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  3. Biogas: a Solution to Many Problems In almost all developing countries, the lack of adequate supplies of cheap, convenient a

Biogas: a Solution to Many Problems In almost all developing countries, the lack of adequate supplies of cheap, convenient a

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问题                 Biogas: a Solution to Many Problems
    In almost all developing countries, the lack of adequate supplies of cheap, convenient and reliable fuel is a major problem. Rural communities depend largely on kerosene, wood and dung for their cooking and lighting needs. But kerosene is now priced out of reach of many people and wood, except in heavily forested areas, is in short supply. The search for firewood occupies a large part of the working day and has resulted in widespread deforestation.
    Dung is in constant supply wherever there are farm animals and, when dried, it is convenient to store and use. But burning dung destroys its value as fertilizer, thus depriving the soil of a much needed source of humus and nitrogen.
    Rural areas of developing countries are also plagued by a lack of adequate sanitation. Improper waste disposal spreads disease, contaminates water sources and provides breeding grounds for disease-carrying insect.
    The problems of improving environmental hygiene, conserving resources and finding alternative sources of fuel may be unrelated. Their solutions, however, are not, as many countries experimenting with biogas technology are discovering. Biogas, a mixture of methane and carbon dioxide, is produced by the fermentation of organic matter. The process of anaerobic fermentation is a natural one occurring whenever living matter decomposes. By containing the matter—and the process—in a digester or biogas plant, the combustible gas can be trapped and used as fuel for household lighting and cooking. The digested slurry that remains can be used on the land as a soil conditioner and fertilizer.
    Biogas plants have attracted much interest in recent years and they are in use in several Asian countries: 36,000 are reported in rural areas of India, 27,000 in Korea and more than 80,000 in China. In most countries the value of the gas has been the prime factor leading to their adoption: 70 per cent of India’s plants, for instance, were built during the energy and fertilizer crisis of 1975-1976—although their use in that country dates back to 1951. Similarly in Thailand and Korea, biogas is being investigated as an alternative to costly charcoal and to save compost materials from being burned.
    In Japan and China, reducing pollution from animal wastes has been an important factor. Privies, hen houses and pigpens are built in proximity to the fermentation chamber in China. Examinations of the digested slurry have shown that the total number of parasite eggs was reduced by 93. 6 per cent, hookworms by 99 per cent and no schistosome flukes were found.
    The greatest benefits from biogas systems, however, are probably to be derived from the ma-nurial value of the slurry, although it is not widely used outside of India and China. Vegetable farmers near Calcutta found that the digested slurry produced bigger and better tasting peas than did other fertilizers and the weight of root vegetables increased by nearly 300 per cent.
    The production of biogas by fermentation of animal and vegetable wastes is a technology that has been largely developed and used in the【R1】______countries. Only very recently have scientists in the industrialized nations begun to show an interest—presumably because of the "energy【R2】______".
    Family-sized-biogas【R3】______first came into widespread use in India in the 1950s in an effort to make a cleaner and more efficient use of cattle dung. The programme really expanded in the 1970s, and today there are as many as 100,000 plants throughout the world. Most are in domestic use to provide fuel for plants, but some larger units are operated in order to recycle wastes, supply fertilizer, control pollution and improve【R4】______One Chinese study has shown that digestion of animal【R5】______in the airtight digesters greatly reduces health hazards from parasitic diseases. One Indian study has estimated that the value of the fertilizer obtained is in itself greater than the cost of producing the biogas. Thus, the system is economically sound, in addition to other benefits such as a cleaner, healthier environment.
【R5】
选项
答案waste(s)／slurry
解析 (倒数第二段，Examinations of the digested slurry have shown that the total number of parasite eggs was reduced。即检查沼池中已分解的粪便，发现寄生虫卵的数量减少，即降低了患寄生虫疾病的风险。)
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