How to Make Biogas at Home with a Biogas Digester

John Moore: [00:00:00] Hello and welcome to the Mother Earth News and Friends podcast. Have you ever heard of biodigesters? Whether you’re familiar with this system or not, you’re in the right place to learn more about them. In this episode, Kenny Coogan chats with Dr. T.H. Culhane all about these “domestic dragons.” This is Mother Earth News.

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What Is a Biodigester?

Kenny Coogan: Good day everyone, and we appreciate you for joining us on another exciting Mother Earth News and Friends podcast. I am Kenny Coogan, and joining me today is Dr. T. H. Culhane. Today we are going to learn about biodigesters, how to make one, and why you would want to. Dr. T.H. Culhane is an associate professor at the Patel College of Global Sustainability at the University of South Florida. He is the director of climate mitigation and adaptation concentration. Welcome to the [00:02:00] podcast, T.H.

Dr. T. H. Culhane: Thank you so much. It’s wonderful to be here. And that idea of why you’d wanna make a biodigester is the immediate thought that occurs to me when I hear that after encountering the domestic dragon, as we like to call it, and realizing just how wonderful, safe, playful, and helpful domestic dragons can be.

The question is, why wouldn’t you want to make one? And I think you’re all gonna feel that way once you really start to spend some time with these incredible beasts, if you like, uh, I don’t want to call them technologies because really a biodigester is an animal. And it’s just an animal clothed in cement or plastic or metal or whatever you choose to contain the microorganisms that do the magic.

But just like a dragon skin, uh, and its integument or that of a, of a horse or a cow, or a pig or a human, the, if you look beyond the skin and go inside, it’s the microbial world [00:03:00] that is doing all of the, the wonderful transduction of what we consider bads into goods. And that’s the animal. It’s what’s going on on the inside, not the outside.

Kenny Coogan: So in addition to being a professor, you are the co-founding director of the not-for-profit educational corporation, Solar CITIES, Inc., which helps community stakeholders solve urban ecology and developmental issues surrounding waste water, solid waste, food security, and decentralized clean energy production.

Can you further explain what a biodigester is or your little dragons and how they help combat all those issues?

Dr. T. H. Culhane: Absolutely, and to do that, let me start by explaining our NGO’s name, Solar CITIES, because Cities is an acronym. It’s actually C(cubed)-I-T-I-E-S, and it stands for Connecting Community Catalysts — that’s the three Cs — Integrating Technologies for Industrial Ecology Solutions, and we called ourselves Solar [00:04:00] CITIES because when I was living in Heliopolis, which is Greek for “solar city” in Egypt, the ancient solar city of Egypt, Heliopolis, and formed my NGO, we were trying to tackle all these problems and photovoltaics, which we were working on, and solar thermal or solar hot water systems, which we were working on in teaching people, and small winds and all of the other environmental technologies that were available at the time, and this is going back about 20 years, could not satisfy all of the needs of our communities. And when we started connecting community catalysts from around the world on the, the vision of creating a solar powered civilization that would not depend on fossil fuels, the intermittency of wind and solar as we know them, and hydropower as we know it, which is a derivative of solar power, was always cited as a factor for why this would never work.

And then we realized that what cities had in common, no matter whether they were rich cities or poor cities, or they were in the [00:05:00] north or the south or the middle of the planet, was they all had organic residuals, which you call food waste and toilet waste. And they have them in abundance. And that the microbes that live in our guts beneath this integument, and that’s all of us, all living, uh, organisms. Whether it’s a single cell or it’s a multicellular animal, we all have in our cells, enzymes and processes that break down organic material and extract energy and produce nutrients, and that’s the life cycle. So we thought, let’s find a way to bring and harness these microbes in the city. And we realized that solar energy is most accessible to everybody everywhere in the form of organic residuals, which we call waste.

Like that’s a solar battery. The sun shines and has shined for millions and millions and millions of years, and it’s stored in the chemical bonds that photosynthesis captured and formed. And we call that [00:06:00] food. So we eat it and there’s parts that we don’t eat, we call food waste or food residuals. And that’s still those same chemical bonds that maybe we don’t find tasty, but microbes do.

And the stuff that comes through our bodies that we have ingested but not completely digested, that also is still energy and nutrient rich. And it comes out the other end of us and people go, ew, it’s waste, poo poo. And yet the microbes go, mmm, delicious. And they continue to break it down into fuel and fertilizer that’s clean and healthy and, and starts the lifecycle again, growing new food.

So we realized as an NGO that if we wanted to achieve a solar powered civilization, we’d have to include this one process that it was fundamental to all life on earth and has been for 3.8 billion years since the first microorganisms, uh, appeared on the earth. We’d have to bring that into the city, and realize that what we’re really talking about when we [00:07:00] talk about a biodigester, you can just point to your own gut. It is a biodigester. It’s how you digest. And so we jest in talking about domestic dragons only because those biodigesters breathe fire. And so, okay, it breathes fire. It’s a dragon. But literally every living being is a biodigester. They all have, all of us in our guts, anaerobic microbes that were here since the beginning of life and have found themselves in every nook and cranny of the, of the planet where there’s no oxygen, including our own guts. And when we put them in an artificial gut, then they continue to thrive because they don’t know whether they’re in a cow or a horse or a human, or a dragon or a tank of, uh, of water made of cement or steel or plastic. To them, they’re just, they need, they need water and they need nutrients, and then energy. So, you make a tank of water and you put three pipes in it, and one is the [00:08:00] throat, uh, with a mouth that you feed it. So think about the tank as just a stomach or a digestive system. You feed it. Another tank is like the, um, the, uh, the ureter, which pees out liquid fertilizer is all animals do. And then another one could be considered either the, the nostrils or the anus that belches or farts out the gases that result from these microbes.

Biodigester is nothing more than a fart gas machine. I mean, if you like to think about it, and it’s very useful biomethane, it’s the same natural gas that people are fracking for and people are flaring off in the desert when they don’t find an economic reason to capture it. It’s a, it’s equivalent to the fossil fuel that microbes created, but it’s being created here and now at a rate where it can be used and transformed safely into carbon dioxide and water to feed plants. Or it can be stored, the way we store methane, it’s just biomethane. [00:09:00] So a solar city that doesn’t have this is going to run into some technological and economic problems. But when you look at the city as an organism and see just how much energy and fertility is being wasted in those cities, cuz the farms are all shipping food into the city and, and there’s truckloads of stuff coming in, and then we consume it, and then we flush it down the toilet, pour it down the drain, dump it in the garbage bin. You realize that there’s this untapped possibility for capturing that solar energy that’s in those chemical bonds and that would drive a solar city everywhere. So we’ve decided we’re gonna put the guts back into the system, literal guts. And we call a biodigester the “solar plexus.” I’m pounding on my stomach, kicking my solar plexus. The solar plexus, literally, of the food, energy, water nexus.

Is Biogas a Sustainable Energy?

Kenny Coogan: One great definition for sustainability is the balance of people, profit, and planet. And in order to be sustainable, you must take into account [00:10:00] those three aspects. So how do biodigesters help people achieve sustainability? I know you talked about your getting, um, this liquid fertilizer, so that’s gonna help the planet because you don’t have to import or, you know, truck all this fertilizer, and then it’s good for people because why?

