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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.

All MOTHER EARTH NEWS community bloggers have agreed to follow our Blogging Best Practices, and they are responsible for the accuracy of their posts. To learn more about the author of this post, click on the byline link at the top of the page.


Rendering of the net-zero energy school

The following release is reposted with permission from Friends School of Portland.

Here on a wooded 21-acre lot, about a mile from the coast of the Atlantic, Maine’s first “net-zero” energy school is under construction.

Friends School of Portland is building a 15,000-square-foot school that will not rely on fossil fuels, and is expected to be the first school in Maine to produce as much energy as it uses. The new school also will be only the third in the United States to achieve Passive House certification, a high international standard for energy efficiency.

The decision to use Passive House design reflects a commitment to environmental stewardship, which is one of the guiding philosophies at Friends School of Portland, a Quaker day school for preschool through 8th grade.

Students breaking ground

“The best part of this project has been seeing how excited and engaged the students and their families have been,” said Jenny Rowe, Head of School. “They have helped steer the whole process. We’re all committed to being good stewards of the land, and to creating a home in Maine that will meet our needs for years to come.”

Friends School of Portland leaders, along with Kaplan Thompson Architects, Warren Construction, and other partners, hope the new building will serve as a model for others interested in sustainable design. The only schools in the U.S. with Passive House certification are in Hollis, N.H., and Rocky Mount, Virginia.

Foundation work

Passive House standards, overseen by Passive House Institute U.S., use solar gain and air ventilation to warm and cool buildings. The new home of Friends School of Portland will not rely on traditional fuels such as oil, gas, coal or wood. With the addition of solar electric panels and thermal tubes for hot water, Friends School of Portland will be a net-zero energy building, meaning that it will produce as much energy as it uses.

The school is in the midst of a capital campaign to raise funds for the $5.5 million project. They have raised about $2 million toward a $2.5 million goal. A second phase of the project, including a gymnasium, is envisioned for the future.


So basically, the thought we left you with last time (here, part 1) was that the US climate is not “biogas-friendly.” Recall? Even at the very best “biogas weather” places in the continental US, to attain anything close to optimal temperature for better than 150 days a year, we have to heat the digester. 

Even here, in semi-tropical Oregon, with the highest happiness index in the US (well actually… I just made that up), I cannot escape certain realities. For me, it’s 335 days of heating the digester, and based on what I see outside my window and the weather for the last week, I’m starting that countdown….

Now, some might ask: isn’t it true that you can produce biogas at colder temperatures? And, yes, sure, not only is it possible, but millions of people around the world do just that, every day. (This is not a hyperbole. It’s really, truly millions in India, China, Nepal and other countries. I can prove it to you if you like: You should always ask for proof.) And the vast majority of those folks have underground digesters, which are seldom very warm at all. (Think of lying directly on the ground to sleep. Even if it was dry, you’d get pretty cold, most of the year. Maybe even in Tahiti….)

So how come millions do it… Yet I’m saying that’s not the way to go, huh?

Well, in a biogas digester, as the previous post said, the rate of digestion is dramatically affected by the temperature of the digester. Colder is slower, so a given volume of slurry will produce less biogas for a given period when it is colder. Right? So if you know that, then it seems obvious that the way all those millions compensate for the colder digesters is to make them bigger. Much bigger. Five or ten times bigger.

And bigger, all else being equal, is more expensive, right? (Of course, again, yes.) But in developing countries, larger digesters are necessary because adding a solar heating system and providing good insulation would be (well… might be) more expensive than building that larger digester.

What’s very cool for us (or very warm, actually), is that the economics of large vs. small digesters are generally pretty different in the US and Europe than in developing countries. Things that are relatively cheap for us, such as some kinds of building materials, are relatively expensive in those other countries. And vice versa: labor is very cheap in most developing countries, but not so much in the US and Europe. These things push the economics of large vs. small around differently in these different places.

Different economics? Want an example? Well, imagine you want to have a small (low-cost) swimming pool at your house in Burbank, CA. (Why would you live in Burbank, anyway? Never mind.) Is it cheaper to go and buy a small above-ground (mostly plastic) pool in the US, or cheaper to dig a hole and build a (mostly concrete) pool in the ground?

