Energy Systems

Biodigesters in Reality

Emma and I have been in the process of becoming homesteaders for several years now. Most of that time was spent learning crafts like building, gardening, and designing, as well as looking for the right property. But, over the last year or more, we’ve moved on to the stage of applying these skills to reality, an actual piece of land and a home (in progress).

The hardest pill to swallow in this new phase has been bridging the things we’ve learned in permaculture courses (and the like) with the reality of putting them into practice. For example, we’d been wanting a sustainable, natural home, but building codes in the US don’t just let you do that. In fact, they are more or less pitted against it and, for that matter, a person building their own home at all, by code or not. Nowhere in our courses did we cover how to actually deal with these challenges, other than to acknowledge we were likely to encounter them.

Some of the sustainable measures we’d like to take, such as composting toilets and a wood (cook)stove hot water heater, we’ve designed as retrofits to our cabin, to be installed once we’ve passed building inspection. Others we are still figuring out. That’s where biodigesters come in. We’d love to give it a go, but the reality is that we’ve come to assume inspectors wouldn’t be big fans. Moreover, we still have a lot of questions about just how beneficial biodigesters are in terms of a fuel source.

 

The Short and Simple on Biodigesters

The concept of a biodigester is easy to understand, and though we’ve never built one, it seems the construction is rudimentary as well. Basically, a biodigester is a large container in which anaerobic (deprived of oxygen) decomposition takes place. This is done by promoting certain naturally occurring microbes to perform their biological functions of breaking down organic waste by providing them an oxygen-free space in which to do it, i.e underwater. As anaerobic decomposition takes place, biogas—mostly methane with some carbon dioxide and small amounts of other gases—is released. That mixture of gases can be captured and used for fuel, similar to but not the same as propane or butane.

There are loads of simplistic designs on the internet, specifically from Solar CITIES, for varying scales of biodigesters. After some research, we’ve narrowed our situation down to the recycled IBC tote design or, less likely, the all-in-one-kit offered by companies like HomeBiogas. While information on these DIY biodigesters is abundant (and free) online, it’s been difficult to fully understand the realities of using one. It’s as if the information emphasises the fact that biogas production is possible. Of that, we have little doubt. But, of course, there are a lot of other relevant details. This has become the issue with so many sustainable measures.

The question remains as to how much biogas are we going to get from this thing. Some estimates put the daily at a couple of hours for a single burner, but honest reviews quote much lower. There seems to be little consensus or stability. Because we cook every meal at home from scratch, even with just the two of us, it seems problematic to rely on maybe just half an hour of cooking fuel a day. That’s just not enough to do all the cooking we do.

 

 

The Voice of Experience

In other words, whether true or not, the information we could gather wasn’t supplying us with what we wanted to hear. We were hoping for a reliable cooking fuel to cover all our needs and supply fertiliser as the digital brochures and YouTube videos seem to suggest. But, that’s not the reality. Not even close.

Biogas Production Systems
Photograph courtesy of Nic Donati

 

Luckily, I’ve recently been exchanging emails with Nic Donati from Rancho Mastatal, a sustainability education center in Costa Rica, and they’ve got two biodigesters on site, both of which have been in operation for nine years. So, I tapped him for some information. The site’s digesters are both run exclusively on water and faeces, one on humanure and the other on cow patties.

Photograph courtesy of Nic Donati

 

The digester for human excrement is fed directly by two toilets. It has a brick digestion chamber roughly 2 meters deep and 1.5 meters across with 1.2 meters spanning the inside cavity. There are separate outer and inner walls with a gap of about 15 cm between them. The gap is filled with water, and atop that water floats a fiberglass dome that rises as the inner chamber produces biogas, filling it, and sinks as people use that gas, depleting it. It was sized to function with the average number of people there (and pooping in it) throughout the year: 20.

Biogas Bag
Photograph courtesy of Nic Donati

The digester for cow manure works differently. It involves a series of tanks (55-gallon barrels) that help to filter out solids before manure effluent enters a PVC bag that captures the biogas. The bag can hold 900 gallons of water, and it is given 30 gallons of manure effluent a day. The solids are put into a vermicompost before being used for the vegetable garden, and the manure effluent passively fertilizes tree crops. All the while, the anaerobic decomposition of the effluent is offing biogas.

