|Biogas Plants in Animal Husbandry (GTZ, 1989)|
|5. Biogas technique|
The technical conception of biogas plants is determined by the aim of achieving optimal parameters for the biological process (cf. chapter 5.1).
That being so, the following operating requirements/limitations must be given due consideration:
- type and composition of organic material, which determines the choice of process
- given demand for biogas and fertilizer, in addition to the available substrate quantities, which determines the size of the biogas plant
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.
The advantage of continuous-feed plants is that the bacteria receive a regular supply of substrate and are therefore able to generate a more constant supply of biogas. The problem is that buoyant constituents tend to form a stiff layer of scum that impedes biogas production and may even plug up the plant. That drawback can be countered by installing suitable agitators and lengthening the retention time.
The digester inlet, outlet and, to the extent applicable, the agitator must be designed to work together in ensuring the proper retention time, i.e. to avoid short-circuit flow, because the gas production rate would otherwise stay well below the optimum level.
Continuous-feed biogas plants are sized on the basis of the desired retention time for the organic material, in combination with the digester load, which in turn is a function of the prevailing temperature and type of substrate (cf. chapter 4.3).