The market shows the answer. No one looking for a suitable but low cost small pool builds one out of concrete in the US, largely because it takes quite a bit of expensive labor and even special equipment (for gunite?) to do it. So in the US, using concrete to build pools is usually more expensive than using plastic. Of course you want something that will last for ‘long enough,’ something that has a low hassle factor. Something that works for you. But if you don’t have to, why send a large torpedo fishtailing its merry way into your bank account?

Volumetric Efficiency
95 degrees F (35 degrees C) 1 volume
85 degrees F (30 degrees C) 1.5 volumes
75 degrees F (24 degrees C) 2.2 volumes
65 degrees F (18 degrees C) 3.2 volumes
55 degrees F (13 degrees C) 4.7 volumes
45 degrees F (35 degrees C) 6.9 volumes
…And do you recall that I promised
I would give you an Excel spread-
sheet for calculating temperature?
Well, you can find that here....

The difference is even more pronounced with biogas digesters because, as we just indicated a few paragraphs ago, a warmer digester is a smaller digester. So, at least as I see it, the most reasonable economic comparison has to be made on the basis of the amount of biogas being produced. That is, you’ve got to compare the cost of a colder larger digester against the cost of a warmer smaller digester, based on the digester volume required to produce the same daily biogas output. Make sense?

In fact, if you recall that table we showed you in part 1 (here’s a better version), it should be evident that, on the basis of daily output, a unit volume of digester at 95 degrees F is equal to almost five unit volumes of digester at 55 degrees F. So you’d best not compare a cubic meter to a cubic meter, or a cubic foot to a cubic foot of digester space, at least when they’re running different temperatures. You’ve got to compare the two digesters on the basis of what we might call “temperature-based volumetric efficiency.” (Say that five times fast, eh?)

The completed hot water heating coil for the press plastic digesterWant a real-world example? I estimate that the press-plastic digester that I am working on completing right now will have about $350 in materials in it by the time I’m finished. It’s got 2” of rigid polystyrene on every one of its six sides, and it will hold a bit more than 2.5 cubic meters of slurry. Based on my calculations at present, on a 40 degrees F day, keeping all the slurry in the digester at a biogas-comfortable 85 degrees will take about the same constant input of heat energy as you, personally, output as heat energy while standing and having a relaxed talk with a neighbor over the backyard fence. Even if your neighbor is boring. In other words, it doesn’t take much energy at all to keep it warm. (Left image: The completed hot water heating coil for the press-plastic digester.)

Now, you likely don’t know this, but a standard 5 or 6 cubic meter underground digester, built in the tropics and running at ambient temperature underground — we’ll call it 55 degrees Fahrenheit — will also cost about $350 in materials. (Well, actually, for materials and labor, so to comparison is not exact. But still.)

And here’s the thing. If both the small warm and the large cold are fed the same diet, then the US-ready, polystyrene-insulated digester I hope to show you how to build might even produce five times the biogas as the underground digester, if we keep it warm, and feed it better and faster. (But for the sake of argument, let’s say they produce the same amount every day...)

In that case, on the one hand, we have a US-ready, heated digester which at minimum is producing the same amount of biogas as an underground, cold, equatorial belt, 5 cubic meter digester. Without having to dig the hole. 

So which one would you want?

In sum? Well ultimately it’s about getting enough biogas, right?

#1: You’ve got to have enough stuff…

Capiche? We made this abundantly clear in Biogas and Food Waste, part 3 (here). Many people will be stopped by the fact that they don’t have enough (and also choose not to get enough) stuff to put in the digester.

I know there’s been a lot of talk about home biogas: but think about it. According to the United Nations Environment Programme, using figures provided by the US Department of Agriculture and the Natural Resources Defense Council, the average American produces less than a pound of food waste a day. Sure, yes: that’s a lot of stuff when we multiply it by more than 300 million of us, but for one household, that’s just not! enough stuff — even if you add in feces and urine, toss in your lawn clippings and ask your dog and cat to contribute, as we’ll cover in an upcoming blog post.

Let’s be real about this, OK? Biogas is great, and for my money it offers a good many of us the opportunity to actually become carbon neutral, maybe even carbon negative. The possibilities are present and important. If you educate yourself, if you invest the time and energy, then you can pretty well make as much as you want. (Just look in the back of your local fast food restaurant.)

At the very same time, the reality is that the great majority of us will not have enough feedstock within several feet of our front door, and not everyone is going to have enough interest to go get what is out there, just a bit further away. You and I are going to educate ourselves, but will the rest?

