C/N ratio
Microorganisms need both nitrogen and carbon for assimilation into their cell structures.
Various experiments have shown that the metabolic activity of methanogenic bacteria can be
optimized at a C/N ratio of approximately 8-20, whereby the optimum point varies from case
to case, depending on the nature of the substrate.
Substrate solids content and agitation
Substrate solids content
The mobility of the methanogens within the substrate is gradually impaired by an increasing
solids content, and the biogas yield may suffer as a result. However, reports of relatively high
biogas yields from landfill material with a high solids content may be found in recent
literature. No generally valid guidelines can be offered with regard to specific biogas
production for any particular solids percentage.
Agitation
Many substrates and various modes of fermentation require some sort of substrate agitation
or mixing in order to maintain process stability within the digester. The most important
objectives of agitation are:
• removal of the metabolites produced by the methanogens (gas)
• mixing of fresh substrate and bacterial population (inoculation)
• preclusion of scum formation and sedimentation
• avoidance of pronounced temperature gradients within the digester
• provision of a uniform bacterial population density
• prevention of the formation of dead spaces that would reduce the effective digester
volume.
In selecting or designing a suitable means of agitation, the following points should be
considered:
1. The process involves a symbiotic relationship between various strains of bacteria, i.e.
the metabolite from one species can serve as nutrient for the next species, etc.
Whenever the bacterial community is disrupted, the process of fermentation will
remain more or less unproductive until an equivalent new community is formed.
Consequently, excessive or too frequent mixing is usually detrimental to the process.
Slow stirring is better than rapid agitation.
2. A thin layer of scum must not necessarily have an adverse effect on the process. For
systems in which the digester is completely filled with substrate, so that any scum
always remains sufficiently wet, there is little or no danger that the extraction of gas
could be impeded by the scum.
3. Some types of biogas systems can function well without any mechanical agitation at
all. Such systems are usually operated either on substrates with such a high solid
content, that no stratification occurs, or on substrates consisting primarily of solute
substances.
Since the results of agitation and mixing are highly dependent on the substrate in use, it is
not possible to achieve a sufficiently uniform comparative evaluation of various mixing
systems and/or intensity levels. Thus, each such system can only be designed on the basis
of empirical data.
Inhibitory factors
The presence of heavy metals, antibiotics (Bacitracin, Flavomycin, Lasalocid, Monensin,
Spiramycin, etc.) and detergents used in livestock husbandry can have an inhibitory effect on
the process of bio-methanation. The following table ( Tabelle 2) lists the limit concentrations
(mg/l) for various inhibitors.
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