For reasons of operating economy, a somewhat shorter period of fermentation, the technical
retention time (RT, t, measured in days) is selected such as to achieve an advantageous,
temperature-dependent relative digestion rate (Dr, measured in Yo), also referred to as the yield
ratio, since it defines the ratio between the actual biogas yield and the theoretical maximum. The
average agricultural biogas system reaches a Dr-value of 30-60%..
Fig. 5.2: Gas yield as a function of temperature
and retention time (fT,RT-curves). 1 fT,RT:
relative gas yield, serving as a multiplier for the
average gas yields, e.g. those listed in table
3.5, 2 retention time (RT), 3 digester
temperature (T), measured in °C (Source:
OEKOTOP)
Table 5.4: Temperature ranges for anaerobic fermentation (Source: OEKOTOP, compiled
from various sources)
Fermentation
Psycrophilic
Mesophilic
Thermophilic
Minimum
4-10 °C
15 - 20 °C
25-45 °C
Optimum
15-18 °C
28-33 °C
50-60 °C
Maximu
m
25-30 °c
35-45 °C
75-80 °C
Retention time
over 100 days
30-60 days
10-16 days
Volumetric digester charge/digester load
The volumetric charge, i.e. how much substrate is added per unit of digester volume each day (Vc,
measured in m³/m³ Vd x d), is given by the chosen (technical) retention time (RT).
The digester load (Ld, measured in kg digested TS (VS)/m³ Vd x day) serves as a measure of
digester efficiency. The digester load is primarily dependent on four factors: substrate, temperature,
volumetric burden and type of plant. For a typical agricultural biogas plant of simple design, the
upper limit for Ld is situated at roughly 1.5 kg VS/m³ x day. Excessive digester loading can lead to
plant disturbances, e.g. a lower pH. In practice, the amount of TS/VS being added is frequently
equated to the digester load.
Specific biogas yields / specific biogas production
The specific gas yield (Gy, measured in m³ gas/kg TS (VS)) tells how much biogas can be drawn
from a certain amount of biomass (cf. table 3.5 for empirical values). The rate of gas generation is
naturally dependent on the digester temperature and retention time (cf. fig. 5.2).
The term specific gas production (Gp, measured in m³ gas/m³ Vd x day) supplements the above
expression by defining the digester's biogas output.
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