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There is no wrong way to build a first biogas digester, so long as it results in the unforgettable experience of seeing the miraculous blue flame of biogas for the first time. Soon afterwards, however, most peoples’ thoughts turn to a continuously operating system for regular daily energy at their home or small farm. If your dream homestead or community happens to lie in a cold region, it used to be believed biogas was not a viable option for you unless it was buried deep underground. Our understanding of the biogas process has improved a lot in recent years together with the introduction of new technology such as evacuated tube solar water heaters, so with a little bit of planning and DIY know-how, it is possible to thumb your nose at Old Man Winter and enjoy biogas in subzero temperatures.

We all know there is no free lunch, heating energy has to come from somewhere. There are a number of different options for heating a biodigester with or without fossil fuels, but before we consider them, it is important to keep in mind two important considerations for wintertime biogas. In the winter and early spring there will naturally be less organic waste available and accessible, requiring lower operating temperatures. Put simply, yes, biodigesters do benefit from heating to operate in wintertime, however, they do not need to be anywhere near peak performance temperatures of 100 degrees Fahrenheit. They need only minimal operating heat between 55 degrees and 75 degrees. Secondly, what heat that is produced needs to be focused in the “active zone” in a biogas digester, which is located on the bottom of the digestion chamber, half-way between the center and the inlet.


Some of the most common methods for heating home biogas digesters are on the chart below. We want to utilize waste heat if it is available and avoid dedicated electric heaters if possible. The following chart is divided by what heating options are appropriate where utility rates are low and where utility rates are high, which includes off-grid applications. The perfect cold climate off-grid combined heat and power system in my opinion would be a home gasifier or boiler burning annually-renewable wood chips or pellets together with a biogas digester. Waste heat from the gasifier heat exchanger or gen-set could be used to heat the biodigester, and in return biogas could provide year-round cooking energy, as wood gas has too much CO to safely burn indoors.

Methods for Heating Biogas Digesters

Where utility prices are low.

Where utility prices are high + off-grid

Home hot water system

Dedicated Electric

(Hot tub heater)

Dedicated Solar

Combined heat & power

Connected to the home hot water system.

Circulates heated water directly back into digester on

Flat panel for warm regions

Waste heat from gen-set, boiler or gasifier.

Evacuated tube for cold regions

If you intend to use your home hot water system to heat your biodigester, you must have your heat exchanger outside the digestion chamber to avoid the possibility contaminating your fresh water supply through a leak or ruptured pipe. For dedicated heater applications, a 50/50 water to planet and pet-friendly glycol is recommended to avoid freezing.

Biogas and solar thermal form a winning team to help reduce fossil fuel use, whether it is with a dedicated solar heater or solar heat supplements your home hot water system. For our two cubic meter home biogas units, I recommend 20-inch x 48-inch evacuated tube heater for Zones 4 & 5, or 10 tubes per cubic meter of digester volume. Provided the heat exchanger is located directly beneath the active zone. In summer months, solar heaters can be covered or disconnected.

In addition to heating, there are some additional steps you can take to maximize your heating energy.

Additional Steps to Improve Wintertime Performance


Thermal blanket

Hoop house


I hope everyone has a happy holidays and has the opportunity to enjoy a fossil fuel-free Christmas dinner prepared with 100 percent natural, clean burning biogas from ordinary household waste.

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.


I will continue the step-by-step introduction to home scale biogas for those interested in learning how to make it with my next post. In the meantime, I thought it might be helpful to mix things up with what I call my Biogas All-Stars series. These are posts where I will highlight one of my colleagues in the international biogas community and let them to answer questions about their projects in their own words.

I believe MOTHER EARTH NEWS readers will find this series useful, as it will offer examples of different types of regionally-appropriate digester designs. These All-stars inspire us all with their resourcefulness and dedication, working with locally available materials in often inhospitable – sometimes even dangerous - conditions to build biogas digesters to transform the lives of people who need it most. The Biogas All-Stars do not appear in any order. I would like to begin with one of the most likeable people I have ever met, Marcello Ambrosio, with the Studio Ambrosio Agricultural Consulting, from Italy.

