254
M. M. Maroneze et al.
To eliminate these limitations, closed photobioreactors (PBRs) were developed,
that support the growth of a much wider selection of strains, under precisely
controlled conditions, with a low risk of contamination. These systems are especially important when working with sensitive species such as H. pluvialis. The major
drawbacks of PBRs are related to high construction and operation cost, difficulty in
scaling up, and high shear stress. Due to the high productivity and the high operational control provided by the PBRs, a considerable effort is being invested into the
development of a wide range of configurations to reduce these limitations and thus
increase the technical and economic viability of microalgae-based processes and
products. However, until now, for large-scale commercial use, tubular photobioreactors are almost exclusively used (Table 11.1) (Borowitzka 2018). Another type
of closed system that deserves special mention is the vertical bioreactors used by
AstaReal to induce astaxanthin production in H. pluvialis mixotrophic cultivations
(AstaReal 2019).
It is essential to consider that all culture systems have advantages and limitations,
and there is no perfect system suitable for all applications and microalgae species.
Therefore, the choice of culture system and culture mode for commercial production
depends on several factors, including (i) reliability of the culture; (ii) type and quality
of the desired product; and (iii) operating and capital costs (Chang et al. 2017;
Maroneze and Queiroz 2018).
The first criterion to be considered is the reliability of the culture, which means
a guaranteed supply of biomass. To achieve reliability, microalgal strains must meet
specific requirements, such as (i) rapid growth in growing conditions (exceptionally
light and temperature); (ii) broad temperature and irradiance tolerance; (iii) high
shear and oxygen tolerance; and (iv) growth in a selective environment (e.g., high
salinity or high pH) to reduce the likelihood of contamination (Borowitzka 2018).
The type and quality of the desired product will define the price of the final product
and the required purity and is, therefore, a deciding factor in choosing the cultivation
system. Here the final biomass must contain a high content of the target product; for
this, often, the reaction system must allow the use of stress conditions that induce the
synthesis of these compounds, such as irradiance, pH, nutrient content, or salinity. As
already mentioned, to associate high biomass productivity with the high intracellular
contents of the product of interest, two-stage systems are often used.
Finally, influenced by the parameters mentioned above, capital and operating
costs will determine the economic viability of commercializing microalgae-based
products. Consideration should be given to the costs of land (open pond systems
require a much larger land area than closed photobioreactors), construction, installation, maintenance, labor, energy and nutrients requirements, and labor. Besides this,
should be considered the climate conditions and geographic position, that will reflect
in potential costs for cooling or heating the cultures.
M. M. Maroneze et al.
To eliminate these limitations, closed photobioreactors (PBRs) were developed,
that support the growth of a much wider selection of strains, under precisely
controlled conditions, with a low risk of contamination. These systems are especially important when working with sensitive species such as H. pluvialis. The major
drawbacks of PBRs are related to high construction and operation cost, difficulty in
scaling up, and high shear stress. Due to the high productivity and the high operational control provided by the PBRs, a considerable effort is being invested into the
development of a wide range of configurations to reduce these limitations and thus
increase the technical and economic viability of microalgae-based processes and
products. However, until now, for large-scale commercial use, tubular photobioreactors are almost exclusively used (Table 11.1) (Borowitzka 2018). Another type
of closed system that deserves special mention is the vertical bioreactors used by
AstaReal to induce astaxanthin production in H. pluvialis mixotrophic cultivations
(AstaReal 2019).
It is essential to consider that all culture systems have advantages and limitations,
and there is no perfect system suitable for all applications and microalgae species.
Therefore, the choice of culture system and culture mode for commercial production
depends on several factors, including (i) reliability of the culture; (ii) type and quality
of the desired product; and (iii) operating and capital costs (Chang et al. 2017;
Maroneze and Queiroz 2018).
The first criterion to be considered is the reliability of the culture, which means
a guaranteed supply of biomass. To achieve reliability, microalgal strains must meet
specific requirements, such as (i) rapid growth in growing conditions (exceptionally
light and temperature); (ii) broad temperature and irradiance tolerance; (iii) high
shear and oxygen tolerance; and (iv) growth in a selective environment (e.g., high
salinity or high pH) to reduce the likelihood of contamination (Borowitzka 2018).
The type and quality of the desired product will define the price of the final product
and the required purity and is, therefore, a deciding factor in choosing the cultivation
system. Here the final biomass must contain a high content of the target product; for
this, often, the reaction system must allow the use of stress conditions that induce the
synthesis of these compounds, such as irradiance, pH, nutrient content, or salinity. As
already mentioned, to associate high biomass productivity with the high intracellular
contents of the product of interest, two-stage systems are often used.
Finally, influenced by the parameters mentioned above, capital and operating
costs will determine the economic viability of commercializing microalgae-based
products. Consideration should be given to the costs of land (open pond systems
require a much larger land area than closed photobioreactors), construction, installation, maintenance, labor, energy and nutrients requirements, and labor. Besides this,
should be considered the climate conditions and geographic position, that will reflect
in potential costs for cooling or heating the cultures.