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 (D_d):
30 kg/day
Mixing ratio: dung: water = 1:1
Fermentation slurry amount (S_d):
30 kg/day x 2 = 60 l/day
Retention time (RT): 80 days
Digester volume (V_D):
60 l/day x 80 days = 4800 l
Digester temperature (t): 26 - 28 °C
Specific -gas production (G_d) 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 (V_G):
1200 l x 0.60= 720 l

Digester/gasholder ratio:

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

Specific gas production (G_d) 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 (V_G):
2240 lx 0.60 = 1344 l
Digester/gasholder ratio:
V_D: V_G = 4800 l 1344 l= 3.6: 1