About five years ago, writer and renewable energy aficionado Warren Weisman 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.
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.
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.