Biogas Plants (GTZ, 1988) 
4. Scaling of biogas plants 

The size of the gasholder  the gasholder volume (VG, see Figure 6)—depends on gas production and the volume of gas drawn off.
Gas production depends on the amount and nature of the fermentation slurry, digester, temperature and retention time (Figures 7,8).
The curves represent averages of laboratory and empirical values. The values vary a wide range owing to differences in the solids content of the dung, animal feeds and types of biogas plant. Regular stirring increases gas production. The 2628 °C line is a secure basis for scaling in the majority of cases.
The curves represent averages of laboratory and empirical values. The measured values show an even wider range of variation than in the case of cattle dung. Particularly large variations occur if antibiotics are added to the feed. The 2628 °C curve is a realistic guide for the planning of a plant.
Gas production is encouraged by high, uniform temperatures (e.g., 33°C), long retention times (e.g., 100 days) and thorough mixing of the slurry.
Gas production is adversely affected by low and fluctuating temperatures (1525 °C), short retention times (e.g., 30 days) and poor mixing.
Example:
1 kg of cattle dung yields only 15 lof biogas in a retention time of 30 days at a digester temperature of 20 °C. If the retention time is increased to 100 days and the digester temperature to 33 °C, 1 kg of cattle dung gives 54 lof biogas (Figure 7). The size of the gasholder is determined, primarily by the amount of gas drawn off and when it is drawn.
Examples:
A refrigerator operating round the clock consumes all the gas
produced on a given day. The gasholder merely has to compensate for fluctuations
in the,daily volume of gas produced.
A water pump consumes the entire daily
gas production in a few hours. The gasholder must every day collect the entire
daytime and nighttime production and compensate for daily production
fluctuations.
The ratio of gasholder volume (VG) to daily gas production (G) is called the gasholder capacity (C).
Example:
Gasholder volume (VG): 1.5m³ (1500l)
Daily gas production
(G): 2.4 m³
Gasholder capacity (C):
1.5 m³ 2.4 m³ = 0.625
= 62.5 %.
The required gasholder capacity and hence the required gasholder size is an important planning parameter. If the gasholder capacity is insufficient' part of the gas produced will be lost. The remaining volume of gas will not be enough. If the gasholder is made too large, construction costs will be unnecessarily high, but plant operation will be more convenient. The gasholder must therefore be made large enough to be able to accept the entire volume of gas consumed at a time. It must also be able to accept all the gas produced between consumption times. Furthermore, the gasholder must be able to compensate for daily fluctuations in gas production. These fluctuations range from 75 % to 125 % of calculated gas production.
Calculation examples for gasholder size:
Daily gas production: 2400 l
Hourly gas production: 2400 : 24
= 100 l/h
Gas consumption
from 0600 to 0800 hrs 
=2h 
from 1200 to 1400 hrs 
=2h 
from 1900 to 2100 hrs 
=2h 
Duration of gas consumption: 
6 h 
To simplify the calculation, uniform gas consumption is assumed.
Hourly gas consumption:
2400 l : 6 h = 400 l/h
Gas is also produced during consumption. For this reason, only the difference between consumption and production is relevant to the calculation.
D_{G} = 400 l/h  100 l/h = 300 l/h
The necessary gasholder size during consumption is therefore:
V_{G}(1)=300l/h x 2h=600l.
The longest interval between periods of consumption is from 2100 to 0600 hrs (9 hours). The necessary gasholder size is therefore:
V_{G}(2) = 100 l/h x 9 h = 900 Q.
V_{G}(2) is the maximum relevant gasholder size. With the safety margin of 25%, this gives a gasholder size of
V_{G} = 900 l x 1.25 = 1125 £.
The required gasholder capacity is thus:
C = 1 125 l : 2400 l= 0.47 = 47 %
Daily gas production: 2400 l
Hourly gas production: 100 l/h
Gas consumption
from 0530 to 0830 hrs 
=3h 
from 1830 to 2000 hrs 
=1.5h 
Duration of gas consumption: 
4.5 h 
Gas consumption per hour:
2400 l : 4.5 h = 533 l/h.
Difference between gas production and consumption:
D_{G} = 533 l/h 100 l/h = 433 l/h.
Hence the necessary gasholder size during consumption is:
V_{G}(1)= 433 l/h x 3 h = 1299 l.
The necessary gasholder size in the intervals between consumption results from the period from 0830 to 1830 hrs (10 h). The necessary gasholder size is therefore:
V_{G}(2) = 100 l/h x 10 h = 1000 Q.
V_{G}(1) is the larger volume and must therefore be used as the basis. Allowing for the safety margin of 25 %, the gasholder size is thus
V_{G} = 1299 l X 1.25 = 1624 Q.
The required gasholder capacity thus works out as
C = 1624 l
: 2400 l= 0.68 = 68 %.
A gasholder capacity of 5060% is normally correct for peasant households in Third World countries. A capacity of 70 % or even more must be allowed only where not more than one meal a day is cooked regularly or where eating habits are highly irregular.