Biogas Stove
Photograph courtesy of Nic Donati

 

Together, these two systems supply gas for the Rancho Mastatal kitchen, which accounts for about 50 breakfasts (eggs and gallo pinto) each morning prepared on biogas alone. Nic says that generally the gas gives out somewhere into lunch, at which time the cook switches over to a fuel-efficient, wood-burning stove. By the next morning, the biogas has replenished. The kitchen is roughly 30 meters away, and the system uses gravity to create enough pressure to use the gas for cooking. Ultimately, the gas comes through a specially designed stove that equates to a ½” (1.27cm) metal pipe with a ball valve. As we already know, biogas requires a different, more free-flowing stove design than what is used with pressurised tanks of propane.

 

Adaption and Application

While interesting and enlightening, in truth, the biodigesters at Rancho Mastatal won’t work for us.  For one, local codes would likely not allow us to use the biogas toilet system, though sewage treatment plants do use similar large-scale systems and there is plenty of information for using a home septic tank to create a biodigester. This, of course, would be better for the environment, which alone would make it worthwhile, but in terms of cooking with gas, our two butts just wouldn’t provide enough effluence. In the case of cow manure, we could probably find some from neighbours with cattle, but we don’t have a reliable source for 30 gallons of effluent a day. Being vegans, animal husbandry won’t be part of our homestead. So, we are left with a different path.

Many biodigester systems do run on kitchen scraps (or claim to), though using some cow manure to inoculate the digester helps. In the case of kitchen scraps (as well as grass clippings and similar nitrogen-rich matter), they must be ground up and added as a slurry rather than straight food scraps. Nic mentioned that they’d tried fruit peelings in their system, but yeast contaminated the fermentation and produced too much carbon dioxide, ruining the biogas.

Thus, we are left with the similar situation: Due to the restraints of environmental and building codes, the reality of practicing this has to be done on the sly, with risk of penalty, and consequently has to be implemented after the fact, a kind of preconceived retrofit to our kitchen. While there is still gobs of experimenting we’ll have to do, at least we do know (and have accepted) that our future biodigester is more likely to provide enough gas for a small fraction of our cooking.

In the winter, with a wood-burning cookstove doubling as a heater and water heater, that’ll be just fine. In the summer…we’ve got some more planning to do.

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Jonathon Engels

The financially unfortunate combination of travel enthusiast, freelance writer, and vegan gardener, Jonathon Engels whittled and whistled himself into a life that gives him cause to continually scribble about it. He has lived as an expat for over a decade, worked in nearly a dozen countries, and visited dozens of others in the meantime, subjecting the planet to a fiery mix of permaculture, music, and plant-based cooking. More of his work can be found at Jonathon Engels: A Life About.

12 Comments

  1. I think the appeal of the bio digester (other than from environmental reasons) is the instant cooking. Wood cooking if generally doesn’t off that.
    Although I haven’t made a bio digester I think I’d prefer to use the “feedstock” to grow plants that could be burnt as a wood source.
    Bio digesters seem fairly complex and rely on very clean gas (like really really clean ;)
    And the potential for blockages seems fairly high.
    Like anything with many moving parts, things will fail.

    I’d suggest you look into rocket stoves for cooking.
    Matt Walker has several designs for cooktops (and ovens)
    That would work equally as well indoors as well as outdoors (for summer).
    His plans are super simple and he will talk you through any issues (as part of the plan cost, consulting is included).
    That would save you a small fortune over the cost of a decent kitchen wood heater.
    http://Www.walkerstoves.com

  2. Thank you for this great article. I have often considered something similar and have been concerned that with only two of us it just wasn’t viable. Part of living a permaculture life has been reducing our food waste to next-to-nothing and what little we do create goes to worms or compost. We don’t have green waste because we compost in-situ (let the weeds grow, chuck on any pruning and mulch straight over it) and ultimately hope to achieve a self-mulching forest system. Our simple bucket system composting toilet is, for us, an example of using energy at it’s highest. The compost from this system is returned to the soil. We have a huge zone 5, so gathering sticks from the edge for a rocket stove is no problem. Given the time, effort and cost of a biodigester I have wondered why we would bother. And then there’s the maintenance!

  3. Jonathon, I’m going to diverge from the main topic here because I was triggered by your opening salvo,- dealing with building code realities. Though up-cycling animal and humanure is among the pillars of self-sufficiency in permaculure, and biogas can be a major component of a complete energy production scheme, it is not included in my short list of priorities for the uninitiated home builder. The first thing we do involves planning a (word choice) ‘program’ that addresses the anticipated users––anticipated needs. Likely, YOU won’t be among the last persons to evaluate those creature-comfort needs. This precedes the specifications for your foundation and getting the thermal envelope exactly right. For the most, yesterday’s (20th C.) developer models were not designed or built right.