Dr. T. H. Culhane: So let’s break it down in, in, into the, into the prime values. The absolute first profit value that benefits people directly is the elimination of very, uh, dangerous pathogenic, uh, at times nuisance, wastes. When you look at the amount of material going to landfill, it can vary from 30% to 60%, depending on the operation. 30% to 60% of that can be organic residuals, and it’s food waste and it’s yard waste, and it’s, uh, it, it animal waste. And, and as you have [00:11:00] all these, these, these high energy nutrient rich molecules that are being shipped to the landfill and there, they’re decomposing under anaerobic conditions in the landfill.

And they say that food waste alone accounts for, uh, being the third largest source of greenhouse gases in the world coming out of landfills. That’s huge. They say that if, uh, if food waste was a country, it would rank just behind China and the United States in terms of greenhouse uh, emissions. So immediately by bio- digesting it and turning it into a useful form, you’ve eliminated that burden. So there’s a cost savings with that. There’s no trucking to the landfill, no burying. There’s a huge labor investment and technology investment in, in, in, in, uh, sequestering things in landfills. Then there’s dealing with the burden, the cost, burden of those greenhouse gases and the, the terrible consequences of global warming.

There’s also the idea that on route to the landfill, you have the presence of [00:12:00] vermin so-called. So there’s all sorts of so-called pests, and I say so-called because I recognize the value of all life forms, but whether it’s cockroaches or rats or flies or mice or, or, uh, certain birds like seagulls, um, uh, pigeons, uh, any, any possible disease carrying organism that is coming to see that energy-nutrient value in the waste we’ve thrown out. Uh, raccoons, uh, feral dogs, feral cats. I mean, just think about the area where people throw their garbage. They’re not after the plastic. They’re not after the metal, they’re not after the ceramics or the glass. They’re after the food waste and they see value in it. And so they’re knocking cans over and causing — and people are throwing their, their food waste to keep the animals away. They’re tightening it up in plastic bags, which are then entering our oceans. I mean, most of the plastic waste you’ll see if you do a survey, uh, in the Pacific Ocean gyre of plastic and beaches on, in rivers, it’s either plastic [00:13:00] bottles or plastic bags.

And if you tear those bags open, which I often do, what’s in it? Food waste. Cuz if people have clean plastic bags, they tend to reuse them. They’re not really single use plastic if they’re clean, but people throw their food waste in trying to keep the animals off of it, keep the flies away, and then it ends up in the river, in the stream.

So there’s so many tangible benefits from keeping your food residuals and your organic residuals in C2, in the city, or in the suburb, and having them rapidly decompose in the biodigester into safe, clean fuel and fertilizer. So there’s a, there’s a cost offset there that benefits your bottom line and benefits people.

Biodigesters also, in terms of people, eliminate 98% to 99% of possible pathogens. So when we installed them in Haiti, for example, we were all aware that the terrible cholera epidemic that claimed so many lives and continues to was because fecal material from, um, some peacekeepers after the [00:14:00] earthquake got into the water supply and it spread a, an epidemic of cholera. Biodigesters kill cholera. So if you poop into a biodigester, the cholera organism, Vibrio… I’m not sure the species name, but Vibrio I think is the genus…. they can’t outcompete the hydrolytic microbe clades, the acidogenic and acetogenic microbe clades, and the methanogenic microbe clades. When a biodigester is functioning, this ecosystem is so diverse and so competitive that cholera, typhoid, dysentery, all these bad microorganism functions, they can’t establish a foothold.

So you’ve already made it much, much easier to eliminate the disease burden. And when you feed these biodigesters both animal waste and human waste and food waste, there’s such a richness of nutrients and energy that that ecosystem just grows more and more and more robust.

So [00:15:00] on the terms of the people side and the profit side, let’s go to the profit for a second. It is the liquid fertilizer that results, that is the high value output. And that is because a biodigester being anaerobic doesn’t lose any nutritional value the way an aerobic composting pile would because it’s not oxidizing. It’s a hundred percent retention of whatever you put in it. It’s not going anywhere. I mean throw it in. It forms methane gas and carbon dioxide, a little bit of hydrogen sulfide. So that’s in the gaseous portion that it’s bubbling, which you store. And then in the liquid portion, everything else is there. Nitrogen, phosphorus, calcium, molybdenum, zinc, you name it, micronutrients, macronutrients, NPK, it’s all in that solution. So you’ve lost nothing. So then when you take it that liquid and put it into soil, you have done complete nutrient recovery and then the soil microbes can further take care of it to turn it into the forms that plants can uptake. So there’s no downside to [00:16:00] this.

So you have people, planet, and profit. Planet, of course, is all about soil. Planet is about a living network of microorganisms with macroorganisms that have formed this incredible tapestry that supports co-evolution of further life forms. And when we break it apart and desertify by stripping the, the land of, of those nutrients and, and baking out and, and, and desiccating and, and, and removing all of the living beings that create that soil organism that covers the surface of the planet Earth. And when we also poison the oceans and get rid of that living liquid, I guess I could call it soil, I mean the ocean in a sense. And all water bodies are a living liquid organism. Soil is just the land form of what the microbiologist Lynn Margulis and Carl Sagan, her husband, the astronomer, called Hyper Sea. They said that the sea is the living organism [00:17:00] earth, Gaia, it’s this ocean planet, it shouldn’t be called the Earth. It should be called the ocean, right? And then on the land masses, the sea crawled onto the land in the form of living beings, principally soil. And then all the fruiting bodies of soil that we are, that, you know, from the earth, we come into the earth, we return, but we we’re walking hyper seas. Our bodies under this integument are cells of water with these processes and this energy and these nutrients, and we just move water around the planet. So from that perspective, when you, when you create these domestic dragons, if you like these, these ecosystem beings that have these processes so that they can work at the rate of our waste production, then you have enabled planet to come back together as a cohesive system of exchange.

And so I guess what I, what I really want to say about this is that, we have to [00:18:00] work with nature now to keep up with the rate of energy and nutrient packages that we are producing, which I was just saying, the cockroaches and the flies and the dogs and the right, they get it, but they’re at the wrong place at the wrong time for us.

And we like to say in our class at USF, our waste not want not, reconsidering refuse as resource. We like to say that pollution is the right thing in the wrong place, at the wrong time, in the wrong concentration. But when you have a biodigester, you’ve suddenly eliminated pollution because all those rules are the right now, in the right place, at the right time, at the right concentration in the belly of that domestic dragon.

And you have more, you just build a bigger dragon. If you have less, you can use a smaller dragon.

How Much Biogas is Produced from Food Waste?

Kenny Coogan: That brings me to my next question. Your motto on Solar CITIES is “bringing clean, free fuel and nutrient dense fertilizer to people around the world.” So how much free fuel are we talking about and what sizes can you make a [00:19:00] biodigester? Keeping in mind that our readership, you know, is usually a one family, they have one to 100 acres, but how much does a family of four, you know, how much space do we need to make work?

Dr. T. H. Culhane: This is always being improved and over evolutionary time, uh, the efficiency of these processes has been optimized for the planet pre-hominid appearance. So those efficiencies are relative to our civilizations seem to be a little low, but are good enough for families. And what I mean by that is, let’s just use buckets cuz it’s easier than using kilograms or, or, or, or liters. Paint bucket worth of food waste digested in a biodigester 40 times the size of that paint bucket will give you one kilowatt worth of energy from that one bucket.

Kenny Coogan: Like a one gallon bucket, maybe?

