Parameters and process optimisation
The metabolic activity involved in microbiological methanation is dependent on the following
factors:
• Substrate temperature
• Available nutrients
• Retention time (flow-through time)
• pH level
• Nitrogen inhibition and C/N ratio
• Substrat solid content and agitation
• Inhibitory factors
Each of the various types of bacteria responsible for the three stages of the methanogenesis
is affected differently by the above parameters. Since interactive effects between the various
determining factors exist, no precise quantitative data on gas production as a function of the
above factors are available. Thus, discussion of the various factors is limited to their
qualitative effects on the process of fermentation.
Substrate temperature
Temperature range of anaerobic fermentation
Anaerobic fermentation is in principle possible between 3°C and approximately 70°C.
Differentiation is generally made between three temperature ranges:
• The psychrophilic temperature range lies below 20°C,
• the mesophilic temperature range between 20°C and 40°C and
• the thermophilic temperature range above 40°C.
Minimal average temperature
The rate of bacteriological methane production increases with temperature. Since, however,
the amount of free ammonia also increases with temperature, the bio-digestive performance
could be inhibited or even reduced as a result. In general, unheated biogas plants perform
satisfactory only where mean annual temperatures are around 20°C or above or where the
average daily temperature is at least 18°C. Within the range of 20-28°C mean temperature,
gas production increases over-proportionally. If the temperature of the bio-mass is below
15°C, gas production will be so low that the biogas plant is no longer economically feasible.
Changes in temperature
The process of bio-methanation is very sensitive to changes in temperature. The degree of
sensitivity, in turn, is dependent on the temperature range. Brief fluctuations not exceeding
the following limits may be regarded as still un-inhibitory with respect to the process of
fermentation:
• psychrophilic range: ± 2°C/h
• mesophilic range: ± 1°C/h
• thermophilic range: ± 0,5°C/h
The temperature fluctuations between day and night are no great problem for plants built
underground, since the temperature of the earth below a depth of one meter is practically
constant.
Available nutrient
In order to grow, bacteria need more than just a supply of organic substances as a source of
carbon and energy. They also require certain mineral nutrients. In addition to carbon, oxygen
and hydrogen, the generation of bio-mass requires an adequate supply of nitrogen, sulfur,
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