heterotrophic cultures is the demand for exogenous organic
carbon, since, in these types of cultivation, the carbon source
represents about 80% of the cost of the culture medium
(Francisco et al. 2015). In this sense, the obtainment of
organic carbon and other nutrients from industrial wastes
may offer an inexpensive alternative for microalgae cultures,
with parallel wastewater treatment (Queiroz et al. 2018).
Another option is the mixotrophic cultivation that is a
variant of the heterotrophic growth regime. In this case, the
microorganisms simultaneously assimilate organic carbon
and CO 2 and use both photoautotrophy and heterotrophy
(Mohan et al. 2015). Since photosynthesis is not the only
route available for obtaining energy, microalgal growth is
not strictly dependent on light. This eases the geometry of
the photobioreactors, making the scaling-up easier. The
differential of this mode of cultivation is that it is possible to
use both wastewaters as culture medium and CO 2 from
industrial wastes (Wang et al. 2014).
Regardless of the cultivation method, on a commercial
scale, the algal cultures require an enormous amount of
freshwater and compounds like carbon, nitrogen, phosphorous, and several other trace nutrients (Pandey et al. 2019).
Thus, the production of microalgae-based products in an
economical way depends on the source of water and nutrients used. As already discussed, industrial wastes are a
source of nutrients useful to support the microalgae growth,
nonetheless still has some bottlenecks that need to be considered. The main setbacks are the possibility of the presence
of biotic or abiotic growth inhibitors and complicated harvesting processes. These issues will depend on the source of
wastewater, and for this reason, they must be washed into
consideration when choosing the waste for biotechnological
use (Osundeko et al. 2019).
The biotic factors can be present in the form of viruses,
fungi, bacteria, zooplankton, and predators. Once established, herbivorous consumers can reduce or inhibit the
microalgae growth within just a few days. Besides, the
contamination with fungi and viruses can negatively affect
microalgal growth and induce changes in microalgal cell
arrangement, diversity, and succession (Park et al. 2011). To
overcome these biological barriers, the integrated pest
management that involves the application of chemical herbicides and pesticides has been identified as a viable solution, on the other hand, it will result in an augmentation in
the costs of the process, and with the prolonged use, the
microbiota may acquire resistance to these substances
(McBride et al. 2014). Other options with great potential for
success include ecological engineering strategies of aquatic
communities to promote beneficial interactions and genetic
and metabolic engineering techniques to improve the resistance of the microalgae strains (Bagwell et al. 2016).
The abiotic contaminants that can be present in wastewater include heavy metals, nitrogen oxides, sulfur oxides,
and ammonia, in which high concentrations can inhibit
microalgae growth. Clijsters and Assche (1985) demonstrated that in the presence of several heavy metals, the
chloroplast ultra-structure was disorganized. Besides this,
these compounds can inhibit microalgae photosynthesis at
physiological levels by blocking the prosthetic metal atoms
in the active site of important enzymes (Arita et al. 2015). At
the same time, when concentrations of essential nutrients in
wastewater are low, they need to be supplemented so that
there is no reduction in growth rates and lipid productivity
(Osundeko et al. 2019).
6 Biodiesel Characteristics
The biodiesel properties depend on the fatty acid profile of the
feedstocks used, which may vary from one another. Table 2
shows the fatty acid profile of distinct biodiesel feedstocks.
The biodiesel from the different feedstocks must meet a
specifications series. The biodiesel properties established by
ASTM International (ASTM D6751), European Union (EN
14214), and Brazil (ANP 45) are shown in Table 3 (ASTM
2002; European Standard (EN) 2003; ANP 2014).
The biodiesel quality is influenced by the fatty acid
profile, contaminants presence of the feedstock, production
process, and storage. The properties of biodiesel related to
fatty acid profile and contaminants inherent to the feedstock
include the iodine value, viscosity, cloud point, cetane
number, and phosphorus content. On the other hand, the
properties of biodiesel directly related to the production
process include free and total glycerin, carbon residue, ester
content, methanol content, and flashpoint, while those related to storage include oxidative stability, acidity value, and
content of water (Cavalheiro et al. 2020; Lobo et al. 2009).
As shown in Table 3, the biodiesel from microalgae
biomass grown in agro-industrial wastewater has an ester
content of about 99%, a cetane number of 55, an iodine
value of 73.5gl
2 100g
−1 , and a degree of unsaturation of 75%
(Maroneze et al. 2014). Noteworthy, the microalgae appear
to be the most realistic biodiesel feedstock, capable of
replacing traditional fuels in way more environmentally
friendly. These microorganisms can be capable of compensating and balance the growing demands for bioenergy
(Maroneze et al. 2019).
Many countries of Europe and America have begun to
assess the possible commercialization of biofuels from the
microalgae biomass. Many microalgae are favorable to the
production of biofuel due to the high content of lipids. The
current unfeasibility of microalgae biodiesel is due to the
elevated production cost (Khan et al. 2018). However, this
cost can be reduced considerably with the use of industrial
residues as a source of nutrients and water for cultivation
(Jacob-Lopes and Franco 2013).