    Fortunately, (whether you buy into the ‘Net-zero,’ PassivHaus,’ and ‘LEED’ ideologies, or not,) those of us who live in relatively mild climates now have two excellent ‘go-to’ resources for definitive insight on today’s building codes and essential material needs. The most comprehensive, yet easy to understand is Matt Risinger’s “the Build Show.” For example, he just featured a mind blowing example of efficiency in a system designed to heat in entire home in Vancouver with a single hair drier. (see: http://mattrisinger.com/topics/ ) Those interested in working with ‘rocket stoves’ would definitely be excited to venture there.

    The other MAJOR player is Building Science, Inc. They are mentioned throughout the current energy sections of the California and Florida State building codes. (see https://www.buildingscience.com/events/building-science-fundamentals?sku=BSF-19-001) Lesser discussed ‘programs’ could enable you to ‘do your own thing’ in the bush without ruffling feathers down at City Hall. For example, you might start out by pitching simple non residential temporary workspaces, where a simple porta-potty will suffice. :)

  4. I don’t see why being vegan prevents you from animal husbandry.
    Seems an unnecessary limitation. I’m not saying this lightly – and would happily discuss.

    Also regarding code, at some point in your area, someone will have to bite the bullet and begin the “fight”.
    Sepp Holzer and likely many others have suggested and proven that if the local law is nonsense, then do the right thing and when someone comes to complain or remove, have them test your solution – ideally they’ll find it works, can’t prove a danger; in fact likely the opposite – and then change the law :)

    Good luck, whatever you decide.

    And yes, never rely on one solution to a “problem” – have a bigas digester, efficient wood stove (and copse to fuel it), some solar and wind and water energy (if possible all three) that give you a backup of electricity in a worst case scenario.
    All the best

  5. I run a one cubic metre IBC biogas plant in my suburban Brisbane backyard. Have done for 7 years or so. Had to replace the original IBC because the bottom gate valve through which I extract the sludge when it builds up too high, failed. Got a black uv resistant IBC with a high quality ball valve to replace it.
    Fed via a toilet pan, the gas is plumbed up via half inch irrigation poly pipe, to a queen sized air mattress to store the gas. An old internal door placed on top of this, provides just enough pressure to run one of those cast iron low pressure propane gas burners.
    When the temperature falls below 18C, no gas gets produced in any usable quantity. Mid summer, it is possible to produce an hour or so of low flame cooking per day. Grass clippings tend to be too high in non digestible fibre and tends to clog up the works too quickly. Fruit and vege scraps, urine and palatalized fish and seaweed feeds my digester, the liquid effluent being mixed about 1 part to 5 parts water for liquid manure to the garden.
    The good news, is if you’re a good scrounger, it should be possible to build this system for around AU$100. No law against doing this in OZ.

  6. Recently, the International Code Council approved and passed the proposed Cob Construction Appendix for inclusion into the 2021 International Residential Code!!!! Currently, the International Building Code has been adopted in all 50 United States as well as by USA federal government agencies, a number of U.S. territories and some other countries (International and the International Building Code Post-Frame Buildings Code Council, 2015a).
    NOW is the crucial moment in time for radical positive change in the Building Culture towards natural, healthy, durable, fire-proof, earthquake-friendly, sustainable and BEAUTIFUL building … so everyone, please keep being appropriate housing activists. Sometimes, it only takes pressure from several people to change the rules.

  7. I’d suggest you do something of an energy balance, how much energy will be going in, against what you hope to get out. Then you may need to heat it which Costa Ricans won’t have to. Heating might discourage yeasts and encourage bacteria to produce methane.
    Years ago there was a guy in France/Belgium, who heated his home from a heap of wood shreddings, by running a hose pipe through, and connecting to radiators indoors. He also produced some biogas. I can’t remember his name, something short.

  8. Therefore the need to be part of a community (even online) that together can lobby to change building codes. Working together as permaculture suggests rather than struggle alone. Check out quailspringspermaculture, they have mobilized the community to change cob building codes in California and beyond. Take part and lead the way. Permaculture is about independence as much as it is about interdependence.

  9. Jonathan,
    I am friends with the Solar CITIES people. They are wonderful people here in Chester County PA and are full of helpful info and support. If you are interested I can connect you with them as I’m sure they can give you some insight. I did a workshop with them on building the tote biodigester. We even checked out a project a friend of theirs was doing converting a propane heater, for a greenhouse if I remember correctly, to run on biogas.

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