Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 68

5.3.3.3 Hybrid Production Systems
As open and closed systems offer different advantages and disadvantages, it seems
practical that a combination of the two could provide the best of both worlds. This
idea has been investigated in various configurations, either by circulating culture
between open and closed reactors through a single growth stage, or by having a
two-stage culture regime where cells are moved from one to the other at a certain
point. A simple single-stage intermediate design is produced by enclosing or semi-
enclosing open ponds in tunnels or greenhouses to improve temperature control and
reduce evaporation and contamination. This is very effective in improving produc-
tivity, particularly in colder seasons, but comes at increased capital cost (Vonshak,
1997). Pushparaj et al. (1997) described a system where an alveolar panel oriented
toward the sun was coupled with an open raceway for gas transfer. Adding the panels
improved the productivity of the pond from 0.18 to 0.31 g L −1 d −1 .
In two-stage configurations, culture is usually grown initially in closed PBRs to
optimize the growth rate and minimize contamination of the inoculum, which is
then moved to an open pond for the second growth stage. Integration of open and
closed PBRs in this way could provide sufficiently large, clean inoculants to limited-
duration culture in outdoor raceways in order to significantly limit adverse events
(Greenwell et al., 2010).
The second cultivation stage often involves nutrient stress for accumulation of
a metabolite such as lipids or pigments. The nutrient stress stage is suited to open
ponds because the growth rate is naturally low and therefore not affected by the low
light availability (Brennan and Owende, 2010). Initial culture in closed reactors also
implies that the culture entering the pond is relatively dense and therefore less likely
to be contaminated, particularly in a nutrient-deprived environment (Singh et al.,
2011). This sort of system has been used for the production of astaxanthin from
Haematococcus (Huntley and Redalje, 2006) and described for the production of
biodiesel from Nannochloropsis (Rodolfi et al., 2009).
5.4 COMPARISON OF REACTOR TYPES
The microalgal reactors described above differ in features such as surface-to-volume
ratio, freedom to adjust orientation and inclination, efficiency of mixing and gas
supply (related to hydrodynamics and mass transfer), ease of maintenance, tempera-
ture regulation, and construction materials. Table 5.4 presents a comparison of these
design features in six major types of reactor. No reactor design is able to effectively
control all these parameters simultaneously; therefore, any choice will be a compro-
mise between the advantages and disadvantages of each system (Table 5.5).
5.4.1 t he o pen versus C losed s ysteM d ebate
The relative merits of closed and open systems have been extensively debated in
the microalgal literature (Pulz, 2001; Carvalho et al., 2006; Grobbelaar, 2009;
Mata et al., 2010). There is no doubt that open ponds are the primary systems used
in large-scale, outdoor microalgal cultures, but their commercial use has been
limited to species that can be maintained using an extreme cultivation environment