|Biogas Plants (GTZ, 1988)|
|2. The dgestion process|
Biogas is produced by putrefactive bacteria, which break down organic material under airless conditions. This process is called "anaerobic digestion".
The digestion process consists of two main phases:
- acid formation,
- methane formation.
In the first phase, protein, carbohydrate and fat give rise to fatty acids, amino acids and alcohols. Methane, carbon dioxide and ammonia form in the second phase. The slurry becomes somewhat thinner during the process of digestion.
The better the two phases merge into each other, the shorter the digestion process. The conditions for this are particularly favourable in the "fermentation channel" arrangement (Fig. 27,b).
The following types of digestion are distinguished according to the temperature in the digester:
- psychrophilic digestion (10-20 °C, retention time over 100
- mesophilic digestion (20-35 °C, retention time over 20 days),
- thermophilic digestion (50-60 °C, retention time over 8 days).
Thermophilic digestion is not an option for simple plants.
The pH of the fermentation slurry indicates whether the digestion process is proceeding without disturbance. The pH should be about 7. This means that the slurry should be neither alkaline nor acid.
Biogas can in principle be obtained from any organic material. Cattle manure can be used as a "starter". Feed material containing lingnin, such as straw, should be precomposted and preferably chopped before digestion. More than ten days' preliminary rotting is best for water hyacinths. Gas production is substantially improved if the preliminary rotting time is twenty days.