Home Cookin' with Homemade Biogas

Intermediate

Inside this Article

Biodigester with inflated rubber bladder.
As the biomass decomposes, methane and carbon dioxide are created, inflating the rubber bladder to create pressure.
Biodigester
The Maitreya Ecovillage biodigester turns 15 pounds of kitchen scraps and garden clippings into a day’s worth of cooking fuel for the community’s kitchen.
Biodigester Educational Sign
Loading kitchen scraps into the digester.
Maitreya resident Christa Stark loads kitchen scraps into the digester.
Kitchen scraps and garden waste.
Biodigester hopper filled with kitchen scraps and garden waste.
Stirring the organic scraps into the muck.
Stirring the scraps into the muck exposes them to active methanogenic bacteria.
Replacing water in the biodigester.
Cleaning up and adding water to replace what is drawn off as liquid compost.
Extracting the nitrogen-rich liquid fertilizer.
At the output, the nitrogen-rich liquid fertilizer is extracted from the digester, which produces 5 to 10 gallons per day.
Nitrogen-rich liquid fertilizer.
5 to 10 gallons of nitrogen-rich liquid fertilizer is produced per day.
The liquid fertilizer can be added directly to plants and trees.
The liquid fertilizer can be added directly to plants or covered with dirt or mulch to keep beneficial ammonia in the soil.
Cooking with biogas
Now we’re cooking with (bio)gas.
Biodigester with inflated rubber bladder.
Biodigester
Biodigester Educational Sign
Loading kitchen scraps into the digester.
Kitchen scraps and garden waste.
Stirring the organic scraps into the muck.
Replacing water in the biodigester.
Extracting the nitrogen-rich liquid fertilizer.
Nitrogen-rich liquid fertilizer.
The liquid fertilizer can be added directly to plants and trees.
Cooking with biogas

About five years ago, writer and renewable energy aficionado Warren Weismann was researching ancient Greece for his novel when he stumbled across information that the Greeks had built anaerobic digesters to produce methane. He then read about similar archaeological evidence in ancient Syria and China. But it was the modern biogas boom in China that got him most excited and distracted him from his writing career: Tens of millions of home-scale biodigesters have been built in China over the last century, with the pace of construction still accelerating. Warren wanted one for himself.

After a few years of further research, including conversations with colleagues in India and Nepal, where small-scale biogas production is prevalent, Warren modified traditional designs to create a plan for his own  700-gallon biodigester. He was living at Maitreya Ecovillage, a three-block community and green-building-oriented neighborhood near downtown Eugene, Oregon. After building his first biodigester last year, he’s become increasingly excited about the possibilities for home-scale biogas, and has established Hestia Home Biogas to build biodigesters locally and consult on biodigesters across the globe. 

Back from Obscurity

Biogas has been used for lighting for at least a century, and possibly millennia. But it was mostly abandoned in the United States after cheap and abundant fossil fuel was harnessed in the early 20th century. Home-scale biodigesters have remained on the sidelines in the developed world, but are poised for a comeback as interest in a replacement fuel increases.

There are good reasons to consider building biodigesters for a community, small farm, or even home. Biodigesters yield two products that are extremely useful for the home and garden—high-nitrogen compost and flammable gas. 

Biodigesters anaerobically (without air) break down organic matter in a slurry held in a tank. The nitrogen remains in the composted slurry as ammonia, a vital plant nutrient. The flammable gas produced by biodigesters is about two-thirds methane and one-third carbon dioxide—very similar to natural gas—making it a good cooking fuel. Cooking requires intense direct application of heat on demand, and renewable options for accomplishing this are limited. Solar energy is dispersed and not consistently available, making solar cooking challenging, and burning wood contributes to particulate pollution and further depletes diminishing resources in the developing world. Cooking is not a huge consumer of energy in the industrialized world, but doing it more sustainably is challenging. Unlike cooking with solar electricity, biodigesters can be assembled with readily available materials by a handy homeowner. Any type of propane or natural gas stove will run on biogas. For maximum efficiency, propane stoves will require a larger air inlet. 

Inside a Biodigester

A biodigester is a sophisticated way to harvest fuel from the complex carbon chains of organic matter—energy collected by plants from the sun as they grow—without combusting them directly. Direct combustion of carbon causes air pollution, a loss of much of the nutrient value of the biomass, and a poor energy harvest—especially when used for cooking or lighting, as most of it goes up in smoke. Burning wood, even in an EPA-certified woodstove, can produce more than 500 times the fine particle emissions of burning natural gas. 

