LFG: Some Essential Facts

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Controversy has swirled around the effectiveness of the piping systems used to collect landfill gas, for there currently is no verifiable method to measure emissions. In a recent article, Amy Van Koken Banister and Pat Sullivan argued that critics of the landfill industry’s impressive claims—that gas capture “approach[es] 100%”—are engaged in unsupported rhetoric. However, in view of the thousands of odor complaints against landfills, the industry representations appear on their face to be hyperbole, even before examining the technical details that follow.

IPCC Report
Far from being debunked, the major concern raised by independent experts mirrors the analysis followed by the Intergovernmental Panel on Climate Change (IPCC) when it finalized consideration of peer review comments on its 2007 Fourth Assessment Report (FAR). Their methodology provided the basis to recognize that a landfill’s capacity to collect gas is congenitally poor, in contrast to the industry’s studies that only conclude otherwise by focusing on the wrong metric.

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The draft waste chapter in FAR was largely written by a US landfill consultant, Dr. Jean Bogner. In reliance on the same study by Kurt Spokas that Banister and Sullivan cite, and which she coauthored, her draft claimed capture rates greater than 90%.

However, during that review of the draft chapter, a Dutch engineer, Hans Oonk, submitted seminal comments that questioned Spokas’ reasoning. Oonk explained that Spokas’ attempt failed to recognize the relationship between gas generation and capture over the entire period that gas is generated, which he referred to as the “integrated rate.”

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Underlying Facts
To understand Oonk’s point, four noncontroversial facts need to be considered:

  • High moisture levels, only intermittently present, are a prerequisite for gas generation.
  • A final cover is necessary for gas collection to work properly.
  • Gas generation tapers off after the site is capped.
  • Gas capture is largely nonexistent when there is sufficient moisture to generate gas.

The Oonk Paradox
From this, Oonk presented a paradox to the IPCC. Gas collection only works properly when the site is, and for as long as it remains, sealed. But this period, which is the time that Spokas focused on, is also when relatively little gas is generated, because of the absence of critical moisture. When gas is generated—which is before and after the dormant middle period—there is either no gas collection or poorly performing capture systems.

IPCC Adopts Oonk Interpretation
In the final version of the FAR, the IPCC continued to state that the best systems could achieve more than 90% collection efficiency during the time a landfill is sealed. But the panel also inserted essential qualifiers to that statement. First, it first pointed out the obvious fact that not all landfills perform optimally and “may have less efficient or only partial gas extraction systems.”

Second, adopting Oonk, it added “there are fugitive emissions from landfilled waste prior to and after the implementation of active gas extraction.” Therefore, the IPCC concluded, “estimates of ‘lifetime’ recovery efficiencies may be as low as 20%.”

There are three phases in a landfill’s life:

  •  Before gas capture is functional
  • The middle period when it is
  • After the collection systems are shut down

Key is the fact that most gas generation tends to occur at a time when collection efficiency is zero or quite low, and vice versa. Thus, there is no need to debate collection efficiency in the second phase, because so little gas is generated then.

The following discussion will help explain the poor gas performance in the first and third phases.

First phase—Gas collection is absent or not fully functional in the first phase of a landfill’s life (generally up to about the first 10 or more years and until the entire site is capped). While gas starts being generated a few weeks after garbage is buried, the collection systems are not required to be installed for five years. But merely installing gas wells then does not, by itself, mean the system will work as designed. In reality, operational practices have fundamentally changed in the last decade, driving up fugitive emissions.

The 1991 federal landfill rules were predicated on dry-tomb precepts. They required liners and covers, along with liquids and gas-removal systems, to keep the site dry and biologically inactive for as long as possible in order to minimize mobilization of pollutants into the atmosphere or groundwater.

However, by the end of the 1990s, the landfill industry determined to reverse course by deliberately boosting moisture levels. These wet-cell practices include recirculating leachate and delaying installation of the final cover to allow more years for rain to infiltrate the waste mass.

By augmenting moisture, the operators sought to capture the financial and operational benefits from accelerating decomposition. Two or more times the air space could be recovered from greater subsidence and resold. Also, leachate treatment costs could be lowered.

There is no real controversy that these wet-cell practices also shift gas generation from the future to the present, as well as increase the concentration of methane in that gas. That means not only is near-term methane generation being increased, but so is, cumulatively, the aggregate methane produced over the landfill’s biologically active life. Together, methane generation in the next 10 to 20 years, is three times what would be the case had the shift to wet cell practices not occurred.

Widely acknowledged, also, is the fact that these changes significantly degrade near-term collection efficiency. Thus, much more methane is produced in the short-term, and more of that escapes, at a time when we confront irreversible climate tipping points. While there are no field measurements of the impact on gas capture, the inference from the EPA studies suggests collection efficiency in wet cells is 45% less than in a dry landfill. In consequence, in the near-term, 10 times more methane will be released, with 250 times the warming impact, than from traditional landfills. This is of especial concern because the EPA “does not anticipate that [dry-tomb landfills] will be in the majority” in the future.

Third phase—The third phase points to the impact on gas emissions in the final chapter of a landfill’s life, after a 30-year period of maintenance ends following closure.

After the buried trash reaches final grade, the landfill is to be sealed with some final cover to minimize infiltration of precipitation and keep the site biologically inactive. Then, the EPA’s rules designate the 30 years that follow as the postclosure period. During that time, the owner is also to provide some financial assurances for, and to perform the work involved in, maintaining the site. But, after postclosure ends, those obligations generally cease. This implies care will ultimately end.

Without maintenance, the cap will deteriorate and, the EPA acknowledges, “ultimately fail,” permitting rainfall to re-enter the site. This will reignite a second wave of gas generation, even while the site remains a threat to the environment.

Finally, even before that second wave of gas is generated, at some time during the 30-year postclosure period, the gas system will have been removed from service. Therefore, those second wave gases will escape unabated.

Banister and Sullivan argue that there will be no uncontrolled emissions following closure because, during “late phases of a landfill’s lifetime, LFG generation is at its lowest;” even if there are any remaining organics left to decompose, there “is no evidence of landfill covers failing in United States in postclosure to any substantial degree;” and, even if there were failures, “that methane [might be] oxidized in the landfill cover.” But these claims conflict with observations and reasonable projections.

In fact, as detailed in the notes online, the first-order decay model that they use to conclude there is no late-life gas produces dramatically inaccurate results. It fails to account for whether moisture is prevalent, while actual field data demonstrate that moisture, which flows mostly through narrow channels, does not come in contact with most of the wastes while the site is still open. The EPA’s own inspector general has found that many covers fail even before maintenance ends, and most agencies that addressed this issue anticipate future cover failures. As regards the claimed oxidation effects, they ignore the fact that it does not occur in the high-flux conditions at landfills.

Unfortunately, the spacial and temporal challenges at today’s megafills have defeated efforts to develop field measurements or models that could inform us with an accurate estimate of how effective landfill gas collection actually is. But the life cycle framework incorporated into the IPCC’s Fourth Assessment tells us more than enough to know that only a trivial fraction is captured. Msw Bug Web

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