Dr. T. H. Culhane: A five gallon bucket, five gallon paint bucket that [00:20:00] you used to get from Lowe’s or Home Depot or wherever. Hardware store. That amount of food waste, which a family can produce in a day or two, depending on the size and consumption patterns with the family. That can produce one cubic meter, which is 275 gallons. So think of those white IBC tank tote tanks. The white plastic tanks with the cages around them, they ship goods all over the world in, that is a one cubic meter or 275 gallon tank. It can take one bucket of food waste per day and create one cubic meter its size in gas, which is equivalent to one kilowatt of energy. And that can happen every day when the temperatures are around Florida type temperature. Plus if you’re in colder areas, so you want to keep them, uh, indoors or heat them up. Uh, find ways to, uh, put them in a greenhouse, uh, in a windowed area.

But that’s the, the usual rate. So a family can produce one kilowatt. Now what does that mean? One kilowatt? Well, it means that if you ran it through a [00:21:00] generator, you could run the generator for about 45 minutes to an hour, produce a kilowatt of energy and run with LED lights. I mean, the comparison’s crazy. It used to be run a hundred wat light bulb for 10 hours. But now we’re talking about LEDs. You can light your place all day, but you could use your microwave, which uses about a kilowatt for three quarters of an hour to an hour. Okay? And that’s not what we use biogas for. We use it for cooking primarily cuz it’s easier than generating electricity cuz then you need a generator.

So if you cook on that gas, you can get a flame that is a medium height flame like you have on your stove top when you’re making soup or making, um, you know, making pancakes. You can cook on that for about two hours from that one cubic meter that came from that one bucket. Now, that’s very useful for most families because most families, particularly in developing countries, aren’t cooking for more than two hours a day.

So you have that daily available fuel. [00:22:00] You have the liquid fertilizer coming out, which is equivalent to the amount you put in. So if you put in a bucket, you get a bucket out, and then it’s what did you feed it? If you fed it, Twinkies, maybe it’s not so healthy to put on your soil, but if you’re feeding it balanced nutrition, as I mentioned, all those nutrients are still in that solution. They’re coming out the other side, and then you’ve completely recovered them and you can grow that food cycle again.

What to Feed a Biogas Digester

Kenny Coogan: So let’s say we, we have, um, a couple, and they’re, they’re eating a lot of food, but they’re very conscious about it. So they don’t have a lot of food waste, but they’re raising chickens and goats. They can put that, uh, fecal material from those animals. We don’t wanna say waste , we don’t use the word waste, but they can use that fecal material, add it to their five gallon bucket and that’s how you can get a bucket’s worth a day. Correct?

Dr. T. H. Culhane: Yes. Although I should say that the old idea of biodigesters when they first appeared in [00:23:00] India and China and got, uh, I mean not first appeared cuz they appeared there first thousands of years ago, but when they became part of the development scenario in the 1940s and 50s when Mahama Gandhi in India was pushing the idea of biodigesters for development, when, um, when Mao Tse-Tung in China was sending extension agents throughout China and their so-called Great Leap Forward building biodigesters all over the countryside, they knew that manure produced biogas. Everybody had observed that, just like when we fart. So they were building rural biodigesters and feeding them on animal dung, but they didn’t realize is that animal manures are spent fuel. The cow jumped over the moon, right? And the pig went to the disco and, and the chickens uh, went on the chicken run. And so by the time that they poop, most of the energy is gone. The microbes are there. It’s great, there’s great nutrients in there, but the energy content is gone cause that’s why we eat. So if you use poop, you need to build a digester [00:24:00] anywhere from 10- to a 100-, depending on what kind of poop you’re using, times larger than if you consider the digester the animal itself. So if the animal is the pig or the goat or or, or the or or the chicken, then the animal is eating primary solar energy, using the energy to go to the disco and then pooping it out. And yes, you can then put that poop right back in another digester cuz it is not very efficiently digested. That’s what I meant about evolutionary history. It’s like animals didn’t need to get every gram of, of nutrition out and every gr, every, uh, firm of energy out. So poop is great to put back in. It’ll always reinoculate with a good, uh, microbial consortium. It’s great to put it back in to get those nutrient recovery and then get the liquid out to the soil, and it can kill if there’s any, any bad microbes in the animal poop, biodigester will further process them and kill them cuz it’s more efficient.

Um, but the real value [00:25:00] is in primary photosynthetic energy stored in leaves and fruit and, and fats and oils and carbohydrates and the stuff that we and other organisms consider food. There are organisms that eat poop, but the bigger ones are eating primary stuff from plants and animals. So what do you want to put in? You want to put….

Kenny Coogan: What about, you know, what about yard waste?

Dr. T. H. Culhane: Absolutely. So the yard waste you don’t want to put in would be brown yard waste. If it’s already decayed and lost its greenness or its yellowness or its, you know, for flowers and purple and, and, and the color indicates that there’s still complex molecules there. When it starts turning brown and goes on to being black, you’re losing the chemicals that create the color that have the energy in them, and you’re getting to carbon black basically, which is the end result. So you put in green stuff and, and, and colored stuff all you want, and it’s gonna break that down just as the decay process does with your leaves as they begin to [00:26:00] turn brown. But you’re gonna get that energy stored as the methane.

But also think about like when we were with the Maya people in Mexico, uh, there were biodigesters being fed on pig waste and we said, you should put food waste in and get 10 to a 100 times more energy and nutrition out than just using pig waste. And they said, with all due respect, we’d thought of that. But the pigs eat all the food waste. There is no food waste in our community. And I said, well, what about that? And I pointed to an area that was under a tree where there was a whole pile of fruit that a tree had just dumped down. And they said, the pigs don’t eat that because that’s poisonous. I said, well, the biodigester will. Similarly in Kenya, we were in the boma with the Maasai people and we had the same situation. “In our boma, we have all these animals. This is what we do. We’re we’re herders and we have the cows, and then we’ve got the chickens that walk with us and we’ve got goats and all these different animals. They eat every scrap of everything.” And I said, but look at that pile over there. And they said, oh yeah, but that’s stuff that no animal eat. It’s the rotten parts of the food [00:27:00] with the flies buzzing all over them. It’s stuff that’s fermented and it’s got onion skins in it and, and I said, that’s what the biodigester will eat.

And they said, oh, but you were describing a biodigester as being a kind of cow, an artificial cow. And I said, then I used the wrong metaphor and that’s why we use dragons now, cuz the dragon will eat everything that is not decayed to the point where it is just carbon. So yeah, put it all in meat, fat, grease, you know, moldy stuff, everything that you are told not to put in your compost, the biodigester will eat.

How Safe are Biodigesters?

Kenny Coogan: Well, that’s very promising.

Can you explain, I just wanna go back a little bit. When we talked about clean free fuel, can you explain to the listeners what clean fuel means?

