Many of us probably do not realize the impact that models have on us, yet they affect our lives every day. From predicting the weather, to programming of traffic lights, to streamlining our Internet searches, it’s models that help us be more efficient. Similarly, models are used in the solid waste industry for everything from optimizing collection routes to designing landfill gas collection systems.
In addition, models are frequently used to guide policy making. For example, the Intergovernmental Panel on Climate Control (IPCC) and the U.S. Environmental Protection Agency (EPA) have used greenhouse gas (GHG) models to develop estimates for GHG emissions, which in turn have been used to guide the development and refinement of global and national policies on reporting and controlling said emissions. Landfills, as one of the top three U.S. emitters of anthropogenic methane, are subject to these estimates.
Given the relative importance of these efforts, one would think these models are closely tied to the real world, thus making them highly accurate predictors of GHG emissions from landfills, right? While efforts have been made to improve their accuracy in recent years, current GHG inventory models still have some significant limitations. In order to understand these limitations, it’s important to note that the backbone of these models is a simple mass balance, which for landfill emissions can be shown by the following equation:
Landfill Methane Emissions (E) = Methane Produced (P) – Methane Collected (C) – Methane Oxidized (O)
As can be seen, there are only three variables in this equation that are used to estimate landfill emissions. When we talk about emissions, we are referring to methane that is not captured but rather is emitted from the landfill in an uncontrolled fashion, usually through the landfill cover. Because these gases are escaping control and collection, they are typically referred to as “fugitive emissions.”
The methane produced (P), is a function of the waste placed in the landfill and the relative health of the microorganisms that degrade the organics in the waste and make the methane. The methane collected (C) is based in the extent and type of gas collection system installed. Methane oxidized (O) represents the amount of methane that diffuses up through the landfill cover soil and is consumed by another class of microbes called methane-oxidizers, which consume the methane.
Of these three variables only one can typically be determined with a reasonably high level of accuracy: the volume of methane collected (C). The methane produced (P) cannot be directly measured easily in the field and is itself an estimate based on a model. Yet many factors influence methane production in a landfill, including waste composition, temperature and the availability of moisture (e.g., rainfall, leachate recirculation, etc.).
The amount of methane oxidized through soil cover (O) is also hard to nail down. Historically, EPA has suggested 10 percent as a guideline based on a couple of studies done nearly two decades ago. Yet recent research funded by the Environmental Research and Education Foundation (EREF) conducted by Dr. Jeff Chanton at Florida State University suggests this value is incorrect by a factor of at least three; that real-world methane oxidation is at least 30 percent and can be as high as 60 percent or more depending on climate and locale.
Additionally, it has been reported that the major factors that influence and control landfill emissions are (1) properties of the landfill cover material (i.e., soil thickness, type of cover), (2) Seasonal/climatic effects (i.e., rainfall, temperature) and (3) the extent of landfill gas collection. These factors in combination can substantially affect the level of landfill emissions and have been attributed to variations of five to six orders of magnitude, so the effect can be huge.
Yet as you can see in the aforementioned equation none of these factors is included in today’s GHG models. Recent research funded by EREF hopes to change this. A current study being conducted by the University of Illinois-Chicago aims to refine GHG inventory models to include factors such as landfill cover properties, climate effects and gas collection efficiency, better reflecting real-world conditions. This will yield more accurate estimates of GHG emissions, ensuring that subsequent regulatory guidance is science-based and reasonable.
The UIC study is expected to be complete early next year. Results from the study done by Florida State are available through EREF. For more information on both studies, please visit www.erefdn.org or call 919-861-6876.