As new plant material is fed into the digester, it is first attacked by acidogenic bacteria, which break the chains holding together some of the more complex plant matter, especially cellulose—the structural backbone of most plants. Ammonia and acetates (mostly acid) are produced, lowering the pH and using up any oxygen in the process. Acetates are the perfect food for methanogenic bacteria, as long as the slurry they reside in is not too acidic and all oxygen has been removed. They consume the acetates and produce methane (CH4) and carbon dioxide (CO2), along with a lesser amount of other gases and residues depending on the original feedstock.

Comments (11)

Warren Weisman's picture

If you would be seriously interested in a portable, heavy-duty poly model of these home biogas digesters available Spring 2014, please complete the brief contact form on our website here...
http://hestiahomebiogas.com/home-bi...

stevemac's picture

Enjoyed article very much. Read about using a 1 kW Honda EU1000i generator for a short amount of time(dvd's). Any info for using this generator for longer periods of time possibly for heating or fridge with a 700 gallon digester? Have 1 1/2 acres plus work for a grade-school that waste unbelievable amount of food that I believe is ample to supply this digester. All the best.

Michael Welch's picture

Hi there. That is a super-lightweight, portable generator. While I do not know much about it other than that, I seriously doubt if it is up to snuff as anything other than an occasional-use, emergency type generator. But that is just my gut level feeling, I have no experience with it. I suggest checking out its warranty, and doing an online search to see what other users say about it.

Thomas Henry Culhane's picture

Lovin' it Warren, just lovin' it! Your answer about safety was also great; I've been asked the same questions endlessly, particular as we were building digesters this week at the US Embassy compounds and UN compounds in Baghdad and Erbil in Kurdistan; naturally folks want to know if it is a security risk (they are banning regular compressed gas bottles in case of rocket fire from insurgents) so I take pains to explain why biogas systems of this size are actually the safest thing we can have for reliable energy while dealing with wastes. Glad to be on the global team of biogas apostles with you, brother!

Rayzer's picture

How about safety? If you were to construct a biodigester for your home, are they safe? Would your neighbors have a legitimate complaint about the safety of one of these? My next question is about the smell. Would your neighbors be complaining about the smell of a biodigester? This is very intriguing.

Warren Weisman's picture

Wow, didn't realize this article was online. This was SUCH A GREAT ARTICLE! I'm the guy who built the digester featured. Safety is not major concern, when full like the pictures there's only the equivalent of 1 liter of gasoline worth of BTUs in there. So, it's not going to blow anything up. Odor is a non-issue. If it's maintained correctly there's only a slight odor when loading that is contained within the lid. Hundreds of people walk past this one and have no idea what it is. The liquid byproduct when it comes out has a neutral, earthy smell.

We didn't have any problem with the City of Eugene with the digester, their concern was with using the imported Chinese stovetop indoors, since it wasn't UL certified. So, that's the regulatory hold-up now.

Michael Welch's picture

Thanks for the kind words and response, Warren. Keep us updated on the stove top.
- Michael, Home Power

Warren Weisman's picture

I thought Stephen and that photographer did AN AMAZING job! The layout in the print edition was just awesome. My biogas colleague Prof. Thomas Culhane, Ph.D. who also posted a comment here said Home Power is one of his favorite magazines, it was my first exposure to it. Great magazine.

Ben Root's picture

I think this project is really intriguing. But there's no way my 2-person household will come up with that much compost/waste per day.
I wonder if it would be feasible to do this on an even smaller scale, or if the amount of gas produced would be moot.
Or, if we could mooch compost from friends, neighbors, restaurants, horse ranches, etc. One man's dung....

Warren Weisman's picture

Ben, biogas is scalable all the way down to a soda bottle if needed. Many countries around the world use a 1 cubic meter unit or one made from 55-gallon drums, I went with the two cubic meter (700 gallon unit) because most American households are going to want to run a generator for a little while in addition to cooking. The 70 cubic feet of gas per day allows 3 meals per day and probably 2-3 movies on a TV/DVD set-up with a 1 kW Honda EU1000i generator.

If you feed it less there will be less gas production, but most people will be pleasantly surprised how little waste it really takes once it has a healthy culture going in it. The biodigester in the article has been functioning just fine for going on 1-1/2 years now.

Ian Woofenden's picture

A sustainable learning center I work with in Costa Rica does buy cow manure to supplement the kitchen waste and humanure that their facility generates. They are committed to reducing or eliminating their use of fossil fuels, and their purchase of local manure helps a local family generate some income while their kitchen gets more cooking gas!

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