Responding to changing regulatory and societal expectations on landfill gas emissions is driving a demand for a better understanding of landfill gas emissions. But how do you determine which tools and systems are right for the conditions at your landfill?.
Landfill gas has the potential to migrate into the air through landfill soil caps and cover, releasing potent greenhouse gas emissions, causing negative impacts on the environment and public health. Direct measurement of these emissions is emerging as a solution that helps make reporting more accurate and helps landfill operators make informed, data-driven decisions to improve management practices.
Historically, landfill owners have self-reported using modeling techniques, such as the Scholl Canyon or LandGEM first-order decay models, to estimate and report landfill gas emissions and calculate compliance costs. However, a model is a simplification of actual conditions and may not accurately represent actual landfill gas emissions year over year.
Six ways to measure and quantify landfill gas: Choosing the right solution.
Specialized consultants can help determine which methodology best suits the needs of each landfill. Regardless of the methodology selected, direct measurement can help landfill operators enhance their management systems and, in some cases, reduce compliance costs that may result from model overreporting. Direct measurement helps to accurately understand landfill gas resources for upgrading to renewable natural gas and supports net-zero pathway planning.
There are several different tools available to measure and quantify gas emissions and reduce the uncertainty often associated with models. Since every landfill site is unique, a holistic approach should be taken to determine which monitoring tools and technologies will provide the best results for a given site and purpose. Size, types and placement of waste, climatic differences, moisture retention, and age all factor into landfill gas generation. In addition, it may be important to understand potential impacts on ambient air quality of landfill gas and its associated constituents that may cross over the property boundary.
The rate at which vapor-phase chemicals, such as those found in landfill gas, cross the soil-air interface is called the landfill gas "flux" rate, which is measured as mass per unit area per unit time (e.g., micrograms of landfill gas per square meter of soil surface per minute). Direct measurement using the isolated flux chamber method is often used for landfills to quantify this rate of flux. Landfill gas is captured by flux chambers placed on the landfill surface so that the changes in concentrations of methane (and other constituents) in the landfill gas can be measured.
Advantages:
Considerations:
Eddy covariance technique (EC) is the most widely used methodology to measure carbon fluxes at a landscape level and is one of the most direct and defensible methods for flux measurement. EC uses an assembly of sensors arranged in a tower to measure emission flux based on the number of molecules moving upward and downward over time, as well as the travelling speeds of these molecules.
Advantages:
Direct, near real-time measurement
Capability to measure flux variation over time
Considerations:
Higher capital cost due specialized equipment
Improved performance hardwired power connections
Sensitive to low wind speeds conditions (<1 m/s, specifically during nights).Sensitive to low wind speeds conditions (<1 m/s, specifically during nights).
Suited for permanent or semi-permanent installations.
This method is widely used to measure carbon at a landscape level
The radial plume mapping technique is a relatively new technique and is based on the U.S. Environmental Protection Agency test method (OTM 10) for characterizing emissions from area and fugitive sources. An open path, path-integrated Optical Remote Sensing (ORS) system is employed to identify “hot spots” of emissions and quantify emission fluxes. Open path-Fourier-transform infrared (OP-FTIR) spectroscopy is the most common system using this technique with these common uses:
1. Horizontal radial plume mapping: Used for mapping gas concentrations in a horizontal plane. Hot spots near the ground can be located.
2. Vertical radial plume mapping: Designed to determine the emission flux of contaminants in a vertical plane downwind from an emission source.
3. One-dimensional radial plume mapping: Suitable for obtaining the profile of pollutant concentrations along a line-of-sight (e.g. fence line monitoring ).
Advantages:
Characterizes fugitive emissions from large area sources
Provides 3D gas profiles
Considerations:
Suitable for diffuse sources and does not able to quantify point emission sources
The capital costs for the system are expensive due to specialized equipment
Detailed technical understanding, training, and experience is required
The tracer correlation method is used to produce two-dimensional profiles of landfill gas emissions. It relies on the controlled release of a known rate of a tracer gas, compared with a suspected gas. The concentration of the tracer is measured downwind with a mobile monitor and simultaneously measures the source gas concentration. By knowing the emission rate of the tracer and the downwind concentrations of both tracer and target, the target emission rate can be calculated.
Advantages:
Detailed accurate meteorological measurements are not required.
Good for quantifying emissions from area sources and provides insights on emission pathways.
Considerations:
Careful selection of tracer gas requires balancing considerations background conditions project emissions and health and safety.
Useful for understanding emissions from smaller areas; sampling complexity increases for with area size.
Remote sensing techniques including drones, aircraft, and satellites are very promising for detecting emission hotspots over large area sources from a distance such as well site emissions. Using remote technologies coupled with modeling software provides a high-resolution overview of the landfill and path map of the emissions which is then used to determine the total emissions and/or emission rates.
Advantages:
Less labor intensive than other methods
Reduces the time necessary to complete quarterly monitoring; improving efficiency and lowering labor costs.
Considerations:
May not accurately detect small scale changes in emissions.
Ambient air monitoring measures the air quality conditions around the landfill or at the facility fenceline. It can help to estimate the emission rates of LFG constituents through inverse plume modeling (“reverse modeling”), can help confirm dispersion modeling predictions, or to quantify the concentration of individual chemical compounds. Ambient monitoring can be carried out by using either fixed monitoring stations or using mobile platforms. Fixed stations are generally suitable for long term studies, while mobile platforms are more flexible and allow sampling at a variety of downwind distances and plume directions but are constrained by the limits of vehicle operations.
Advantages:
Determines actual concentrations of pollutants that can cause impacts to public health.
Helps understand and confirm if air quality standards are being met.
Considerations:
Often requires permanent or semi-permanent station. Best suited for long-term monitoring programs.
Better data and a better understanding of actual emissions can help with landfill and emission management in a number of ways. For example: it could be used to determine compliance obligations, potential impacts on neighbors and other stakeholders; for feasibility studies for future landfill gas utilization; to identify target emission reductions and inform net-zero pathway decisions; to provide data for odor management; or enable better decisions for optimal LFG management. Whatever your goal, improved identification and quantification of landfill gas emissions leads to better landfill management.
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