Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 110

Biogas

Digestate

CH 4 , CO 2

Hydrogenotrophic
methanogenesis

Aceticlastic
methanogenesis

H 2 , CO 2 , or HCOO - CH 3 -COO -

Syntrophic
acetogenesis

Non-acetic SCFA
(propionate, butyrate,
lactate, etc)

Fermentative
acidogenesis

Fermentative
acidogenesis

Fermentative acidogenesis

Monomers and oligomers
(sugars, AA, LCFA, peptide)

Hydrolysis

Polymeric feedstocks

Feedstock

Fig. 1 The four phases of anaerobic digestion process

containing large amounts of readily fermentable carbohydrates (e.g., sugars and
starch) are digested at high organic loading rates, the production of SCFA can
exceed their consumption, leading to SCFA accumulation and consequential AD
upset or even failure [10].
The final phase of AD involves methanogens of the Archaea domain.
Methanogens are strict anaerobes and produce CH 4 as the major end-product of
their catabolism. Most methanogens are fastidious microbes and only grow on a few
substrates within a narrow spectrum of environmental conditions (neutral pH, Eh <-
300 mV, etc.). Methanogens use a unique methanogenesis pathway to produce CH 4
[36]. Hydrogenotrophic methanogens produce CH 4 via the reduction of CO 2 by
H 2 or by the conversion of other C 1 substrates (e.g., methanol and methylamines),
while acetoclastic methanogens convert acetate to CH 4 . It should be noted that the
former accounts for approximately one third while the latter accounts for two thirds
of the CH 4 produced in anaerobic digesters. This is because acetate is the major end
product of the acidogenesis step in all anaerobic digesters [86]. In spite of this, only
a few species of acetoclastic methanogens have been known and they are within
genera Methanosaeta (formerly Methanothrix ) and Methanosarcina . Methanosaeta
spp. are obligate acetoclastic methanogens, while species of Methanosarcina
also use C 1 substrates. Hydrogenotrophic methanogen species are found in gen-
era Methanobacterium, Methanospirillum, Methanobrevibactor, Methanococcus,