other biofuels production process can be used in biogas production process. The
factors affecting biogas production consist of retention time, organic loading, pH,
temperature, quality of the substrates (characteristic of cell wall), pretreatment of
substrate, and the presence of methanogenesis inhibitors (Jankowska et al. 2017).
The digestibility of cell wall can be improved with the help of pretreatment which
further increase the biogas yield and help in intensification of the process. The
different pretreatment processes include mechanical (ultrasound, high pressure
homogenization, and microwave), thermal, chemical (use of alkali, acids, and ionic
liquids), and biological (enzymes). Ultrasound pretreatment can increase the
methane yield by up to 91% (Park et al. 2013). Microwave irradiation also has an
effect on the cell wall protein which results in the disruption of the cells leading to
easy access to the cellular material. Irradiation of microalgae with MW has been
reported to increase the production of biogas up to 79% (Passos et al. 2013).
Microwave irradiation can be a efficient technique for pretreatment as the pretreatment time required is less but high energy requirements might be an issue when
employed on large scale. Thermal pretreatment of microalgae cells is typically
performed using autoclaves, heat chambers, or water bath. González-Fernández
et al. (2012a) reported an increase in biogas yield by 123% with help of thermal
pretreatment. Disadvantage of this method is it consumes large amount energy but
the energy available after heating can be employed to maintain the temperature of
reactor during anaerobic fermentation and hence some heat integration approaches
can be thought of. Enzymes (mostly cellulase) can also be employed in biological
pretreatment of microalgae as they are rich in cellulose. The lipid extraction efficiency can be increased by up to 56% with help of enzymes (Fu et al. 2010). Cost of
enzymes is the major hindrance in the use of enzymes in pretreatment process. Acid
and alkali pretreatment come under the category of chemical pretreatment which
mostly uses sulfuric acid as the acid and sodium hydroxide as the alkali. With the
chemical pretreatment, the biogas yield can be increased by threefold to fourfold
(Jankowska et al. 2017). The summary of effects of different pretreatment processes
on biogas production has been reported in Table 3. Combinations of pretreatment
process like employing dilute acid pretreatment with microwave or
ultrasound-assisted approach can further result in significant increase in biogas
yield. Such combined processes also help in reduction of process cost and overcome the disadvantages of individual methods.
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S. Joshi and P. Gogate
factors affecting biogas production consist of retention time, organic loading, pH,
temperature, quality of the substrates (characteristic of cell wall), pretreatment of
substrate, and the presence of methanogenesis inhibitors (Jankowska et al. 2017).
The digestibility of cell wall can be improved with the help of pretreatment which
further increase the biogas yield and help in intensification of the process. The
different pretreatment processes include mechanical (ultrasound, high pressure
homogenization, and microwave), thermal, chemical (use of alkali, acids, and ionic
liquids), and biological (enzymes). Ultrasound pretreatment can increase the
methane yield by up to 91% (Park et al. 2013). Microwave irradiation also has an
effect on the cell wall protein which results in the disruption of the cells leading to
easy access to the cellular material. Irradiation of microalgae with MW has been
reported to increase the production of biogas up to 79% (Passos et al. 2013).
Microwave irradiation can be a efficient technique for pretreatment as the pretreatment time required is less but high energy requirements might be an issue when
employed on large scale. Thermal pretreatment of microalgae cells is typically
performed using autoclaves, heat chambers, or water bath. González-Fernández
et al. (2012a) reported an increase in biogas yield by 123% with help of thermal
pretreatment. Disadvantage of this method is it consumes large amount energy but
the energy available after heating can be employed to maintain the temperature of
reactor during anaerobic fermentation and hence some heat integration approaches
can be thought of. Enzymes (mostly cellulase) can also be employed in biological
pretreatment of microalgae as they are rich in cellulose. The lipid extraction efficiency can be increased by up to 56% with help of enzymes (Fu et al. 2010). Cost of
enzymes is the major hindrance in the use of enzymes in pretreatment process. Acid
and alkali pretreatment come under the category of chemical pretreatment which
mostly uses sulfuric acid as the acid and sodium hydroxide as the alkali. With the
chemical pretreatment, the biogas yield can be increased by threefold to fourfold
(Jankowska et al. 2017). The summary of effects of different pretreatment processes
on biogas production has been reported in Table 3. Combinations of pretreatment
process like employing dilute acid pretreatment with microwave or
ultrasound-assisted approach can further result in significant increase in biogas
yield. Such combined processes also help in reduction of process cost and overcome the disadvantages of individual methods.
74
S. Joshi and P. Gogate