Design a Low-Cost, DIY Biogas Digester for Cold Climates

Of course, we both know that even though plants want to grow, it takes real skill and deep knowledge to grow healthy food, and to do it year after year while keeping the land in good heart, protecting the earth. And even though fire wants to burn, just because you have a book of matches does not mean that you can design a good wood stove, one that will provide warmth without smoke, do it with real efficiency, and throw off lovely warmth without crisping the cat if wanders near.

I’m saying, in other words, that to really understand farm-fresh fruit, fireplaces, furniture, fingers or fusion (that little thing the sun does) requires some science. You’ve got to learn stuff. At the same time, drop a seed, strike a match: To grow a garden or build a fire is not all that complex. How hard is it to use satellite TV? How hard is it to design the satellite?

So it is with biogas and biogas digesters. Making biogas couldn’t be much simpler than it is. Making a properly-designed biogas digester is solidly somewhere toward the other end of the spectrum. Make a digester for a science fair? Easy. If you’ve got a something-or-other that keeps the air out and the water in, well sure, that’s a digester. And yes, it may well generate biogas. Some.

OK, but there are two critical questions that divide real success from relative failure: 1) Can you demonstrate you’re getting what you should out, given what you’re putting in? And 2) can you keep the thing warm outside on a cold night, with minimal energy input? The problem is the same if you buy a digester. After all, there’s no Institute of Good Gaskeeping, and it may take quite a while before Consumer Reports turns its spotlight in a biogas direction. Is the digester you’re thinking of buying well-designed and worth its cost? Does it look good? Does that mean it works well?

Perhaps someday, digesters will be a consumer item, and Popular Biogas will publish an article with colorful charts, highlighting the one you’ve had your eye on — the Flatulence 1000 Turbo. Their commercials feature Crocodile Dungdeep, a sassy Aussie dung beetle with a great accent. What’s not to like? You won’t have to run the tests that Popular Biogas ran because… well, because they already ran the tests. Your digester got the best score. Nothing else to figure out. Sears carries it, and they have layaway.

Meanwhile, you don’t need to know all that scary and probably complicated stuff some old guy mentioned, in that article you thought about skimming all those years ago. Isn’t the future great? Yeah. Only we’re not there yet. Solar is there, more or less. They have magazines. But small-scale biogas is still … maturing. Meanwhile we — the few, the crazy, the Archeanaunts — we have to design, build or buy something in the hobbyist space, having applied as much knowledge as we can muster.

No doubt it’s a hassle to actually have to learn things, but as my friend T.H. Culhane says (paraphrasing), wait until you get the ‘first flame’, until you set fire to that near-sacred biogas that you produced. Magic. The first hit is free, and then you’ll be hooked. Ergo, this series.

Design Principles for a DIY Biogas Digester

For me, and in this context, I would say that our as-yet mythical good design of a small-scale digester for the U.S. might have certain characteristics, some necessary, some highly desirable. Among them:

• • It should be large enough to be practical.
• It should accommodate whatever substrate you intend to use.
• It just absolutely, positively, should be well-insulated. (Did I say, “on all six sides” enough times? No? Yes? Regardless: On all six sides.)
  • Whatever is put in should digest. (Now, that may seem blankly obvious— we’re talking about a digester, after all– but what I mean is that each morsel fed in should yield its proper bit of biogas before it comes out. A good digester has got to be designed as if it were, at least functionally, an intestine, whereas not all are. We’ll talk.)
• It should provide some way to get those critical measures (such as, [best case] a numeric measure of daily production), and have some minimum of automatic control. (For example, it must maintain a stable temperature in a selected range against changes in ambient. You can’t do that unless you have a thermostat, at the very least.) All that, except unless you want to wait for Popular Biogas and the Turbo to show up and save your bacon.
• It should be as simple as possible, but no simpler. Einstein’s dictum.
• And finally, it should be as low-cost as is allowed by those other items just above.

Now I have to tell you that for a long time — after all, I’ve been associated with biogas for a long time — I didn’t know whether it was actually possible to satisfy this whole list for a U.S. digester. Yet earlier this year I changed my mind, definitively, because, for the first time really, I started trying to imagine designs for low-cost digesters that would work in the US in a hammer-and-nails practical way. And I actually came up with something, or so it seems to me. (You’ll get to judge if I succeeded or failed, if you keep reading.)

A Lot of Trial and Error

It was the latest stopover on a mostly mental journey I’ve recently been on, after years of working on other things; pretty much the same journey I’m going to take you on. Let me start with the spur, the inciting incident: Some time ago a friend from Sri Lanka came to visit me. I had recently returned to biogas. Visualize. We’ve had tea and talked. He’s sitting in my office at the back of the house where we can look out on the meadow, and he catches sight of The Complete Biogas Handbook down low on the bookshelf. What’s that, he asks; and I explain. His eyes get bigger. “We need this in Sri Lanka!” He’s gotten very excited, and with a dawning illumination, catching sight of the village in which he was born, I had a sort of epiphany. They need this.

