In nature, anaerobic decay is probably one of the earth's oldest processes for decomposing wastes. Organic material covered by a pool of warm water will first turn acid and smell rank, then slowly over about six months will turn alkali. The methane bacteria, always present, will take over and decompose it, and gas bubbles will rise to the surface.
Anaerobic decay is one of the few natural processes that hasn't been fully exploited until recent times. Pasteur once discussed the possibilities of methane production from farmyard manure. And (according to a report issued from China, April 26, 1960) the Chinese have used "covered lagoons" to supply methane fuel to communes and factories for decades. But the first attempt to build a digester to produce methane gas from organic wastes (cow dung) appears to have been in Bombay, India in 1900. At about this time, sewage plants started digesting sewage sludge in order to improve its quality. This started a mass of laboratory and small-scale experiments during the 20's and 30's (many of them summarized by Acharya, Ref. 3).
During World War II, the shortage of fuel in Germany led to the development of methane plants in rural areas, where the gas was used to power tractors. The idea spread into Western Europe, until fossil fuels once again became available (although, today, many farmers in France and Germany continue to use home digesters to produce their own methane fuel gas).
Currently the focus of organic digester/bio-gas research is in India. India's impetus has been the overwhelming need of a developing country to raise the standard of living of the rural poor. Cows in India produce over 800 million tons of manure per year; over half of this is burned for fuel and thus lost as a much needed crop fertilizer. (Ref. 4) The problem of how to obtain cheap fuel and fertilizer at a local level led to several studies by the Indian Agricultural Research Institute in the 1940's to determine the basic chemistry of anaerobic decay. In the 1950's, simple digester models were developed which were suitable for village homes. These early models established clearly that bio-gas plants could:
provide light and heat in rural villages, eliminating the need to import fuel, to burn cow dung, or to deforest land;
provide a rich fertilizer from the digested wastes; and
improve health conditions by providing air-tight digester containers, thus reducing disease borne by exposed dung.
More ambitious designs were tested by the Planning Research and Action Institute in the late 1950's. Successes led to the start of the Gobar Gas Research Station at Ajitmal where, with practical experience from the Khadi and Village Industries Commission, two important pamphlets (Ref. 5, 6) were published on the design of village and homestead "bio-gas" plants in India.
In America, where the problem is waste disposal, rather than waste use, organic digesters have been limited to sewage treatment plants. (Ref. 7, 8.) In some cases sludge is recycled on land or sold as fertilizer (Ref. 9, 10), and methane gas is used to power generators and pumps in the treatment plants (Ref. 11). The Hyperion sewage treatment plant in Los Angeles generates enough methane from its primary treatment alone to power its 24-2,000 hp. diesel engines. Usually, however, both sludge and gas are still regarded as waste problems.
Much information on digestion and small-scale digester operations comes from experiences in India, Western Europe and South Africa and journals such as: Compost Science, Water Sewage Work, Soils and Fertilizer, Waste Engineering, Sewage and Industrial Wastes and recent publications of the U.S. Environmental Protection Agency and Solid Waste Conferences (see Bibliography at end). An excellent book to learn from is called: Manual of Instruction for Sewage Plant Operators, put out by the New York State Dept. of Health and available from the Health Education Service, P.O. Box 7283, Albany, New York 12224.
A great deal of information can be found in pre-WW II sewage journals, especially Sewage Works Journal. After WW II, as with most other kinds of science and technology, waste treatment research became a victim of the trend to make machines ever bigger, and information increasingly incomprehensible.
3 Biology of Digestion
Bio-Succession in the Digester
Perhaps the most important thing to remember is that digestion is a biological process.
The "anaerobic" bacteria responsible for digestion can't survive with even the slightest trace of oxygen. So, because of the oxygen in the manure mixture fed to the digester, there is a long period after loading before actual digestion takes place. During this initial "aerobic" period, traces of oxygen are used up by oxygen-loving bacteria, and large amounts of carbon dioxide (C02) are released.
When oxygen disappears, the digestion process can begin. That process involves a series of reactions by several kinds of anaerobic bacteria feeding on the raw organic matter. As different kinds of these bacteria become active, the by-products of the first kind of bacteria provide the food for the other kind (Fig. 6). In the first stages of digestion, organic material which is digestible (fats, proteins and most starches) are broken down by acid producing bacteria into simple compounds. The acid bacteria are capable of rapid reproduction and are not very sensitive to changes in their environment. Their role is to excrete enzymes, liquefy the raw materials and convert the complex materials into simpler substances (especially volatile acids, which are low molecular weight organic acids -- See 4 Raw Materials). The most important volatile acid is acetic acid (table vinegar is dilute acetic acid), a very common by-product of all fat, starch and protein digestion. About 70% of the methane produced during fermentation comes from acetic acid (Ref. 12).
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