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Figuring Out Subsurface Landfill Reactions

What triggers heat-generating reactions and how can they be prevented?

Elevated temperature landfills (ETLF) are garnering the industry’s attention, with questions arising such as, is the landfill on fire? What triggers heat-generating reactions and how can they be prevented?

Researchers are working to better understand these reactions’ origins and how to best prevent and manage them. New developments are evolving that may bring them closer to answers. And this is critical as these subsurface reactions pose challenges, from regulatory compliance, to low methane gas content, to odor complaints.

The problem can lead to tremendous investments of time and money. Republic has spent about $200 million to control odor and manage a subsurface reaction discovered at the Bridgeton Landfill near St. Louis just over six years ago.

(Elevated temperature landfills will also be the subject of an upcoming Waste360 webinar on Dec. 14, 2016. To register for the webinar, click here.)

The question to ask is not so much where is heat generated? The question is: where does it accumulate, says Morton Barlaz, head of the department of Civil, Construction and Environmental Engineering at North Carolina State University.

“We are trying to understand how and why [heat] accumulates and at what point there is enough to trigger self-propagating, heat-releasing reactions,” Barlaz says. “Only once we understand what causes heat to accumulate can we recommend appropriate methods to prevent it.”

Unlike landfill fires, these reactions occur in the absence of oxygen, beginning beneath the surface and sometimes when waste is saturated with water.

Identifying triggering mechanisms is not a straightforward task. Whether heat accumulates is specific to a landfill. For instance, aluminum will generate heat, but the amount of heat and rate of heat accumulation is not well understood.

Among the newest developments is the understanding that some forms of aluminum and ash, and some other materials emit considerable energy, leading to heat.

“The real question is does enough accumulate to raise temperatures and trigger these self-propagating reactions? That’s what we are trying to understand,” says Barlaz.

Landfill operators may avoid potential problems in two ways: rigorous operation of gas and leachate collection systems and close monitoring, primarily for trends in methane-to-carbon-dioxide ratios.

Controlling moisture in gas collection systems is critical because when gas wells are flooded with water, gas cannot be sucked out and instead accumulates in landfills. And organic chemicals in gas can stimulate heat-generating reactions.

Monitoring methane-to-carbon-dioxide ratios in gas wells matters because decreased methane-to-carbon-dioxide signals that temperatures may start to increase.

What red flagged Republic to look very closely at Bridgeton was elevated carbon monoxide levels. Ultimately, the corporation invested in a 46-acre synthetic liner; temperature monitoring probes; a robustly upgraded gas extraction system; an upgraded flare system to destroy captured gas; and cooling points to remove subsurface heat in strategic locations.

Today, the reaction is isolated to a portion of the landfill’s South Quarry, and Missouri Department of Natural Resources’ publicly available data confirms a steady decline in odor over the past year.

“For now, researchers, would tell you that much is still unknown about subsurface reactions,” says Michael Beaudoin, director of remedial projects at Republic Services. “But, we do know that [they] do not constitute a fire, nor are they a smoldering combustion. And only a small fraction of landfills with elevated temperatures experience subsurface reactions,”

The industry is challenged to get regulators on the same page with regard to how to manage these reactions.

The New Source Performance Standards (NSPS) state each landfill well can’t exceed 131 degrees Fahrenheit. But the rules were written under the assumption that exceeding this temperature indicates a fire, and to ensure efficient gas collection in these cases, says Anne Germain, director of waste and recycling technology for the National Waste and Recycling Association.

“In the event of fire, it is good to shut down the gas collection system in the affected area. But it’s the wrong thing to do in the event of elevated temperature. Collecting gas is important; otherwise you can exacerbate the problem.

“We need to work with regulators to get an exemption to operate at higher temperatures.”

Within its own operations, Republic focuses on prevention by gathering and evaluating data in near-real time, reviewed at multiple levels.

“This establishes varying levels of management oversight and peer review, which facilitates collaboration on decision making, ongoing training and best practices,” says Beaudoin.

But industry leaders are still working to increase understanding and finetune best practices to identify and avert potential reactions.

“We have to remember that the modern, subtitle-D landfill is still a relatively young phenomenon, so we should expect to learn more over the next few decades,” says Beaudoin.

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