Biorefineries using waste – production of energy and chemicals from biomasses by micro-organisms - Biomass as Energy Source: Resources, Systems and Applications - page 252

production with mesophilic and thermophilic processes, the product yield was 1.3 times higher
in the latter case (Kangas et al ., 2011). However, the quality of thermophilic process reject water
was improved from the sewage treatment plant's point of view. Its COD value was 3.4 times the
one deriving from the mesophilic process, and ammonium nitrogen level 1.3 times the one of the
mesophilic origin. Thus, the method of choice has to be selected after careful overall comparison.
In addition to mesophilic bacteria, thermotolerant bacterial species such as Clostrid-
ium thermocellum , Clostridium thermohydrosulfuricum , Clostridium thermosaccharolyticum ,
Clostridium thermosulfurogenes and Thermoanaerobacter ethanolicus can also produce ethanol
(Chandel et al. , 2011).
For example, coupling the thermophilic organisms with the production of ethanol with low
heat of vaporization is resulting in a process where end-product inhibition caused by the product
is avoided by its continuous evaporation from the process fluid. In these temperatures, it is
also possible to run the process without contaminating microbes occurring. Such case has been
demonstrated with Thermoanaerobacter BG1L1 bacterium in the production of fuel ethanol from
dilute sulfuric acid pretreated corn stover (PCS) (Georgieva and Ahring, 2007). This production
organism was able to exploit the xylose effectively from the substrate yielding ethanol by rate of
0.39-0.42 g/g-sugars consumed. This thermophilic anaerobic bacterium was not inhibited by the
toxicity of the corn stover hydrolysate in this process.
Another example of ethanol production with thermophilic organisms is the use of anaerobic
clostridia. The fermentations of several saccharides derived from lignocellulosics were investi-
gated with a co-culture consisting of Clostridium thermocellum and Clostridium thermolacticum
(Xu and Tschirner, 2011). In this case, mixed cultures of two bacterial species provided a solution
for the exploitation of lignocellulose-based raw materials. The co-culture was able to actively
ferment glucose, xylose, cellulose and micro-crystallized cellulose (MCC). The ethanol yield
observed in the co-culture was up to two times higher than in monocultures. The highest ethanol
yield (as a percentage of the theoretical maximum) observed was 75% (w/w) for MCC and 90%
(w/w) for xylose. In this case low levels of initial ethanol addition resulted in an unexpected
stimulatory impact on the final ethanol productions. Examples of this kind underline the impor-
tance of extensive experimentation in the production scale, on the basis of understanding and
also challenging the microbial physiology. Especially, using the controlled mixed cultures can be
extremely rewarding regardless of their complexity.
In an experiment where the compost of Napiergrass and sheep dung were used as a sub-
strate for selecting effective mixed cultures for degrading lignocellulosic wastes in thermophilic
(60 ◦ C) conditions, the basic microflora components after stabilization were five main organisms
( Clostridium strain TCW1, Bacillus sp. THLA0409, Klebsiella pneumoniae THLB0409, Kleb-
siella oxytoca THLC0409, and Brevibacillus strain AHPC8120) (Lin et al. , 2011). Acetic acid,
ethanol, and butanol were the main biochemical products produced by biological fermentation.
Under optimal conditions, ethanol yields fromAvicel and Napiergrass reached maxima of 0.108
and 0.040 g/g, representing ethanol productivities of 0.00055 and 0.00028 g/g/h, respectively.
This example is again an indication of the increasing development in the field of mixed cul-
tures, which reflect the nature's own methods in generating new substances via degradation. In
another study, co-cultures of cellulolytic Clostridium thermocellum with non-cellulolytic Thermo-
anaerobacter strains (X514 and 39E) significantly improved ethanol production by 194-440%
(He et al. , 2011). In this reaction, the importance of vitamin B12 for the ethanol production was
also documented.
13.10 VOLATILE PRODUCTS
In the rumen, glucose undergoes a bacterial fermentation with the production of volatile fatty
acids (VFA): acetic, propionic and butyric, and the gases carbon dioxide and methane (Madigan
et al. , 2003). In addition, hydrogen results from several fermentations. The same substances are
potential products (or raw materials) for biochemical engineering processes. It is possible, as