Fundamental science and engineering of the anaerobic digestion process for biogas production - The Biogas: Science, Production and Applications - page 105

the fermentative bacteria ferment the resultant monomers to a wide range of
fermentation end products. End products of the acidogenic stage include
acetic acid, hydrogen and carbon dioxide. However, the majority of the
products are higher carbon number volatile fatty acids (VFAs) such as
propionate, butyrate and alcohols.

5.2.2 Acetogenic bacteria

The obligate proton-reducing (OPR) acetogenic bacteria are unique as they
are obligate syntrophs (i.e. they must act together with bacteria in a different
trophic group to digest a substrate). They cannot be cultivated in a pure
culture; their existence was not discovered until 1967, by Bryant and co-
workers (McCarty, 1981). Their energy is derived from substrates if the
hydrogen partial pressure is maintained at a very low level. Hydrogen is a
product of their own metabolism and is toxic to them. These acetogenic
bacteria require the syntropic action of a H 2 -utilising species. The role of the
OPR acetogenic bacteria is crucial to the overall anaerobic process as they
convert the fermentative intermediates (VFAs) to methanogenic substrates,
H 2 ,CO 2 , acetic acids and unicarbon compounds.

5.2.3 Methanogenic bacteria

This group consists of hydrogenotrophic methanogenic bacteria and
aceticlastic methanogenic bacteria. The hydrogenotrophic bacteria utilise
the H 2 which the OPR acetogens produce. H 2 uptake by the methanogens is
very efficient, having an affinity of parts per million, which ensures very low
hydrogen partial pressure. The relationship between OPR acetogens and
hydrogenotrophic methanogenic bacteria is an excellent example of
syntropic mutualism: bacteria in different trophic groups converting
propionate, butyrate and long-chain fatty acids to methane and water.
Species of only two genera Methanosarcina and Methanothrix can
produce methane from acetic acid and be termed aceticlastic.
Approximately 70% of methane produced comes from acetate and
aceticlastic methanogens. It is usual in the literature to read that
methanogens have slow doubling times (reproduction rates), but this is
not entirely true: hydrogenotrophic methanogens are very efficient and have
relatively fast doubling times (Pfeffer, 1979). Aceticlastic methanogens are,
however, relatively inefficient in acetate uptake and as a result have slow
doubling times.

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