The carbon(C)/nitrogen(N)-ratio of animal and human excrements is normally favorable for the
purposes of anaerobic fermentation (9 - 25:1), while that of plant material usually indicates an
excessive carbon content.
In many cases, various substrates should be mixed together in order to ensure a favorable gas yield
while stabilizing the fermentation process and promoting gas production. The following formulae
can be used to calculate the C/N-ratio and total-solids content of a given mixture:
MC/N = [(C/N1 x Wl) + (C/N2 x W2) + . . . + (C/Nn x Wn)]/(W1 + W2 + . . . + Wn)
MTS = [(TSI x Wl) + (TS2 x W2) + . . . + (TSn x Wn)]/(W1 +W2 + ... + Wn)
MC/N = C/N-ratio of mixed substrate, MTS = TS-content of mixed substrate, C/N = C/N-ratio of
individual substrate, W = weight of individual substrate, TS = TS-content of fresh material.
3.3 Agricultural/operational prerequisites and stock-farming requirements
In order to fulfill the prerequisites for successful installation and operation of a biogas plant, the
small farm in question must meet three basic requirements regarding its agricultural production
system:
- availability of sufficient biomass near the biogas plant,
- use for digested slurry as fertilizer,
- practical use(s) for the biogas yield.
Farms marked by a good balance between animal husbandry and crop farming offer good
prerequisites for a biogas tie-in. Unfortunately, however, such farms are rare in tropical countries. In
numerous Third World countries, animal husbandry and stock farming are kept separate by
tradition.
As the world population continues to grow, and arable land becomes increasingly scarce as a
result, the available acreage must be used more intensively. In wet savannas, for example, the
fallow periods are being shortened, even though they are important for maintaining soil fertility. In
order to effectively counter extractive agriculture, animal husbandry must be integrated into the crop
farming system, not least for its fertilizing effect. On the other hand, systematic manuring is only
possible as long as collectible dung is allowed to accumulate via part-time or full-time stabling.
The installation of a biogas plant can be regarded as worthwhile, if at least 20-40 kg manure per
day is available as substrate. This requires keeping at least 3 - 5 head of cattle, 8-12 pigs or 16-20
sheep/goats in a round-the-clock stabling arrangement. The achievable gas yield suffices as
cooking fuel for a family of 4-6 persons. That, in turn, means that the farm must be at least about 3
hectares in size, unless either freely accessible pastures are available or extra fodder is procured.
Crop residue like rice straw, sorghum straw, cornstalks, banana stalks, etc. should be chopped up,
partially composted and mixed with animal excrements for use in the fermentation process (cf.
chapter 3.2).
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