Fig. 5.3: The batch-feed principle (1) vs the
continuous feed principle (2)
(Source: OEKOTOP)
Fig. 5.4: The fermentation channel (1) vs the
complete -mixed digester (2) (Source: OEKOTOP)
Fig. 5.5: Slurry flow for various configuration of feed,
discharge and stirring. 1 Low inlet, outlet at top (beside the
gasholder); 2 High inlet, low outlet (normal); 3 Low inlet,
low outlet (with partition wall); 4 Vertical agitator; 5 Fixed-
dome plant; F: Quality factor for thorough mixing and
favorable throughflow conditions, normal situation = 100%
(Source: OEKOTOP)
The range of simple biogas plants includes the following basic types:
Batch-type plants are thus referred to because they are charged with successive batches of organic
material and a certain amount of seeding slurry to serve as starter. The digestion process is
interrupted as soon as the rate of biogas production has slowed down to the point that continued
digestion would be uneconomical. Then, the plant is cleaned out and refilled. To achieve a more or
less uniform rate of biogas production, several digesters must be operated in parallel, i.e. filled at
staggered intervals. Differentiation is made between semi-dry plants (operating on a total-solids
content of more than 15%) and liquid plants.
Batch plants are suitable for digesting strawy, fibrous material with a high solids content, usually in
areas with low annual precipitation, and for use as simple demonstration plants.
Continuous-feed plants are those in which there is a continuous throughflow of biomass, resulting in
a near-constant volume of slurry in the digester. In practice, such plants are fed once or twice each
day. There are three main sub-versions:
- complete-mixed digesters
- fermentation channels and
- combinations of the two.
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