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.
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.
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