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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


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 days),
- 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.

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

2.2 Fermentation slurry as fertilizer

During the digestion process, gaseous nitrogen (N) is converted to ammonia (NH3). In this water-soluble form the nitrogen is available to the plants as a nutrient. A particularly nutrient-rich fertilizer is obtained if not only dung but also urine is digested.

Compared with solid sludge from fermented straw and grass, the liquid slurry is rich in nitrogen and potassium. The solid fermentation sludge, on the other hand, is relatively richer in phosphorus. A mixture of solid and liquid fermented material gives the best yields. The nutrient ratio is then approximately N:P2O5:K2O= 1:0.5:1. A fermented slurry with a lower C/N ratio has better fertilizing characteristics. Compared with fresh manure, increases in yield of 5 - 15 % are possible. Particularly good harvests are obtained from the combined use of compost and fermentation slurry.

The fertilization effect depends on the type of crop and on the soil. Information given in specialized literature is seldom applicable directly. Tests of one's own are always better. Reliable information is possible only after three to five years.

When fermentation slurry is used as fertilizer for years, the soil structure is improved. The proportion of organic materials in the soil is increased, enabling the soil to store more water.

If fermentation slurry is to be stored before spreading on the field, it should be covered with earth in layers. This reduces evaporative nitrogen losses even further.

2.3 Biogas

Biogas is somewhat lighter than air and has an ignition temperature of approximately 700 °C (diesel oil 350 °C; petrol and propane about 500 °C). The temperature of the flame is 870 °C.

Biogas consists of about 60 % methane (CH4) and 40 % carbon dioxide (CO2). It also contains small proportions of other substances, including up to 1% hydrogen sulphide (H2S). See also the table in Fig. 38 on page 55.

The methane content and hence the calorific value is higher the longer the digestion process. The methane content falls to as little as 50% if retention time is short. If the methane content is considerably below 50 %, biogas is no longer combustile. The first gas from a newly filled biogas plant contains too little methane. The gas formed in the first three to five days must therefore be discharged unused.

The methane content depends on the digestion temperature. Low digestion temperatures give high methane content, but less gas is then produced.

The methane content depends on the feed material. Some typical values are as follows:

Cattle manure


Poultry manure


Pig manure


Farmyard manure








Kitchen waste




Water hyacinths