Creating a Bio-Digester
Welcome to the Thunderdome! Just in case there are those among you who are too young, or perhaps are just not familiar with the classics, this is a screenshot from Mad Max: Beyond Thunderdome. While I would highly recommend anything in the franchise for the entertainment value alone, it’s this particular title I draw attention to because of “Bartertown” itself. First, a little background, this is a post-apocalyptic story where the world (Australia in-particular) has run out of natural resources and everyone uses the limited resources left to have car-chase gunfights. It makes sense if you don’t think too hard about it.
Bartertown is powered entirely by methane gas. That begs the question, or at least it should, how do they get this resource, especially considering we just said they were out of everything? They use pigs, lots and lots of pigs, what they feed them or how they mechanically extract the gas is a tad murky but the science is basically sound. Pigs do produce methane gas as a by-product of eating, for that matter every animal I know of does on some level including humans (just ask my wife). Which brings me to the point of this whole trip down bad ass movie memory lane, methane capture for daily use.
The number of homes that use natural gas for heat, cooking, or are powered by a natural gas power plant is substantial. Beyond that, in a SHTF situation, having a renewable, sustainable and easy-to-maintain source of methane gas you can use for all these things is incredibly appealing. So on to the science, I promise you won’t have to raise pigs or try to catch your farts (as amusing as that would be for those watching).
The most interesting thing about building a methane capture system, at least to me, is just how very easy it is. By just understanding the basics of digestion, especially in animals that produce high methane content gas, we can basically just do the same thing artificially in a box. Essentially, We’re building a big stomach and can cram just about anything organic in nature inside it, unlike a compost pile, which is a bit more picky. The easiest way to start is with the plastic bin itself, for this I recommend something called an IBC tote tanks (Intermediate Bulk Container) because it is readily available, relatively cheap, and easily transportable. Also, because these things are standardized the measurements are available online so that you can plan your project out with exacting detail.
With the tank in hand, or rather on the ground, drill (3) three-inch diameter holes in the top in the corners. I have put a picture of the hole placement below so that this isn’t confusing and you're not drilling too close to the edge. The point of drilling three holes and putting them in the configuration I have is that it maximizes the space inside with the longest digestion time and the highest output for the space. It is really important to either drill the holes with a hand drill or using a red hot 3” pipe to melt the holes with smooth sides. This is vitally important for the next step which is simply placing 2” uniseals in the holes we have just made in the top of the IBC tote. Before putting them in place, it’s a good idea to put some silicon plumbing sealant on the outside ring to make it more airtight.
Now we have that done the next step is to take three 2” diameter pipes and cut them so that approximately 20” of pipe extends above the uniseals. The precise amount isn’t so important at this stage but having more now can save you a lot of trouble on the back end as you cut them to their specific purposes.
The first pipe will be the “feeding” pipe (colored green) and the first cut we want to make is a 45-degree angle at the bottom. Sanding this down so that we remove burs and jagged edges ensures the waste we feed into the tank flows smoothly into said tank. Soaping the pipe up saves you a lot of effort as you push the pipe down into the tank… twisting also helps but try to keep it straight as you push, as to avoid warping the area around the seal, which may lead to leaks in the final product.
The second pipe will be for the gas outlet (colored blue) and for this one a little measuring is required. It’s easier to measure this before you insert the uniseal by placing the pipe in the cut-out hole and using a sharpie or other similar permeant marker to draw a line about a half an inch below where the top of the tank touches the pipe. You will want to cut a hole or drill several smaller holes directly below that line but no farther down than three inches. Remember, we want gas, not liquids or solids from this pipe. Sanding to remove burs and jagged edges is important as we don’t want to damage the uniseal as we slide this pipe into place.
The last pipe will be a “slurry” pipe (colored copper), which is where we will get our wonderfully rich organic fertilizer as a finished product of all this digestion. We will start by cutting a large hole in this pipe, the hole should be approximately in the middle of where it sits in the IBC tank. With an equal amount of pipe above and below the new hole as the pipe sits in the tank. We do this because we want to leave the active digestion on the bottom to its business while skimming the most digested stuff from the top. By placing the slurry pipe opposite the feed pipe we give the material inside the best chance of digesting the higher energy feed while removing the most processed material for use elsewhere.
Now comes the finishing work, we need to have add components to the tops of these pipes to get the finished products. For the gas pipe, we will start by putting a 2” to 1/2” PVC reducer, a 1/2” ball valve, an elbow and a 1/2” barb connector. It is best to not glue them on in case you ever need to remove them for maintenance or cleaning.
For the slurry pipe we want to remove about half of the exposed pipe from the top before adding a 2” T-fitting, a piece of 2” pipe will be placed in the T-fitting so that it is perpendicular to the slurry pipe, a 2” elbow connector and a final piece of 2” pipe pointing to the ground with a bucket so that you can reclaim the fertilizer. You will want to leave the top of the T-fitting uncovered so that you won’t get vapor lock which will lead to a siphon effect spilling the contents of your digester on the ground in short order.
Any and all organic waste can be put in this biodigester, human waste, animal waste, rinds, peels, meat, grease, whatever you want as long as it is ground up in some kind of processor (masticator) so as not to cause clogs. It is also helpful to add water to this biomaterial to help it go down, grey or soapy water is fine and will not affect the biodigester negatively in anyway. It is also important to keep in mind that climate will affect these systems greatly. You have built an animal’s stomach and the primary anaerobic bacteria that you will be counting on work best in hot, dark and oxygen free environments.
As a final step, you will want to spray paint the exterior of the IBC tote all black for warmer environments, the cooler your area the more insulation is required. Spray foam insulation works best but cheaper Styrofoam and stretch wrap (maybe even black garbage bags) can work in some cases depending on your situation. You are aiming for internal body temperatures so whatever methods achieve this best in your area are what you should use. Light will damage the internal environment as well causing algae growth that will kill the bacteria you want and leave you with suboptimal fertilizer and useless CO2 rather than methane.
With this simple and expandable design, you too can power your own Bartertown in the post-apocalyptic wasteland without the hassle of having to keep all that delicious bacon around! The methane gas recovered can power generators, heat buildings, or cook food. The fertilizer that comes from this is rich and ready to be used right away to grow the healthiest produce you can grow, never again over pay for what you can make at home! Stay tuned and be sure to check out www.americansocietyofpreppers.com for future articles, where we will go greater in depth on the efficient use of this biogester. Stay alert, stay alive!