Make Homemade Biogas Energy

Making Biogas Make Sense

The good news is that biogas is easy to make. It’s the only biologically based renewable energy, besides combustion — simple fire — that happens and then persists in nature. (That’s not true of alcohol, not true of biodiesel.) But if you think about it, you already know that, right? ‘Cause we said: Toss food waste into a hole and cover it up, it will generate biogas/methane. So, it can’t be that hard, eh?

Well, yes and no. Yes, it’s blood simple in nature — it happens all by itself. But also no, the biogas biology can be fragile and finicky, although there are ways to make it more stable, if you know what you’re doing.

But the biggest difficulty is that the biogas biology very much prefers to be toasty warm, and the rate of biogas production depends on the internal temperature of the digester. (This spreadsheet will give you some numbers.)

I can’t tell the whole story here and now, but the take-home is that you need an insulated, heated digester where the contents can be stirred or agitated. Otherwise, if the contents of the digester are “at ambient” —  outside air temps — then, even in lovely Florida during the winter months, an unheated digester will stop producing biogas.

Now, if this were still 2016, that would be pretty much the end of the story. 1) Here’s a problem. 2) Here’s a potential solution, but, oops, sorry, 3) you will have to figure out how to make a digester, because there was no low-cost, small-scale, DIY biogas digester widely available which was insulated and heatable.

Learn to Build Your Own Biogas Digester in This Workshop

Homemade Biogas Digester Exploded

[The_Cube] home-scale biogas digester.

Well, now it seems there is. This new wunderkind is called [The_Cube].

About The Cube | The Complete Biogas Handbook

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[The_Cube] with IBC sump

[The_Cube], professional version (deep base, bottom drain), as it might be seen at your neighborhood fast food outlet. The IBC next to it will collect effluent until the biogas entrepreneur that is leasing it to the restaurant returns once every week or two to replace the full IBC with an empty one.

Note: The majority of the images in this page are ‘clickable’, and thus can be seen in larger sizes.   Try clicking on the image above, for example.  Click here for printable/­sharable/­emailable PDF of this page.  [The_Cube] is the first example of a family of what we call ‘poly-panel’ digesters. These are digesters that make use of some of the properties of rigid polystyrene foam insulation (the pink stuff).

To help you understand what that means, we should explain a few things:

Containers: Convenience vs. flexibility

The first thing to think about when designing a digester is the container. And some may ask, Why not just use an existing container? Why not use a septic tank or a cistern or an IBC?  Well, there are several reasons, actually: good ones. I won’t go into too much detail, but I suggest you think about two questions.

  • First, how do you put an agitation device in an IBC or some other closed container? (Short answer: With enormous difficulty, or more likely, not at all.)
  • And second, how do you put superior (and air-tight) insulation on (and all around and under) an existing container? (My short answer: Probably with very limited success. I would not use fiberglass insulation, since it loses 50% or more of its insulating value when tested at normal levels of humidity. So rigid insulation is better, and yet in turn, rigid insulation is almost impossible to properly put around, on top, and under an existing container so that it does not leak warm air from cracks and gaps near the top, and pull cold replacement air into the bottom cracks and gaps.)

Our conclusion was that we needed to have a container that we could build, so we could put whatever we wanted in it (agitation, heat exchange, gas recirculation, instrumentation), and, if properly designed, make sure that it was well insulated.

In addition and not coincidentally, having a box that we build out of six panels means that we can build the individual panels, pack them together flat and as efficiently as possible, add the various other parts and pieces, and ship a kit that can be built on-site to produce a sophisticated, small-scale digester. [The_Cube], in other words, can be kitted and shipped. (Coolio, eh?)

Polysty welded to polysty

We started with a different design (the “press-plastic” digester, now little but a historical footnote), and in our research and experimentation, we did a substantial number of materials tests, trying to figure out how we could work with and ‘glue’ certain materials to other materials of interest: rigid polysty, polyethylene plastic sheets, wood, plywood, PVC sheets and pipes, and so on. One thing we found out which surprised us was that one bit of rigid polystyrene insulation can be welded to another bit by using sprayable urethane foam. (Here in the NW corner of the US, the most common version of sprayable urethane is “Great Stuff”.)  But just a note here: ‘Great Stuff’ is terrible stuff. It sticks to everything, and when it gets on most things— like clothing— it is impossible to remove. Wear nice clothes when working with this ‘stuff’ and afterwards you won’t. Have nice clothes, I mean.

