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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.
In consuming these acids, the methanogenic bacteria raise the pH and keep it hospitable for both the acid-formers and themselves, both of which would perish if the pH dropped too low. The high-nitrogen ammonia (a byproduct of the breakdown of plant proteins) remains dissolved in the slurry, unlike in aerobic composting where it is released as a gas. Although both the acid-formers and the methanogens can suffer from rapid changes in living conditions created by the addition of feedstock, the methanogens are especially vulnerable to low pH and the introduction of too much oxygen. For this reason, biodigesters generally work on the principle of steady applications of new feedstock in regular intervals, rather than adding large amounts of biomass at once.
There are several different types of methanogenic bacteria that will colonize a digester, depending upon the slurry’s temperature range. The two ranges of interest to home-scale biodigesters are the cryophilic (50°F to 80°F) and the mesophilic (95°F to 125°F). There is a dead zone between these two temperature ranges that must be avoided. Warren’s biodigester operates in the cryophilic range. While mesophilic methanogens can break down material several times as quickly as their cryophilic counterparts, consistently maintaining high temperatures consumes a great deal of energy—which can make a net energy loss for smaller biodigesters.
The released CO2 and CH4 gases percolate through the slurry to the top of the tank. Once enough biogas accumulates, the pressure created inside the expanding rubber top reaches 0.25 to 0.5 psi, enough to move the gas through the delivery pipes and use it for cooking or lighting. As new material is added to the tank through the inlet, it displaces an equal amount of slurry through the outlet, which can be applied directly to the garden. Once applied, covering the slurry with soil helps keep the ammonia from turning to gas and losing its coveted nitrogen.
Operating the Digester
Warren’s biodigester at Maitreya primarily uses kitchen waste once in operation. Getting it up and running, however, requires a whole mess of ruminant manure—about 300 pounds’ worth. There happened to be an alpaca farm nearby, and so he used alpaca manure, but any ruminant manure will be loaded with plenty of methanogenic bacteria. He also added a few gallons of kitchen compost and tree leaves, and then filled the tank to 600 gallons.
It takes a minimum of two to three days before a biodigester begins to produce gas, since the acid-forming bacteria need to do their work before the methane-producing bacteria can go to work. About 10 to 15 pounds of kitchen scraps are collected and added to the biodigester daily, producing an average of about 70 cubic feet of biogas, which is the only source of cooking fuel for the community’s kitchen. When the methane content begins to get low, the flame will begin to burn orange. This is remedied by feeding the digester.
The scraps are shoved into the digester with a pole, and this slight agitation helps mix the slurry, exposing the material to the bacteria so it can be thoroughly composted. The tank can also be topped off with water at this time if the slurry level is low. As new feedstock is added, it displaces an equal volume of composted slurry through the outlet, which is captured in a 5-gallon bucket and then added to nearby gardens. Biodigesters prefer a carbon-to-nitrogen ratio similar to a conventional aerobic compost pile, with about 25:1 being ideal. Too much carbon-rich material (grass clippings, newspaper, etc.) will slow the digester. Manure is the most common nitrogenous material to add to rebalance a slow digester.
The biodigester at Maitreya has been operating for more than a year with consistent results. Warren and Hestia Biogas are in the final stages of getting city permits for new biodigesters, to bring an official stamp of approval to a renewable energy technology with great promise for any homestead.
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Author and builder Stephen Hren lives in Durham, North Carolina. His latest book is Tales from the Sustainable Underground: A Wild Journey with People Who Care More About the Planet Than the Law.
Hestia Home Biogas • www.hestiahomebiogas.com
Biodigesters in China • www.bit.ly/ChinaBiogas
Methane: Planning a Digester by Peter-John Meynell