An optimal gas yield from the fed substrate requires an optimal composition of the microorganisms. These may be different in any biogas digester. It is important to know this microbial state in order to compare it with measured deviations.
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Microorganisms can be distinguishe by their respective DNA. Molekular analyses are based on the inspection of nucleic acids, i.e. DNA or RNA. These analyses may answer sveral questions.
The methanogenic archaea present in biogas digesters may be assigned mostly into four families, which are able to handle different pathways of methanogenesis (v. Tab.).
| Family | Methanogenesis |
|---|---|
| Methanobacteriaceae | hydrogenotrophic only (CO2 + 4H2 → CH4 + 2 H2 O) |
| Methanomicrobiaceae | hydrogenotrophic only (CO2 + 4H2 → CH4 + 2 H2 O) |
| Methanosarinaceae | hydrogenotrophic as well as acetoclastic |
| Methanosaetaceae | acetoclastic only (CH3 COOH + H+ → CH4 + CO2 ) |
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The identification of species allows to assign the methanogens to a family. By assigning all methanogenic archaea a distribution (in percent) of methanogens can be calculated. This allows to determine the route of methanogenesis actually present in the biogas digester (acetoclastic/hydrogenotrophic).
A change in the distribution of bacteria at constant feeding may indicate problems with the hydrolysis. Alternatively it could be monitored if the dominant specis change after a change of substrates.
A qPCR technique for absolute quantification is used to measure the concentration of DNA. This allows to investigate relevant groups of microorganisms or biomarkers in biogas digester, e.g.:
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The concentration is typically given in [ DNA copies per gram​ ] or as exponent n in [ 10n DNA copies / g ].
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