Characteristics that set anaerobic fermentation apart from aerobic fermentation (e.g. composting)
include:
- fixation of biochemical energy in biogas
- little formation of new biomass
- low heat development
- fixation of minerals in the digested slurry.
It is important to know that anaerobic fermentation involves a steady-state flux of acetobacters and
methanobacters, with the methanobacters, being more specialized and, hence, more sensitive,
constituting the defining element. Any biogas plant can develop problems during the starting phase
and in the case of overloading or uneven loading of the digester, and as a result of poisoning. This
underlines the importance of cattle dung, which is rich in methanobacters and therefore serves as a
good "starter" and "therapeutic instrument" in case of a disturbance.
With regard to technical exploitation, anaerobic fermentation must be regarded from a holistic point
of view, since the "organism" is only capable of operating at optimum efficiency under a certain set
of conditions. The process of anaerobic fermentation is quite variable and capable of stabilizing
itself as long as a few basic parameters are adhered to.
Parameters and terminology of biomethanation
Feedstock/substrate:
As a rule, all watery types of biomass such as animal and human excrements, plants and organic
wastewater are suitable for use in generating biogas. Wood and woody substances are generally
unsuitable.
The two most important defining quantities of the biomethanation process are the substrate's solids
content, i.e. total solids (TS, measured in kg TS/m³) and its total organic solids content, i.e. volatile
solids (VS, measured in kg VS/m³ ). Both quantities are frequently stated as weight percentages.
The total-solids and water contents vary widely from substrate to substrate (cf. table 3.2 for
empirical values). The most advantageous TS for the digester of a continuoustype biogas plant is 5-
10%, compared to as much as 25% for a batch-operated plant. A TS of 15% or more tends to inhibit
metabolism. Consequently, most substrates are diluted with water before being fed into the
digester.
Substrate composition
All natural substrates may be assumed to have a nutritive composition that is adequately conducive
to fermentation. Fresh green plants and agroindustrial wastewater, however, sometimes display a
nutritive imbalance.
An important operating parameter is the ratio between carbon content (C) and nitrogen content (N),
i.e. the C/N-ratio, which is considered favorable within the range 30 :1 to 10: 1. A C/N-ratio of less
than 8: 1 inhibits bacterial activity due to an excessive ammonia content.
Fermentation/digester temperature
As in all other microbial processes, the rate of metabolism increases along with the temperature.
The fermentation/digester temperature is of interest primarily in connection with the time required
for complete fermentation, i.e. the retention time: the higher the temperature, the shorter the
retention time. It has no effect on the absolute biogas yield, which is a constant that depends only
on the type of biomass in the digester.
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