What's inside the Domestic Dragon


What's going on inside the domestic dragon?
By T.H. Culhane


Want to be a knight in shining armor, and save your mother earth, the damsel in distress? Want to know how to tame a dragon and make it work for you instead of against us?

In this course, as you've been navigating the food/energy/water nexus, you've heard a lot about how we can and are using the simple process of anaerobic digestion to turn food wastes back into healthy food while producing clean renewable energy and helping keep our freshwater fresh and clean. We've lovingly  talked about our biodigesters as “Domestic Dragons”, metaphorical but literally fire breathing dragons that can safely be domesticated and brought into homes and communities to magically transform the most troublesome organic wastes from our kitchens, restaurants, yards, farms and toilets, into fuel and fertilizer without a single puff of smoke (our condolences to Puff the Magic Dragon!). Did you ever wonder what really goes on inside the belly of the beast, what goes on inside the stomach of the biodigester to make this closed virtuous circle work so well?

This animation gives you a window into the mysterious world of the outwardly synthetic but inwardly completely natural world of biogas!

First of all, a biodigester really is just a stomach, a kind of artificial solar plexus, the literal guts of a biogas system.  It is a simple chamber that uses biomimicry to replicate what goes on in any animal's guts as it digests its food, whether it is a person, a cow, horse or pig, or a dragon.  The principle is simple: the dragon eats, and it pees, and it farts.  Yup, biogas is simply fart gas – in this case a very desirable flatulance that we design our digesters to create for our benefit.

A biodigester, like these Solar CITIES IBC tank based do-it-yourself biogas systems, is designed, like ALL biodigesters, to eat, pee and fart by taking a simple water tight/air-tight tank and plumbing it with three pipes: one for it to eat, one for it to pee and one for it to fart.  The stomach is completely filled with water and an innoculant of microbes that we get from real animal manures that came from real animal stomachs and intestines.

The feeding pipe, or “mouth and throat” of the dragon, has a mouth and neck above the stomach, like we do, and the tube that mimics the esophagous of an animal extends down into the bowels of the tank so the food can get to the bottom.  It has an opening on the side so food waste can slide into the center of the stomach.  Most proteins and carbohydrates and other food solids sink to the bottom to be eaten by microbes, while fats and oils and grease float on top.  As they get broken down through anaerobic fermentation they tend to get to the same density or buoancy as water and a liquid compost begins to accumulate around the center of the tank.  The dragon tank has another pipe running down to the bottom of it with a hole in the center where completely digested food, now a nutrient-rich liquid about the same density as water, can flow out from.  It also rises up above the tank to a level just below the feeding tube's mouth, and then bends so that excess liquid can spill out into the garden or into a bucket.  Every time we feed the dragon's mouth it thus pees out the same amount of compost tea.

However, in this case it isn't “garbage in/garbage out” but rather “garbage in/liquid treasure out!” Every gallon of ground up food waste you put in you get a gallon of the best fertilizer imaginable – an NPK rich slurry that can be used to grow vegetables and fruits hydroponically, completely without soil.
Of course, as anyone who has ever eaten beans knows, eating and drinking doesn't just make you pee, but fart as well.  For that we have a gas out pipe sticking out from the very top of the tank, where the bubbles of gas rise and accumulate, and it sticks up above the feeding pipe so that as we release the gas, which we use to fill balloons that we connect to our stoves or generators, no liquid comes out with the gas.

Unlike most animals, biogas dragons rarely if ever poo. They don't need to because they do such a thorough job of digesting the food waste that sludge hardly ever builds up – almost all the nutrients come out in the liquid pee.  But the tank does have a drain at the bottom in case you feed it stuff it can't break down, like straw or grass or leaves or wood shavings, or paper, and in that case you just remove the sludge like they do a septic tank (where it builds up from the toilet paper used) and dry it and use it as a solid fertilizer and soil conditioner.

