Methane digesters are systems that use anaerobes to produce methane through fermentation. Methane is a main constituent of natural gas and can be readily substituted for that nonrenewable resource.
The anaerobes used in methane digesters are methanogenes, bacteria belonging to the genera Methanobacterium ; Methanosarcina ; and Methanoccus. They can be found in the gastrointestinal tracts of animals such as cows and other ruminants, as well as in soil , water, and sewage. In septic tanks, bacteria liquefy some of the organic matter; which releases energy for the bacteria and by products such as methane and carbon dioxide .
Methane digesters are also known as biogass digesters and organic digesters. The central portion is an airtight drum, called a digester unit, which contains the methanogenes. Raw material is place into the drum, and the unit is kept at a constant temperature of about 95°F (35°C). Some of the methane produced by the digester heats the water. Outlets on the digester unit take away the various products of the system. Liquid and solid fertilizers are collected to be used for crops and other plants. Methane is stored in a tank, from which it can be drawn off for fuel for a variety of purposes. Carbon dioxide and hydrogen sulfide can be filtered out of the methane and put under pressure for use in turning turbines.
Common household organic wastes can be put into a digester, and the methane produced can be used to make electricity. It can also be used for cooking, illumination, heating, and automobile fuel. One system at the University of Maine produced over $8,000 worth of power a year in the early 1990s; the digester also produced a sludge that could be used as a nutritious and relatively odorless plant fertilizer . Some sludges, however can contain high level of metals if the original material is unsorted municipal waste.
Methane is a clean and nontoxic automobile fuel, and it produces no pollutants when burned. It has an octane number of 130. Italy has used it as a motor fuel for over 40 years, and Modesto, California, has a small fleet of methane-powered cars. Because of its cleanliness, it extends the life of engines as well as making starting easier.
The organic matter this system converts into other uses would otherwise have decomposed in a landfill , leaching into the surrounding environment and contaminating groundwater supplies. In a digester, it becomes a useful resource.
[Nikola Vrtis ]
Trans, W. B. "Just Plug It Into That Cherry Tomato Over There." Sierra 75 (May-June 1990): 20–21.