When I made the time to look into it, I was stunned by what had been carefully demonstrated, written up in the literature and peer-reviewed, during those years I was not paying attention, about the catalytic power of biogas to provide benefit to the poor. An escape from energy poverty. More time to pursue a better income. Greater health. Lower expenses. Less deforestation. A reduction in GHGs. Greater gender equality. Improved education, particularly for the eldest girl, the first teacher of her children. Better light in the evening for making crafts for sale, for reading books, and for more safely giving birth away from the stifling darkness. What a jaw-dropping list. Seven of the eight UN Millennium Development Goals and a lot more, bubbling out of a literal hole in the ground. Biogas wants to astonish you.

I opened my eyes, and found an enormous great lever of benefit in my hands. What an amazing privilege. You’ve just got to bow down. My father always told me that with every responsibility there is a privilege, and with every privilege there is a responsibility. So: chase that responsibility. Where to find a fulcrum?

My friend and I finally made a strong and direct contact with the founder (father) and the head (son) of a huge NGO in Sri Lanka which provides services to 15,000 villages. Their organization had been a fountain of good works for 50 years. They were also very interested in seeing the potential benefit of biogas unfolding in those villages. They had built a conduit of trust, and we were seeking the opportunity to move this astonishing technology through it. Everybody was agreed. Yes. Yes.

But the organization had no money. If we were going to offer biogas to villagers in Sri Lanka, not to say the rest of the world, then we would have to get the funding for any project ourselves.

Overcoming the Temperature Challenge

If you want to make biogas, and you like things really, really simple — as simple as possible — then you should move to the tropics.

Why? It’s because the rate of biogas production–all else being equal–depends on the temperature of digestion. Within a certain range, the warmer the digester is, the better. By contrast: Sweater weather? No biogas from an unheated digester. So colder is not nearly as good. Bad, bad, bad, in fact, at least as far as biogas is concerned.

Now why should biogas production be so dependent on temperature? Well, the reason is that heat is just a kind of jittering motion of molecules. And the warmer they are, the faster and further they jitter. You can even see evidence of this if you have enough small particles (pollen, for example?) in a jar of water, and you look really closely. You’ll see them move in a sort of random dance, a jitter… bug? (It’s called Brownian motion. Back in 1905, Einstein proved that atoms existed by drawing certain conclusions based on that motion. You could look it up.)

So when these molecules are banging around, the faster and further they bang, the more likely they are to encounter other molecules, and to break apart and recombine to form new molecules, meanwhile (generally speaking) releasing just a bit of heat. Love at first sight happens more often in the tropics, no? (After all, some folks maintain it’s just chemistry.) Love at first sight, and biogas. In the tropics. Add in Tahiti and buy me a ticket, please.

But hey, I assume you’re like me, minus the beard. To be more specific, I assume you live in the U.S. or maybe Europe. For what concerns us here, it’s much the same, because it’s about climate.

For those of us who are living in a place where, at least some of the year, it actually gets cold enough that long pants, socks, and Pendleton shirts make sense, what that means is that we really can’t expect to put a simple, simple, simple biogas digester in the backyard and have it do very much in the winter. You’ll need insulation. You’ll need a heat source. And of course, as we explained in the series about food waste and biogas (part 1, part 2, and part 3), you’ll need enough of the stuff that makes good biogas.

• 95 degrees Fahrenheit/35 degrees Celsius: 100 percent
• 85 degrees Fahrenheit/30 degrees Celsius: 68 percent
• 75 degrees Fahrenheit/24 degrees Celsius: 46 percent
• 65 degrees Fahrenheit/18 degrees Celsius: 32 percent
• 55 degrees Fahrenheit/13 degrees Celsius: 21 percent
• Colder than 55 degrees Fahrenheit: zero percent

Just to keep it simple so as far as temperature is concerned, we’ll call the rate of biogas production at 95 degrees “100 percent,” and compare other (lower) temperatures to that. Every time Mother Nature drops the temperature by as little as 10 degrees, the rate of the production of biogas also drops, pretty steeply, by about a third. (See the table above)

Now, if you have a garden or farm, then you’re familiar with the USDA Plant Hardiness Zone maps, which revolve mostly around how cold it might get where you live. But at least first draft, what we want to know is how warm it might be for how long, and so for the purposes of biogas, the American Horticultural Society (AHS) Plant Heat Zone Map is what we want.

What this map or these maps tell us, according to the AHS website, is “…the average number of days each year that a given region experiences “heat days” — temperatures over 86 degrees (30 degrees Celsius).” Eighty six degrees ambient. Is that good enough?