Biodigesters are very efficient when they are kept warm and every thousand liters, like these IBC tanks, can take a 5 gallon bucket worth of food a day when kept at mammalian  body temperature or 37 C. At these temperatures a bucket of ground up food waste can give up to a thousand liters of gas within 24 hours, enabling us to cook for about 2 hours on a single burner at a medium flame or run a 2 kW generator for about 45 minutes to charge batteries. The gas, of course, can also be used for gas lamps or on demand gas water heaters. Beat that!   When the temperature is lower, say 20 to 25 C, which is average room temperature and the average temperature of a reptile like a dragon without heating, they can take about half that amount of food and produce half the gas.  The lower the temperature the less they can take and the less they produce, which is why in our experiments in northern climates we have placed our domestic dragons INDOORS, right in the home, usually in the family basement! That way we keep them from hibernating in the winter!

So the technique is very simple for making a domestic dragon.  In order to improve their ability to digest we also take a tip from nature and radically increase the surface area inside the stomach or intestines. Nature uses all sorts of invaginations and microvillage to “rough up” the inside of animal stomachs – they are anything but smooth, and a biodigester follows that principle. In ours we put in lots of things that can fill up the stomach with surface area without clogging the pipes.  Often we will throw in a ton of plastic bottle caps.  Other times we will take mesh laundary bags and fill them up with shredded water bottles or pinecones or wood chips or biochar or lava rock or plastic pond filter blocks – any thing with lots of surface area – and then put a small stone at the bottom of the bag and a piece of styrofoam at the top so it floats in the center of the tank.  We make as many as we can and shove them inside the tank so they are floating around like jelly fish inside. 

The idea is to have the inside act something like a coral reef with plenty of places where food particles can get trapped and eaten by the microbes living inside. The more of these surfaces we have, the more food we can give the digester and the more gas it will make.  We get pretty creative about how to do that, depending on where we are and what's available. It is the principle that matters, not the materials themselves. Into larger digesters we've put in brick and concrete block walls, old radiators, broken ceramic toilet bowls, fence posts, you name it.  The idea is to give the microbes a place to live and form stable biofilms.

So you want to know what really is going on INSIDE the domestic dragon?  It is the magic of the microbe!  On the first day that we set up our digesters we not only fill them with water all the way up but with as many ANAEROBIC microbes as we can.  For each 1000 liter digester we generally try to put in at least 100 kg of some kind of fresh animal manure.  In the city, because it is easier to find, we tend to use horse manure, but almost any animal will do, cows, pigs, chickens, llamas, alpacas, goats, sheep, elephants and hippos (yes, we've started digesters on manure called “Zoo-Doo” from the local zoo!) and we have even started systems on my baby's washable diaper wastes.  It provides a great incentive for Dad's to enthusiastically change the diapers!

This “innoculation” only needs to be done once, although you can always add more manure any time you like because it always adds fresh microbes.

And who are those microbes?

Well, you've got 4 main groups of different tiny critters in these dragon stomachs.
The first group are called “hydrolytic bacteria”. Their name means that they use water (“hydro”) to break apart food molecules (“lytic”).  They are the first creatures to get to work in the process of biodigestion.  They work underwater but most  can survive with or without air.  After they have done their work they make the much smaller molecules available to the second group of microbes called the “Acidogenic” bacteria. These critters take smaller energy rich molecules and turn them into larger chain organic acids like butyric acid and propionic acid.  You are familiar with these creatures because they also exist in your stomach and are responsible for that horrible burn of “acid reflux” that you get when you overeat and start burping up incompletely digested parts of your last meal.  They give you what we call “heart burn” and they also produce a sick stench, which is why vomit smells the way it does.  They also can mostly work with or without air. If that is as far as your digestion goes, you have a mess on your hands, and that is why your garbage pail stinks so bad and attracts flies and vermin.  The byproducts of hydrolitic and acidogenic bacteria digestion is still filled with energy but is broken down enough that lots of other creatures are attracted to it. 

Fortunately there are two other groups of microbes in the biodigester to take care of things before the rats and flies can even get a chance to know there is food about.

