Imagine a world where every food scrap, farm’s cow manure or any discarded organic matter could be made into electricity, hot water and fertilizer.
Anaerobic digestion means what it sounds like:
Digest stuff without air, or specifically not in the presence of oxygen. This is what happens when you eat food, or most any vertebrate or invertebrate eats food: It goes into a tube, lined with friendly bacteria (we’ve all heard how important gut bacteria are!). And many living beings extract energy from the food as it travels through passageways – our intestines – that allow energy to be extracted through the actions of microorganisms and the linings of our “plumbing.”
Much like solar, wind and renewable energy in general, the key magic to take from the story of anaerobic digestion is that it hits all the high marks of regenerative practices:
Take something we used to landfill, and turn it into energy and produce high quality soil amendments to restore agricultural land.
AD (anaerobic digestion) was actually “the first” way of doing digestion on our planet.
Anaerobic heterotrophic* bacteria did this in our early oxygen free oceans about 3 billion years ago. And, one of the most common questions to newcomers to the “AD” or anaerobic digestion space is the basic:
“Yeah… but how does it really work?”
[*Note: Heterotrophic is an important concept in the 4 stages we’ll explore below – since it means an organism “that cannot synthesize its own food – therefore must consume complex molecules.” We (humans) in other words, are heterotrophs. Lucky for us, however, many microorganisms that perform valuable deconstruction of complex organic molecules – food waste – do the same.]
AD is a marvel in some ways, but in terms of explaining it simply, it is biomimicry. Much like a cow has four stomachs, mechanical / biological anaerobic digesters that produce electricity have four stages for turning organic waste into methane gas, warm water and digestate.
These four stages gradually work the complex molecules into simpler ones, all the while re-arranging carbon and hydrogen to create CH4 (methane) to produce power, water and heat.
To make it super simple from an energy production (power generation) perspective, let’s summarize it with these questions and answers:
- Who is doing the work? Mostly the beneficial microbes, demolition and construction experts when it comes to a variety of complex molecules available from food waste, cow manure and other organic matter.
- What’s the exchange? They get food, their colonies thrive, and we get methane (CH4) derived from much complex arrangements of carbon, hydrogen, oxygen, and nitrogen.
- Who are the players? In terms of the periodic table, the ones we care about are primarily hydrogen, carbon and oxygen.
Step One – Hydrolysis
Biomass is typically made up of large polymers that are organic. Bacteria in anaerobic digesters access energy potential of the material by breaking down large molecular chains into smaller constituent parts. These monomers (simpletons, like us reading this article!), such as sugars (yum, sugar!), are then readily available to other friendly microorganisms or beneficial bacteria.
Hydrolysis is that – the process of breaking long chains into smaller molecules.
So think of step one as, let’s tear apart this old car, take all the parts, and see what we can make of it. But before we can take the valuable things from the car, we need to first take big things and make them smaller. This image from Khan’s Academy shows us how maltose (big complicated sugar) can be made into two simpler glucose molecules by the addition of water.
So when you think “water” think “hydro”… or “add water to break stuff down.”
Step Two – Acidogenesis
Acidogenesis takes on the next step – breaking down what has already been started. Since glucose, as in our case above, is a little easier to digest than predecessor maltose, for certain “fermentative bacteria,” you get the creation of volatile fatty acids, or VFAs. These fatty acids are created along with ammonia (NH3), carbon dioxide (CO2) and hydrogen sulfide (H2S). This reaction is one you may know well – it is how milk sours in your fridge. (Or even faster not in your fridge!)
Step Three – Acetogenesis
In the third act, the acetogens come into play. Think of them as those who can make acetic acid – or what you might know as vinegar. While this is an overly simplistic overview of this third stage, the take away is this: The molecules now present in the process are food for bacteria that will produce largely acetic acid, CO2 and H2. Note: We are now only one step away from methane (CH4).
Step Four – Methanogenesis
In the last act the available ingredients from the prior processes are consumed by methanogens to deliver methane, carbon dioxide and water. If you look at the resulting re-arrangement of the elements carbon, hydrogen and oxygen, in their new order, you’ll notice a couple of things:
- Simpler arrangement – the big messy organic chains have been re-arranged by friendly microbes into something useful – CH4 – methane!
- Energy available! – The key part now is final rearrangement of Carbon and Hydrogen into a gas that has energy locked in, or stored. The resulting product can then be captured as renewable natural gas, or burned to create electricity via a combined heat and power system. A bit more about them here.
What isn’t digested is known as “digestate” which can be repurposed as plant or field fertilizer.
Why this Story
The story of AD is worth telling since it will not only dramatically turn around a lot of our agrarian and wasteful food practices, but it is an elegant story of biomimicry.
While cows have four stomachs and can generate energy from symbiotic bacteria, it is worth noting that anaerobic digestion leverages some of these same principles.
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