Can the Eye in the Sky Help to Reduce Greenhouse Gas Emissions?

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Accurately accounting for the various greenhouse gas (GHG) emission sources on Earth is essential to mitigating their contributions to climate change. Satellite and aerial techniques to measure GHG have become less expensive and more accurate and are being deployed more to detect “hotspots” of high GHG and track emissions over time. GHG emissions from the waste sector, represented chiefly by methane created from decomposing materials in landfills, are one of ten critical global sectors targeted for more aggressive emission reductions, and recent studies have focused on measuring these emissions sources with new satellite and aerial technology mainly because of challenges in estimating and measuring waste emissions on the ground. A new study quantified GHG emissions from 250 landfills using planes and remote sensors equipped with methane detection technology, and although the results did not suggest fundamental errors in existing estimates that landfills already report to the US Environmental Protection Agency, the work highlights how aerial and satellite technology can provide more frequent and (potentially) more focused measurement of “problematic” emissions from specific landfills. These measurement technologies will likely be most beneficial in international contexts where lax regulations or a lack of performance data inhibit knowledge of methane emissions from landfills and, therefore, the greatest opportunities for emission reduction.

Quick Primer on GHG Emissions from Waste 🗑️ 

GHG emissions come from many natural and human-caused sources around the world. Recent years have seen new global collaboration (like the Paris Agreement in 2015) to pinpoint the largest emission sources and work to reduce these emissions in line with the scientific consensus of how much carbon should be in the atmosphere to avoid some of the more dramatic, negative consequences of climate change. So, where are GHG emissions coming from? The figure below from Our World in Data is one of my favorites as it cleanly shows GHG emission trends over time, by sector, so you can see both the absolute and relative magnitude of each sector:

We can see that the Waste sector is not the most significant contributor to GHG emissions, but I think it’s among the most interesting because of why it happens. When stuff gets “thrown away”, odds are that no matter where you are in the world, this stuff will go to a landfill, to the tune of around 2 billion tons annually. A paper I published as part of my PhD dissertation found that each person in the US contributes about a ton of waste to landfills yearly. Methane (CH₄) is a significant GHG and starts being produced in landfills about a year after waste is put there. Bacterial colonies then form, consuming and digesting the solid material, producing gas. The amount of methane produced in a landfill is most extensive shortly (about a year) after waste is put into the landfill, then declines over time. This shows the typical relationship of methane production at landfills, taken from my book:

Waste gets landfilled for both good reasons (e.g., some discarded stuff is nasty and it’s better off being contained in a landfill than littered into the environment) and not-so-good reasons (lots of discarded stuff has value today in recycling commodity markets, to the tune of around $11 billion annually in the US). There are also fascinating “nexus” issues for some wastes, like food, because when food waste is sent to landfills, it usually degrades quickly, thus making it challenging to capture the methane produced. What’s more, a lot of food discarded to landfills could have been consumed by people or animals, part of the staggering statistic that about one-third of all food produced worldwide is wasted.

Measuring and Reducing GHG Emissions from Waste 

Researchers have produced estimates of GHG emissions for major sectors for years, and these estimates have improved over time for different reasons like more and better direct measurements from various sources, improved computing power, new software programs facilitating data collection and analysis, etc.

Directly measuring emissions on the ground from landfills is surprisingly tricky, mainly because landfills cover a reasonably large area (dozens if not hundreds of acres, or hundreds of thousands to millions of square meters for metric system fans), and technology to measure methane over the large area from ground level historically did not perform well and was expensive and clunky to deploy. Thus, emissions have historically been estimated by leveraging equations that tell us how much methane would be produced (based on the type and amount of waste put into the landfill) and then applying some factor that assumes how much is collected (if the landfill has a system to collect the gas, otherwise nearly all of the gas is thought to be emitted into the atmosphere). In the US, nearly a thousand landfills have been required to estimate their emissions this way since 2010 and report annually under the US Environmental Protection Agency’s (US EPA’s) Greenhouse Gas Reporting Program law.

Now that we’ve covered traditional methods of estimating methane emissions from landfills, we’ll briefly cover how GHG emissions from landfills are reduced. Since the mid-1990s, US landfills have been required to collect and destroy collected methane once they reach a specific size under laws called the New Source Performance Standards and Emission Guidelines. These laws require landfills to have methane collection systems designed and installed throughout much (but not all!) of the landfill, and the laws include allowances to let landfills “phase in” new collection infrastructure to give time for new waste to be put in and to enable waste disposal operations to continue. Methane is usually collected by a network of wells (like the one below (source)) that extract the methane and route it to a “destruction device” like a flare or energy conversion system. We’re not really “destroying” methane, per se, but rather converting it to carbon dioxide, which is 25x less potent of a GHG than methane.