And besides, not everyone has the right situation. Is someone really going to try to put a two-ton digester in their apartment? On the fifth floor? (If they do, and you live on the fourth floor, try very hard to make sure your dining room table is not directly underneath the digester....)

Hey: it’s not heresy to say that biogas is not for everyone. So what? More for the rest of us.

#2: Keep it warm…

The newest digester on which I’ve been working (the poly-panel digester) uses the digester insulation as the digester container.

This insulation-as-container idea not only keeps the cost per volume low, but it insures that added heat will not be wasted, and thus that it’s going to be practical to heat the digester. Here in the US. Even in rainy, presently-bracing-and-chilly Oregon.

You want to know more about the digester I’m building (the press-plastic digester), or that even newer and more exciting (poly-panel) digester project we’ve got going? Well... keep reading, ‘cause I’m going to keep posting.

Get Temperature and biogas production: a spreadsheet. Find a table that shows how biogas production is impacted by changes in temperature. Or, to look at it another way, get a table that shows how, at lower temperatures, you will need a larger digester. And hey: those links offer some pretty cool pictures, too.

Do you want to keep up-to-date on progress on this new digester? Well, if you sign up for our newsletter at The Complete Biogas Handbook’s website, we’ll let you know what’s happening with biogas, with these new digesters, and with upcoming Beginner’s Biogas workshops, and Build-a-Kit Biogas workshops.

All MOTHER EARTH NEWS community bloggers have agreed to follow our Blogging Best Practices, and they are responsible for the accuracy of their posts. To learn more about the author of this post, click on the byline link at the top of the page.


As we head out the door on our road trip to the 2014 Mother Earth News Fair in Topeka, Kansas, I wanted to give an idea of how much energy can be generated at home using biogas while using the example of our home biogas digesters we will be exhibiting at the event. Home biogas and solar – both photovoltaic electric and solar thermal – would complement one another quite well. Biogas provides excellent, clean burning fuel for year-round cooking energy and a small amount of stand-by electric, while PV can do the heavy lifting for electric and solar thermal for heating and hot water.


Just as solar panels depend on the amount of available sunlight, the amount of biogas that can be produced depends on the amount of organic waste available. A typical American household with a lawn or garden will generate enough energy to cook three meals a day. Our two cubic meter (525 gallon) home digesters are intended to be fed between 10 and 30 pounds of mixed waste per day, such as table scraps and garden waste, animal manure, grass clippings and tree leaves. This works out to be between a third and a full 5-gallon bucket per day, depending what temperature they are operated at. The units are fully insulated and have a heat exchanger filled with pet-friendly glycol beneath the digestion chamber intended to connect to an evacuated tube solar heater. Naturally, a wintertime hoop house would help improve performance.    


Temperature (degrees F)

Daily Waste

Energy Equivalent per Month

(25 pound LPG cylinders)


10 pounds



20 pounds



30 pounds


Under the lid is the key to Hestia’s simple operation, indicator lines show when the water level is too high (yellow arrow) and too low (blue arrow). Normal operating water level will be between these two lines. If you cannot see any lines, you know it is time to remove liquid biofertilizer; if you can see two lines, you know to add some water.


This baffle separating the inlet from the digestion chamber serves a further function as a built-in pressure relief system. If gas is allowed to build up for a few days, say, during a family vacation, the gas will push down the water level in the digestion chamber until it “burps” out through the inlet. Eliminating the need for any pressure relief valves that could become clogged and ensuring a failsafe pressure relief.

Recommended Uses

Our company, Hestia Home Biogas, is named after the Greek goddess of the hearth. Hestia was also quite naturally the goddess of the family. Just as it was in ancient times, the kitchen remains the center of American life. Everyone we have ever talked to who loves to cook prefers gas over electric ranges, and cooking without fossil fuels or wood adds a level of enjoyment that really makes mealtime a celebration again. Any LPG stovetop or barbecue can be converted to run on biogas by either removing the jets or drilling out the orifices to allow more air flow. Our units ship with a free double burner stovetop from Puxin of China. The Puxin is specifically jetted for biogas and has the “whirlwind” type burners to provide even heating. puxinstovetop

We are often asked about operating electric generators with biogas. This is where the two cubic meter system has its advantages over a one cubic meter system. The 70 cubic feet of biogas per day, it provides enough cooking fuel for three meals per day, while also allowing enough energy to run a generator to watch a couple of movies, while recharging phones and electronics. For home units we recommend 1 kW (1,000 watt) generators or smaller. It is not going to be enough energy to run a clothes dryer or a chest freezer, so the smaller generators will operate much longer. Any generator will require a fuel-gas conversion kit from US Carburetion or similar supplier for around $200. 




