Methane Digesters

Methane Digesters

(Aka Anaerobic Digesters)

The Methane Problem

Methane gas was discovered in 1776 by Italian physicist Alessandro Volta, as he observed what appeared to be flammable air rising over muddy Lake Maggiore. However, it wasn’t until a century later that scientists would finally understand the link between decaying vegetation and the formation of methane gas- Volta’s flammable air.  

Today, methane gas is known as a “greenhouse gas” that contributes to global warming by trapping incoming radiation from the earth’s surface. Methane is in fact 34 times more potent than carbon dioxide in the atmosphere over a 100-year period. Therefore, to keep climate change at bay the world must control both CO2 and methane emissions, which come predominantly from organic waste created by agricultural, industrial and human digestion processes (see figure below). Some methane is also emitted from wetlands and other natural sources- hence the “marsh gas methane” discovered by Volta.

The methane source you’ve likely heard about is “cow burps”- which is a very real and significant contributor to global warming. When cows burp they emit methane, which then rises and remains in the atmosphere as a greenhouse gas. When you think of how many millions of cows are raised every year for human food, that’s a lot of burps that are harmful to our climate.

 Methane concentrations in the atmosphere have increased by 150 percent from pre-industrial times, and continue to grow. Finding ways to reduce or remove methane will, therefore, have an outsize and fast-acting effect in the fight against climate change”.

The Conversation academics & researchers, First Post , May 2019
lmage source: Global Carbon Project

Fortunately, as with other climate mitigation strategies, innovation and technology for reducing and capturing methane emissions is coming swift and sure (though importantly, as with CO2, these methods are not any substitute for government methane reductions policy). Education is also improving steadily and surely, on how fossil fuel use and large-scale agricultural practices effect our planet.

There is an incredibly simply answer to the question of reducing methane emissions from livestock such as cattle: the world can simply stop eating as much meat. Luckily for the climate, this is already a growing trend as the ratio of vegetarians verses meat-eaters globally is growing, in tandem with the number of meat-alternatives. This trend will only continue as more and more people shift their protein choices for ethical, environmental or personal reasons. However, at the same time we also will not wake up tomorrow, or in five years or even 30, to find that no longer are livestock raised for human consumption. Therefore, as is the case with CO2, new agricultural practices and technologies must be embraced by the industry- and by consumers– to keep methane emissions at bay. Watch for my followup post on the highly intriguing and potentially groundbreaking “The Kelp Diet” to come. The Methane Problem is a big one and like all other climate change challenges, there is no one silver bullet to solve all. For the purposes of this post, I will overview one such exciting and promising strategy for reducing methane emissions, that involves turning waste into a fuel of the future.

Anaerobic Digestion

As organic waste- whether food waste, agricultural waste, or human waste- decays, fugitive methane is emitted into the air. Yet it is also possible to harness the process of decomposing waste for good use. As waste decays, microorganisms within soils can also break down the biodegradeable material, in the absence of oxygen, and transform it into something completely different. This weird and wonderful process is called anaerobic digestion. Fermentation used to make food and drink products uses anaerobic digestion. Anaerobic digestion also occurs naturally in some soils and in lake and ocean sediments. As Paul Hawken puts succinctly in Project Drawdown,

“Anaerobic digestion provides thoughtful management for organic wastes as they decomposeshit happens“.

Project Drawdown

Technological advancements have also allowed for the development of special tanks called “anaerobic digesters” that literally transform scraps and sludge from organic material into useable byproducts. Digesters are very efficient and can function continuously, as long as the waste feedstock is maintained and the microorganisms are healthy. The useable byproducts created in this process are biogas, and digestate.

Image source: New Green Business Ideas

Byproducts of Anaerobic Digesters: Biogas and Digestate

As the above diagram of a digester shows, once the waste is broken down by anaerobic bacteria, it is transformed into 1. A mixture of (captured) methane, CO2 and other gases called biogas, and 2. High-nutrient sludge called digestate. Now, these may not seem like very appealing byproducts, but both are in fact incredibly useful. The gas mixture can be used to create renewable energy, acting as a fuel for electricity generation. For instance, this biogas can be used to power vehicles that would otherwise use natural gas (much less climate-friendly). Biogas can also be used for heating in homes.

“Biogas can reduce demand for wood, charcoal, and dung as fuel sources and therefore their noxious fumes, which impact both planetary and human health.

Paul Hawkin, Project Drawdown
Biogas Production- Abyewardena Graphics

The second product of anaerobic digestion, digestate, can be used as fertilizer in place of fossil fuel-based fertilizer. Moreover, it is actually healthier for the soil, as it provides moisture retention and organic content, which can break down even further in the soil. Digestate also protects soils against erosion and provides nutrients for plants. Now that’s some good news to digest! Recent trials have shown positive results of digestate (as a product of anaerobic digestion of waste) being used as a fertilizer for crops.

Digestate produced by members of the “Digestate Certification Program” in the US– (check them out!)

Due to technological advances and lower capital costs, many countries have began experimenting with anaerobic digesters to harness organic waste as an energy resource. Germany leads the world today with almost 8,000 methane digesters as of 2014. North America is increasingly following suite- especially as governments and the public become more and more aware of how climate-threatening methane emissions are.

Paul Hawkin and the Project Drawdown team estimate that “the cumulative results [of methane anaerobic digesters] would range from 6.2-9.8 gigatons of greenhouse gases emissions avoided, at marginal first costs of $173-285 billion“. And,

“By 2050, small digesters can replace 57.5 million inefficient cookstoves in low-income economies.”

Project Drawdown