The insulation is the container!  As soon as we saw that polystyrene could be welded to polystyrene, we realized that we could use it to make a welded box, one that could actually hold water, and maybe even be hermetic, “gas-tight”. And so we did make one. Several really. What we learned was that the promise of our experiments was borne out in practice: cut sheets of rigid polystyrene foam insulation can be welded together to make boxes (of many different sizes), and if those boxes are carefully made, they are hermetic.  We usually want to be able to remove the top to check on the digester or fix something that may have gone wrong: so in this design, we do not weld the top in place. This means that one challenge is making a top that is not welded to the sides gas-tight, so the biogas won’t leak out; but it can be done.

Dealing with the static pressure of water/slurry

What our experimental boxes showed us, even when working with a welded polystyrene box that was only 18 inches high, was that the pressure of water in any box of a useful size was great enough that it caused the poly to be pushed out: the whole box wanted to ‘become spherical’. That, in turn, puts all the welds under tension, so they will crack and the box will leak or even fall apart completely.  The upshot is that the poly-only panels have to be supported with wood. (It doesn’t take much.)

Toxic? Degradable?

Those who understand biogas know how easily harmed the methanogens are. (These are the micro-beasties which actually produce the methane in biogas.) Those folks might wonder: Will polystyrene give off chemicals that harm anaerobic life? After all, what’s the point of having a neat-o new micro-beastie house if it poisons everyone who walks in the front door?

Well, according to the literature (e.g. [1]), polysty beads have been used as an “inert” material in anaerobic digesters for better than 30 years.

Ok, fine: but what about the opposite problem? Will the polysty be attacked? Will it ‘biodegrade’ inside the digester? Will it get holes ‘eaten’ into it?

Well, again, not according to the literature (or at least, not very much, not anaerobically, yada yada). After all, consider: there would be no reason to use polystyrene as ‘carrier’ in your digester if it regularly disappeared: you want it to stay there and support your local biofilm, no? (And it does. Yet for those who want to know at least one way to deal with the polysty after the digester has served its time, see this article.)

So… Same questions about the wood, hey? Will the wood inside the digester get consumed like the organic materials fed in to produce the biogas? Well again, not enough to make any difference, according to the literature,[2] (or at least not for wood in truly anaerobic, zero free-oxygen environments, as contrasted with conditions that are merely ‘low-oxygen’).

Some parts of [The_Cube]

The front panel of [The_Cube], shown with the effluent pipe bung (top) and the drain bung (bottom). The image on the right is of that same panel with the polystyrene missing, showing the wood support inside.

“As for ‘waste disposal,’ we’ve got two mis-defined terms resulting in an abominable oxymoron. In nature there is no such thing as waste. All residues serve as resources for further growth— there is nothing to be disposed of. Nothing is thrown away. Indeed, there is no ‘away’. Everything must go somewhere.”

Bob Hamburg of Omega-Alpha Recycling Systems

Supported by wood, featuring bungs

To the left are two images of the front panel for [The_Cube]: the first shows the front panel with polysty, and the second shows the panel without polysty. You can see that the polystyrene is supported by wood support members. (These are screwed together, front to back, right through the polysty.) On the inside wooden framework in the image on the right we can also see the slotted piece that supports the axis for the agitator (click to see a larger picture; and there’s more about the agitator below). Also notice the ‘bungs’, those short pipes or holes in the poly panel. For any container, we need to put things in and get things out, and that means making holes in it. These bungs allow us to insert pipes into the poly panels and to seal around the pipes so that things don’t leak there either….

In sum, these are the three main items that make up any poly panel digester: polystyrene panels, wood supports, and bungs (more commonly called ‘bulkhead fittings’).


The Agitator

The inside of a digester, speaking biologically, is a very, very complex place. The materials fed into the digester go through many stages: breakdown into smaller particles and then into full liquefaction, followed by any of several different pathways breaking down complex chemicals into simpler and simpler chemicals. Each stage of this process is really accomplished by a different kind of life, as if, in this factory in reverse, we had the microlife from Noah’s Arc lined up along a disassembly line, or somewhat more accurately, along diverging disassembly lines.

So if you think about it you’ll see a rather strange thing: each group uses, as its food or energy source, what the preceding group left behind as its ‘waste products’. If the next group in line is missing for some reason, and therefore is unavailable to take the current group’s ‘waste’ and break it down to gain some of the energy stored in the chemical bonds, then the accumulation of ‘waste’ materials will tend to poison the current group. (Swimming in your own waste products, right?) So everyone has to be lined up, right there, cheek by jowl, ready to take whatever is left behind by those ahead in line, and to hand off whatever results from their disassembly to those next in line. It’s a biological bucket brigade, where the buckets start off filled with food waste, and by the end of the line, have almost nothing in them but liquid fertilizer, water, carbon dioxide and methane.