Marcello (pronounced March-ello) and I met in New York City during a conference for Solar CITIES, an international non-profit biogas education and training organization we both belong to. He is a big fan of Western movies, and once worked as a cowboy in Wyoming during a visit to the U.S. When it comes to building biogas digesters, however, he is definitely the Lone Ranger. Usually working by himself, he has single-handedly built digesters as large as 100 cubic meters (26,000 gallons).  


Marcello specializes in the most common type of biogas digester in the world, the Chinese underground pit-type digester. There are an estimated 50 million of these type digesters in China. They are usually built underground for gas production throughout the cold Chinese winters. The advantage of this type is it allows those willing to get their hands dirty an opportunity to trade labor for material costs. With a readily available supply of bricks and mortar, this type of digester can be built for very little money. Building and operating plans for these types of digesters are available in ‘A Chinese Biogas Manual,’ which can be purchased through online book retailers or a free PDF Copy.


Weisman: When did you discover biogas?

Ambrosio: I first learned about biogas in 2006, as a student at the Polytechnic of Turin. After graduation, I went on to work at large, commercial biogas plants in Germany and Luxemburg. These large plants were using a lot of dedicated crops for feedstocks – mostly corn – and I knew there was no way this was sustainable.

Weisman: Describe the first time you created flammable biogas:

Ambrosio: In 2007 I built a small 100 liter (45 gallon) home plant and produced my first flame. I then built a one cubic meter plant (275 gallons), but it did not work very well, especially in winter. I live in the Alps. I then made my first 12 cubic meter plant, which was big enough to produce sufficient gas throughout the winter.



Weisman: What has been your favorite project so far?

Amrbosio: My favorite size is the 25 to 40 cubic meter plants. They are ideal for small farms with a few cows or horses.



Weisman: What advice would you give young people interested in biogas?

Ambrosio: I would say be careful not to think about biogas by itself, but to consider it as one link in a chain of closed loop sustainability. It is just the first step in the management of the organic wastes and soil ecology. It is important to consider every step of this cycle, for example utilization of the byproducts from the biogas process in agriculture. This multifunctional approach has far more value than the flame itself.

Weisman: Anything else you’d like to add for the American DIY community?

Ambrosio: Just be careful with methane from a climate change perspective. If you generate it, it is okay to burn it, but if you release it into the atmosphere 1 cubic meter (35.3 cubic feet) of methane is equal to 23 cubic meters (811 cubic feet) of CO2. The same impact of driving 60 km (27 miles) in a medium-size car. So, when you build it is important to have proper systems with no leakage and to have the digester sized correctly so when we are not home the gas is not released. Otherwise we are solving the problems of waste disposal and nutrient recycling, while adding to a bigger problem, emitting GHG gases.

Weisman: What is your favorite Western movie and why?

Ambrosio: The Good, the Bad, and the Ugly (1966) directed by Sergio Leone. There is just something magic about Western movies. They commemorate a time when there were still some uncharted lands in the West. Going West was a great adventure for the pioneers in search of prosperity and happiness.

I think people today can learn a lot from that pioneering spirit in our thinking, only instead of heading West we now must look outside the box canyon we have become trapped in to find a better way of doing things.

If you have any home or small farm biogas projects in Europe or non-profit projects anywhere in the world you would like to talk to Marcello about, you can email him at

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.


