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Comparing the Advantages and Limitations of Different Techniques Used to Quantify Methane Emissions From Municipal Solid Waste Landfills - 1. Introduction

1. Introduction 

In the United States, Subtitle D landfills are used to dispose of municipal solid waste (MSW).  Solid waste is defined by the Environmental Protection Agency as “any garbage, refuse, sludge from a wastewater treatment plant, water supply treatment plant, or air pollution control facility and other discarded material, including solid, liquid, semisolid, or contained gaseous material resulting from industrial, commercial, mining, and agricultural operations, and from community activities.”  Furthermore, municipal solid waste consists of the household and commercial solid waste streams (Sharma and Reddy 2004).  Generally, MSW in the United States consists primarily of organics (such as food waste), paper (such as cardboard and office paper), plastics (such as shopping bags and plastic packaging), and durable goods (such as appliances).  Additionally, glass, residues, and metals constitute the remaining portion of the MSW stream.

Organics and paper alone account for more than 65% of the MSW stream, and these are the materials that are capable of producing biogas through the process of methanogenesis when degraded in the anaerobic conditions provided by a landfill setting.  The three materials in the MSW stream with the highest mean methane yield are food waste, office paper, and old corrugated cardboard with yields of 300.7, 217.3, and 152.3 cubic meters of methane produced per megagram of dry refuse, respectively.  These three materials represent over a quarter of the entire waste stream (Staley and Barlaz 2009).

The majority of the waste disposed of in Subtitle D landfills have methane production potential.  Methane is a greenhouse gas, along with water vapor, ozone, carbon dioxide, nitrous oxide, and chlorofluorocarbons.  An increase in the atmospheric concentrations of greenhouse gases trap thermal energy in the atmosphere, a phenomenon known as global warming.  In terms of global warming potential, methane is 25 times more potent than carbon dioxide; therefore, it is important to measure how much methane landfills are emitting (Gardner et al. 1993).

Currently, methane emissions from landfills are modelled using a tool developed by the Environmental Protection Agency (EPA) called LandGEM (Landfill Gas Emissions Model).  The model is based on a simplified first-order decay equation with input parameters that cannot change with time (Dillah et al. 2013), and its predictions have not been tested against actual field measurements; therefore, it is critical that methane emissions from landfills are quantified directly to validate the LandGEM predictions and to understand the effects landfills have on global warming.

The purpose of this paper is to highlight and compare the most common methods currently available that are capable of measuring methane emissions from a landfill.  Techniques that are available to obtain point-source methane measurements are flame ionization detection (FID), photoionization detection (PID), and flux chambers.  More advanced optical remote sensing (ORS) technologies exist that are capable of measuring concentration over a spatial path.  Examples of these technologies are Fourier transform infrared (FTIR) spectroscopy, tunable diode laser (TDL) absorption spectroscopy, and cavity ring-down spectroscopy (CRDS).  These ORS technologies also require emission estimation models, such as tracer gas correlation, radial plume mapping (RPM), or differential absorption light detection and ranging (DIAL), to convert the ORS methane concentration data into spatial emission data.

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