How are landfill gas collection systems operated? Once a gas collection system is installed, it operates continuously. Each collection well is typically adjusted by a human operator at least monthly (in line with the US EPA gas collection regulations), increasing and decreasing how much gas is extracted at each location and checking for any unusual conditions that may affect performance. A design thumb rule is that you have one collection well per acre of landfill surface, so many landfills will have dozens if not hundreds of individual wells that are operating. Depending on the landfill size, collecting a reading from every gas well can take anywhere from one to several days. Yours truly has designed and operated many gas collection systems. Still, it doesn’t take on-the-ground experience to know that if you only check on performance monthly, there will likely be periods when the system is not operating optimally.

New Study: Using Planes and Remote Sensors to Measure Methane Emissions from Landfills

An interesting new study published earlier in 2024 in the (excellent, very tough to get published in) scientific journal Science reported on new methods for measuring methane emissions from landfills by flying a plane equipped with methane detection technology about 3 to 5 km above around 250 landfills, pinpointing emissions for each landfill down to a resolution of about 3 to 5 m. Here’s an image I adapted from the study that gives a good visual of what they did:

Over the 6-year study period, the authors quantified methane emissions at 250 municipal solid waste landfills in 18 US states, aiming to address these two key questions:

1. Do landfills show so-called “point source” emission behavior, meaning are they showing spikes of large amounts of methane being emitted? And if so, are these significant emission events episodic, or do they occur frequently?

2. Do emission estimates using this new method compare favorably to the estimation methods landfills already use when reporting their emissions to the US EPA Greenhouse Gas Reporting Program?

Let's first put aim (1) in context. The authors defined the significant emissions “point source” threshold at 10 kg of methane (CH₄) per hour.  Is that a lot? Let’s put this threshold into terms that may answer that question. One of the things you measure when monitoring landfill gas wells is the flow rate of gas being collected by the well - a typical value you might see may range from 10 cubic feet of methane per minute to many dozens of cubic feet of methane per minute. OK, the study threshold is in kg/hr, and the standard unit of measure is in cubic feet per minute…so let’s convert the low end of what you typically see flowing through one landfill gas well to see how it compares to the study threshold:

OK, so that means a gas well extracting 10 cubic feet per minute (again, this is at the low end of what you’d typically extract in one well) is the same as it collecting 12 kg methane per hour. Thus, one would expect this threshold to get pretty quickly exceeded, as in theory, it would only take one malfunctioning well to lead to enough methane being emitted rather than collected (remember, a typical landfill will have dozens or even more than 100 operating gas collection wells).

The authors reported that of the 250 landfills examined in the study, 52% exhibited “point source” levels of methane emissions at some time.  However, the converse of this conclusion is somewhat telling - another way to couch the results is “48% of the 250 landfills did not have a detected amount of methane greater than a fairly low threshold of 10 kg/hr”.

So, are these results surprising? Not really. Because US federal landfill gas collection system regulations explicitly allow for monthly gaps between well field adjustments, periods of higher emissions should be expected. For example, a landfill with gas collection wells operating within the confines of federal regulations could potentially have up to 30 days where it is operating sub-optimally, so if we assume that an operator made their monthly measurements on the first of the month, a flyover measuring methane emissions anytime between that day and the subsequent measurement could capture instances where landfill gas performance is sub-optimal. You might think the solution is to “just require more frequent monitoring”, but as described above, the monthly monitoring process is already pretty lengthy. It’s probably safe to assume the federal requirement to monitor wells monthly was a compromise that acknowledged the realities of how long it takes to actually make the measurements.

Beyond federal regulation compliance, are landfill operators incentivized to operate methane collection systems more efficiently? In short - yes. Hundreds of landfills in the US and around the world convert collected methane into energy (e.g., burning the methane and using an engine to create electricity, piping the methane to a nearby industrial facility to burn in a boiler, or cleaning up impurities in the gas and using the cleaned-up gas to power vehicles that can run on natural gas). Landfills get revenue from the sale of the converted energy, so any uncollected methane represents lost revenues that can never be recaptured.

The study identified 113 landfills with high (“point source”) methane emission levels. I cross-referenced this list against a database maintained by the US EPA of landfills converting collected methane into energy. I found that 64 of the 113 landfills have an active energy conversion project. Put another way, the other 49 landfills (43%) with high methane emission rates have no energy conversion system and, therefore, less incentive for aggressive methane collection practices.

The authors also rightly point out that several factors contribute to a landfill exhibiting higher-than-expected methane emissions, including the constant placement of new waste and reconfiguring the landfill. Many of the landfills studied here were open and taking in new waste (most landfills are designed and built to take in waste for several decades or more). Recall how federal regulations specify timing requirements for installing new methane collection systems - in brief, the regulations say you don’t need to collect methane from newly placed waste for up to five years. You might wonder, “Why so long?” Well, the regulation reflects some immutable realities about how landfills operate. First, methane won’t be produced until about a year after the waste is placed, but you wouldn’t put in new gas wells in that area because you’re still filling the area with new waste. Landfills are busy places, and you may have hundreds of trucks daily delivering new waste to the site. Again, partly because of federal guidelines for installing methane collection systems, we’d expect methane emissions to be higher at some point in a landfill’s life than others, owing to the deployment of the collection infrastructure relative to the entire site.

