In contrast, the aeration system energy requirement is a significant cost in
bioreactors and also contributes to the carbon footprint of heterotrophic cultivations. So, for a viable biofuel production, a trade-off between the operating costs
related to energy required for aeration and the productivity of the bioprocess
(Santos et al. 2015). In this sense, Santos et al. (2015) concluded that for a heterotrophic bubble column bioreactor, the aeration of 0.5 VVM (volume of air per
volume of medio per minute) is an equilibrium between kinetic performance and
power requirements in bioreactor.
Mixing and Viscosity
Like in the cultivation systems already discussed, mixing is one of the most
important operations in heterotrophic microalgal cultivation. This operation is
necessary for uniformly distributing nutrients and for gas exchange. The adequate
mixing can be provided by impellers and baffles or by aeration with airlift or bubble
column systems (Perez-Garcia and Bashan 2015).
The viscosity of the medium is closely related to the mixing, where high viscosity in cultures requires higher impeller speed or airflow, which increases power
consumption and operational costs. The viscosity comes mainly from the exogenous carbon source used, but is also increased with the high cell concentration and/
or with the production of viscous cellular material.
2.3.2 Biomass Productivity in Heterotrophic Bioreactors
In terms of biomass production, the heterotrophic cultivations can present higher
values of productivity, when compared to the other large-scale systems discussed in
this chapter, as shown in Table 3, which summarizes the microalgal biomass
productivities in heterotrophic cultivations with different carbon sources, bioreactors type, and microalgae species reported in the literature.
2.3.3 Costs in Heterotrophic Bioreactors
The production costs of the heterotrophic microalgae production depend on variables such as bioreactor, carbon source, microalgae strain, downstream processing
operations, type and quality of the end product, among others. Tabernero et al.
(2012) evaluated the production of microalgal biodiesel from C. protothecoides
biomass grown heterotrophically. The cost estimated to produce one kilogram of
biomass was US$ 1.29 per kg (corrected for 2017). This value was estimated for a
biorefinery producing biomass in 465 continuously stirred bioreactors each of
150,000 L and producing 10 million/L/year of biodiesel.
A value below this (US$ 0.06/kg, corrected to 2017) was found by Roso et al.
(2015) to produce P. autumnale biomass, in a techno-economic analysis of a
16
M. M. Maroneze and M. I. Queiroz
bioreactors and also contributes to the carbon footprint of heterotrophic cultivations. So, for a viable biofuel production, a trade-off between the operating costs
related to energy required for aeration and the productivity of the bioprocess
(Santos et al. 2015). In this sense, Santos et al. (2015) concluded that for a heterotrophic bubble column bioreactor, the aeration of 0.5 VVM (volume of air per
volume of medio per minute) is an equilibrium between kinetic performance and
power requirements in bioreactor.
Mixing and Viscosity
Like in the cultivation systems already discussed, mixing is one of the most
important operations in heterotrophic microalgal cultivation. This operation is
necessary for uniformly distributing nutrients and for gas exchange. The adequate
mixing can be provided by impellers and baffles or by aeration with airlift or bubble
column systems (Perez-Garcia and Bashan 2015).
The viscosity of the medium is closely related to the mixing, where high viscosity in cultures requires higher impeller speed or airflow, which increases power
consumption and operational costs. The viscosity comes mainly from the exogenous carbon source used, but is also increased with the high cell concentration and/
or with the production of viscous cellular material.
2.3.2 Biomass Productivity in Heterotrophic Bioreactors
In terms of biomass production, the heterotrophic cultivations can present higher
values of productivity, when compared to the other large-scale systems discussed in
this chapter, as shown in Table 3, which summarizes the microalgal biomass
productivities in heterotrophic cultivations with different carbon sources, bioreactors type, and microalgae species reported in the literature.
2.3.3 Costs in Heterotrophic Bioreactors
The production costs of the heterotrophic microalgae production depend on variables such as bioreactor, carbon source, microalgae strain, downstream processing
operations, type and quality of the end product, among others. Tabernero et al.
(2012) evaluated the production of microalgal biodiesel from C. protothecoides
biomass grown heterotrophically. The cost estimated to produce one kilogram of
biomass was US$ 1.29 per kg (corrected for 2017). This value was estimated for a
biorefinery producing biomass in 465 continuously stirred bioreactors each of
150,000 L and producing 10 million/L/year of biodiesel.
A value below this (US$ 0.06/kg, corrected to 2017) was found by Roso et al.
(2015) to produce P. autumnale biomass, in a techno-economic analysis of a
16
M. M. Maroneze and M. I. Queiroz