main component found in biogas (approximately 25–50%). Biogas can be upgraded
up to >97% methane content and used as a substitute for natural gas
(Toledo-Cervantes et al. 2017) to generate electricity.
The first mention of using microalgal biomass to produce biogas was long ago
(Golueke et al. 1957), but the idea was taken to the modern times with the work of
Sialve et al. (2009) and mainly due to the efforts to improve the economy and
sustainability of biodiesel production from microalgae lipids (Harun et al. 2011)
using the waste defatted biomass. Biogas production from anaerobic digestion of
microalgae biomass is primarily affected by organic loads, temperatures, pH, and
retention times in the reactor used. Besides, it was demonstrated that biogas
potential is also strongly dependent on the microalgae species and biomass
pre-treatment (Alzate et al. 2012; Jankowska et al. 2017; Harun et al. 2011;
Mussgnug et al. 2010).
As was previously mentioned, biomethane from microalgae biomass can be used
as gaseous fuel and to generate electricity, whereas the spent biomass can be used to
make biofertilizers or a wide range of biofuels and chemicals in a thermochemical
approach. Although microalgae biomass offers good potential for biogas production, industrial production has still not been fully implemented.
3.3.3 Fermentation
Bioethanol is usually obtained by alcoholic fermentation from carbohydrates, such
as sugars, cellulose, or starch (Harun et al. 2014; Ho et al. 2013; Tan et al. 2014) or
previously hydrolyzed lignocellulosic feedstocks. Microalgal bioethanol can be
produced through two distinct processes: via dark fermentation or yeast
fermentation.
The dark fermentation of microalgae consists of anaerobic bioethanol production
by the microalgae themselves through the consumption of intracellular starch (Ueno
et al. 1998). The yeast fermentation process of microalgal biomass is well known
industrially, and to achieve higher yields, it is necessary to screen microalgal strains
with high carbohydrate content or induce accumulation of intracellular starch. On
the other hand, polysaccharides on the microalgal cell walls are not easily fermentable for bioethanol production by microorganisms. For fermentation, an acid
pre-treatment has been proposed as the best option compared to other pre-treatment
methods, namely in terms of cost-effectiveness and low energy consumption (Harun
and Danquah 2011). During the bioethanol fermentation process, the pH is maintained in the range of 6–9, because a pH below 6 or above 9 could slow down
bioethanol production. The fermentation process consumes less energy, and the
process is much simpler in comparison with the biodiesel production system. In
addition, the CO 2 produced as a by-product from the fermentation process can be
recycled as carbon source for microalgae cultivation, thus reducing greenhouse gas
emissions as well.
Hydrogen can be also produced by dark fermentation (DF) through the
spore-forming bacteria, such as Clostridium. There are comprehensive reviews on
112
P.-L. Gorry et al.
up to >97% methane content and used as a substitute for natural gas
(Toledo-Cervantes et al. 2017) to generate electricity.
The first mention of using microalgal biomass to produce biogas was long ago
(Golueke et al. 1957), but the idea was taken to the modern times with the work of
Sialve et al. (2009) and mainly due to the efforts to improve the economy and
sustainability of biodiesel production from microalgae lipids (Harun et al. 2011)
using the waste defatted biomass. Biogas production from anaerobic digestion of
microalgae biomass is primarily affected by organic loads, temperatures, pH, and
retention times in the reactor used. Besides, it was demonstrated that biogas
potential is also strongly dependent on the microalgae species and biomass
pre-treatment (Alzate et al. 2012; Jankowska et al. 2017; Harun et al. 2011;
Mussgnug et al. 2010).
As was previously mentioned, biomethane from microalgae biomass can be used
as gaseous fuel and to generate electricity, whereas the spent biomass can be used to
make biofertilizers or a wide range of biofuels and chemicals in a thermochemical
approach. Although microalgae biomass offers good potential for biogas production, industrial production has still not been fully implemented.
3.3.3 Fermentation
Bioethanol is usually obtained by alcoholic fermentation from carbohydrates, such
as sugars, cellulose, or starch (Harun et al. 2014; Ho et al. 2013; Tan et al. 2014) or
previously hydrolyzed lignocellulosic feedstocks. Microalgal bioethanol can be
produced through two distinct processes: via dark fermentation or yeast
fermentation.
The dark fermentation of microalgae consists of anaerobic bioethanol production
by the microalgae themselves through the consumption of intracellular starch (Ueno
et al. 1998). The yeast fermentation process of microalgal biomass is well known
industrially, and to achieve higher yields, it is necessary to screen microalgal strains
with high carbohydrate content or induce accumulation of intracellular starch. On
the other hand, polysaccharides on the microalgal cell walls are not easily fermentable for bioethanol production by microorganisms. For fermentation, an acid
pre-treatment has been proposed as the best option compared to other pre-treatment
methods, namely in terms of cost-effectiveness and low energy consumption (Harun
and Danquah 2011). During the bioethanol fermentation process, the pH is maintained in the range of 6–9, because a pH below 6 or above 9 could slow down
bioethanol production. The fermentation process consumes less energy, and the
process is much simpler in comparison with the biodiesel production system. In
addition, the CO 2 produced as a by-product from the fermentation process can be
recycled as carbon source for microalgae cultivation, thus reducing greenhouse gas
emissions as well.
Hydrogen can be also produced by dark fermentation (DF) through the
spore-forming bacteria, such as Clostridium. There are comprehensive reviews on
112
P.-L. Gorry et al.