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close this bookBiogas Plants (GTZ, 1988)
close this folder2. The dgestion process
View the document(introduction...)
View the document2.1 The fermentation slurry
View the document2.2 Fermentation slurry as fertilizer
View the document2.3 Biogas

2.1 The fermentation slurry

All feed materials consist of

- organic solids,
- inorganic solids,
- water.

The biogas is formed by digestion of the organic substances. The inorganic materials (minerals and metals) are unused ballast, which is unaffected by the digestion process.

Adding water or urine gives the substrate fluid properties. This is important for the operation of a biogas plant. It is easier for the methane bacteria to come into contact with feed material which is still fresh when the slurry is liquid. This accelerates the digestion process. Regular stirring thus speeds up the gas production.

Slurry with a solids content of 5-10% is particularly well suited to the operation of continuous biogas plants.


Fresh cattle manure is made up of 16 % solids and 84% water. The cattle dung is mixed with water in the proportions of 1:1. The prepared fermentation slurry then has a solids content of 8% and a water content of 92%.

All feed materials consist to a great extent of carbon (C) and also contain nitrogen (N). The C/N ratio affects gas production. C/N ratios of 20:1 to 30:1 are particularly favourable. Mixtures of nitrogen-rich feed material (e.g., poultry manure) and carbon-rich feed material (e.g., rice husks) give high gas production.

If there is any suspicion that the digestion process is impaired by pollutants (Fig. 2), water or "clean" feed material must be mixed in. This reduces the concentration of toxic substances.

(Table 1) Fig. 2: Feed material tables Straw, leaves and, in particular, water hyacinths can be digested only in certain types of plants or using special conditioning techniques. For this reason, reliable information of general validity concerning gas production cannot be given. *Intense surface scum formation