There’s a good deal more to the story, but the upshot for now is that we have to keep the contents of the digester stirred so that all those lovely biochemicals and astonishingly complex systems of anaerobic life are completely mixed together. What often happens in a digester that is not stirred is that the incoming slurry— the only slurry in most small digesters which is in motion— ‘short-circuits’ and takes the shortest path to the nearest exit. The remaining contents of the digester ‘go stale’ and stop producing methane.

Not having a way to stir up the contents a digester— not having the option of agitation, in other words— may well mean that your digester’s “effective volume”— the volume of slurry that is in active digestion— is some fraction of the actual volume.

The primary agitator for this digester is like an upside-down waterwheel. When enough biogas has been generated under one of what we call it’s “wings”, the buoyancy of the gas lifts and turns the agitator. So this agitator is intended to be self-powered.


The Gas Collection System

But will enough biogas be produced to cause the agitator to turn often enough to adequately stir the digester’s contents? I don’t know, as of December 2016….

Because we are still in a research and experimental phase, as we see it, with this digester design, we have added the ability to actively recirculate biogas into the digester: into the middle, to power the agitator, and into the corners to get more of the contents moving. We will be doing experiments, as time unfolds, to figure out how well the ideas work, and whether having gas recirculation is necessary or irrelevant— or something between.

HEx panel, detail

The PVC sheets in these images are rendered as semi-transparent. In practice, the sheets are opaque, and dark grey is the cheapest color.

HEx panels, complete

The Heat Exchange (HEx) units

We experimented with a number of different ways to pump heat into the digester. One of the most popular options for doing that is to run flexible pipe around the inside of the digester wall. I built and tried some alternatives using flex pipe, but I concluded after these experiments and my heat exchange calculations that it was not a great approach: too expensive. Too fussy. Too complex and time-consuming. Just… not elegant.

Meanwhile I had been working with PVC sheets to make the shields for my bungs, and one day I realized that I could glue a thin PVC sheet on either side of a slotted PVC pipe and run hot water through the pipe. The water would exit through the slot into the space between the two PVC sheets, and la voilá!: a heat exchanger.

According to my calculations, these HEx units are half the cost and twice the efficiency of a “ring around the digester” flex pipe solution that would take up about the same space.

The heater core has a small, cheap ‘fountain’ pump on the right end, which is all that is required to push water through the HEx units.

The hot water sump

At its present stage of development, the only heating option is electric. As the design matures, as further resources become available, as we have more and more of the million and one details (of the design and marketing and plans and calculations and learning new software and workshops) nailed down, then we will be working on solar and biogas heating options. Hey, there’s just one of me, after all.

reed switch

A reed switch, mounted on a piece of PVC sheet in the shape of a tongue depressor, offers a count of revolutions on the agitator. As the unit rotates, a magnet embedded in one of its arms rotates around and closes the reed switch, one per full revolution.


Obviously there are more parts and aspects of [The_Cube] which we might discuss, but this page is already top-heavy, and prolix, and this is a world with limits. But it does seem important to mention instrumentation: those units that offer counts and measures to be collected and aggregated. The key point about instrumentation amounts to a philosophy of design with regard to digesters and many other things: “What we do not measure, we cannot improve.”

Consider: These digesters might be used in many ways, but to have the greatest impact, they must be used in applications where the numbers are important. How can an installer say to a restaurateur, “These digesters can help your bottom line”, if she has no numbers to show? Any claim of function or benefit, of outcome or production, must surely be likewise supported by the numbers.

The importance of The Numbers

There’s room for discussion, but in the small-scale biogas realm, it seems to me that progress at the grassroots is impeded in part because (speaking generally) we do not have a bias towards the numbers, a bias towards measurement, a bias towards proof. I might go so far as to say that, in the small-digester space, the difference between biogas as a business and biogas as a hobby (like golf or collecting stamps; hobbies are good things, mind you), pivots on the determination to collect good and sufficient data.

Say I want to promote this digester as “a solution for homeowners”. (I don’t, but suppose I do.) Well, shouldn’t I be able to show— if I’m selling it that way, don’t I have what amounts to a moral obligation to show— that my proposed solution actually provides the implied or stated benefits? For example, shouldn’t I bend my energies to do the research that would allow me to say (something like) “Based on the data we have collected from W homes, a significant fraction (X%) of them are able to provide themselves with cooking fuel for Y hours per day, using their wasted food (which amounts to Z pounds per day, on average).”? And if I do not have data sufficient to make that or a similar statement, should I really be saying or implying that this digester is good for homeowners?

