Suitability of climatic zones
Tropical Rain Forest: annual rainfall above 1.500 mm, mean temperatures between 24 and
28°C with little seasonal variation. Climatically very suitable for biogas production. Often
animal husbandry is hampered by diseases like trypanosomiasis, leading to the virtual
absence of substrate.
Tropical Highlands: rainfall between 1.000 and 2.000 mm, mean temperatures between 18
and 25°C (according to elevation). Climatically suitable, often agricultural systems highly
suitable for biogas production (mixed farming, zero-grazing).
Wet Savanna: rainfall between 800 and 1.500 mm, moderate seasonal changes in
temperature. Mixed farming with night stables and day grazing favor biogas dissemination.
Dry Savanna: Seasonal water scarcity, seasonal changes in temperatures. Pastoral
systems of animal husbandry, therefore little availability of dung. Use of biogas possible near
permanent water sources or on irrigated, integrated farms.
Thornbush Steppe and Desert: Permanent scarcity of water. Considerable seasonal
variations in temperature. Extremely mobile forms of animal keeping (nomadism). Unsuitable
for biogas dissemination.
Firewood consumption and soil erosion
A unique feature of biogas technology is that it simultaneously reduces the need for firewood
and improves soil fertilization, thus substantially reducing the threat of soil erosion. Firewood
consumption in rural households is one of the major factors contributing to deforestation in
developing countries. Most firewood is not acquired by actually cutting down trees, but rather
by cutting off individual branches, so that the tree need not necessarily suffers permanent
damage. Nonetheless, large amounts of firewood are also obtained by way of illegal felling.
In years past, the consumption of firewood has steadily increased and will continue to do so
as the population expands - unless adequate alternative sources of energy are developed. In
many developing countries such as India, the gathering of firewood is, strictly speaking, a
form of wasteful exploitation. Rapid deforestation due to increasing wood consumption
contributes heavily to the acceleration of soil erosion. This goes hand in hand with
overgrazing which can cause irreparable damage to soils. In the future, investments aimed at
soil preservation must be afforded a much higher priority than in the past. It will be
particularly necessary to enforce extensive reforestation.
Soil protection and reforestation
The widespread production and utilization of biogas is expected to make a substantial
contribution to soil protection and amelioration. First, biogas could increasingly replace
firewood as a source of energy. Second, biogas systems yield more and better fertilizer. As a
result, more fodder becomes available for domestic animals. This, in turn, can lessen the
danger of soil erosion attributable to overgrazing. According to the ICAR paper (report issued
by the Indian Council of Agricultural Research, New Delhi), a single biogas system with a
volume of 100 cft (2,8 m3) can save as much as 0.3 acres (0,12 ha) woodland each year.
Taking India as an example, and assuming a biogas production rate of 0.36 m3/day per
livestock unit, some 300 million head of cattle would be required to produce enough biogas
to cover the present consumption of firewood. This figure is somewhat in excess of the
present cattle stock. If, however, only the amount of firewood normally obtained by way of
deforestation (25.2 million trees per year) were to be replaced by biogas, the dung
requirement could be satisfied by 55 million cattle. Firewood consumption could be reduced
to such an extent that - at least under the prevailing conditions - a gradual regeneration of
India's forests would be possible.
According to empirical data gathered in India, the consumption of firewood in rural
households equipped with a biogas system is much lower than before, but has not been fully
eradicated. This is chiefly attributable to a number of technical and operational short-
comings. At present,
• many biogas systems are too small to handle the available supply of substrate;
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