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close this bookBiogas Plants (GTZ, 1988)
close this folder4. Scaling of biogas plants
View the document4.1 Definitions
View the document4.2 Scaling of the digester
View the document4.3 Scaling of gasholder
View the document4.4 Digester/gasholder ratio
View the document4.5 Measuring and test programmes

4.4 Digester/gasholder ratio

The form of a biogas plant is determined by the size ratio between the digester and the gasholder (see Figures 11 - 13).


Fig. 11: Digester/gasholder ratio The ratio of the digester volume (VD) and gasholder volume (VG) substantially determines the shape and design of a biogas plant. These two parameters must be calculated before any project is planned. For a digester/gasholder volume ratio of VD:VG = 6:1, a spherical shell is far more economical than a cylinder even in floating-drum plants.


Fig. 12: Dependence of shape on retention time on a floating-drum plant (cattle dung above; pig manure below) Filling volume and gasholder capacity (C = 55 %) are the same in each case. The differences in digester/ gasholder ratios result solely from the differing retention times (RT).


Fig. 13: Dependence of shape on retention time on a fixed-dome plant (cattle dung above; pig manure below) The filling volume and gasholder capacity (C = 55 %) are the same in each case. The differences in digester/ compensating tank ratios result solely from the different retention time (Gd as a result of RT, Figures 7 and 8).

For floating-drum plants with a low digester/ gasholder ratio (1:1 to 3:1), the best shape for the digester is a cylinder. If the ratio is larger, shell and vault structures are worthwhile.

The digester/gasholder ratio depends primarily on:

- retention time (RT),
- specific gas production (Gd),
- gasholder capacity (C).

The digester/gasholder ratio chosen must be correct regardless of the type of plant, otherwise the biogas plant will not serve its purpose.

In a fixed-dome plant, the digester/gasholder ratio corresponds to the size ratio between the net digestion space and the compensating tank above the zero line (see Figure 6): VD: VG corresponds to VD: VO

The examples given below show the importance of the specific gas production for the scaling of the plant and for the digester/ gasholder ratio.

For extensive biogas plant construction programmes, a knowledge of the specific gas production and the necessary gasholder capacity is particularly important. It is then a good plan to carry out measurements and tests of one's own (see Section 4.5).

Examples for the Calculation:

Feed material: cattle dung, amount (Dd):
30 kg/day
Mixing ratio: dung: water = 1:1
Fermentation slurry amount (Sd):
30 kg/day x 2 = 60 l/day
Retention time (RT): 80 days
Digester volume (VD):
60 l/day x 80 days = 4800 l
Digester temperature (t): 26 - 28 °C
Specific -gas production (Gd) from Fig. 7:
40 l/kg
Daily gas production (G):
40 l/kg x 30 kg/day = 1200 l/day
Gasholder capacity (C): 60 %
Gasholder volume (VG):
1200 l x 0.60= 720 l

Digester/gasholder ratio:

VD:VG= 4800l: 720 l =6.67: 1
Feed material: pig manure, amount (Dd):
20 kg/day
Mixing ratio: manure: water = 1: 2
Fermentation slurry amount (Sd):
20 kg/day x 3 = 60 l/day
Retention time (RT): 80 days
Digester volume (VD):
60 l/day x 80 days = 4800 l
Digester temperature (t): 26-28 °C

Specific gas production (Gd) from figure 8:
112 l/day
Daily gas production (G):
122 l/kg x 20 kg/day = 2240 l/day
Gasholder capacity (C): 60
Gasholder volume (VG):
2240 lx 0.60 = 1344 l
Digester/gasholder ratio:
VD: VG = 4800 l 1344 l= 3.6: 1