the process employed. When hydrolysis and fermentation are
carried out in a separate reactor, it is known as separate
hydrolysis and fermentation (SHF). On the contrary, when
both processes are offered inside the same bioreactor, the
method is known as simultaneous saccharification and fermentation (SSF). When hydrogen production takes place
through an only step using combined hydrolysis and hydrogen fermentation, this novel approach is called consolidation
bioprocessing (CBP) (Kumar et al. 2015).
3.1 Separate Hydrolysis and Fermentation
SHF, a commonly used, two-stage process, in which cellulosic biomass is solubilized by hydrolysis in the first step,
followed by fermentation of H 2 in the next (Nagarajan et al.
2019). Hydrolysis and fermentation are provided in two
distinct apparatus, enhances the overall H 2 production performance since the process is carried out at individually
optimized experimental conditions. However, a significant
barrier of SHF, hydrolysate, which consists of pentose and
hexose, needs to be treated further collectively into hydrogen. Though hexose can be easily utilized by microbes,
pentose utilization is highly complicated. For complete
biomass utilization, microorganisms fermenting lignocellulosic hydrolysate, particularly pentose, is essential (Ren et al.
2016). Moreover, due to the accumulation of monomeric
sugars as end-products, hydrolysis may get inhibited in the
process (Kumar et al. 2015). This can be overcome in SSF
by accomplishing hydrolysis and fermentation in a single
apparatus, which is a promising alternative (Nasirian et al.
2011; Zhao et al. 2013).
3.2 Simultaneous Saccharification
and Fermentation
In SSF, hydrolysis and fermentation are offered in the same
reactor by employing microorganisms or enzymes for H 2
production. SSF improves bacterial-assisted H 2 production
while reducing equipment cost and operation time. Further,
it also enhances hydrogen yield while eliminating inhibition
by end-products during cellulose hydrolysis (Ren et al.
2016). Though the process has several advantages, the
commercial feasibility of the process is limited. Cellulose
degradation and hydrogen fermentation could have distinct
optimized conditions as an individual process, which makes
the process less optimized altogether. During the SSF process, the substrate–cellulose concentration, pH, and temperature have a significant impact on H 2 generation (Ren
et al. 2016). Moreover, in SSF, efficiency and the production
rate are limited by enzymatic saccharification concerning
rigidly covered cellulose and hemicellulose with lignin.
Several types of wood and agricultural wastes are commonly
used in SSF to produce hydrogen directly. Recently,
microalgae (third-generation feedstock) have also drawn
attention (Cheng et al. 2011).
3.3 Consolidated Bioprocessing (CBP)
A combination of enzymatic saccharification (hydrolysis) of
biomass to fermentable sugars, accompanied by biological
transformation to hydrogen using a consortium or a single
organism, is known as consolidated bioprocessing (Lynd
et al. 2005). For hydrogen production, CBP could effectively
decrease the expense in installation and lead to more competitive and economically feasible technology. Hydrogen
production in CBP is unusually known as the direct microbial conversion. It is a one-stage and integrated design for
the utilization of lignocellulosic residues (Parisutham et al.
2014). Fermentation, combined with cellulose solubilization,
decreases the expenses in procuring industrial enzymes for
separate saccharification makes the CBP a valuable process
from the economic viewpoint (Olson et al. 2012). The
microorganism used in consolidated bioprocessing can efficiently hydrolyze the lignocellulosic residue while converting it into the aspired products. However, most organisms
cannot achieve a satisfying fermentation performance
(Nagarajan et al. 2019). Choosing a functional culture with
hemicellulose-/cellulose-degrading and hydrogen-producing
capacity was regarded as the most critical criteria in the CBP
process (Ren et al. 2016). However, hydrogen production
experiments are carried out under different pH, and temperature with varying pretreatment methods limits the efficiency of the process. Therefore, in order to assess different
production processes for hydrogen, utilizing a single type of
substrate with a uniform pretreatment technique may be
required (Ren et al. 2016).
4 Bioconversion of Lignocellulose
to Hydrogen
The production of H 2 employing a cleaner sustainable route
has always been a significant challenge to the scientific
community. The biological way has effectively achieved
bioconversion of biomass into H 2 . Various agricultural
wastes have been used as feedstock for the production of H 2 .
Among them, half of the bioenergy production processes
used lignocellulosic biomass. The conversion of simple or
complex sugars into hydrogen production can be carried out
through
biophotolysis,
photo
fermentation,
and
dark-fermentation or by the integration of dark- and photo
fermentation (two-stage process), or bio catalyzed electrolysis, etc. (Manish and Banerjee 2008).