Dr. T. H. Culhane: Yeah, sure. So clean in our sense is the same metabolic process that the drag in the human, the cow goes through. When we eat, we emit from our different pores, we emit [00:28:00] carbon dioxide, we emit water vapor. We emit some hydrogen sulfide and some methane. That’s what we do when we belch, when we fart, when we sweat, when we pee. Um, the biodigester releases this liquid fertilizer that has all these nutrients in them, and they are safe and clean as what you put in. If you put in something that’s toxic, it’ll break down most of the toxins. But if you put in medicines, antibiotics, um, if you put in any, any poisons that, you know, yard chemicals or, or household chemicals, it might not remove all of those. So you want to feed it stuff that’s not toxic, except for as a toxic fruit. I mean, if it’s, if nature made those toxins, they’re likely to be completely degraded in it. But if it’s some chemical toxin that we came up with in a lab, then the fertilizer might have that in it. If it’s the methane gas, the methane gas is CH4 methane, the simplest of all hydrocarbons, and then carbon dioxide, CO2, and a little tiny amount of [00:29:00] H2S hydrogen sulfide, just like fart gas. And when you burn that biogas because you want to, because you wanna get the energy out to cook or to do a water heater or to, uh, run a generator or a gas lamp, same things you’d use natural gas for, when you burn it, it turns into carbon dioxide and water. The tiny trace amounts of hydrogen sulfide turn into water and a little bit of sulfur dioxide, which is rapidly, uh, assimilable by the plants in the area which need the sulfur.

But those are tiny, tiny, tiny trace amounts, I have to mention. So you get out carbon dioxide and water, which is exactly what plants use to grow new plants. You’re not adding fossil carbon to the atmosphere. You’re bringing carbon dioxide out of the atmosphere to grow the plant. You eat it, you throw away maybe the parts of the plant that you don’t eat. They go in the biodigester and they liberate that CO2 right back for the plants to take back, to grow a new plant for you to eat, to put back in the biodigester. So it’s a closed loop. The CO2 isn’t toxic. Please don’t [00:30:00] misunderstand and think, well, you’re burning methane and creating CO2. Doesn’t that add to global warming? It’s, it’s non fossil reserve CO2 that’s rate of release is the same as the rate of capture. So there’s, it’s, it’s no different than using a regular aerobic composting pile. Uh, that’s also emitting the same amount of CO2 during the life cycle. You just don’t get to capture it and hold onto the energy value.

The water vapor is of course clean. There’s no carbon monoxide to speak of. It depends on how you burn it, but the studies have shown that most situations there’s zero CO carbon monoxide. Sometimes it’ll be a tiny, tiny, tiny, tiny, tiny percentage that is not hazardous in any way. Uh, all combustion has the potential of, of releasing carbon monoxide, but we’ve never had an issue. So we burn biogas in our RV on the stove with no worries at all. The hydrogen [00:31:00] sulfide is not a worry. So that’s what I mean by clean.

Temperature Control for Small Scale Biodigesters

Kenny Coogan: Earlier you mentioned that you’re in Florida. What would the temperature range be for a biodigester? Because we have people all over North America. And, and we, you just said that it was safe to store the gas and then the liquid fertilizer. So is, are these inside, outside?

Dr. T. H. Culhane: So the biodigester movement, which started I guess in ancient Assyria in ancient China thousands of years ago, and which Marco Polo came back from China talking about in the 13th century. And it could’ve stopped the bubonic plague by the way, because the rats, like “Ratatouille,” came from the countryside to Paris in search of food waste from the restaurants and homes. Then they never would’ve had any incentive to come if they’d had the biodigesters Marco Polo talked about, they were most extant in tropical and subtropical areas because the microbes that people use came from the guts of cows and sheep and, and pigs and, and other animals [00:32:00] and operate best at body temperature, which is, uh, I, I always use, uh, Celsius. But, uh, it’s 37, uh, degrees Celsius. I guess it’s 98.6 Fahrenheit. That’s their optimum range, but they can chill down to about 13 degrees Celsius, which somewhere in the fifties, sixties of Fahrenheit, and they could operate up into the range of about 40, 45 Celsius, which we’re talking about. “I got a fever of 103,” you know. So they have this, this wider range, and because they have this wider range, biodigesters will work literally everywhere where humans feel comfortable.

But if you leave them outside, then when it’s too cold for people and our guts start to freeze up, they’re gonna freeze up and then they go to sleep. So you didn’t see a lot of traction in northern climate or extreme southern climate biodigesters for centuries because people didn’t see a way to make a profit [00:33:00] if they slow down in the winter.

But it’s not to say that people didn’t use them. We have examples of biodigesters being used in Germany and England to make town gas in the 1800s, but they were solving a problem, getting rid of waste. And then they had the side effect of producing gas, which in the spring and the summer you could use to light buildings. And then in the winter, sort of it, it stopped and they used other sources of, uh, of gas to get it. But profitably, biodigesters have always been used in tropical and subtropical climates.

We started doing some pioneering research in Alaska in 2009, 2010 with National Geographic when our colleague Dr. Katie Walter Anthony from U Fairbanks showed that as the permafrost is melt, biogas is being released from all the lakes that are forming in the former permafrost in the Arctic Circle because the microbes there are coming awake after their slumber, and they’re metabolizing down as low as, [00:34:00] you know, just above freezing.

So we started working with those microbes. They’re called psychrophilic microbes, and they will work from just above freezing all the way up to body temperature, and then they sort of die off. We mixed them with the, they’re called psychrophiles. We mix ’em with the mesiphiles, which are the ones that we have. And then there’s high temperature biodigester microbes called thermophiles. And when you mix ’em together, they can start to co-evolve together and start exploiting this niche space. That’s why I said that things were limited in op, in in their optimization because human beings hadn’t tried to push the envelope, and nature just did what it did. It didn’t need to.

What we also found is that not only can we increase the amount of digestion by mixing these different microbes and letting them co-evolve. Cause remember, they reproduce every 20 minutes or so. So their generation length is really fast. So they’re gonna start adapting to the human civilization presence if we allow them and we encourage them. We’re doing symbiosis with them. The other thing is, is that they’re [00:35:00] really good domestic companions. So we started doing work in Alaska and in New York and in Pennsylvania and in Germany, where we put the biodigesters in the house, in the basement, behind the stairs, in the bathroom, in the kitchen. We did one in Ireland, and found that if you’re keeping your house warm, then the biodigesters stay warm and they keep working. And people thought, well, isn’t that dangerous? Well, no, they’re just tanks of water. That’s all they are. The worst that could happen is that you get a pipe that breaks and you get a little smelly spill. It’s not that smelly cuz it’s been digested. It smells like sewer water in a way. So as long as you do good plumbing, they’re perfectly safe. They do bubble out gas every five seconds, [makes bubbling sound] as they do the reaction, but that’s safely stored in a balloon. So for example, in Kathy Puffer’s house, she helped us form Solar CITIES, up in Tillson, New York, up the Hudson River. She’s gone through several polar vortices with the world’s first indoor household [00:36:00] biodigesters that we built out of IBC tanks. Just outta those one cubic meter tanks you can get anywhere in the world. Uh, we built two in her basement. They’ve been running for 7, 8, 9, almost a decade now. Um, and because they’re in the basement by the boiler that heats the house and they’re insulated and she’s got an aquarium heater if the boiler goes down, they’ve been burping out gas, and then the gas goes out the window into a balloon that’s underneath her porch outside.

We’ve also done it inside, but most people don’t wanna keep it inside, but you can. I mean, it’s not explosive. It’s, it’s flammable. Um, and then, uh, she pipes it into the kitchen, into the stove. And so the gas, doesn’t matter how cold it is, the biodigester’s kept nice and warm in the house, so there’s no reason not to do them in the North.

