Biorefineries using waste – production of energy and chemicals from biomasses by micro-organisms - Biomass as Energy Source: Resources, Systems and Applications - page 248

Table 13.2. E. coli amount (cfu/mL) in every syringe at the end of the
experiment and gas flow from syringe to syringe (mL/min).

cfu/mL Gas flow from syringe to syringe

2 × 10 9 Syringe 1 72mL/min

10 8 Syringe 2 4 × 60mL/min

10 9 Syringe 3 2 × 45mL/min

5 × 10 9 Syringe 4 36mL/min

10 9 Syringe 5 3 ×

are sufficient for promoting the growth of obligate anaerobic bacteria (Fung, 1988). Actually,
in human skin a majority of bacteria are obligate anaerobes. This is illustrating the situation in
natural microenvironments, where aerobes and facultative anaerobes rapidly consume the oxygen
thus preparing the conditions suitable for the strict anaerobes. Similarly, it could be well supposed
that in natural conditions the exchange of substances occur within short distances by various sects
of microorganisms. These interactions could in future offer interesting possibilities for exploiting
the microbial world for biotechnical purposes with mixed cultures.
Many bioprocess productions require the use of anaerobic microbes (Fung, 1988). In bio-
chemical literature, it is widely emphasized the rapidity of the aerobic reactions, such as glucose
utilization, in comparison with the anaerobic fermentation. However, when the diffusion limita-
tion has been overcome, for example, in the PMEU both the aerobic and anaerobic metabolism
has been operated at equal speed (Hakalehto et al. , 2007). This corresponds with the observa-
tion that during the wintertime in Finland many landslides, not only the composts, develop heat,
which prevents them from freezing. Therefore, a novel thinking in the bioreactor design and in
the exploitation of the anaerobic organisms could provide fruitful improvements in the produc-
tivity of the biochemical processes. In our laboratory, we have successfully accomplished e.g.
acetone-butanol fermentation in a few hours of time with novel bioreactor arrangements (results
not shown here).

13.8.4 Some exploitable biochemical pathways of bacteria and other microbes
The fermentative reactions are carried out usually in anaerobic conditions, in which the ultimate
electron acceptor is usually CO 2 . Simultaneously, the cell metabolism is accumulating some
molecular species as end products. In fact, the CO 2 is also initiating the bacterial growth (cell
division) by some unknown mechanism. With suboptimal CO 2 concentration, the growth rate of
E. coli is controlled by the CO 2 concentration in otherwise adequate growth medium (Repuske
and Clayton, 1968). It has also been documented that the CO 2 is a necessity for bacterial growth:
“The removal of carbon dioxide from an environment, which is otherwise favorable, results in
complete cessation of bacterial growth”. This illustrates the sensibility of microbial cultures (and
biological systems in general) to the environmental conditions in the bioreactor. In practice, this
means mainly the circumstances surrounding the cells.
Many bacterial fermentations lead to the accumulation of organic acids. These products can be
purified and exploited as such or they can further processed by other microbes, or by chemical
reactions. For example, the succinic acid has been suggested as a platformchemical for biorefinery
applications (Kamm and Kamm, 2004).
Some other fermentation products, such as ethanol, acetone, butanol or 2,3-butanediol have a
wide variety of potential applications in bioindustries. Besides the uses as fuel components, they
are typical bulk chemicals for the industries. For example, the 2,3-butanediol is easily converted
into butadiene, which serves as new material for synthetic rubber and plastic monomers, as well
as anti-icing substances (American Chemistry Council, 2011).
Besides the direct production of bulk chemicals microorganisms produce many hydrolytic
enzymes, which are exploited in the biomass pretreatments. Often it has to be decided which type