Environmental Engineering Reference
In-Depth Information
with highly efficient cellulases that were developed in-house. However, different
enzymes are required for other biomass types.
7.3.5.3
Fermentation Process
The production of ethanol undergoes glycolysis, a common biochemical pathway
that is present in animals as well as yeast and bacteria. The overall reaction scheme
for the conversion of 6-carbon sugars into ethanol is:
CHOCHOH O
6 26 25
→
2
+
2
2
Similarly, the stoichiometric equation for the reaction with a 5-carbon sugar is:
3
CHOCHOH O
→
5
+
5
5 05 25
2
There are at least three different processes that can be used to convert cellulosic
biomass into ethanol. In separate hydrolysis and fermentation (SHF), the pre-
treated biomass is treated with a cellulase that hydrolyzes the cellulose to glucose
at 50°C and pH 4.8. The resulting material is then fermented. In simultaneous
fermentation and saccharification (SSF), the hydrolysis and fermentation occur in
the same bioreactor. Third, in direct microbial conversion (DMC), the microor-
ganisms that produce the cellulase also perform the fermentation.
Commercial ethanol production is primarily done by microorganisms such as
S. cerevisiae
,
Zymomonas
sp., and
Candida
sp. Of these, the well-known fungi
S. cerevisiae
(also known as baker's yeast) is the most widely used. Natural
S. cerevisiae
may be used, but genetically modified or “recombinant” strains are
also available. Interestingly, several authors have described the genetic modifica-
tion of
S. cerevisiae
using different techniques [31]. It has been demonstrated
that the efficiency of fermentation by
S. cerevisiae
can be dramatically increased
by cloning three exogenous genes into it [31].
In addition, a very interesting patent recently described a novel mix of fungi
that can ferment both 6- and 5-carbon sugars into ethanol. This patent also states
that the mixture of fungi enhances ethanol production by
S. cerevisiae
and can
therefore be used in existing industrial set-ups [32, 33].
7.3.6
Production of Biodiesel from Plants and Algae
Biodiesel is a fuel that is derived from vegetable oils or algae. It is a biodegradable
and non-toxic diesel equivalent. Diverse feedstocks can be used for biodiesel pro-
duction, including rapeseed and soybean oils. In addition, animal fats including
grease and chicken fat and sewage waste can be used as substrates for algae that
produce biodiesel (Table 7.1).
Biodiesel is made from vegetable oils consisting of glycerol esters of fatty
acids. A process of transesterification is used to exchange the glycerol components
