Consider, for example, the planning of a GTZ project in Cote d'Ivoire. The project included the
development of a model farm intended to exploit as efficiently as possible the natural resources soil,
water, solar energy and airborne nitrogen.
The integral agricultural system "Eco-ferme" (ecofarm) comprises the production elements
gardening, crop farming (for food and animal fodder), stock farming (for meat and milk) and a
fishpond. A central component of such closed-loop agricultural production is the biogas plant, which
produces both household energy and digested slurry for use in the fishpond and as a fertilizer.
The average family-size "eco-ferme" has 3 ha of farmland with the following crops:
Fodder plants
Panicum (for the rainy season)
Sugar cane (for the dry season)
Leucaena and brachiaria (mixed culture)
Panicum, brachiaria and centrosema (mixed culture)
Food plants
Manioc
Corn
Yams
Potatoes - beans
Vegetables
Rice and miscellaneous crops
0.15 ha
0.50 ha
0.50 ha
0.50 ha
0.20 ha
0.40 ha
0.10 ha
0.10 ha
0.20 ha
0.17 ha
Four milk cows and three calves are kept year-round in stables. The cattle dung flows via collecting
channels directly into a 13 m³ biogas plant. The biogas plant produces 3.5-4 m³ biogas daily for
cooking and lighting. Part of the digested slurry is allowed to flow down the natural gradient into an
800 m² fishpond in order to promote the growth of algae, which serves as fish food. The remaining
digested slurry is used as crop fertilizer.
4. Balancing the energy demand with the biogas production
All extension-service advice concerning agricultural biogas plants must begin with an estimation of
the quantitative and qualitative energy requirements of the interested party. Then, the biogas-
generating potential must be calculated on the basis of the given biomass incidence and compared
to the energy demand. Both the energy demand and the gas-generating potential, however, are
variables that cannot be very accurately determined in the planning phase.
In the case of a family-size biogas plant intended primarily as a source of energy, implementation
should only be recommended, if the plant can be expected to cover the calculated energy demand.
Since determination of the biogas production volume depends in part on the size of' the biogas
plant, that aspect is included in this chapter.
29