Microalgae: A Renewable Source of Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 220

production. Inclined systems have not received much attention, although they have promise
because of highly turbulent flow and very thin culture layers, thus promoting high cell
densities and a higher surface area to volume ratio compared with other open systems
(Richmond, 1999; Tredici, 2004). Although circular ponds require high energy input for
mixing, they are widely used in Asia (Japan, Korea and Indonesia) for Chlorella mass
production (Lee, 2001 ).
Raceway ponds are popular algal pond systems and are the preferred production system
for many of the commercial operations, including Cyanotech Corporation in Hawaii and
Earthrise in California, both of which produce Arthrospira (S pirulina ), as well as Seambiotic
in Israel (Plate 9.1c). Excessive evaporation is a problem in these regions and a minimum
depth of at least 15 cm is required to avoid a reduction in flow which can result in light
limitation (Tredici, 1999 ).
9.5.3 Photobioreactors
Photobioreactors are bioreactors where phototrophic microalgae and other microorganisms
and plant cells are grown under conditions that aim to ensure that algal growth can proceed
without light limitation. They are usually considered to be closed systems, that is, in contrast
to the low technology systems considered above, they do not allow the direct exchange of
gases such as carbon dioxide and oxygen or contaminants (other microorganisms,
atmospheric particles etc.) between the culture and the atmosphere (Tredici, 1999).
The status of photobioreactor technologies has been reviewed a number of times (Tredici,
1999 ; 2004 ; Pulz, 2001 ). While being more expensive to operate than low technology
systems such as ponds and raceways, photobioreactors ensure high productivities along with
quality control with low contamination rates. Some advantages of these systems and
technical issues have been discussed by Grobbelaar (2009). The dense cultures obtainable
using photobioreactors are easier to harvest than is the case for dilute open pond cultures.
Indeed, it is feasible to grow a dense culture that is already a slurry and, therefore, requires
little effort to harvest. Closed systems allow efficient utilisation of carbon dioxide and other
gases, such as flue gases, for microalgal production. Moreover, photobioreactors can use
both natural sunlight as well as artificial illumination. There are many different designs that
have been developed (Plate 9.1d), but most are not amenable to scale up to the levels required
for commercial production of microalgae.
The increased productivities achievable in photobioreactors versus open ponds were
demonstrated by Tredici and Materassi (1992). However, comparison of open pond systems
and various photobioreactors for a number of species, including Chlorella pyrenoidosa ,
Tetraselmis chui and Spirulina spp., made by Lee (2001) showed that productivities from
photobioreactors need not surpass those of open pond systems. More recent estimates of
productivities (Chisti, 2007; Wiffjels and Barbosa, 2010) suggest that photobioreactors will
be essential to achieve the high production necessary for commercial production of bioenergy
and other bioproducts.
Geographic location can play an important part in deciding whether microalgae culture
will be effective. A temperature of 15 °C or above is considered essential for sustained algal
production; outdoor productivities are affected by environmental variables such as low
seasonal and night-time temperatures as well as variable irradiance (Von Hamelen and
Oonk, 2006). Chini Zittelli and co-workers (2003) demonstrated that a combination of
natural and artificial illumination gave optimum productivities of Nannochloropsis between
December and May in the continental Mediterranean climate of Italy.