Measured approach

As most waste industry professionals are aware, landfills rank among the top three sources of human-induced methane emissions, following the oil and gas industry and agriculture.

As Figure 1 on Page 31 illustrates, those top two sources comprise well over half of all methane emissions, with oil and gas at 28 percent and enteric fermentation from agriculture at 25 percent, while landfills are at 16 percent. legislators and industry stakeholders given their emissions footprint.

Still, landfills garner increasing attention from legislators and industry stakeholders given their emissions footprint. An ongoing challenge, however, is that these reported numbers are based on modeled estimates of emissions.

Modeled estimates can be helpful in many ways, but they do not always reflect reality. Accurate emission quantification is essential for building effective climate change strategies and sustainable materials management policies.

Measuring landfill emissions is complex given factors such as the variable nature of their sources (Is it a point source or a diffuse source?) and landfill geometry, which can cause complex movements of air on and around the landfill, creating microclimates or orographic lifting. Unlike other industries, such as oil and gas, landfills emit gases across large areas and in ways that behave as both point and nonpoint sources.

The array of technologies deployed to directly quantify emissions includes satellites, aircraft, drones and ground-based approaches. Each of these technologies include sensors that detect methane. Because of the different configurations of these technologies, though, each has advantages and disadvantages, some of which are a function of the technology itself and others are related to site-specific variables.

These differences also make it challenging to compare technologies with one another, even at a single site, because of uncertainty regarding whether differences can be attributed to measurement accuracy or fluctuations in the source and rate of methane emissions.

Emission rates also can vary dramatically over short distances because of cracks or holes in the temporary soil of landfill covers or leaks around gas wells or other areas, which can create hot spots, and low-level emissions that can seep slowly out of cover systems. One strategy to evaluate these technologies involves releasing methane at a known rate, otherwise known as a controlled release. Comparing what is measured to that known rate allows for the accuracy of a particular technology to be measured under specific conditions.

With funding from the Environmental Research & Education Foundation (EREF), St. Francis Xavier University’s FluxLab in Nova Scotia conducted a controlled methane release study at a closed landfill in Ontario.

Nine methodologies were assessed in the study, including those specializing in localization to identify where emissions are originating; those focusing on quantification to determine how much is being emitted; and those with localization and quantification capabilities.

The study sought to answer three main questions:

1. How do different methodologies perform under various meteorological conditions?
2. What are the quantification accuracies of these different methodologies?
3. What are the localization accuracies of these different methodologies?

Researchers tested nine methodologies for quantification and detection performance during 71 experiments. Methods included satellite (satellite imaging sensor emission assessment), aircraft (airborne point sensor emission assessment and gas mapping light detection and ranging [LiDAR]), drone (unmanned aerial vehicle column sensor emission and point sensor emission assessments), ground-based truck (mobile tracer correlation emission assessment, gaussian plume assessment and Lagrangian emission assessment) and fixed ground-based (remote point sensor emission assessment).

FluxLab established a temporary above-ground pipeline network to facilitate the controlled release of methane at various rates and locations on the landfill. Three weather stations were installed to collect meteorological data, including wind speed and direction, barometric pressure, relative humidity, air temperature and dew point.

The vendors submitted results based on their own sources of weather data. They were provided data from the on-site weather stations and had the opportunity to submit revised measurements for increased accuracy.

The site is closed and has no neighboring methane sources, which greatly reduced the potential for background methane concentrations, a factor that can make such studies more challenging to manage. All methodologies tested can detect methane emissions, but each has different dependencies, costs, speeds and uncertainties.

Key findings

Satellite-based methods were effective for identifying emission hot spots but less suitable for detecting lower-level, day-to-day emissions. In this study, satellite methods were unable to detect the known release rates, which ranged from 1 kilogram per hour (kg/hr) to 300 kg/hr.

However, these higher-altitude methods are deployed much further from the emissions source than other methods tested. This typically means much lower resolution compared with other methods, particularly when emissions rates are lower.

These methods tend to be more appropriate for “hot spot” detection and large-scale surveys rather than localized, low-detection thresholds and/or high-frequency monitoring efforts.

Aerial and drone-based methods provided variability in detection. Aircraft-mounted LiDAR, for instance, was effective at localizing leaks, with deviation from the release rates that ranged from 6 percent to 128 percent. Drone-mounted systems showed a higher number of false positives, with deviations from the known release rates that ranged from 2 percent to 96 percent.

Fixed sensor methods showed substantial variability, and their deviation from the known release rates ranged from 1 percent to 3,597 percent.

Such variability is likely because fixed sensors are less mature in their development compared with other technologies, and their accuracy can be substantially affected by spatial configuration, number deployed and type of sensors used.

While these sensors are appealing for their low-maintenance potential, more research is needed to increase accuracy across various weather and site conditions.

The Tracer Correlation method, a mobile (truck-mounted), ground-based method, provided the most reliable and accurate results. This technique has been deployed for more than two decades, and its accuracy and use are relatively mature compared with other technologies.

When site-specific characteristics do not limit its use, Tracer Correlation increasingly is viewed as a benchmark to compare against other technologies. It consistently has provided the least variability in measurement technologies and the highest accuracy in detecting emissions.

Overall, this research is a significant first step in measurement technology “stacking.” The results show areas for further research, which could improve consistency and accuracy for the complex task of landfill emissions measurement.

The full report is available on EREF’s website at https://bit.ly/3Z431UX.

Stephen Aber is communications manager for the Environmental Research & Education Foundation, or EREF, a Raleigh, North Carolina-based nonprofit that advances scientific research in sustainable waste management practices. He can be emailed at saber@erefdn.org.