114
R. R. Dias et al.
carbon, since, in these types of cultivation, the carbon source
represents about 80% of the cost of the culture medium
(Francisco et al. 2015). In this sense, the obtainment of
organic carbon and other nutrients from industrial wastes
may offer an inexpensive alternative for microalgae cultures,
with parallel wastewater treatment (Queiroz et al. 2018).
Another option is the mixotrophic cultivation that is a
variant of the heterotrophic growth regime. In this case, the
microorganisms simultaneously assimilate organic carbon
and CO 2 and use both photoautotrophy and heterotrophy
(Mohan et al. 2015). Since photosynthesis is not the only
route available for obtaining energy, microalgal growth is
not strictly dependent on light. This eases the geometry of
the photobioreactors, making the scaling-up easier. The
differential of this mode of cultivation is that it is possible to
use both wastewaters as culture medium and CO 2 from
industrial wastes (Wang et al. 2014).
Regardless of the cultivation method, on a commercial
scale, the algal cultures require an enormous amount of
freshwater and compounds like carbon, nitrogen, phosphorous, and several other trace nutrients (Pandey et al. 2019).
Thus, the production of microalgae-based products in an
economical way depends on the source of water and nutrients used. As already discussed, industrial wastes are a
source of nutrients useful to support the microalgae growth,
nonetheless still has some bottlenecks that need to be considered. The main setbacks are the possibility of the presence
of biotic or abiotic growth inhibitors and complicated harvesting processes. These issues will depend on the source of
wastewater, and for this reason, they must be washed into
consideration when choosing the waste for biotechnological
use (Osundeko et al. 2019).
The biotic factors can be present in the form of viruses,
fungi, bacteria, zooplankton, and predators. Once established, herbivorous consumers can reduce or inhibit the
microalgae growth within just a few days. Besides, the
contamination with fungi and viruses can negatively affect
microalgal growth and induce changes in microalgal cell
arrangement, diversity, and succession (Park et al. 2011). To
overcome these biological barriers, the integrated pest
management that involves the application of chemical herbicides and pesticides has been identified as a viable solution, on the other hand, it will result in an augmentation in
the costs of the process, and with the prolonged use, the
microbiota may acquire resistance to these substances
(McBride et al. 2014). Other options with great potential for
success include ecological engineering strategies of aquatic
communities to promote beneficial interactions and genetic
and metabolic engineering techniques to improve the resistance of the microalgae strains (Bagwell et al. 2016).
The abiotic contaminants that can be present in wastewater include heavy metals, nitrogen oxides, sulfur oxides,
and ammonia, in which high concentrations can inhibit
microalgae growth. Clijsters and Assche (1985) demonstrated that in the presence of several heavy metals, the
chloroplast ultra-structure was disorganized. Besides this,
these compounds can inhibit microalgae photosynthesis at
physiological levels by blocking the prosthetic metal atoms
in the active site of important enzymes (Arita et al. 2015). At
the same time, when concentrations of essential nutrients in
wastewater are low, they need to be supplemented so that
there is no reduction in growth rates and lipid productivity
(Osundeko et al. 2019).
6 Biodiesel Characteristics
The biodiesel properties depend on the fatty acid profile of the
feedstocks used, which may vary from one another. Table 2
shows the fatty acid profile of distinct biodiesel feedstocks.
The biodiesel from the different feedstocks must meet a
specifications series. The biodiesel properties established by
ASTM International (ASTM D6751), European Union (EN
14214), and Brazil (ANP 45) are shown in Table 3 (ASTM
2002; European Standard (EN) 2003; ANP 2014).
The biodiesel quality is influenced by the fatty acid
profile, contaminants presence of the feedstock, production
process, and storage. The properties of biodiesel related to
fatty acid profile and contaminants inherent to the feedstock
include the iodine value, viscosity, cloud point, cetane
number, and phosphorus content. On the other hand, the
properties of biodiesel directly related to the production
process include free and total glycerin, carbon residue, ester
content, methanol content, and flashpoint, while those related to storage include oxidative stability, acidity value, and
content of water (Cavalheiro et al. 2020; Lobo et al. 2009).
As shown in Table 3, the biodiesel from microalgae
biomass grown in agro-industrial wastewater has an ester
content of about 99%, a cetane number of 55, an iodine
value of 73.5gl
2 100g
−1 , and a degree of unsaturation of 75%
(Maroneze et al. 2014). Noteworthy, the microalgae appear
to be the most realistic biodiesel feedstock, capable of
replacing traditional fuels in way more environmentally
friendly. These microorganisms can be capable of compensating and balance the growing demands for bioenergy
(Maroneze et al. 2019).
Many countries of Europe and America have begun to
assess the possible commercialization of biofuels from the
microalgae biomass. Many microalgae are favorable to the
production of biofuel due to the high content of lipids. The
current unfeasibility of microalgae biodiesel is due to the
elevated production cost (Khan et al. 2018). However, this
cost can be reduced considerably with the use of industrial
residues as a source of nutrients and water for cultivation
(Jacob-Lopes and Franco 2013).
114
R. R. Dias et al.