66
P. D. Patil et al.
carried out in a separate reactor, it is known as separate
hydrolysis and fermentation (SHF). On the contrary, when
both processes are offered inside the same bioreactor, the
method is known as simultaneous saccharification and fermentation (SSF). When hydrogen production takes place
through an only step using combined hydrolysis and hydrogen fermentation, this novel approach is called consolidation
bioprocessing (CBP) (Kumar et al. 2015).
3.1 Separate Hydrolysis and Fermentation
SHF, a commonly used, two-stage process, in which cellulosic biomass is solubilized by hydrolysis in the first step,
followed by fermentation of H 2 in the next (Nagarajan et al.
2019). Hydrolysis and fermentation are provided in two
distinct apparatus, enhances the overall H 2 production performance since the process is carried out at individually
optimized experimental conditions. However, a significant
barrier of SHF, hydrolysate, which consists of pentose and
hexose, needs to be treated further collectively into hydrogen. Though hexose can be easily utilized by microbes,
pentose utilization is highly complicated. For complete
biomass utilization, microorganisms fermenting lignocellulosic hydrolysate, particularly pentose, is essential (Ren et al.
2016). Moreover, due to the accumulation of monomeric
sugars as end-products, hydrolysis may get inhibited in the
process (Kumar et al. 2015). This can be overcome in SSF
by accomplishing hydrolysis and fermentation in a single
apparatus, which is a promising alternative (Nasirian et al.
2011; Zhao et al. 2013).
3.2 Simultaneous Saccharification
and Fermentation
In SSF, hydrolysis and fermentation are offered in the same
reactor by employing microorganisms or enzymes for H 2
production. SSF improves bacterial-assisted H 2 production
while reducing equipment cost and operation time. Further,
it also enhances hydrogen yield while eliminating inhibition
by end-products during cellulose hydrolysis (Ren et al.
2016). Though the process has several advantages, the
commercial feasibility of the process is limited. Cellulose
degradation and hydrogen fermentation could have distinct
optimized conditions as an individual process, which makes
the process less optimized altogether. During the SSF process, the substrate–cellulose concentration, pH, and temperature have a significant impact on H 2 generation (Ren
et al. 2016). Moreover, in SSF, efficiency and the production
rate are limited by enzymatic saccharification concerning
rigidly covered cellulose and hemicellulose with lignin.
Several types of wood and agricultural wastes are commonly
used in SSF to produce hydrogen directly. Recently,
microalgae (third-generation feedstock) have also drawn
attention (Cheng et al. 2011).
3.3 Consolidated Bioprocessing (CBP)
A combination of enzymatic saccharification (hydrolysis) of
biomass to fermentable sugars, accompanied by biological
transformation to hydrogen using a consortium or a single
organism, is known as consolidated bioprocessing (Lynd
et al. 2005). For hydrogen production, CBP could effectively
decrease the expense in installation and lead to more competitive and economically feasible technology. Hydrogen
production in CBP is unusually known as the direct microbial conversion. It is a one-stage and integrated design for
the utilization of lignocellulosic residues (Parisutham et al.
2014). Fermentation, combined with cellulose solubilization,
decreases the expenses in procuring industrial enzymes for
separate saccharification makes the CBP a valuable process
from the economic viewpoint (Olson et al. 2012). The
microorganism used in consolidated bioprocessing can efficiently hydrolyze the lignocellulosic residue while converting it into the aspired products. However, most organisms
cannot achieve a satisfying fermentation performance
(Nagarajan et al. 2019). Choosing a functional culture with
hemicellulose-/cellulose-degrading and hydrogen-producing
capacity was regarded as the most critical criteria in the CBP
process (Ren et al. 2016). However, hydrogen production
experiments are carried out under different pH, and temperature with varying pretreatment methods limits the efficiency of the process. Therefore, in order to assess different
production processes for hydrogen, utilizing a single type of
substrate with a uniform pretreatment technique may be
required (Ren et al. 2016).
4 Bioconversion of Lignocellulose
to Hydrogen
The production of H 2 employing a cleaner sustainable route
has always been a significant challenge to the scientific
community. The biological way has effectively achieved
bioconversion of biomass into H 2 . Various agricultural
wastes have been used as feedstock for the production of H 2 .
Among them, half of the bioenergy production processes
used lignocellulosic biomass. The conversion of simple or
complex sugars into hydrogen production can be carried out
through
biophotolysis,
photo
fermentation,
and
dark-fermentation or by the integration of dark- and photo
fermentation (two-stage process), or bio catalyzed electrolysis, etc. (Manish and Banerjee 2008).
66
P. D. Patil et al.