Number 30 in this series is large methane digesters[i], a comparatively low-tech climate solution designed to capture the methane produced when human and animal waste and other organic matter decompose. Doing so prevents this greenhouse gas, which is as much as 86 times more potent[ii] than CO2, from entering the atmosphere. Moreover, the biogas[iii] (usually 50-75% methane) can replace fossil fuels.


No official definition exists to distinguish between large and small methane digesters, though BiogasWorld[iv]—which bills itself[v] as “the largest commercial, technical, and information network of the biogas industry”—uses 80kW[vi] as its threshold. For comparison, the average American household uses 914kWh of electricity per month[vii].

Given how small these digesters are, they are most commonly used to replace traditional stoves in developing rural areas of the world. According to Drawdown[viii], 1.9 gigatons[ix] of CO2 can be saved if 60 million more households adopt biogas stoves by 2050. This comparatively low number of gigatons explains why small-scale methane digesters are only ranked 64th[x]on Drawdown’s top 100 list of climate change solutions.


However, the impact of biogas stoves extends far beyond the carbon dioxide or methane removed from the atmosphere. Because traditional cooking stoves burn wood and charcoal inefficiently, it releases soot particles and carbon monoxide (along with carbon dioxide) fumes that pollute the lungs. Known as household air pollution (HAP), this phenomenon has been linked to a variety of diseases[xi] from pneumonia and chronic obstructive pulmonary disease (COPD) to stroke and heart disease. HAP causes as many as 4 million deaths worldwide every year—more than any other environmental factor[xii], and more than HIV/AIDS, malaria, and tuberculosis combined[xiii].


Thus, it is primarily in their potential to replace traditional cooking fuels that small methane digesters have the most sustainable and widespread impact.


The Benefits of Small-Scale Digesters


Nearly half the world’s population—3 billion in 2016[xiv]—still burn wood or charcoal for fuel, and 95% of these them[xv] live in developing or rural areas of Sub-Saharan African and Asia. Of all the clean alternatives available, biogas has an efficiency equal to LPG (liquid petroleum gas) and ethanol stoves[xvi] and an environmental footprint equal to that of solar-powered stoves[xvii]. Like these other fuels, using biogas reduces deforestation by removing the need to collect firewood. It also has the added benefit of using household and livestock waste that would otherwise release methane into the atmosphere as it decayed. While the biogas is siphoned off to provide heat and electricity, the remaining slurry or digestate can be used as fertilizer.


Some of the most exiting benefits of biogas-powered stoves apply to the women and girls who traditionally do the cooking. A 2015 study[xviii] of 199 households in southwestern rural India found that women who cooked on biogas stoves spent on average 2 fewer hours a day cooking while managing to prepare a more diverse diet. Greater control over cooking temperate enables women to multitask more, enabling them to use more ingredients and prepare more dishes. In addition, the women surveyed in the 2015 study appreciated not having to spend time collecting firewood—in part because doing so exposed them to pests from venomous snakes to catcalling men.


While the study did not establish what women did with their extra 2 hours of time, it’s crucial to remember that educating women and girls is #6[xix] on Drawdown’s list of solutions. If future generations can make use of biogas stoves to carve out time for anything from income generation to education—or even much needed relaxation—then biogas can play a much bigger role in sustainability beyond capturing methane and using it to replace fossil fuels.


The Challenges of Small-Scale Digesters


In fact, it’s actually when it comes to capturing methane that the sustainability of small methane digesters comes into question. A 2011 study[xx] focusing on China’s rural Sichuan province found that biogas stoves reduced GHG emissions by anywhere from 22-54% compared to traditional stoves. Moreover, the study found that implementing biogas technology and infrastructure improved local sanitation by making use of organic human and animal waste, thereby addressing a significant public health concern.


The study does point out, however, that as many as three-fourths of the lower emissions rates from biogas stoves can be negated if poorly-maintained systems permit too much leakage. As a separate 2014 study[xxi] notes, the benefits of replacing traditional fossil fuels with biogas can outweighed if anywhere between 3-51% of the methane is lost through digester leaks or intentional release. Similarly, a 2015 study[xxii] of digesters on small pig farms in Vietnam found that as much as 36% of captured methane was intentionally released because it was of no use to the farmers. Moreover, solid waste that normally would have been applied directly to fields was instead turned into a liquid slurry that instead went into ponds and the ground as watery runoff; as a result, this study concluded that unless digesters were patched to prevent leakage and excess biogas was transported to other potential consumers, any GHG emissions reduction would be severely undermined if not negated entirely.


It’s important to remember, however, that this latter study focuses on single-family pig farms and not on cooking stoves. In our post on large methane digesters, we emphasized that capturing methane isn’t as sustainable as reducing global meat consumption and shifting to other renewable energies like wind and solar. Thus, we should greet an article[xxiii] celebrating Vietnam’s 250,000 biogas digesters with some skepticism. We all need to consume less meat if we want to reverse climate change.


However, the benefits of household biogas stoves are too important to ignore. Independent of any reductions in GHG emissions, they reduce the toll of one of the most important and ironic environmental catastrophes of our time: being killed by your food. While we can put a number on the amount of lives saved, eradicating such a needless cause of death for our most vulnerable populations is truly priceless.




[i] Insert link to Methane Digesters (Large) post when live.











[xii] Drawdown, 44.