Well, how about this: Let’s assume first of all that you carefully studied The Complete Biogas Handbook. That gave you all the tools you need so that you can design your digester to use one of those really good substrates (like food waste) and to be large enough so that when it’s warm and cozy at 95 degrees, it gives you 150 percent of your daily biogas needs: for cooking or whatever it is that you have in mind.

Well, it turns out that if that if you can get 150 percent of what you want at 95 degrees, then at 85 degrees, the rate of production will peg at just about 100 percent of what you want, just by sheer and astonishing coincidence. So if the average daily outdoor (ambient) temperature is 85 degrees or better, then without heating your digester on such days, you can make all the biogas you need, and maybe even a bit more, assuming you keep feeding your digester what it wants, what it’s designed to consume. Got the picture?

Cold Weather Biogas Design

Now I live in Oregon, between Portland and Salem — just above the 45th parallel — and the AHS map for Oregon tells me that, at very best, I should expect only 30 to 45 days a year with temperatures over 86 degrees, right? In other words, if I expect to keep getting at least as much biogas as I had planned to get from my digester, I’d have to heat my digester for 335 days a year! (Ouch. That’s a bit discouraging, hey. Now where’s that Tahiti ticket when I need it?)

My digester would be better off in Florida, as you might expect. (I’d have a better tan as well. It’s a win-win, eh?) West and a little south of Miami there’s an area where I would experience better than 210 days a year of biogas weather, but that still means that I would need to heat the digester for in excess of 150 days a year.

In any case, of course, all that AHS Plant Heat Zone stuff is far from the whole story, because you may not have to heat the digester very much even on colder days, particularly with proper insulation and the proper approach.

In fact (spoiler alert), on a day that is 55 degrees, the new digester I am working on (see the picture?) can be heated to 85 degrees using less energy in an hour than you generate as heat just by sitting down and watching internet videos for an hour… Even if you’re not laughing!

October first, 2014: Jeffrey Ironwood-Hunt tightens the main bolt on “The Compressor” a tool developed by David William House so that his new, low-cost, kit-able & shipable, well-insulated biogas digester could utilize very low cost ‘bungs’ (holes in the wall of a container). These bungs David has developed cost less than a dollar apiece, and replace purpose-made bungs costing $20 or $30 each. The digester being built as shown here is larger than 2 m3, and the materials cost is less than $350.

Ideals are important, to be sure. And some say that we are either idealistic or realistic, as if becoming aware of injustice somehow necessarily prevents us from addressing it. But at least in my experience, that’s just not the way the world works. Just look around: there are a fair number of people who are both idealistic and realistic — but we could always use many more.

No doubt talk is cheap, and many things are difficult. That means that whether our goals are noble or selfish, failure to achieve those goals is not uncommon. But hey: Surely it’s better to fail when trying to help people escape poverty than it is to fail to add another zero to our personal wealth, no? (The zero is supposed to go on the right-hand end of the number, for any who were unsure.)

My father taught me a lot about how to find ways to have high ideals and achieve goals. (He invented the cochlear implant, if anyone can be said to have done so. Dr. William House; look it up.) And after all, I reasoned, I’ve done my service to capitalism, established a strong marriage, raised a family, and launched them into their amazing success stories. So, what was preventing me from trying for this brass ring– helping the poor worldwide — except a desire to sleep late on Sunday?

Well, naturally then, when my friend from Sri Lanka woke me up to the potentials of biogas for global benefit, I began to pursue this dream of helping many, many others, with limited resources and all by myself. How does one do that in a practical, step-by-step, realistic manner?

I first tried a very conventional approach, developing a darn spiffy, multi-page, gee-whiz spreadsheet, a timeline and other accoutrements, and using these to pursue grants. But after shopping it around and doing the standard hey-there-fund-this-grant two-step, I was left with the feeling that I was standing on a field — along with 10,000 other folks — where all of us were waving our particular fistful of paper about our particular Good Idea, trying to gain some funding, then maybe some attention. Up close maybe you could hear one of us. From a few yards farther away, where I imagined the closest grantors were standing, it would have been just sort of a lot of noise.

The experiences left me with the thought that, well, this would be a lot easier if I was able to get the attention first; the funding will follow. So I began to plan and build a solar-heated greenhouse with some fairly revolutionary features, intended to house a 10-cubic-meter digester fed entirely with food waste from a local restaurant. I got some modest funding from a few good friends, developed the design, spent months going up and down a ladder, and gradually the thing began to take shape. Here was to be something that people could point cameras at, to come and watch biogas in action.

Then, months into the build, a huge winter storm came through and brought it all down. Wham! I had neither the heart nor the funds to start such a large project again. Meanwhile, I had developed a small, very cheap digester suited to the tropics and intended to be manufactured in quantity. Materials cost: $10.

