, bottom ash, tires and lime kiln dust, among others. He says elevated temperatures can develop in most U.S. landfills depending on the type of waste, operation of gas collection system, and whether leachate is recirculated.
“Elevated temperatures related to certain industrial wastes may be linked to specific regions or locations. For example, landfills in Kentucky, Ohio, Tennessee, and Indiana have experienced aluminum production waste reactions because of the presence of primary and secondary aluminum processing industries in these states,” says Jafari. “In particular, hydropower electricity in the Ohio and Tennessee River valleys is necessary for the energy intensive primary aluminum production process, while secondary aluminum producers are located where recycled aluminum is easily accessible, near densely-populated urban centers. As a result, the wastes generated from both industries are typically disposed in nearby municipal solid waste (MSW) landfills because this waste does not usually classify as hazardous.”
Elevated temperatures have been observed at several landfills in the U.S. since the early 2000s, maybe even as early as the 1990s, according to Kerfoot, however Jafari cites a specific landfill in Ohio as the catalyst for this issue.
“The Countywide Landfill in Stark County, Ohio brought elevated landfill temperatures to the attention of the waste industry, consultants, regulatory agencies, and public starting in 2005, he says. “Temperatures significantly increased to greater than 200 degrees Fahrenheit when the facility recirculated leachate in areas where aluminum production wastes were disposed in the MSW landfill. As the changes in landfill behavior due to induced elevated temperatures became readily apparent, other landfill owners and operators noticed the same trends and behaviors occurring at their facility.”
Some of the measures landfill operators are using to remedy the increased temperatures include leachate removal and gas removal to remove heat and are developing measures to mitigate the occurrence of heat-generating processes, according to Kerfoot.
“At CEC, we are currently evaluating cooling measures to stop or isolate the heat-producing reactions and we are modeling heat generation and transport in waste. We have also found geochemical fingerprints of the process in the gas and leachate to obtain an early warning,” he says.
Barlaz says that because every landfill is different, there is not one solution.
“Operators are working to collect and treat gas and leachate, and to control leachate breakouts when they occur, just like at typical landfills. In some cases, more specialized equipment must be added to the gas collection and control system. The landfills are being monitored carefully,” he says.
Landfill operators may want to adjust their operations and the way they accept and manage waste in the future.
Jafari suggests landfill owners and operators evaluate incoming industrial, non-hazardous wastes for heat generation potential that may damage engineered barriers and landfill infrastructure.
“In the past, landfill design and operation has evolved to deal with new challenges. The development of measures to manage heat is likely, and work is under way in that area,” says Kerfoot.