The next step in the process is the great work of the “Acetogenic” bacteria.  These critters take the bigger stinkier acid molecules and break them down into much simpler Carbonic acid (which is basically carbon dioxide dissolved in water) and hydrogen and lots of acetic acid, a.k.a. Vinegar.  You know your digester has gotten to this stage when you smell a vinegary acid smell but it doesn't make you feel sick or give you the gag reflex.  But it still smells “sour”.

Even more fortunately, there is another final group of microbes in the tank – the METHANOGENS. The methanogens are very special.  They aren't bacteria.  They are what we now know as “The Archaea”, the most ancient life forms on the planet Earth. They were the first creatures here and will be the last, they are the alpha and the omega, they are our direct ancestors and that of all living things, they have always been with us, once upon a time they were the ONLY creatures on Earth, and they now live inside us in our stomachs and intestines and those of every living creature and in every nook and cranny of the modern earth, wherever the sun don't shine, wherever they isn't any oxygen.  You see the Archaea are completely anaerobic microbes, and they ruled the earth until their descendents, the cyanobacteria, a type of early algae, started making oxygen and poisoned them and drove them into the anaerobic or “air free” spaces.  But they never went away, and their speciality is to take carbon dioxide and hydrogen and acetic acid and turn these energy rich molecules into biogas, into biomethane, or CH4, the simplest organic molecule with its single carbon and four hydrogen atoms.

Methane is a tiny molecule, so it is lighter than air.  When it burns it turns into nothing more than carbon dioxide and water as the oxygen combines with the carbon (C + O2 = CO2) and with the Hydrogen (2 H2 + O2 = 2 H2O).  So biogas burns clean and safe, so smoke or soot or pollution. And because the biomethane still contains about 30 to 40% carbon dioxide from that left over from the acetogens, biogas isn't explosive – after all, carbon dioxide is a flame extinguisher that is heavier than air and puts out fires while the methane rapidly rises up and spreads out to the point where it can no longer ignite.  Only at the source of a biogas flame can you get ignition, right where you want to cook or run a generator or light or heater or gas refrigerator.

The four types of microbes work together efficiently to turn almost all organic wastes, with the exception of woody lignocellulose or straw like materials, into a safe clean fuel and rich organic fertilizer.   On the surfaces we create in our artificial stomachs and intestines inside the digester they form what are called “biofilms”, a kind of surface scum layer like you find on your teeth in the morning before you brush them, which are like microscopic cities.  In the biofilm the microbes create symbiotic structures that efficiently divide the labor. 

Imagine if you will a tiny city where in one district, like a warehouse district, food wastes are being broken down by the hydrolytic workers into smaller molecules that are then shipped or trucked over to the acidogenic district filled with vats and test tubes where they get turned into seething green and yellow acids.  Then you can imagine them being piped over to the acetogenic district with its big  fizzy fermenting tanks and wine and vinegar barrels where they are turned into acetic acid and bubbles of fizzy carbon dioxide and hydrogen, and finally these swirl through glass tubes into the methanogenic district where our most ancient relatives (as different from other bacteria genetically as we are according to DNA analysis!) carefully take apart the vinegar and combine it with the CO2 and hydrogen to make bubbles of dancing fart gas, hoppity poppitty bubbles of methane that rises up through the watery world of the digester tank and collects in the smoke free chimney leading to the gas valve. 

The entire process happens each and every day and night continuously in a process that never sleeps. 
Biogas is a form of solar energy that we will never run out of because it is solar energy that was trapped through photosynthesis into plants, eaten by animals, shipped to our human cities , whose uneaten or undigested parts we have traditionally simply but tragically thrown away, incompletely broken down, creating bad odors that make you sick and attract disease carrying rats and insects and pollute the air and water.  With biodigesters that solar energy is rendered and transduced within 24 hours by this incredible consortium of microbes from a life threatening problem into a life-enhancing solution.
So that is what goes on inside a biodigester, inside the belly of the beast, inside the heart of the domestic fire-breathing dragon, a dragon that far from trying to slay, we have now turned into a house pet that can help save the kingdom rather than destroy it.

Want to know how to tame a dragon?
Build a biodigester. 
Now THAT … is chivalry!

Add new comment