In Sweden, we built one with the Suderbyn Ecovillage in a geodesic dome, and we built one that’s like we have here at Rosebud, made of cement, 10 cubic meters. So it’s, it’s a big dragon, nine foot tall by nine foot wide, and it’s producing the gas for the community. [00:37:00] And there’s a big grinder. They take all their food, they dump it in, it grinds it in, feeds the dragon. The dragon goes into these big storage balloons. Uh, so it absolutely can be used indoors.

Kenny Coogan: Now we’re going to take a quick break in our conversation to hear a word from our sponsor, and when we return, we will learn how folks can build their own biodigesters step by step.

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How to Make Biogas at Home

Kenny Coogan: We are back with Professor T.H. Culhane, and we are talking about biodigesters.

So T.H., for a household we kind of talked about IBC uh, intermediate bulk container, which people probably are familiar with, like a cistern type tank. Um, how many of those would you need for a household?

Dr. T. H. Culhane: We always start with the minimum size so that people can get in the game and it’ll work well enough that they’ll be encouraged to continue and go, go larger.

You can build a biodigester out of anything. You can, we’ve built them out of, uh, soda pop bottles. We build them in workshops around the world out of five gallon water jugs. Uh, you, we’ve built [00:39:00] them out of five gallon water, uh, sorry, uh, paint buckets. We’ve built them out of pickle barrels in Palestine and built them out of, uh, 55 gallon drums in Hungary.

No matter where we go in the world, we’ll find a container and we’ll turn it into a biodigester. But what we found is that one cubic meter, 275 gallons, is just the right size to produce a useful amount of gas for doing your cooking needs. If you do a 55 gallon drum, you might be able to cook for 15 minutes to a half an hour. That may be sufficient. My wife in Palestine, when she went to Israel to the Arava Institute for the environment, built a pickle barrel size and cooked for 15 minutes every morning to make her egg and her tea in her dorm. She kept the biodigester in her dorm and everybody loved it. And of course, the temperature in Israel is favorable to having that. She didn’t have to insulate it or heat it. But most families are gonna produce up to a bucket of food waste today, and they want to cook for a couple hours. So it just turns out that that IBC [00:40:00] tank that you mentioned is the perfect size. It’s 40 times the size of a paint bucket and that we use because it takes 40 days and 40 nights, like Noah, uh, and the rain for the digestion to complete.

And so if you feed it 1/40 of the volume of the digester today, after 40 days and 40 nights, the liquid fertilizer that comes out the other sides will be more or less fully digested. So that’s this 1/40 figure. If you overfeed it, then the stomach, because it can’t keep up with that digestion rate, can get an upset stomach. It can go acidic, you know, acid indigestion. And that’s not a big deal. You just have to give it the equivalent of Tums, you know, you have to get it, uh, put sodium bicarbonate or, or, or something to bring the pH up. But that acid event can discourage people cause they go, oh, I built this little digester with my kids and we were using it and they were being able to cook an egg, and then somebody dumped a whole bunch of sugar in it and the remains of the birthday cake and it went acid and it stopped making biogas and then you have to reinoculate and, [00:41:00] and and all that. So we found that the one cubic meter IBC tank avoids most of those problems. Build it too small, there’s a danger, you’ll overfeed it.

Kenny Coogan: You just need one. Once you get that, what uh, do you do to the top or the sides to get it to work?

Dr. T. H. Culhane: Yeah, so a biodigester is really simple. So if you take your hand and make the, “I love you,” sign from sign language. It’s sort of like the “Go Bulls” sign from USF, but it’s, you just stick your pinky up and your index finger and your thumb out and you can look that one up. That’s “I love you,” “I-L-Y” in sign language. That represents a biodigester and teaches you all you need to know to build one. Your hand, the palm of your hand is the tank. Your index finger is the gas out, coming right out the top cuz gas is light and rises. So that goes at the very top of the tank. You have a pipe where your pinky is, that goes down one side of the tank all the way down to the bottom of the tank where you have to have a cut to let the food in so it doesn’t get smashed in there. You have to have some way for the food to slide in. So you make a cut at the bottom of the pipe, and that [00:42:00] pipe is the feed pipe, that’s the, the mouth and throat of the digester, and then the thumb sticking out to the side is your fertilizer pipe or your ureter that pees out liquid fertilizer. It needs to come out of the middle of the tank, but we don’t put it in the middle because over many years of experimenting, we found somebody can walk into it and knock it off, and then the pipe tank spills. So we found that if we put it through the top of the tank, but extend it down to the middle, then the liquid that’s in the middle will come out the top, and then you never have to worry about a spill because all three pipes are in the top of your tank. That was our innovation when Solar CITIES started designing urban home biodigesters to put three pipes through the top of any tank.

And then you have a biodigester.

Kenny Coogan: For the first pipe that you’re feeding it, the pipe touched the bottom of the tank.

Dr. T. H. Culhane: It doesn’t have to, but it’s easiest. We found if two of the pipes, because the the gas out pipe we put into the lid of the tank and so we can screw that on and take it off and it’s got a [00:43:00] bulkhead fitting. And so that one is right at the top of the tank on the. The two pipes, the fertilizer out pipe and the feed in pipe, it’s easiest for us using this rubber gasket called a uniseal. You look up uniseal, that’s the one of the great innovations of our time. There’s this, these, these gromits that you can buy any size and you carve a hole in the tank and you’ve put the uniseal and just shove the pipe in. We found if we just put the uniseal in the hole and then shove the pipe all the way down, it was very easy. We didn’t have to worry about how high it was, so we put a, we cut a cut in the bottom of the feed pipe, which usually nowadays is a four inch pipe, so you can put a lot of food in without grinding if you don’t want to, you can just stuff the food in. So four inch pipe. A four inch unile, a five inch hole. You shove that pipe all the way down to the bottom, but it’s got a cut that’s about, uh, I would say eight inches of the bottom of the pipe is sort of cut out as sort of like a…

Kenny Coogan: On the one side.

Dr. T. H. Culhane: One side. So it can, the food can slide in and then you [00:44:00] just shove it down and don’t worry about it. And then you can always cut, lop the top off if, if the height is the wrong one, then the feed pipe out. We want the liquid to come out the center, but we don’t have to measure the pipe and put it to the center of the tank. We get a long enough pipe. We use a two inch pipe for this cuz the liquid comes out a lot faster than food goes in. Two inch pipe, two inch uniseal, three inch hole, put the uniseal in, shove the pipe all the way down to the bottom. But what we did before we put the pipe in is we drilled a hole in the center of the pipe. So now when we shove it down, there’s a hole at the center of the tank for the liquid fertilizer to come out. The bottom we leave open, but then in the future it’s gonna clog up with all sorts of, um, fertilizer solids. There’s gonna be a sludge layer and that’s gonna keep the liquid from coming out. Most of the liquid comes out the center. That’s good enough for us. So we don’t bother to cap it or anything. We’re just gonna shove that one in, shove that one in, screw the one in for the type and you’re done. And you can build a biodigester, golly, in an hour.

Biodigesters for Fuel and Fertilizer

Kenny Coogan: How does the liquid [00:45:00] fertilizer come out? From gravity, from gas, pushing it out, or….

