Environmental Engineering Reference
In-Depth Information
The majority of research has been directed towards expensive pure substrate
or to a much lesser degree solid waste or wastewaters. However, more sustainable
feedstock will be needed for a sustainable process. These could be achieved by
sugar-containing crop such as sweet sorghum and sugar beet, starch based crops
such as corn or wheat, or ligno-cellulosics such as fodder grass and Miscanthus .
Ligno-cellulosic biomass is a complex of biopolymers that makes up the struc-
tural components of plant material. The approximate composition of lingo-cellulose
found in most biomass feedstocks is roughly 45-60 % cellulose, 20-40 % hemicel-
lulose, 25 % lignin, and 1-5 % pectin (Demain et al. 2005 ; Desvaux 2005 ; Lynd
et al. 2005 ). Cellulose consists of linear, insoluble polymers consisting of up to
25,000 repeating β-1,4 linked β-D-glucopyranose units. Cellulose is a highly or-
dered molecule consisting of 15-45 crystalline microfibril chains, which in turn
associate to form cellulose fibers. In nature, cellulose is found primarily in plant cell
walls and is associated with varying degrees of other biopolymers, including: (i)
hemi-cellulose, a random, amorphous hetero-polysaccharide composed of typically
β-1,3 linked xylans, arabinoxylan, gluco-mannan, and galactomannan; (ii) lignin, a
complex hydrophobic network of phenyl-propanoid units; (iii) pectins, composed
of α-(1-4)-linked D-galacturonic acid; and (iv) proteins. Ligno-cellulosic biomass
is renewable, inexpensive, constitutes a large fraction of waste biomass from mu-
nicipal, agricultural, and forestry sectors, and thus offers excellent potential as a
feedstock for renewable biofuels. Cellulose is, however, difficult to hydrolyze due
to its crystalline structure. Current strategies that produce fuel ethanol from lingo-
cellulosic biomass (or “second-generation” biofuels) use simultaneous saccharifi-
cation and fermentation (SSF) or simultaneous saccharification and co-fermenta-
tion (SSCF). Both SSF and SSCF require extensive pre-treatment of the cellulosic
feedstock by steam-explosion and/or acid treatment, followed by addition of exog-
enously produced cocktails of cellulolytic enzymes to hydrolyse cellulose chains
and release the glucose monomers required for fermentation. These pre-treatments
are costly, and some of the by-products generated, for example furfurals, can in-
hibit downstream processes. Steam-explosion of corn stover, with or without acid
treatment, can be a suitable substrate for H 2 production. Hydrogen production from
fermentable biomass has the advantage over ethanol production that the microflora
is able to use a wider range of cellulosic substrates than the yeast.
Fermentative H 2 production using cellulose as the sole carbon source is un-
der extensive investigation (Wang et al. 2008 ; Lo et al. 2008 ). The mesophilic,
cellulolytic bacterium, C. termitidis strain CT1112 has displayed a cell generation
time of 18.9 h when grown on 2 g L −1 α-cellulose (Ramachandran et al. 2008 ).
The major soluble fermentation byproducts were acetate and ethanol. Maximum
yields of acetate, ethanol, H 2 , and formate on α-cellulose are 7.2, 3.1, 7.7 and
2.9 mmol L −1 culture, respectively. Although, the generation time was longer when
cultured on α-cellulose than on the soluble cellulodextrin cellobiose, acetate and
H 2 synthesis were favored over ethanol synthesis, indicating that carbon flow to
ethanol and formate was restricted. During log phase, H 2 was produced at a specific
rate of 2.79 mmoles h −1 g −1 dry weight- of cells on α-cellulose.
The thermophilic, cellulolytic bacterium Clostridium thermocellum strain 27405
produced greater amounts of H 2 when cultured (in Balch tubes) on cellulosic
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