To put this idea in numbers, I analyzed the methane collection efficiency (as reported to the US EPA’s Greenhouse Gas Reporting Program) for each of the 113 landfills exhibiting a large methane emission amount. The figure below shows that only a handful of sites have a reported efficiency near 100% - the rest have far lower values. Not because their gas collection systems are not functioning properly, but simply because the infrastructure hasn’t been overextended (or even most, in a few cases) of the landfill. Again, this mostly reflects regulatory timelines, not explicitly poor performance.

As for the study’s second aim - it turns out that the aerial method they used to measure methane emissions widely disagreed with self-reported methane emissions estimates at each of the studied landfills sent to US EPA as part of annual Greenhouse Gas Reporting Program submissions, even in cases where the research team measured emissions from the same landfill 10+ times. Interestingly, the discrepancy happened roughly equally on both sides of the self-reported estimates, with about half of the aerial measurements exceeding the self-reported emissions estimates and the other half less than the self-reported emissions estimates.

I don’t think it matters much that the aerial measurements and the self-reported estimates did not agree. If this study’s conclusion was “We flew over 250 landfills, and the emissions we measured were greater than all of the self-reported estimates”, that would be a big deal. However, the techniques used in this study signal a shift toward more frequent and ever-improving monitoring of the waste system. The fact that researchers can now take measurements at multiple landfills at any time with functioning detection technology signals even greater scrutiny on the world’s waste management practices. Further, if we consider landfills in a non-US context, aerial and satellite technologies measuring methane means we can pinpoint large emission sources in parts of the world where landfill details and performance data are not available. You may not be surprised to know that the regulations compelling landfills in the US to gather large amounts of data and report on emissions is pretty unique in the world, so you can see how new measurement techniques can help drive emissions reduction from the waste sector globally. I also think the continued attention and reporting of results in the US from aerial and satellite measurement campaigns will likely spur more aggressive methane collection practices, even in the absence of any significant regulatory shift in the US.

Insights and Implications

The study and its results have implications for a variety of initiatives, sectors, and stakeholders:

Landfill Operators Should Expect to See Additional Aerial Measurement Studies in the Future. The study is the latest research article where measurements are taken at landfill sites using methods that do not explicitly require the researchers to access a landfill site or get permission to take measurements. The authors of the study I analyzed here mentioned they plan additional future satellite-based measurement campaigns with an expected continued focus on landfills, including sites in the US and outside of the US, so I hope we will see more studies of this type (perhaps with improved accuracy and more extensive comparison to other operational conditions to understand the “why” behind specific sites emitting more than others beyond that discussed here). I don’t see aerial methods replacing landfills' traditional estimation methods, though. Still, aerial measurements will probably be an essential check on self-reported emission estimates and help better grasp emission amounts internationally, particularly in areas where good records of waste types and quantities are unavailable.

We may see increased deployment of automation at landfills to better monitor and manage methane collection systems. Even when aligned with regulatory operational standards, manual operation of gas collection systems leaves long periods where gas well performance is unchecked, likely resulting in excess methane emissions. Some landfills have begun to experiment with automated landfill gas collection systems, where logic is programmed into each gas collection well and its settings can be continuously adjusted, automatically, without direct human intervention. Landfills incorporating automation capabilities at individual wells should have a better opportunity to reduce emissions simply because of more significant and frequent visibility on individual well conditions and gas collection rates (Disclosure: In the past I have advised sites using automation systems for landfill gas collection along with the technology providers).

Food Waste Diversion from Landfill Should Remain a Priority, but Infrastructure is Lacking. The simple answer for food waste reduction is this: “We need less edible food going to landfill, and food waste that cannot be avoided should probably go somewhere other than landfill so that we avoid the hard-to-collect methane emissions”. Easier said than done! Many great organizations are working on this very problem - as you can imagine, several factors ranging from simple to complex contribute to food from households and commercial businesses being wasted (I’m only mentioning household and commercials here because those are the food waste sources most likely to go to landfill - on-farm and supply chain losses are another story!). There are technological solutions (namely digesters and compost facilities) that can intake food waste with a far better GHG emissions profile compared to landfills. Still, the infrastructure is lacking, with these facilities only handling 4% of the total annual food waste produced in the US (Source: Center for the Circular Economy’s Composting Consortium). Enhanced efforts to reduce the creation of food waste while increasing diversion from landfills should reduce the amount of methane produced from food waste decomposition at landfills.

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