Dr. T. H. Culhane: Oh, I love the, the way you’re thinking, uh, the physics, uh, that you’re, that you’re, um, articulating. Um, it’s a little of both, but even if there was no gas, water seeks its own level. So we make the four inch feed pipe about two inches, three inches taller than the fertilizer out pipe. And we then put a tea on it and we put a horizontal pipe and then another elbow going down to a bucket outside. And as soon as the water level as we’re feeding it goes up above that, the fertilizer spills out passively. Now we are using uniseals and we’ve screwed the lid on the pipe, and the gas out pipe is taller than the feed pipe as well.

So we’re always keeping a positive pressure. We overfill these, don’t get me wrong, you, you, your IBC tank is completely filled with water when you start, and then over that the tank is over about eight [00:46:00] inches is the height that works best. So you have about eight inches on the feed side, and you’ve got about six inches on the overflow side. And so every time you feed it, it’s just passively over spilling.

Now if you seal the gas out, see the valve on it, so the gas pipe is coming up about 10 inches, and then you’ve got a valve. If you seal that and the gas starts being produced, it’s gonna drive the water level down inside the tank cuz it’s gotta, gas has somewhere to go. It’s gonna fill up the top of the tank, it’s gonna force that water out the spillover pipe, the fertilizer pipe. And so you know when your biogas digester is working by sealing the gas thing and just watching. And one day, as the microbes start doing their job, it’s gonna start pushing liquid out by gas pressure and go, oh, it’s working. And then you open that valve and release that gas and then replace it with water.

Kenny Coogan: Okay. So you get the IBC tank. I understand you put the three pipes in. And then how do you feed the tank? How do you [00:47:00] get it to start?

Dr. T. H. Culhane: So the coolest one I ever did about 14 years ago was I had a baby and I was responsible for changing his diapers. And because I don’t believe there’s any such thing as waste, or at least waste shouldn’t be a noun, it’s a verb or an adjective, “you wasted that.” I didn’t wanna waste the, uh, material that was in my baby’s diapers. We used cloth diapers. So I decided, I built a biodigester in the bathroom, and I decided that I was gonna just go up there when I changed the diaper and scrape that off into the feed pipe.

And this was a smaller digester. It’s, uh, but you can always feed a tiny digester poop because as I said, it’s spent fuel, it doesn’t add too much, um, sugar or, or lipid, uh, fat. So it’s not going to create acid indigestion. You can always feed poop to any size digester, so we don’t have to worry about the pH going off.

So I just had this little digester. It was about, I guess, um, it was the size of a trashcan, like 40 gallons. [00:48:00] And I would just scrape that in and it would do the digestion in there and it would produce gas enough to sometimes make a cup of tea. And then the over spill would go into a nutrient bucket that I had and I put an airstone in that. And, uh, that took care of any odor. And then I had liquid fertilizer, so that was great cuz I didn’t have to seed it with anything other than my baby’s diapers. I’ve done it with my own waste. We’ve done it with dog poop. We built a dog poop digester in Brazil, in Rio, in a garage. Uh, I’ve used cow manure, horse manure, I’ve used chicken, I, I’ve literally used every poop. But you can also use lake mud.

How to Start a Biogas Digester

Kenny Coogan: So for a 275 gallon IBC tank, how much do you fill it up with just water?

Dr. T. H. Culhane: The entire thing.

Kenny Coogan: The entire thing. And then, um, on day one you can add, um, lake mud and waste up to five gallons, or not necessarily?

Dr. T. H. Culhane: Right. Not yet. So it’s a good question [00:49:00] you’re asking, and many people do this wrong.

You inoculate it, you seed it the first day with just a source of microbes, poop works really well.

Kenny Coogan: And like one gallon?

Dr. T. H. Culhane: No. In Palestine, we were running out of time and all we could find was one dried clod of horse poop on the road, and it was really dry and we thought, this is not gonna work. But we had to run to get through the border crossing in time.

So we just gathered this one, one handful and we dropped it in. And then about a month and a half later, I got an email from my colleagues in the West Bank, uh, in, and it was in, uh, in Bethlehem. And they said, you know, um, uh, it, it’s making gas. So we know that it takes some time. If there’s one living microbe in that piece of poop, it will eventually colonize the whole tank. It just takes longer. We generally like to put in about, you know, 10 buckets full of poop, which is, you know, it’s about 50 gallons. And that way we know that we’ve got enough starter material that it [00:50:00] more rapidly will fill the tank with microbes. So you give it a head start, but you can, you, we’ve even used RID-X powder. You go down to the Home Depot or Lowe’s, buy a thing of RID-X powder for your septic tank, septic powder that has living microbes, and just dump that box in and wait. And there’s enough living microbes in that starter culture. It’s like making yogurt or making sourdough bread. And then you can give a bottle to your neighbor once your biodigester is going and they don’t have to go get poop from anywhere, you just give them a liquid bottle.

Kenny Coogan: And how do you know when to start adding your food waste?

Dr. T. H. Culhane: When it produces its first flammable gas when you can light it the first time. So in the beginning, the microbes are disoriented and they’re competing with a lot of other stuff in that tank. Uh, maybe some chlorine in the water that’s causing them to weaken.

So you’re trying to establish an ecosystem, and in the beginning when the bubbles start to appear as the microbes colonize and start eating what’s in that residual poop, they are gonna make carbon dioxide. And you’ll get gas and you’ll try to light it and [00:51:00] it will not light, just the same way you blow out a candle, it’s carbon dioxide, it’s flame extinguisher. Over time, and it’ll take somewhere from three to five weeks, you’ll come back, you’ll still see gases being formed. You’ll try to light it and they’ll start to light like a little bit like, ooh. It almost lit. It almost lit. You’re like, okay, take it easy. It’s gonna get there. There’s gonna be a day when you open that valve and light that and it’s gonna go, [makes sound of flame being lit] the beautiful blue flame is gonna appear like, like Moses saw that the, at the burning bush. It’s, it’s an epiphany. It’s a great experience, almost a religious experience. And when you see that blue flame, you’re like, ah, you guys are ready and you’re hungry. That’s their signal to you that they said, “Feed me,” like in “Little Shop of Horrors.” But this is a little shop of delights.

Once your biogas plant is ready to be fed. And I, I know I was looking right behind you, that’s why I saw that. Right behind Kenny is this, uh, is this “Little Shop of Horrors” plant with big teeth . I love it. It’s a carnivorous plant. That’s your, your specialty.

So this domestic dragon, uh, doesn’t have any teeth and, uh, but it is hungry.

Feeding Your Small Scale Biodigester

Kenny Coogan: Okay. So speaking of which, you’re, you’re [00:52:00] gonna see the flame. You’re gonna know that it’s hungry. And then you mentioned that the, it’s a four inch pipe to feed it. And then you’ve, you’ve, you’ve alluded to, oh, you can grind up the food, but sometimes you don’t have to. So what is the benefit of you processing the food before you feed it to the biodigester?

Dr. T. H. Culhane: Well, and as we just said, the, the dragon doesn’t have any real teeth. So if you, if you, well, let me go backpedal for a second. If you start feeding a lot of food waste the day that it says, feed me, you can overwhelm it and it’ll get acid indigestion. So remember it’s a baby dragon. Even though you may have built it really big, it’s stomach is still out of a baby. It’s just come into the world, just established itself. So you wanna slowly work your way up to that five gallon bucket. So you might start with a cup of food waste and see how it does. It starts producing gas. You’re like, okay, we’re good. Next day, put two cups, then go to four cups, then go to eight cups, then go to 16 cups and you can double each day. But slowly get it up to its maximum, which is [00:53:00] 1/40 of its total volume. So five gallons for a IBC tank size. Right?

