Figure 13: Concrete digester with integrated plastic gas-holder
Photo: TBW
Figure 14: Steelvessel fermenter with seperate ballon gas-holder
Photo: TBW
Selection of appropriate design
In developing countries, the design selection is determined largely be the prevailing design in
the region, which, in turn takes the climatic, economic and substrate specific conditions into
consideration. Large plants are designed on a case-to-case basis.
Typical design criteria are:
Space: determines mainly the decision if the fermenter is above-ground or underground, if it
is to be constructed as an upright cylinder or as a horizontal plant.
Existing structures may be used like a liquid manure tank, an empty hall or a steel
container. To reduce costs, the planner may need to adjust the design to theses existing
structures.
Minimizing costs can be an important design parameter, especially when the monetary
benefits are expected to be low. In this case a flexible cover of the digester is usually the
cheapest solution. Minimizing costs is often opposed to maximizing gas yield.
Available substrate determines not only the size and shape of mixing pit but the digester
volume (retention time!), the heating and agitation devices. Agitation through gas injection is
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