In other words and more specifically, my hope and intention with this digester (and any others I design) is to provide reliable information: the facts. Imagination is essential, but when it comes to evaluation of outcomes, the numbers are irreplaceable. And as of December of 2016, I do not have the numbers. Now, please realize that by saying that, I definitely do not mean that it’s all up in the air or that nothing is known. [The_Cube] is a closed container. I will without doubt produce biogas. The questions needing careful examination and testing have to do with how much biogas, with such things as how much food waste we can feed it (more than a ton a week?), and so on.

Again, there’s only one of me, and the implied effort promises to consume a great deal of someone’s time going forward. Regardless of obstacles or difficulties, I intend— gradually perhaps, but with determination— to get those numbers.

Meanwhile, would like to see it, every stick of wood and polysty bit, in it’s nearly-completed form? You’re in luck, then: We have put up a 3D rendered model here. Just a note though: this model is huge, @ 17mB, so it is likely to take some time to load into your browser. If you want, you can download a zip file with everything you need to use the model locally, from here.

In sum, these are the first kittable, shippable, insulated, heatable, agitated (not to mention DIY) small-scale anaerobic digesters of which we have heard, worldwide. Do you know of any others? (We know of several kit digesters that are unheated and essentially unheatable, but none that are agitated.)

And here’s another big thing which makes these digesters absolutely unique: In our workshops, we will teach you how to make them from materials you can purchase from Home Depot, Lowes, and/or some other hardware store. Learn how to make them, and you will have in your hands and head and heart the means of making them yourself forever, without ever purchasing so much as a #10 nail from us, ever again.

  • The importance of temperature
  • Refer to the amount of food it requires; not a ‘home solution’
  • Max/min loading rate

I’ll be writing a lot more about all aspects of this new digester, using this blog to tell you when plans are ready and other Pretty Neat Stuff, so keep coming back to MOTHER’s site for more juicy details. As well, you may want to sign up for email notifications and further information, here.

Meanwhile, we will be teaching workshops — including a Complete Biogas Workshop May 4-8, 2017, in Aurora, Oregon —so that anyone who wants to find out how to build these digesters from parts you can get at your local hardware store: polystyrene insulation (the pink stuff, rigid boards), various bits of lumber, some pipe and such, a small resistance heater, a fountain pump, and a few other items. The first day will be “all about biogas”, and thereafter, we will spend time building one of these digesters, and experiencing some hands-on manufacturing techniques. More details in the link just above.

We won’t be holding anything back, mind you. Come to a “builder’s” workshop and you will learn everything you need to know about how to produce complete digesters from locally purchased materials: as many as you want.

As well, this auspicious year of 2017, one of our main goals will be to work hard to put together everything we need to enable folks to use these digesters (and upcoming designs) to make a business out of saving the planet, one soggy French fry at a time. Further, we will be releasing plans, for those who cannot (yet) attend a workshop, and likewise selling kits. Stay tuned.

In sum, that’s the 411: Food waste, bad. Methane, worse. Biogas, good. You can make it, burn it, gain energy (and high quality fertilizer!), using [The_Cube] and follow-on designs, and you will then be responsible for withdrawing food waste from the landfill and thus doing a very good thing.

You can be a global warming warrior. You can become personally carbon negative!

Homemade Biogas Digester Plan

David William House is the founder of Earthmind, an educational nonprofit that teaches about ecological living, as well as Computer Classroom, ici (computer sales), and an eponymous computer consultancy. He’s the author of Methane Systems, The Complete Biogas Handbook, Journey (a book of poetry), and numerous articles. David is also the designer of a low-cost, plastic-bag-based biogas digester for equatorial belt countries and inventor of a patented new technology for cochlear implants. Find him at The Complete Biogas Handbook and read all of his MOTHER EARTH NEWS posts here.

[1] The… fluidized bed [digester] utilizes floatable particles… [as a ‘carrier’ or support for anaerobic biofilms in digesters using very watery or dilute slurries]. Synthetic materials are the most usual carriers in these studies, especially foamed polystyrene. [emphasis added.]

Garcia-Calderon, D., et al. “Anaerobic digestion of wine distillery wastewater in down-flow fluidized bed.” Water Research 32.12 (1998): 3593-3600. [link]

[2] “[The] degradation of wood under completely anaerobic conditions has not been unequivocally demonstrated.”

Bowens, Amanda, ed. Underwater Archaeology: the NAS guide to principles and practices. John Wiley & Sons, 2011. [link]

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