Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 66

b)

a)

FIGURE 5.6 Schematic diagram of (a) flat plate and (b) alveolar panel reactor.

5.3.3 a lternative d esiGns
5.3.3.1 Stirred Tank Fermenter
Conventional heterotrophic fermenters (used routinely for cultivation of nonphotosyn-
thetic microorganisms) have been used for the production of microalgae, particularly
for high-value products such as fine chemicals and pharmaceuticals (Mata et al.,
2010). The area-to-volume ratio of a stirred tank is low; therefore, some form of inter-
nal illumination (e.g., artificial lighting or sunshine directed through optical fibers) is
necessary, or cultures must be grown heterotrophically (Lee, 2001; Carvalho et al.,
2006). Some algae are able to grow mixotrophically or heterotrophically on organic
substrates such as glucose, acetate, or peptone (Grobbelaar, 2009). In this case, part
or all of the carbon and energy is supplied by the organic substrate, thereby reducing
the dependence of growth rate on light and CO 2 provision. Mixotrophic growth rates
(where cells utilize both light and organic substrates) are often greater than purely
photo-autotrophic or heterotrophic (e.g., Chlorella and Haematococcus ) (Lee, 2001).
The main advantages of stirred tank reactors are the precise control over operating
parameters, the ability to maintain sterility, and the wealth of experience in their oper-
ation and scale-up with yeast and microbes that exists. Maintaining sterility of cultures
is crucial for the production of certain high-value metabolites (e.g., pharmaceuticals).
Chlorella is routinely grown in stirred tanks up to high cell density (45 g L −1 ), with a
volumetric productivity of up to 20 g L −1 d −1 (Lee, 2001). When an organic substrate
is added to the medium, sterility becomes a priority as bacteria readily compete with
algae for the dissolved nutrients (Lee, 2001). Stirred tanks of up to 250 L have been
run (Carvalho et al. 2006). Ogbonna et al. (1999) investigated the use of stirred tanks
with a combination of sunlight and internal artificial lighting, which may reduce costs.
Cultivation in stirred tank systems is limited to species able to assimilate organic car-
bon substrates. Not all algae are able to grow heterotrophically (Lee, 2001).

5.3.3.2 Wave/Oscillatory Flow Reactors
Oscillatory flow bioreactors contain equally spaced orifice plate baffles in a tubular
style reactor. This reactor has improved heat and mass transfer due to the oscillatory
motion that is imposed on the net flow of the fluid, which means that the degree of
mixing is independent of the net flow. This results in long residence times in rela-
tively low length-to-diameter ratios. This reactor design, therefore, has the potential
to decrease the energy required for mixing algae cultures, due to decreased pumping
requirements, and also leads to decreased capital costs (Harvey et al., 2003).