And okay, so then you’re saying, okay, I’m, I’m gonna feed it little by little. And now what I want to do is I want to accelerate the rate of decomposition, cuz these guys don’t have any teeth down there. They’re using enzymes. And in a aerobic composting pile, you got fungus with its enzymes, you got worms, you got beetles, you got ants, you got all these critters with teeth and mandibles and jaws, [makes theme music from “Jaws”] right? And they’re all contributing to the breakdown. Here you’ve just got microbes that have no teeth. So if you help them by grinding, you can accelerate the process so that they can get it done within a day.

The problem is you are be tempted to overfeed it. I mean, you can grind an awful lot of food down to a five gallon bucket. You can grind 10 gallon buckets on it.

Kenny Coogan: All right, just like a normal compost. If you can make the food smaller, the breaking down happens quicker.

Dr. T. H. Culhane: Right. [00:54:00] So what we’ve discovered along with our colleagues at HomeBiogas.com in Israel, who we work with very extensively since their origin, you know, over a decade ago, uh, is that if you don’t grind the food waste, it’s not a problem because it, in that 40 days and 40 nights of digestion, the food you put in today will yield gas and fertilizer 40 days from now. And yeah, you’ll have to wait the first 40 days, but then the second day you’re coming out, the 41st day and the third day — on the 43rd day. So once you’re feeding regularly, you have a constant supply of gas. But sometimes, you know, you have a party tomorrow and you haven’t got a digester that’s been working for a long time. So you’re like, shoot, I’m gonna grind up my food, throw it in. I know tomorrow I’m gonna have enough gas to barbecue and make my impossible burgers. So that’s the one of the advantages.

The other advantage of grinding is that if you have excess, cuz you’re only supposed to feed about a bucket a day, let’s have a huge amount of food waste and you grind it up, [00:55:00] you can store it cuz ground up food waste, if you just let it sit, it doesn’t stink. I mean, for weeks. It’s amazing cuz you hydrolyze it and you aerate it, uh, oxygenate it during the grinding process, break up the cell walls. The enzymes are, are spread out from the vacuoles in the, in the, in the cells of the, of the food plants and animals that you’ve ground up. And we’ve found that if you grind it up and you still have more than you can store and that you’re gonna feed your digester, you can simply dump it on the ground and it’ll turn into soil in as little as three to six days rather than three to six months. And you don’t need a composter at all. Um, so the advantages of grinding, considering that, uh, a grinder might use 60 watts of power to grind for five or 10 minutes, and then the bucket of food waste and yield you a kilowatt, it’s a thousand watts. So you’re using 60 watts, you’re getting a thousand watts out within 24 to 36 hours. You’re getting the liquid fertilizer out within 36 hours, 24 hours. There’s [00:56:00] all these benefits, but we wanted people to know that if you don’t have a food grinder and most people around the world don’t, and they’re very expensive outside the U.S., then just go ahead and toss the food in and then wait and, and then get that pulse going. And then every day you will get the, the gas and the fertilizer from it.

Kenny Coogan: To clarify, we wanna put in about 1/40 of the volume.

Dr. T. H. Culhane: Yes.

Kenny Coogan: But, um, if we fill a, a five gallon bucket, it should be like the big food scraps and stuff. We shouldn’t be pulverizing it, like 15 gallons to fit into the five gallons, or….

Dr. T. H. Culhane: Yeah. So, so this is still a, it’s, it’s a variable that we want to continue exploring with our, our biogas communities. So like on our Facebook group, Solar CITIES Biogas Innoventors and Practitioners, we have over 15,000 active members around the world who are all experimenting at this household scale.

Meanwhile, HomeBiogas.com has their website and their Facebook groups and, uh, the, the [00:57:00] home biogas, uh, users groups, and we’re all sharing data with one another and experience. And what we’ve found is that you can in fact feed more than that 1/40 depending on the temperature, because the hotter it is, the faster digestion occurs depending on the surface area inside your biodigester.

So if your biodigester has been running for years, there’s enough surface colonized by those microbes that they can take more. Think about your stomach with all the invaginations. The, uh, you know, your alveoli in your lungs is an example for a gas exchange. But in your stomach you have all these, what is it? Villi, they call them, or microvilli? So you’ve got all these tiny little, you know, cremulations, that surface area that helps you digest, when you have just a tank, then there’s no surface area in there. It’s just the walls, the floor, and the ceiling of this biodigestor. So the microbes don’t have that much place and they hate to digest floating. That’s been proven. Microbes don’t like to do their work in a tank of water. They like to [00:58:00] clinging to your teeth. They like to cling to the sides of rocks and rivers. I mean, microbes like to form what they call biofilms or rafts, where they create these complex cities of microbes that do all this exchange of material.

So you have to give them a city. In Solar CITIES, we said, well, we can put in vertical tubes. We’ve put in bio blocks that people use through aquarium filters. Anything that has enough space, biochar that has little cell, uh, spaces in it. Anything that, that, uh, that the, that the microbes like lava rock, can, can, can, uh, colonize.

But we’ve also found that over time, the food waste you put in does create those little rafts because you might put in pecan shells or, or pistachio shells or ground up bits of shell, you know, seashell or anything you put in that stays in the tank floating around is a raft for the microbes to build it.

So over time, your microbes, uh, are, are expanding in their population presence and they can handle more and more and more food. So I have some [00:59:00] digesters where I’m like, yeah, shoot, I’ll take 15 gallons, grind it down to, you know, to five gallons and pour it in with impunity. Like, yay. But that biodigester has been with me for 10 years.

I’m not telling the the public, Hey, build a biodigestor. Just gonna start to throw in your food waste at it. You need to grow with your dragon. It’ll grow with you. It’ll adapt to what you feed it, it’ll adapt to your temperature conditions. That’s, that’s the thing is they’re living organisms that are rapidly changing and they become your domestic pet and they’ll eat what you throw at them.

Applying Fertilizer from Your Biodigester

Kenny Coogan: Very good. We have about five minutes left, and all of those sites and links that T.H. mentioned are gonna be in the show notes attached, associated with this podcast.

For, um, the fertilizer off pipe or the liquid fertilizers coming out, um, are you just collecting it in a bucket or are you letting it…. right? You’re, you’re collecting it somehow?

Dr. T. H. Culhane: Right. Well, we have 10 biodigesters here at the Rosebud Continuum Eco [01:00:00] Science Center in Land O’Lakes. We’ve got three cement, concrete, 10 cubic meter, nine foot by nine foot digesters, one that looks like a dragon, one that looks like Snow White’s wishing well and is half buried, and one that’s completely buried under a bee pollinator garden. To show that you can build them above ground or below ground or anything in between. We’ve got several home biogas, uh, units, uh, from that company, uh, from various stages of their development, including their latest one, which has a toilet attached to it. Uh, we have IBC tank biodigesters, the Solar CITIES waste scattered all over. They’re under the three RVs as the way to take care of toilet waste and our toilet wasted, um, the materials and shower and, and soaps and, and, you know, detergents from the washing machine and the dishwasher. Uh, we’ve got standalone ones in the greenhouse. We’ve got so many. We’ve got pickle barrel sized ones and 55 gallon drums and five gallon bottle — we have so many digesters, so, every digester is used for different experiments. One will feed just chaya leaves to it, so we know what’s coming out for the [01:01:00] fertilizer. So they’re all producing fertilizer at different heights and different levels. Some we store in an IBC tank, some we store in jugs. Some we let just spill like the toilet ones from the RVs. All that goes in the digester and then goes out a perf pipe into a Hugelkultur. Now, a Hugelkultur is a mound, you probably know from permaculture, the Austrian idea of, of Sepp Holzer. You have a bunch of logs and, and detritus and mulch underneath the pipe. And then above the pipe in a hill, it’s called hill culture, Hugelkultur, that wicks and absorbs the nutrients coming out. And then they have a garden of different ornamental plants planted on top of that and breaking it down into soil. And then we’ll replace the Hugelkultur. That’s been going on for five years and we still haven’t had the need to replace it.

