pretreatments. Mechanical pretreatment disrupts the algal cell wall by applying
shear forces. Pretreatment method such as bead beating or milling is less dependent
on microalgal species and can break the cell wall due to the collision of microalgal
biomass with minute glass, ceramic or steel beads, under high agitation.
Ultrasonication is another pretreatment method for cell wall disruption and solubilization of the organic matter. In this method, the repetitive compression and
rarefaction of the sonic waves cause the formation of microbubbles which grow and
then collapse, generating high pressure and heat, shear forces and free radicals
thereby damaging the cell wall. Cheng et al. (2012) found that bead milling can
disrupt some of the cyanobacterial cells releasing carbohydrate and protein to be
utilized by hydrogenogens. However, through milling lower yield of H 2
(38.5 mLH 2 /g DW) was obtained as compared to ultrasonication pretreatment
(55.9 mL H 2 /g DW).
Thermal pretreatment utilizes heat energy for the solubilization of microalgal
biomass. Optimal range of temperature for the disintegration of organic matter
varies according to substrate characteristics. For microalgal biomass, pretreatment
temperature and time duration range from 65 to 180 °C and 15–60 min, respectively (Wang and Yin 2017). Thermal pretreatment by microwave heating is
favoured for uniform distribution of heat and for achieving higher temperature in
less time. Hydrothermal pretreatment (steam heating) at 100 and 121 °C led to
increase in carbohydrates and proteins solubilization from lipid-extracted
Scenedesmus biomass (Yang et al. 2010). However, thermal pretreatment alone is
not sufficient for efficiently hydrolysing the microalgal biomass. Combination of
heat and chemical pretreatment is commonly applied to improve the hydrogen yield
from algal substrate. For instance, microwave and steam heating with dilute acid
efficiently pretreated the biomass of C. pyrenoidosa with 8.6- and 9.5-fold increase
in H 2 yields, respectively (Xia et al. 2013). Similarly, Roy et al. (2014) obtained
high H 2 production (1.33 m
3 /m
3 ) and reducing sugar concentration (9.6 kg/m
3 )
from HCl-heat pretreated biomass of Chlorella sorokiniana. Thermal–alkaline
pretreatment of lipid-extracted Scenedesmus sp. biomass enhanced the H 2 production up to 168% (Yang et al. 2010).
Chemical pretreatment method involves the use of acid, alkali, solvents and
oxidizing agent for the cell wall disintegration and saccharification of microalgal
carbohydrates. Among the chemicals, acid and alkali reagents, generally in combination with heat, are used for the solubilization of organic matter. Liu et al. (2012)
reported H 2 production of 1.42 L/L from acid (HCl)-pretreated hydrolysate of
Chlorella vulgaris ESP6. In contrast, alkaline (NaOH)-pretreated hydrolysate was
found to be less efficient for biohydrogen production. However, strong acidic
conditions during the pretreatment may generate fermentative inhibitors such as
furfural and HMF due to the dehydration of sugars. Moreover, the formation of
inhibitory by-products can occur during the neutralization of the hydrolysate after
acidic or alkaline pretreatment (Liu et al. 2012; Harun et al. 2014). Oxidizing agent
such as H 2 O 2 generates the nascent oxygen which helps in breaking the glycosidic
bonds of complex sugars and converts it into simpler fermentable form. Roy et al.
(2014) observed better H 2 production from H 2 O 2 -pretreated algal biomass than
10 Biofuels from Microalgae: Biohydrogen
215
shear forces. Pretreatment method such as bead beating or milling is less dependent
on microalgal species and can break the cell wall due to the collision of microalgal
biomass with minute glass, ceramic or steel beads, under high agitation.
Ultrasonication is another pretreatment method for cell wall disruption and solubilization of the organic matter. In this method, the repetitive compression and
rarefaction of the sonic waves cause the formation of microbubbles which grow and
then collapse, generating high pressure and heat, shear forces and free radicals
thereby damaging the cell wall. Cheng et al. (2012) found that bead milling can
disrupt some of the cyanobacterial cells releasing carbohydrate and protein to be
utilized by hydrogenogens. However, through milling lower yield of H 2
(38.5 mLH 2 /g DW) was obtained as compared to ultrasonication pretreatment
(55.9 mL H 2 /g DW).
Thermal pretreatment utilizes heat energy for the solubilization of microalgal
biomass. Optimal range of temperature for the disintegration of organic matter
varies according to substrate characteristics. For microalgal biomass, pretreatment
temperature and time duration range from 65 to 180 °C and 15–60 min, respectively (Wang and Yin 2017). Thermal pretreatment by microwave heating is
favoured for uniform distribution of heat and for achieving higher temperature in
less time. Hydrothermal pretreatment (steam heating) at 100 and 121 °C led to
increase in carbohydrates and proteins solubilization from lipid-extracted
Scenedesmus biomass (Yang et al. 2010). However, thermal pretreatment alone is
not sufficient for efficiently hydrolysing the microalgal biomass. Combination of
heat and chemical pretreatment is commonly applied to improve the hydrogen yield
from algal substrate. For instance, microwave and steam heating with dilute acid
efficiently pretreated the biomass of C. pyrenoidosa with 8.6- and 9.5-fold increase
in H 2 yields, respectively (Xia et al. 2013). Similarly, Roy et al. (2014) obtained
high H 2 production (1.33 m
3 /m
3 ) and reducing sugar concentration (9.6 kg/m
3 )
from HCl-heat pretreated biomass of Chlorella sorokiniana. Thermal–alkaline
pretreatment of lipid-extracted Scenedesmus sp. biomass enhanced the H 2 production up to 168% (Yang et al. 2010).
Chemical pretreatment method involves the use of acid, alkali, solvents and
oxidizing agent for the cell wall disintegration and saccharification of microalgal
carbohydrates. Among the chemicals, acid and alkali reagents, generally in combination with heat, are used for the solubilization of organic matter. Liu et al. (2012)
reported H 2 production of 1.42 L/L from acid (HCl)-pretreated hydrolysate of
Chlorella vulgaris ESP6. In contrast, alkaline (NaOH)-pretreated hydrolysate was
found to be less efficient for biohydrogen production. However, strong acidic
conditions during the pretreatment may generate fermentative inhibitors such as
furfural and HMF due to the dehydration of sugars. Moreover, the formation of
inhibitory by-products can occur during the neutralization of the hydrolysate after
acidic or alkaline pretreatment (Liu et al. 2012; Harun et al. 2014). Oxidizing agent
such as H 2 O 2 generates the nascent oxygen which helps in breaking the glycosidic
bonds of complex sugars and converts it into simpler fermentable form. Roy et al.
(2014) observed better H 2 production from H 2 O 2 -pretreated algal biomass than
10 Biofuels from Microalgae: Biohydrogen
215