Run 55-inch TV on

2 m3 of Biogas


Honda EU1000i


8 hours


Powerhouse 500Wi 


10 hours*

 *estimated. We have not tested this generator at this time.

I hope this brief introduction to our home biogas system encourages everyone to visit our booth at the Mother Earth News Fair this weekend to see one of our home biogas units up close. We look forward to talking to everybody and meeting the Mother Earth News staff and other exhibitors. As always, any technical questions about our products or biogas in general can be posted on the forum on our website. With your help we can build this forum into a central clearinghouse for home biogas users in North American and Europe.

Photo: Hestia Home Biogas 

All MOTHER EARTH NEWS community bloggers have agreed to follow our Blogging Guidelines, and they are responsible for the accuracy of their posts. To learn more about the author of this post, click on their byline link at the top of the page.


Many people have attempted DIY biogas projects and become discouraged after they failed to produce flammable gas. With my first blog entry, I would like to start at the beginning. This article does not get into gas yields or what biogas can be used for, it is a basic introduction to the five necessary conditions to create flammable biogas in the first place and – hopefully – encourages a few folks who have failed before to try again.

I can guarantee the reader on my life biogas works, and it works great. The ancient Assyrians used biogas to heat their baths in 3,000 BC, the famous gas lamps of Victorian England were fueled with biogas, Sweden runs all of its city buses with biogas and today there are an estimated 50 million households in China using biogas. There are no technical reasons every home in the world is not already using biogas for cooking energy and some light electric. The failure of any biogas project big or small are a result of violating one or more of these five easy-to-remember steps.

The microscopic organisms that produce biogas, known as Archaea, are among the oldest life forms on Earth. They predate the planet’s oxygen atmosphere — much less oxygen-breathing and CO2-absorbing plant life — by a cool 3.5 billion years. That’s billion with a “B.” Archaea are not bacteria, they are genetically closer to humans and other animals (eukaryotes), and form their own animal kingdom. As the Earth’s atmosphere became predominantly oxygen about 500 million years ago, archaea became isolated in the few remaining airless places, such as stagnant swamps, deep oceans, caves and hot springs, and of course the stomachs of vertebrates. To create biogas, we must recreate the conditions in which Archaea thrive in nature.

5 Steps to Making Homemade Biogas

The following table outlines the five steps to creating flammable biogas and I will get into further detail with each one. Biogas is reproduced in a special airtight tank called an anaerobic digester. The design of the anaerobic digester determines the first three steps.

How To Make Homemade Biogas 

Step 1. Airtight Environment. A Ziploc baggie can be used for an anaerobic digester. The difficulty arises from trying to add fresh material without allowing oxygen into the system. The most common method of creating a continuous flow digester is the “teapot” or “P-trap” shape. Most biogas digesters are some variation of this teapot shape.

Homemade Biogas Gas Storage 

Step 2. Archaea love water. When loading a digester, the water content in the material put in it should be taken into consideration. A head of lettuce, for example, looks very solid to us, however, it is 98% water. Dried rice is only 14% water. Regardless of the size of your digester, the “40-50-10 Rule” is simple rule of thumb to follow to get the correct volume: Forty percent material, fill the rest of the digester with water except for 10% headspace.

Home Biogas Digester Contents 

Step 3. A good analogy to think about regarding temperature and anaerobic digestion is your temperature is like the gas pedal of your car. The more you step on it, the faster your digester will convert waste into gas. However, also just like stepping on the gas pedal, there are consequences for it. The warmer your digester is, the archaea that decompose your waste get more fragile and susceptible to an unexpected crash.

Home Biogas Generator Chart 

Temperature can be controlled a few different ways. In China, digesters are typically buried underground and built much larger than they need to be. This way they can be overloaded in winter months to maintain consistent gas production. Other designs employ a greenhouses or hoop house over them. More advanced systems integrate some kind of heat exchanger, which can be heated with solar collectors. Regardless of your design, avoid using biogas or any other fuel to heat your digester. Make sure energy you use is excess energy on its way to being wasted.

