Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 111

Methanomicrobium, Methanoculleus, Methanogenium , and Methanothermobacter .
All methanogens contain a unique cofactor, F 420 , that is autofluorescent at a
wavelength of 420 nm [38]. Some methanogens, especially hydrogenotrophic
methanogens, contain so much of it that they appear blue when viewed under a
microscope. Several trace elements, especially nickel and cobalt, are required by
methanogens for methanogenesis and growth. For some feedstocks, supplementa-
tion with trace elements can significantly enhance methane biogas production and
process stability [48]. Because of the low energy yield from the methanogenesis
pathway, most methanogens grow slowly, especially acetoclastic methanogens (e.g.,
Methanosaeta spp. have a generation time of 3.5-9 days) [36]. However, methano-
genesis is typically not a rate-limiting step of the entire AD process because the
low-energy yield of the methanogenesis pathway forces it to run rather rapidly.
Additionally, methanogens are susceptible to a host of factors (e.g., pH, ammonia,
and metals) so they are often implicated in instability or sub-optimal performance
of AD [17].
The small amounts of SCFA with three or more carbons (e.g., propionate,
butyrate, isobutyrate, valerate) and the ethanol produced during the fermentative
acidogenesis as well as the long chain fatty acids derived from lipid hydrolysis can
not be used directly by any known methanogens. A unique guild of strictly anaer-
obic bacteria (referred to as syntrophic acetogens) can oxidize these intermediates
to acetate, H 2 , and CO 2 so that they can serve as the substrates of methanogenesis
[75, 91]. However, the oxidation of these fatty acids and ethanol under fermentative
conditions (referred to as syntrophic acetogenesis) is thermodynamically unfavor-
able; and hydrogenotrophic methanogens are needed to reside in close proximity to
rapidly consume the H 2 produced by the syntrophic acetogens through interspecies
hydrogen transfer [23]. Syntrophomonas wolfei and Syntrophobacter wolinii are
thought to be important syntrophic acetogens in anaerobic digesters, with the former
primarily oxidizing butyrate and the latter oxidizing propionate. With a generation
time of greater than one week, syntrophic acetogens grow extremely slowly [24]. As
a result, the solid retention time (SRT) in digesters has to be long (15 days or longer)
to retain enough syntrophic acetogens. Hence, syntrophic acetogenesis can be a rate-
limiting step during AD, and failure or suboptimal performance encountered during
AD operation often involves this guild of bacteria, which is exemplified by AD fail-
ure when the organic loading rate was too high and the production of non-acetic
SCFA exceeded that of their utilization [47]. Thus, syntrophic acetogens are impor-
tant members of the microbial community of stable AD processes even though the
carbon flux through them is relatively small, and it is critical to maintain a balanced
production and consumption of these non-acetic SCFA by avoiding organic over-
loading. It should be noted that because they cannot be cultured as single cultures,
syntrophic acetogens are not well studied. The recent advancement of genomics and
metagenomics offers new opportunities to better understand this important guild of
bacteria in anaerobic digesters (see [55] for a recent review).
Several features of feedstocks can have profound effects on AD, such as the con-
tent of readily fermentable carbohydrates, particle sizes of insoluble feedstocks (the
hydrolysis step is especially affected by particle sizes), nutrient content and balance,