So that’s a passive spill. Uh, with other ones we’re spilling right on the garden next to the digester and others, we have it piped out in a pipe that goes out to where we want to fertilize. Um, we….

Kenny Coogan: And depending on the size, it’s off [01:02:00] putting up to five gallons of liquid a day, or….?

Dr. T. H. Culhane: It’s putting out whatever you feed it.

So if you put in five gallons, you’ll get five gallons of liquid out. If you put in 10 gallons, you’ll get 10. If you put in a hundred, you’ll get a hundred. And that’s only for the 10 cubic meter ones. You can feed a hundred gallons if you have it. Um, and that’s why we have a IBC tank collecting the fertilizer from the big digester, cuz that’s a thousand, uh, a 275 gallon tank.

When you use it for hydroponics, you cannot, and this is important to emphasize, you cannot put it right into your tower garden or into your aeroponics or hydroponics set up because it’ll burn the roots of the plants. It’s so nutrient dense. You’ve gotta dilute it 10 to one, 10 parts water, one part biodigestate, or 20 to one, depends on the plant and its needs. And we find that it works best, cuz it comes out anaerobic, if you aerate it first. Now if you’re putting it on soil, the soil will take care of that. It’s an aerobic process so you can put it in your garden. Yes. But if you put it directly onto plant roots, [01:03:00] better to put it through a bubbling system like a aquarium bubbler, let it bubble up and let it be in a water solution so it dilutes it.

Do You Have to Clean a Biogas Digester?

Kenny Coogan: So my last question is, um, does a biodigester ever have to be cleaned? Does it, should it ever be fully emptied out and then restarted?

Dr. T. H. Culhane: Yeah. There there’s, there’s some different thoughts on that. In India and China, I visited biodigesters, where after anywhere from five to 10 years of operation, they deliberately aspirate out, like with a shop vac or with a, a septic hose. Just like septic tank people will do, they come in and they suck the sludge out, and then they sell it as a fertilizer amendment, the solid cake, uh, cuz it’s where, where your heavier, uh, elements like your potassium and your phosphorus, the nitrogen stays in solution as ammonia and comes out. But sometimes most of your phosphorus and potassium and calcium is gonna remain at the bottom.

Like you put in eggshells. The calciums in the eggshells, they’re [01:04:00] just gonna digest, but it’s gonna stay at the bottom as a sludge and you’ll lose those nutrients. So if you want it, they will take it out. And if you’re putting a lot of lignocellulose, meaning paper towels and napkins and plates and cups and uh, you know, bamboo and, and, and brown leaves, like I said, if you put that stuff in, it doesn’t hurt the digester. But since it doesn’t fully digest, it’s just gonna build up the sludge layer and clog your digester until your effective volumes have been reduced. Like where my only digestor is working half as well, it, it’s half full of sludge.

So depending on what you’re feeding it. Now, septic people come out to your house because your toilet paper can’t really be digested well. The poop can be fully digested, but the toilet paper binds with that poop, forms a sludge, and they come out and they suck that out. Um, think of the biodigester as just a, a optimized septic tank. And if you throw paper in, you’re gonna have to remove it more often. If you don’t, no, it’s, it’s just going to, we’ve had some running now for, for 15 years and we’ve [01:05:00] never had to empty them, but we might want to, after five years, get those nutrients out if you don’t have a stirer. Another way to do it is put a stirer in and just keep stirring it up so the bottom stuff gets out the liquid fraction. It’s not an issue for a…. A biodigester running at a good temperature and fed properly without lignocellulose, you’ll probably never have to clean out.

Kenny Coogan: Thank you so much, uh, Dr. T.H. Culhane, for speaking with us. We appreciate it and we really enjoyed our conversation on biodigesters. It’s been very insightful.

Dr. T. H. Culhane: Thank you, Kenny, as ever. It’s really great to see you and I, I hope one day we can take some of these carnivorous plants and recycle them into a biodigester and then they can compete with the “feed me,” uh, with the “feed me.” Thank you so much.

Cultivate Kindness with More Mother Earth News

Kenny Coogan: Thank you. And we thank the listener for joining our podcast and encourage you to share it with your friends, colleagues, and family. To listen to more podcasts and to learn [01:06:00] more, visit our website, www.MotherEarthNews.com. You can also follow our social media platforms from that link.

And remember, no matter how brown your thumb is, you can always cultivate kindness.

John Moore: Thanks for joining us and Dr. T.H. Culhane for our conversation about biodigesters. For links to some of the resources we covered and so much more info, go to www.MotherEarthNews.com/Podcast.

Our podcast production team includes Carla Tilghman, Jessica Mitchell, John Moore, and Kenny Coogan. Music for this episode is “Travel Light” by Jason Shaw. The Mother Earth News and Friends podcast is a production of Ogden Publications.

Have you ever wanted to meet our podcast presenters in [01:07:00] person or take workshops from them? You can by going to one of our many MOTHER EARTH NEWS Fairs each year. You can take hands-on workshops, attend information filled presentations, and shop from our many vendors specializing in DIY ideas, homesteading, and natural health.

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Until next time, don’t forget to love your Mother.

Meet Dr. T.H. Culhane

Dr. T.H. Culhane is an Associate Professor as instruction at the Patel College of Global Sustainability at the University of South Florida. He is the Director of the Climate Mitigation and Adaptation concentration. He is also the co-founding director of the not-for-profit educational corporation “Solar CITIES Inc.” which helps community stakeholders solve urban ecology and development issues surrounding waste-water, solid waste, food security and decentralized clean energy production

Additional Resources:

Learn more about Solar CITIES on their website and on Facebook

Visit the Solar CITIES Biogas Innovators and Practitioners Facebook group

Home Biogas website

Biogas Digester Construction by Kenny Coogan

Our Podcast Team:

Carla Tilghman, Jessica Mitchell, John Moore, and Kenny Coogan
Music: “Travel Light” by Jason Shaw

Listen to more podcasts at MOTHER EARTH NEWS PODCAST.

Check out the MOTHER EARTH NEWS Bookstore for more resources that may interest you.

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The Mother Earth News and Friends podcasts are a production of Ogden Publications.

Ogden Publications strives to inspire “can-do communities,” which may have different locations, backgrounds, beliefs, and ideals. The viewpoints and lifestyles expressed within Ogden Publications articles are not necessarily shared by the editorial staff or policies but represent the authors’ unique experiences.

Ogden Publications strives to inspire “can-do communities,” which may have different locations, backgrounds, beliefs, and ideals. The viewpoints and lifestyles expressed within Ogden Publications articles are not necessarily shared by the editorial staff or policies but represent the authors’ unique experiences.