Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 61

10 g L −1 Chlorella were achieved. However, the biomass productivity was compa-
rable to that of raceways (25 g m −2 d −1 ) (Lee, 2001). The system had a sloping base
made of glass, which rendered it very expensive, but the use of cheaper materials
could make it price competitive with raceway ponds. A similar system has been
used in Western Australia, consisting of a 0.5-ha sloping, plastic-lined pond for the
production of Chlorella , achieving similar biomass productivity (Borowitzka, 1999).
5.3.2 C losed s ysteMs
Although they are more expensive to build and run than open systems, the promise
of improved yields, and the possibility of growing a wider range of species, has led
to significant interest in closed reactors. It is much easier to control contamination
and environmental parameters in closed systems, allowing the cultivation of more
sensitive strains and expanding the potential product range. Biomass concentra-
tions obtained are higher than in open systems, thus reducing the cost of harvesting.
However, the capital and operating costs of closed reactors are higher than those of
open ponds (Carvalho et al., 2006).
A large variety of PBR designs have been proposed, only a few of which have been
commercialized (Greenwell et al., 2010). Most designs are based on the premise of
optimizing light provision by maximizing the area-to-volume ratio, while ensuring a
reasonable working volume, cost of reactor material, and mixing pattern (Carvalho
et al., 2006). One of the major problems with closed reactors is temperature control,
and the larger the area-to-volume ratio, the more susceptible the temperature of the
medium is to changes in environmental temperature. The optimum light path length
is 2 to 4 cm (Borowitzka, 1999), but most closed reactors have a larger diameter for
ease of mixing, cleaning, temperature regulation, and to increase the working volume
while reducing the cost of construction materials. Sedimentation is prevented by
maintaining turbulent flow through mixing mechanically or by airlift.
An important and often overlooked feature of closed reactors is the ease with which
they can be cleaned and sanitized (Chisti, 2007). Closed microalgal reactors are often
presented as having the advantage of a decreased risk of contamination. Contamination
can be avoided in closed reactors, but only if they are operated under sterile conditions,
which adds to the cost (Scott et al., 2010). Due to their large size and surface area, closed
reactors cannot be effectively sterilized by heat, and therfore require chemical steriliz-
ers. These are not always 100% effective and sometimes require large volumes of sterile
water to flush out the chemical agent. Most closed PBRs do not satisfy the good manu-
facturing practice requirements for production of pharmaceutical products (Lee, 2001).
The most common designs are tubular (Miyamoto et al., 1988; Richmond et al.,
1993; Borowitzka, 1996; Vonshak, 1997) or flat-plate (Hu et al., 1996; Vonshak, 1997)
reactors. Both types usually operate with culture circulated between a light-harvesting
unit, consisting of narrow tubes or plates, to provide a high surface area, and a reser-
voir or gas exchange unit in which CO 2 is supplied, O 2 removed, and harvesting car-
ried out. The circulating pump must be carefully designed so as to avoid shear forces
disrupting algal cells. A variety of microalgae, including Chlorella and Spirulina ,
have been successfully maintained in both tubular and flat-plate PBRs (Molina
Grima, 1999; Lee, 2001; Pulz, 2001; Carvalho et al., 2006).