Step 4. Neutral pH is an important parameter in anaerobic digestion, just as it is for aerobic composting. If pH is measured at the inlet, it will be slightly lower than neutral — usually around 5.5 — as fresh material is converted into acids. The pH will neutralize as these acids are converted into methane gas. By the time the liquid biofertilizer comes out the digester, it should be 7. If the pH of the biofertilizer is lower than this, it is an indicator the digester has been over-fed and is at risk to “sour,” or stop working due to low pH. If the pH at the inlet goes below 5.5, it is necessary to add some wood ashes or lime to buffer the digester. A soured digester has no bubble activity and instead of producing gas, instead it draws air into it. The top will be sucked in tightly against the surface of the liquid and if a brewer’s airlock is being used, the water in the airlock will be sucked into the digester. Restarting a soured digester is time consuming, and in most cases it is simpler to dump it out and start over. 

Step 5. Biogas production is best at the same 25:1 C:N ratio as aerobic composting. The reason cattle manure is far and away the most common feedstock for biogas is cattle manure is naturally the perfect 25:1 carbon-to-nitrogen ratio. Cattle manure makes an excellent feedstock to begin experimenting with biogas with. Other wastes need to be combined as a compost pile is.

Best Biogas Materials Chart

After these five steps, it is important to know that for the first 48 hours for a small digester or up to a couple of weeks for a larger system, the digester will only produce carbon dioxide (CO2). Carbon dioxide is of course used in fire extinguishers. When you put a match to the gas to test for flammability, it will be blown out with an audible “hiss” and a wisp of black smoke. As the biogas begins to come on, the hiss and black smoke will be gone and you will smell the distinct “rotten eggs” scent of the hydrogen sulfide (H2S). This odor is the signal to begin capturing your gas, as it is either flammable or soon will be. This “CO2 Phase” has caused many people to abandon DIY projects that might have been flammable if they had waited a short time longer.


For additional information, a terrific introductory text to the subject of biogas is A Chinese Biogas Manual, available on Amazon and other retailers. This guide is an English version of the same booklet handed out to Chinese villagers to build their home and village scale biogas digesters. Our company, Hestia Home Biogas, offers a biogas science kit, which includes everything necessary to produce a small but useful amount of flammable biogas for classroom demonstrations. Just as the home brewer brews beer or wine to achieve just the right taste, the best way to learn how to make biogas is practice. The rewards will outweigh the difficulties when you light the blue flame of biogas for the first time. With this magic formula you can create clean burning renewable energy wherever you are.

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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?

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…. The US isn't very “biogas friendly” in terms of climate, is it?

(Hey. Don’t lose hope now. It will be all right. We’ll get there. Together, if you keep reading.)

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!

Want to learn more? Then keep reading… Part 2 will come along real soon now!

Photo: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, kittable & shippable, 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.

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 solar panels

One of my clients, Sun Light & Power, a Berkeley green business just sealed a deal with San Rafael-based solar electric finance specialist SolED to do Power Purchase Agreements (PPA) in the State of California. In addition, this partnership enables both companies to be stronger California Benefit Corporations for the State and taxpayers.

As we all know, this means:

• schools municipal buildings
• village halls
• fire departments
• city halls
• other halls of government in the State.

This well needed energy savings or tax monies will enhance the educational budgets of schools; setting that great example needed while reducing government’s overall carbon footprint. Gary Gerber, founder and president of Sun Light & Power also announced that the deal will give both his public sector and his private sector customers better access to a full range of financing options. ; which can now include the PPA contracts to "pay as you go". This also gives the owner of the system (SolEd and/or Sun Light & Power) the tax advantages of the renewable energy tax credits and accelerated depreciation.

“We wanted to enhance our ability to help our clients navigate the complexities of solar energy financing and incentives,” Gerber says. “A strategic partnership was clearly the best way to achieve that goal, but we took our time finding the right match, with people who truly share our customer-centered values.”

“Our mission is to provide host customers with the lowest lifetime cost of energy,” said David Kunhardt, CEO of SolEd. “Sun Light & Power has been delivering quality for repeat customers for decades, and is also a B Corporation, the perfect partner for SolEd.”

Again, this is news worth noting.

Sources: Sun Light and Power for schools; Sun Light and Power for homeowners

All MOTHER EARTH NEWS community bloggers have agreed to follow our Blogging Guidelines, and they are responsible for the accuracy of their posts. To learn more about the author of this post, click on their byline link at the top of the page.

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