10.1.1 Floating-drumplants
10.1.2 Fixed-domeplants
10.1.3 Earth pit with plastic-sheet gasholder
10.1.4 Estimating the earth-pressure and hydraulic forces
Floating-drum plants
10.1.2 Fixed-domeplants
10.1.3 Earth pit with plastic-sheet gasholder
10.1.4 Estimating the earth-pressure and hydraulic forces
Floating-drum plants
Design calculation
| Sizing factors | Example |
| Daily substrate input, Sd | = 115 l/d |
| Retention time, RT | = 70 days |
| Daily gas production, G | = 2.5 m³/d |
| Storage capacity,Cs | = 60% |
| Digester volume, Vd | = 8 m³ |
| Gasholder volume, Vg | = 1.5 m³ |
Calculating formulae after Sasse, 1984
1. Vg = Cs · G
2. ha = design-dependent
3. Vg= r · p · h
4. rg =
1. Vg = Cs · G
2. ha = design-dependent
3. Vg= r · p · h
4. rg =
5. rd = r + 0.03
6. Vd1 = p · d2 · p · h
7. Vd2 = R3 · p · 2/3
8. R =
9. Vd3 = R2 · p · H/3
10. H = R/5
11. Vd3 = R3 · p · 1/15
12. Vd2 : Vd3 = 10 : 1
13. Vd(2+3) = 1.1 Vd2
14. Vd(2+3) = Vd - Vd1
15. hd = hg
16. hdk = hd + structurally dependent free board (0.1 . . . 0.2 m)
Fig. 10.1: Conceptual drawing of a floating-drum biogas plant
Vd = Vdl +Vd2 +Vd3
= digester volume
Vg = gasholder volume
Index g = gas holder
Index d = digester
= digester volume
Vg = gasholder volume
Index g = gas holder
Index d = digester
| Sample calculation | Results |
| 1. Vg = 0.6 · 2.5 | = 1.5 m³ |
| hg = (specified) | = 0.7 m |
| 4. r =  | = 0.82 m |
| 5. r = 0.85 (chosen) | |
| 6. Vdl = 0.852 ·3.14 · 0.7 | = 1.58 m³ |
| 14. Vd (2+3) = 8.45 - 1.58 | = 6.87 m³ |
| 8+ 14. R =  | = 1.45 m |
Fig. 10.2: Constructional drawing of a floating-drum plant. Vd = 6.4 m³, Vg = 1.8 m³. Material requirements: Excavation 16.0 m³, Foundation 1.6 m³, Masonry 1.1 m³, Rendered area 18.0 m², Sheet steel 5.7 m². (Source: OEKOTOP, Sasse)
Fig. 10.3: Constructional drawing of a water-jacket plant. Vd = 6.0 m³, Vg = 1.8 m³. Material requirements: Excavation 16.0 m³, Foundation 1.6 m³, Masonry 1.6 m³, Rendered area 21 m², Sheet steel 5.7 m². (Source: OEKOTOP, Sasse)
Fig. 10.4: Constructional drawing of a cylindrical floating-drum plant for quarrystone masonry. Vd = 9.4 m³, Vg = 2.5 m³. Material requirements: Excavation 21.0 m³, Foundation 1.0 m³, Masonry 5.4 m³, Rendered area 27.3 m², Sheet steel 6.4 m². (Source: OEKOTOP, KVIC)
10.1.2 Fixed dome plants
Design calculation
| Sizing factors | Example | Sample calculation |
| Daily substrate input, Sd | = 115 l/d | R = (0.76 · 8)1/3= 1.85 m |
| Retention time,RT | = 70 days | r = 0.52 R= 0.96 m |
| Daily gas production, G | = 2.5 m³/d | h = 0.40 R= 0.72 m |
| Storage capacity, Cs | = 60% | p = 0.62 R= 1.14 m |
| Digester volume, Vd | = 8 m³ | |
| Gasholder volume, Vg=G·Cs | = 1.5 m³ | |
| Vd : Vg | = 5.3 : 1 |
| Vg : Vd | 1:5 | 1:6 | 1:8 |
| R | (0.76 · Vd)1/3 | (0.74 · Vd)1/3 | (0.72 · Vd)1/3 |
| r | 0.52 R | 0.49 R | 0.45 R |
| h | 0.40 R | 0.37 R | 0.32 R |
| p | 0.62 R | 0.59 R | 0.50 R |
Fig. 10.5: Conceptual drawing of fixed-dome biogas plant. Vg gasholder volume, Vd digester volume. (Source: OEKOTOP, Sasse)
Fig. 10.6: Constructional drawing of a fixed-dome plant. Vd = 8 m³, V = 1.5 m³. Material requirements: Excavation 25 m³, Foundation 2.2 m³, Masonry 2.0 m³, Rendered area 22.0 m², Sealed area 7.0 m². (Source: OEKOTOP, Sasse, BEP Tanzania)
10.1.3 Earth pit with plastic-sheet gasholder
Fig. 10.7: Constructional drawing of an earth-pit biogas plant with plastic-sheet gasholder. Vd = 11 m³, Vg = 2.2 m³. Material requirements: Excavation 16 m, Rendered area 28 m², Sheeted area 10 m² (Source: OEKOTOP)
10.1.4 Estimating the earth-pressure and hydraulic forces
Fig. 10.8: Schematic diagram of earth-pressure and water-pressure forces
In-depth forces, h (e, w)
pW = wW · hw
pW = hydrostatic pressure at depth hw (m) wW = specific weight of water
= 1000 kp/m³ pW = 1000 · h (kp/m²)
pE = wE · ce · he
pE = active earth pressure, i.e. force of pressure of dry, previously loose but now compact column of earth on a solid vertical wall
wE = specific weight of dry backfill earth
= 1800 . . . 2 100 kp/m³
he = height of earth column (m)
ce = coefficient of earth pressure for the earth column in question
= 0.3 . . . 0.4 (-)
pE = (600 . . . 700) · h (kp/m² )
pW = hydrostatic pressure at depth hw (m) wW = specific weight of water
= 1000 kp/m³ pW = 1000 · h (kp/m²)
pE = wE · ce · he
pE = active earth pressure, i.e. force of pressure of dry, previously loose but now compact column of earth on a solid vertical wall
wE = specific weight of dry backfill earth
= 1800 . . . 2 100 kp/m³
he = height of earth column (m)
ce = coefficient of earth pressure for the earth column in question
= 0.3 . . . 0.4 (-)
pE = (600 . . . 700) · h (kp/m² )
Force acting on a surface
P(E, W) = p · A (kp = (kp/m²) · m²)
Note: The above formulae are simplified and intended only for purposes of rough estimation.
