The process of anaerobic fermentation can be illustrated in the form of a three-stage model, as
shown in figure 5.1.
Table 5.1: Basic criteria for acetobaeters (acid-forming bacteria) and methanobacters
(methane-forming bacteria) (Source: OEKOTOP, compiled from various sources)
Criterion
Dominant microorganisms
Temperature range
Optimum temperature
pH range
End metabolites
Mass transfer by . . .
Medium
Sensitivity to cytotoxins
Requirements regarding
nutrient composition
Special features
Acetobacter
facultative anaerobes
3 °C - 70 °C
approx. 30 °C
acidic (3.0) 5.0-6.5
relatively short duplication
period, usually less than 24
hours
org. acids, H2, CO2
intensive mixing
aqueous (water content >
60%)
low
well-balanced supply of
nutrients
viable with or without free
oxygen
Methanobacter
obligate anaerobes
3 °C - 80 °C
approx. 35 °C (sensitive to temperature
fluctuations of 2-3 °C or more)
alkaline, 6.5-7.6 relatively long
duplication period (20 - 10 days)
CO2, CH4
gentle circulation
substantial
viable only in darkness and in absence
of free oxygen
Table 5.2: Energy potential of organic compounds (Source: Kaltwasser 1980)
Material
biogas
(I/kg)
Carbohydrate
s
Organic fats
Protein
790
1270
704
CH4
CO2
vol. fraction %
50 50
68 32
71 29
Energy content
(Wh/g)
3.78
8.58
4.96
Anaerobic fermentation converts the "volatile solids" (proteins, carbohydrates, fats). The
"nonvolatile solids" are essential to the bacteria as "roughage" and minerals. Water serves
simultaneously as the vital medium, solvent and transport vehicle.
Theoretical/laboratory data on maximum gas yields from various organic materials show that
anaerobic fermentation is just as capable of achieving complete mineralization as is the process of
aerobic fermentation. Note: The theoretical maximum biogas yield can be ascertained by way of the
basic composition of the biomass.
Table 5.3: Energetical comparison of aerobic and anaerobic fermentation (Source: Inden
1978)
Metabolite
Cytogenesis
Heat
Methane
aerobic anaerobic
energy fraction (%)
60%
10%
40%
-
- 90%
40