yield, when mixed biomass of Chlorella pyrenoidosa and cassava starch was used
as feedstock in dark fermentation.
Finally, the economic viability of the hydrogen production from microalgal
feedstock is an important aspect that ought to be contemplated. Therefore, to
increase the economic feasibility of the process, a biorefinery approach where
microalgal biomass residues after lipid and value-added product extraction that are
still rich in sugars can be utilized as feedstock under dark fermentation.
2.2.3 Pretreatment of Microalgal Biomass for Hydrogen Production
Carbohydrates in algae are entrapped within the cell wall in form of complex
polymer or stored intracellularly as starch or glycogen. Therefore, when algal
biomass is used as feedstock, it is necessary to carry out algal cell wall disruption
followed by conversion of polymeric carbohydrates into simple fermentable sugars
(glucose, arabinose, galactose, xylose and mannose), which are readily accessible
for hydrogen-producing bacteria (Ho et al. 2013; Kumar et al. 2013). Efficient
pretreatment of algal biomass is required to enhance the saccharification and
thereafter biohydrogen yield (Xia et al. 2013). For instance, Roy et al. (2014)
reported very low hydrogen production (0.03 m
3 /m
3
) from untreated algal biomass
compared to the pretreated biomass (1.33 m
3 /m
3 ). The effectiveness of pretreatment
process depends upon the cell wall characteristics of the microalgal species.
Microalgae having carbohydrate (cellulose and hemicelluloses)-based cell wall
(Chlorella kessleri and S. obliquus) are difficult to be pretreated. In contrast,
microalgal species having protein-based cell wall (C. reinhardtii, Arthrospira
platensis, Euglena gracilis) are easily degraded (Mussgnug et al. 2010). The
Scenedesmus sp. has one of the most resistant cell walls consisting of trilaminar
structure where inner layer is composed of cellulose covered by hemicellulose. The
outer layer contains sporopollenin-like biopolymer which confers high resistant to
pretreatment (Miranda et al. 2012).
Nevertheless, pretreatment is an indispensable step for efficient production of
hydrogen from microalgal feedstock; unoptimized pretreatment and saccharification
conditions can generate sugar degradation products (furfural, hydroxymethylfurfural (HMF), formic acid, acetic acid, propionic acid and lactic acid) (Harun et al.
2014; Hernández et al. 2015; Xia et al. 2015). The accumulation of such
by-products is inhibitory to the microbial growth and fermentation process thereby
decreasing the overall hydrogen production (Miranda et al. 2012; Xia et al. 2015).
The biomass pretreatment step is associated with high price and significantly
contributes to the overall cost of biohydrogen production process (Roy et al. 2014).
In order to increase the feasibility of biohydrogen production process, the selected
pretreatment procedure must be simple, energy efficient, cost-effective and must
enhance the polymeric carbohydrate conversion into fermentable sugars without the
formation of inhibitory by-products.
To date, the pretreatment methods used for the microalgal biomass hydrolysis
are mechanical, thermal, chemical, biological and combination of any two
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H. Singh and D. Das
as feedstock in dark fermentation.
Finally, the economic viability of the hydrogen production from microalgal
feedstock is an important aspect that ought to be contemplated. Therefore, to
increase the economic feasibility of the process, a biorefinery approach where
microalgal biomass residues after lipid and value-added product extraction that are
still rich in sugars can be utilized as feedstock under dark fermentation.
2.2.3 Pretreatment of Microalgal Biomass for Hydrogen Production
Carbohydrates in algae are entrapped within the cell wall in form of complex
polymer or stored intracellularly as starch or glycogen. Therefore, when algal
biomass is used as feedstock, it is necessary to carry out algal cell wall disruption
followed by conversion of polymeric carbohydrates into simple fermentable sugars
(glucose, arabinose, galactose, xylose and mannose), which are readily accessible
for hydrogen-producing bacteria (Ho et al. 2013; Kumar et al. 2013). Efficient
pretreatment of algal biomass is required to enhance the saccharification and
thereafter biohydrogen yield (Xia et al. 2013). For instance, Roy et al. (2014)
reported very low hydrogen production (0.03 m
3 /m
3
) from untreated algal biomass
compared to the pretreated biomass (1.33 m
3 /m
3 ). The effectiveness of pretreatment
process depends upon the cell wall characteristics of the microalgal species.
Microalgae having carbohydrate (cellulose and hemicelluloses)-based cell wall
(Chlorella kessleri and S. obliquus) are difficult to be pretreated. In contrast,
microalgal species having protein-based cell wall (C. reinhardtii, Arthrospira
platensis, Euglena gracilis) are easily degraded (Mussgnug et al. 2010). The
Scenedesmus sp. has one of the most resistant cell walls consisting of trilaminar
structure where inner layer is composed of cellulose covered by hemicellulose. The
outer layer contains sporopollenin-like biopolymer which confers high resistant to
pretreatment (Miranda et al. 2012).
Nevertheless, pretreatment is an indispensable step for efficient production of
hydrogen from microalgal feedstock; unoptimized pretreatment and saccharification
conditions can generate sugar degradation products (furfural, hydroxymethylfurfural (HMF), formic acid, acetic acid, propionic acid and lactic acid) (Harun et al.
2014; Hernández et al. 2015; Xia et al. 2015). The accumulation of such
by-products is inhibitory to the microbial growth and fermentation process thereby
decreasing the overall hydrogen production (Miranda et al. 2012; Xia et al. 2015).
The biomass pretreatment step is associated with high price and significantly
contributes to the overall cost of biohydrogen production process (Roy et al. 2014).
In order to increase the feasibility of biohydrogen production process, the selected
pretreatment procedure must be simple, energy efficient, cost-effective and must
enhance the polymeric carbohydrate conversion into fermentable sugars without the
formation of inhibitory by-products.
To date, the pretreatment methods used for the microalgal biomass hydrolysis
are mechanical, thermal, chemical, biological and combination of any two
214
H. Singh and D. Das