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Complete Biogas: Designing a Low-Cost, DIY, Biogas Digester for North America, Part 1

biogas digester

There are two things that are true about biogas. At least two. And I mean things at the core of biogas, down in its chambered, beating heart. The first is that biogas wants to happen. It wants to happen like plants want to grow, like fire wants to burn. And that force, that will to happen, has (at least) those two aspects: life, like the plants; and chemistry, like the fire. But the second thing about biogas is standing on the other side of town, on the other side of the lake, on the other side of the ocean. And that second thing is that as simple as biogas is, it’s just as true that biogas is complex, deep, unfathomable.

It’s not just that there’s a world inside that digester; there’s a universe. It’s symbiotic to the 42nd degree, seething, stochastic, and astonishing. It lives in the dark, underground part of the wheel of life, and to fully understand it (I don’t) requires math, physics, chemistry, engineering, biology, ecology, and agricultural sciences. And I left some out. Now these two poles of biogas bear on my present subject, which is— across this new series of blog posts— to show you the parts and pieces that make up the design of at least one low-cost digester made for US latitudes and realities, and to explain why they exist, what they are intended to do. (And to tell a few jokes too. Jokes are important.)

If you’ve been keeping up with your caffeine intake and you have clever hands, by the end of the series I think you’ll be able to build one, just from what I’m going to show you. For free. Just because I like your sister. (She’s great, after all. No offense, but you need a bit more work. Isn’t honesty really the best policy?) Right then: biogas is simple. Keep it wet, keep it warm, and keep it away from the air, and huzzah! Almost anything that was once alive will yield up the energy in its carbon bonds as lovely burnable methane, mixed into marvelous natural biogas. Like plants want to grow, and fire wants to burn, biogas wants to astonish you.

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?

In short, how can you know that what you’ve come up with (or what you’re considering buying) reflects good design? How do you evaluate the quality of a digester, objectively and with some insight? But hey wait a minute. Do you really need to?

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. That’s a great digester, huh? 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, contented sigh, 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.

For me, and in this context, I would say that our as-yet mythical good design of a small-scale digester for the US 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.)

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….

And then? And then? Well, this is where (in the interests of having a tight plot line), I leave out a great many things. So will the made-for-TV movie, I’m sure. (I’m thinking Brad Pitt for the lead. We have a similar sort of twinkle, don’t you think? I may be a little taller, though.)

And I will tell you the parts of the story… next time I post. Keep reading.

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