Table 3.1: Climatic zones and their suitability for biogas plants (Source: OEKOTOP)
Climatic zone
Tropical rain
forest
Wet savanna
Dry savanna
Thornbush
steppe
Dry hot desert
Factors of relevance for biogas generation
Annual rainfall > 1500 mm;unfavorable
As biogas zone:
temperature fairly constant at 25-28 °C; little animal
husbandry due to various diseases, i.e. scarcity of dung;
vegetable waste from permacultures and gardening
Water usually available all year (rainfall: 800-1500 mm),
livestock farming on the increase, integral farms (crop
farming + livestock)
Short rainy season, long dry season; most livestock
pastured, but some integral farming
Short rainy season (rainfall: 200400 mm) extensive-type
pasturing (nomads, cattle farmers), dung uncollectable;
shortage of water
---
favorable
possible
unsuitable
unsuitable
Soil conditions
Since the digesters of simple biogas plants are situated underground, the temperature of the soil is
of decisive importance. It depends on the surface structure, the type of soil and the water content.
The soil temperature usually varies less than the air temperature, e.g. tropical soils show nearly
constant temperature at a depth of 30-60 cm. Due to lower absorption, the temperature amplitude of
light soils is smaller than of dark soils. Since moist soil appears darker than dry soil, the same
applies with regard to temperature amplitude. As a rule of thumb, the region's mean annual
temperature may be taken as the soil temperature in tropical areas.
For biogas plants with unlined digesters and/ or underground masonry, it is important to know the
stability of the soil structure. The stability of a given soil increases along with the bedding density.
Natural soils are generally stable enough for biogas plants. Caution is called for, however, in the
case of alluvial and wet, silty soils. Most of the laterite soil prevailing in the tropics shows high
structural stability and is therefore quite suitable for biogas plants with unlined digesters. Unlined
earth pits usually become more or less impermeable within a short time, but preparatory seepage
trials should be conducted in exploratory holes, just to make sure. Previous experience has shown
that seepage can drop to below 5% of the initial rate within a week. In the case of large-scale
biogas plants, it is always advisable to have an expert check the soil stability.
Biogas plants should never be located in groundwater, areas subject to flooding, or near wells. On
the other hand, an adequate supply of water must be available in the immediate vicinity of the
biogas plant, because the substrate must be diluted. If the direction of groundwater flow is known,
the biogas plant should be placed downstream of the well.
3.2 Suitable types of biomass and their characteristics
Practically any kind of watery organic substance is suitable for anaerobic digestion. The agricultural
residues and waste materials that can be used as substrate for biogas plants consist chiefly of:
- waste from animal husbandry, e.g. dung, urine, fodder residue and manure, .
- vegetable waste, e.g. straw, grass, garden residue, etc. (though such materials do not ferment
well alone),
- household waste like night soil, garbage, wastewater, etc.
16