Carbon dioxide (CO2) tends to grab the headlines when it comes to greenhouse gases, but more and more attention is being paid to the effects of methane (CH4). While it’s true that carbon dioxide lasts far longer in the atmosphere—centuries to methane’s average of 9 years[i]—methane actually warms the planet 86 times as much[ii]. Moreover, methane actually breaks down into CO2 and water vapor, thus increasing CO2 emissions.
When the media does cover methane, it’s often in the form of eye-catching headlines about cow farts (“Scientists Underestimated How Bad Cow Farts Are”[iii]; “Does Alexandria Ocasio-Cortez Really Want to Get Rid of ‘Farting Cows’? Not Yet, At Least”[iv]). However, the science suggests that methane is no laughing matter. Humans are responsible for anywhere from 50-65%[v] of all global methane emissions, primarily from natural gas and petroleum production, human waste, and, yes, livestock agriculture.
Livestock is actually the largest global contributor of methane, comprising 28%[vi] of all emissions in 2014. On average, a cow releases anywhere between 30-50 gallons of methane a day through digestive processes. In 2012, factory-farmed cows, pigs, and poultry generated 13 times more waste[vii] than all 314 million Americans.
The best way to remove methane from the atmosphere, of course, is not to produce it in the first place. Without losing sight of the need to reduce livestock farming and fossil fuel use, however, we can also work to reduce the harm that these practices are currently inflicting on our atmosphere. That’s where methane digesters[viii], sealed tanks that harness the energy and nutrient-rich byproducts of anaerobic digestion, come in.
In this post, we’ll be looking at large methane digesters, digesters used on commercial and industrial scales. With a potential to generate up to 69.8 gigawatts of installed capacity worldwide by 2050, large digesters could potentially lead to a 8.4 gigaton reduction[ix] in CO2. As a result, Drawdown lists this technology as #30[x] on their top 100 list of climate change solutions.
How Digesters Work
All digesters work on the basic premise of anaerobic digestion[xi], or the process by which microbes break down organic matter into biogas, water, and nutrient-rich solid matter—all in the absence of oxygen, hence the term anaerobic.
There is no standard categorization[xii] for methane digesters across the various industries that use them. Some digesters are distinguished by the temperature range at which anaerobic digestion takes place; others by the percentage of solids needed to work successfully.
Methane digesters are effectively methane-capturers that trap the greenhouse gas in order to harness it as two viable resources[xiii]: biogas, used to generate power, and digestate, solids that can be used as fertilizers. Biogas[xiv] is 50-70% methane, 30-40% carbon dioxide, and trace amounts of other gases. In its original form, it is used to generate electricity[xv] in lieu of fossil fuels. When further refined, it can replace natural gas and sometimes even serve as vehicle fuel. Some research suggests that replacing natural gas with refined biogas can reduce GHG emissions up to as much as 91%[xvi] when compared to petroleum.
Some materials break down more easily than others[xvii]: in particular food waste, fats, oils, and greases break down best. While reducing food waste[xviii] should be the main priority—the U.S. produced nearly 67 million tons[xix] of food waste in 2010—anaerobic digesters offer one way to lessen the impact of this problem. Livestock manure, in contrast, is much harder to break down.
Where You Find Them
Water Resource Recovery Facilities (WRRFs)—the current industry term for wastewater treatment plants—are ideal places for methane digesters. In fact, anaerobic digestion at American plants has occurred as far back as the 1900s[xx]. Of the approximately 15,000 WRRFs[xxi] in the U.S. in 2018, about 1300[xxii] (9%) have anaerobic digesters; of those 1300 facilities, 860[xxiii], or two-thirds, use the biogas they produce to power their facility. Twenty percent of these co-digest materials from other facilities, often earning a tipping fee[xxiv] in the process.
In 2019, only 245 livestock farms[xxv] had methane digesters, with 32 more under construction. The vast majority of these are located on farms that have 500 cows or more, a troubling fact considering 95% of all farms in the U.S. have fewer than that.[xxvi] With upfront costs of anywhere from $400,000 to $1.5 million[xxvii], however, it’s understandable that smaller farmers are reluctant to take that kind of financial risk.
Finally, landfills can reduce the impact of the food waste they receive by installing anaerobic digesters. Landfills are the third-largest source of methane in the U.S., with food waste alone comprising 21%[xxviii] of all landfills. While digesters at 352 landfills generated around 11 billion kWh[xxix] of electricity by collecting biogas (also called LFG, or landfill gas) in 2018, that is only 11% of all 3091 active landfills[xxx] in the nation.
Ultimately, large methane digesters could be considered a stop gap: a way of reducing the harm of human waste (that which our bodies produce and that which we make and throw away) and conventional agriculture. That said, it should never supplant the ultimate goal of reducing the harm done in the first place.
[viii] Drawdown, 26.
[x] Drawdown, 26.
[xiii] Drawdown, 26.
[xv] Drawdown, 26.