Now, I happen to know something about design and manufacturing because of my past work experience, and I knew that for something like a digester-for-the-poor– which is supposed to work well where every floor is a dirt floor– one crucial design process is to have people bang on the thing and try to break it. It should be reasonably sturdy, right?

So to provide funding for efforts to improve the design, to get the designs out and about and under stress, and to begin the process of gaining attention, I decided to start teaching workshops about biogas, capped with a half-day segment where we would all manufacture these low-cost digesters from kits and parts.

Creating a Model for Biogas-for-All

That went very well. I love to gab and I had something useful to say. My dear friends Tim and Suzanne of Friendly Aquaponics invited me to come to Hawaii and teach, I got invited to Australia, and the National Center for Appropriate Technology organized a workshop in Iowa, just to mention a few. From California to upstate New York, groups of charged-up folks had a good time and learned all about biogas. And they loved it, at least according to the evaluation forms I gathered from each class.

The favorite part of these workshops for most of the participants, it seemed, was building those low-cost tropical digesters. But at the same time, that was also the key problem, that little word there: “tropical“. The information was good, the folks were enthusiastic, the workshop was humming and the digesters were tropical. In Iowa there was snow on the ground. In New York, it was early spring. Too cold. Even in Hawaii, perhaps surprisingly: it was too cold almost everywhere.

Then this last spring I was invited to do a workshop in Brooklyn, and as I was doing the work to prepare for it, I gradually came to realize two things. The first was that these workshops, as popular and fun as they were, were not going to get me close enough to the brass ring. It was a great job description to add to a number of others, like “scriptwriter” that I had accumulated — “biogas workshop leader” — but all by themselves, these get-togethers probably weren’t going to get me to Sri Lanka, organizing the manufacture and distribution of many thousands of digesters.

It hit me: I had given a lot of thought to low-cost, practical, tropical digesters, but I hadn’t thought at all about digesters that would work in Burbank, upstate New York, or Iowa in the winter during a hard freeze: That is, almost anywhere on this continental land mass here that I’m sitting on just now.

The good news, as I came gradually to realize, was that all the design ideas and testing, all the invention of manufacturing equipment that I had undertaken in my quest for tropical, could help me– it could help you, come to think of it– to produce that utterly rare creature: A low-cost, well-designed, small biogas digester that will work profitably here, where tropical is a travel brochure, not a weather pattern.

The key ideas, as it turned out, were pretty simple. What I knew was that if I took two sheets of plastic and carefully pressed them — compressed them — together along a line, say with a couple of 2-by-4s and a bit of weather-stripping, nothing would leak through that line: not liquid, and not gas (at least not at low pressures). So in fact, with a bit of ingenuity and modest folding, I could create a water- and gas-tight container out of plastic sheets pressed into place by rigid pink polystyrene foam insulating boards, held in place by plywood. The polysty would provide good insulation. I knew from my experiments how to create a very cheap, very strong bung– a pipe-hole through the wall– and that’s all anyone really needs: A tight box with some pipes, minimum three: a slurry inlet, an effluent outlet, and a gas collection pipe.

And that’s it right there, really. That’s an entirely new design for a biogas digester which is cheap and well-insulated: the grail. And for the next many posts in this blog, that’s what I’m going to tell you, in some detail, how to make.

It will take more than a few posts and therefore some time, because there’s only so much you can say in a 1,000 words or so, even with pictures to multiply the syllable count. If you want to learn much more, a lot more quickly, then you are welcome to attend the next Beginner’s Biogas Workshop, which will be held in Washington DC in mid-April. (Read the details.) There and then, we will be revealing all. (Well, almost all. But in any case, enough.)

And if you can’t make it there, then no worries. There will be more workshops (sign up to be notified on the TCBH site), and whether or not it’s practical for you to attend a workshop, keep reading the blog and I’ll tell you everything I can.

Here’s my hope, finally. This isn’t just about providing practical, cheap, small biogas to a small set of dedicated crazies in the US. I firmly believe that this effort can help catalyze greater use of wasted food in this lovely, forgetful-of-its-high-ideals nation, and that in turn, it is my fond hope, can have a measurable impact on the release of climate changing ‘wild’ methane from out of landfills. (Food wasted around the world produces as much greenhouse gas as all the annual emissions of the entire country of India, the third largest GHG emittter! And if we can reduce methane emissions, according to the New Scientist magazine, we can delay the impacts of climate change by 15 years… Hey: that’s worth doing, right?)

Then finally, if I can create enough excitement and pay down my mortgage from these efforts, I’ll bet I can take it all the way to Sri Lanka.

David William House is the founder of Earthmind, an educational nonprofit that teaches about ecological living, as well as Computer Classroom, 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.

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