Environmental Engineering Reference
In-Depth Information
it can become a future source of aromatic chemicals and materials. Alternative
organisms need also to be employed due to the impossibility of the traditional yeast
and bacteria to process the pentose (C5) sugars derived from hemicellulose [114].
Branched alcohol mixtures
. The preparation of a branched-longer chain alcohol
mixture with a potential use as biofuel has been recently reported by Liao et al.
[115]. Such alcohol mixture with high isobutanol content is produced via synthetic
non-fermentative pathway employing metabolic engineered bacteria (e.g.
E. coli
)
and glucose as carbon source. This strategy diverts the 2-ketoacid intermediates
in the aminoacid biosynthetic pathway of
E. coli
for alcohol synthesis, converting
them into aldehydes (by 2-ketoacid decarboxylases) and then to alcohols (by alco-
hol dehydrogenases) [115]. The process has been already licensed to Gevo (spin-off
company from Pasadena, US) that hopes to begin commercial scale production
within a few years [116, 117].
Biogas
Biogas is an environment friendly, clean, cheap and versatile fuel, composed of
a mixture of CH
4
and CO
2
that is usually generated from bacterial digestion of
biomass in absence of air between 10 and 72
◦
C [83, 118]. Almost any type of
organic matter (e.g. sewage sludge, animal wastes, industrial effluents) is suitable
for the production of biogas, which can be directly utilised in cooking and heat-
ing systems [119]. The process is carried out in anaerobic digesters that can vary
in size from 1 m
3
(small household unit) to as large as 2000 m
3
[120], involv-
ing a step-wise series of reactions that require the cooperative action of several
microorganisms. Initially, a group of microorganisms (acidogens) break down the
organic matter into a digestible form (usually simpler fatty acids) that can be utilised
by methane-generating anaerobic bacteria (methanogens) that produce biogas as
metabolic byproduct [121, 122].
The use of biogas as transport fuel has been explored in its application in explo-
sion engines. Biogas has shown a great potential for its uses in Brazil [123] and
in places such as Sweden, has been use in urban buses since 2004 [124]. Also in
Sweden some studies have evaluated the economic and environmental feasibility of
biogas as a renewable source of energy in large scale showing positive results in its
applicability CHP (centralised heat and power) [125].
Biohydrogen
Various authors have recently reviewed the prospects and potential in the produc-
tion of biohydrogen [126-130]. Biohydrogen can be produced by three different
biological pathways: fermentation and direct or indirect (bio)photolysis.
Fermentation
. Dark and photo fermentation are technologies under development
(currently at lab scale) to produce biohydrogen from wet biomass (e.g. molasses,
organic wastes, sewage sludge) using (an)aerobic hydrogen fermenting bacteria
[130, 131]. The advantage of the dark fermentation is that the biohydrogen is pro-
duced directly without formation of methane [127, 130]. During dark fermentation,