Canada Off Grid Home 

“Off-the-grid” is a terribly-abused expression. I (Phillip) have heard people say they’re “off-grid” if they switch off their cell phone for a day. Others think that anyone living far from the city is off-the-grid. Some use “off-the-grid” to describe people who wish to run and hide, to go incommunicado. In Canada, off-grid is a technical expression with a precise meaning defined by the government. Engineers and architects, to whom the government’s definition can be attributed, say “off-grid” to refer to those dwellings (individual homes as well as entire communities) that are disconnected from the electricity and natural gas infrastructure servicing a particular region. This definition makes things clear and simple: a home (not an individual) is off the grid in relation to electricity and natural gas.

The reality on the ground, however, is a bit more complex. Off-grid households capable of generating their own heat and electricity are often also intent on harvesting water, growing food, and disposing of their own sewage and waste without the aid of municipal infrastructure. These homes typically also have a cautious attitude towards communication links, and may therefore be cut off from telephone landlines or television cables. At times, they may be in remote places, even off the road. All these are incredibly interesting lifestyle choices that are simply mind-boggling for most people. I’m one of those people.

I moved to a small island off the British Columbia coast in 2010. Though I was no stranger to small town life, moving away from a municipality and off to a rural island meant becoming responsible for my water supply (and don’t get me started on my septic field). It meant, in other words, living with a groundwater well and monitoring it carefully to make sure I wouldn’t consume too much of my own water. My own water. Those three words, spoken in a row, had never even entered my consciousness until then. What could life be like — I started asking myself — if I had not only my own water, but also my own heat and my own electricity? What could life be like, off the grid?

Finding Life Off the Grid

I took a short trip to find out. I reached out to a friend of a friend who lived off-grid on Vancouver Island and asked for a guided tour of his house. Three hours later —three hours full of amperage, wattages, BTUs, inverters, and all kinds of gadgets that looked like something out of a Steampunk catalogue — I was enchanted, mesmerized, in awe. And utterly confused.

I returned home wondering whether I too could live like that. I am no stranger to a slower way of life, to growing veggies and filtering out unwanted television, social media, or cell phone signals, but I questioned whether I could go the extra mile and sever my ties to the power grid too. I puzzled over whether I had what it takes: the ability to do with less, to rely on myself more, to embrace a little inconvenience. I agonized over my lack of handy skills and my tendency to tackle domestic projects with stress and panic, rather than the required “I-can-fix-it” attitude that I admired in so many tool-box-endowed friends. I tossed and turned, debating ad nauseam whether as a good ethnographic researcher I should indeed “go native” and practice firsthand the life I chose to write about.

The prospect of going off-grid, for most people, rightly feels like taking a leap in the dark. Most of us have become accustomed to flicking on a switch and, in virtue of the technological miracle called electric light, stretching daylight deep into nighttime. Those of us living in the Western hemisphere, and fortunate enough to pay the monthly bills, are spoiled with the historically unique privilege of being able to microwave our food in seconds, tumble-dry our machine-washed clothes in minutes, and power up dozens of digital gadgets all day long. Our homes can be warmed at the twisting of a thermostat and kept cooled during the muggiest of summer days. Our store-bought foods can be preserved effortlessly for months in capacious freezers. Our constitutionals can be flushed away in an instant, out of sight and out of mind.

Why Go Off-Grid?

So, how can we, and why would we, cut off these life lines? Some might say because our lives may have gotten just a tad too boring, too disconnected from the natural world, too comfortable, too lazy, too irresponsible, too crowded, too expensive, too confined, too saturated, too superficial, too fast, too incompetent, and too dependent. Though I didn’t know if those arguments would really compel me and my family to make the leap, I felt the urge to understand them better, to experience and practice them vicariously through the everyday lives of full-time off-gridders.  

For the next three years, together with my then-student and now collaborator Jonathan Taggart, I travelled to all of Canada’s provinces and territories to document the ways of life of people off the grid. In June of 2013, Jon and I ended our travels on Canada’s easternmost point, having travelled 65,000 miles coast to coast to coast to find and interview about 200 off-gridders. Our experiences, now told in a book and a film, will be the subject of this blog in the coming weeks and months.

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.


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.

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