4.1 Definitions

To calculate the scale of a biogas plant, certain characteristic parameters are used. These are as follows for simple biogas plants:

- Daily fermentation slurry arisings (Sd),
- Retention time (RT),
- Specific gas production per day (Gd), which depends on the retention time and the feed material.

The following additional concepts and parameters are also used in the theoretical literature:

- Dry matter (DM). The water content of natural feed materials varies. For this reason the solids or dry matter content of the feed material is used for exact scientific work (see table in Fig. 2).

- Organic dry matter (ODM or VS). Only the organic or volatile constituents of the feed material are important for the digestion process. For this reason, only the organic part of the dry matter content is considered.

- Digester loading (R). The digester loading indicates how much organic material per day has to be supplied to the digester or has to be digested. The digester loading is calculated in kilograms of organic dry matter per cubic metre of digester volume per day (kg ODM/m³/day). Long retention times result in low digester loadings. In a simple biogas plant, 1.5 kg/m3/day is already quite a high loading. Temperature-controlled and mechanically stirred large-scale plants can be loaded at about 5 kg/m3/day. If the digester loading is too high, the pH falls. The plant then remains in the acid phase because there is more feed material than methane bacteria.

Example:

Calculation of digester loading
Digester volume (VD): 48001 (4.8 m³) Retention time (RT): 80 days
Daily amount of fermentation slurry (Sd): 60 kg
Proportion of organic matter: 5 %

R = 5x60/100 x 4.8 = 0.625 kg/m3/day

Retention time (RT or t) indicates the period spent by the feed material in the digester. It is chosen by economic criteria. The retention time is appreciably shorter than the total time required for complete digestion of the feed material.

Specific gas production may be quoted for the amount of fermentation slurry, the dry matter, content or only the organic dry matter. In practice, it represents the gas production of a specific feed material in a specific retention time at specific digester temperatures.

Degree of digestion is measured as a percentage. It indicates the amount of gas obtained as a proportion of total specific gas production. The difference from 100% indicates the proportion of feed material which is not yet fully digested. In simple biogas plants, the degree of digestion is about 50 %. This means that half the feed material is not used.

Biochemical oxygen demand (BOD) is an important parameter in effluent treatment. It indicates the degree of pollution of effluents or sewage. The BOD is a measure of the amount of oxygen consumed by bacteria in biological purification.