Environmental Engineering Reference
In-Depth Information
an area of several hundred to a few thousands square meters. Turbulence is usually
provided by rotating paddle-wheels which create a flow of the algal suspensions
along the channels at a rate of 0.2-0.5 m s
-1
. The adequate supply of carbon diox-
ide is very critical, and it is usually controlled through a pH-stat, thereby ensuring
both provision of carbon and optimum pH of the culture, simultaneously. The open
raceway pond reactor has some drawbacks that limit its use to strains that, by virtue
of their weed-like behavior (e.g.
Chlorella
) or by their ability to withstand adverse
growing conditions, as
Spirulina (Arthrospira)
or
Dunaliella
, can outcompete other
microorganisms. The more recently developed and technologically advanced closed
systems provide better options to grow virtually every microalgal strain, protecting
the culture from invasion of contaminating organisms and allowing exhaustive con-
trol of operation conditions. These photobioreactors are either flat or tubular and
can adopt a variety of designs and operation modes. In comparison to open systems,
closed photobioreactors offer high productivity and better quality of the generated
biomass (or product), although the latter are certainly more expensive to build and
operate than the former systems. On the basis of the two-stage strategy, different
approaches have been developed for the industrial production of astaxanthin, includ-
ing even the use of artificial light and an organic carbon source for mixotrophic
cultivation in closed photobioreactors [32, 40]. Usually, the systems utilized are
based on photoautotrophic growth conditions and involve either a combination of
a closed photobioreactor for the green phase and an open pond for the induction
phase [41] or closed tubular photobioreactors for both stages (Algatechnologies,
www.algatech.com
).
A two-stage system operating under continuous illumination indoors yielded a
product rich in astaxanthin (over 3% of dry biomass) and had a maximal reported
productivity of 11.5 mg l
-1
d
-1
of carotenoids, with astaxanthin representing about
94% of the total [30]. Values for productivity of two-stage systems outdoors are
rarely found in the literature [1]. Olaizola [41] reported a productivity of 2.2 mg
astaxanthin l
-1
d
-1
for large scale commercial facilities, and Aflalo et al. [30]
recently reported about 8-10 mg total carotenoids l
-1
d
-1
for the productivity out-
doors of a combined experimental set-up composed of a flat vertical panel (green
stage) and a horizontal tubular photobioreactor (red stage).
Currently, whereas commercial production of
Haematococcus
' astaxanthin is a
reality, technological advances are required for a substantial reduction of costs. This
would allow the competition of natural with synthetic astaxanthin so as to reach
markets other than the nutraceutical one.
A reduction in production costs requires significant improvement in astaxan-
thin yields. A major factor influencing astaxanthin productivity is the yield at the
growing phase, i.e., the “green stage” [17, 42]. However, most of the studies per-
formed have been focused on the induction phase of pigment accumulation [10, 13].
Recently, García-Malea et al. set and modeled optimal conditions for enhancing
growth in the “green stage” [43, 44]. Continuous culture with a simulated solar
cycle has been performed in bubble-column reactors operated indoors. A significant
productivity of green biomass (0.6-0.7 g l
-1
d
-1
) was obtained as a result of the
combination of high irradiance and nitrate concentration.
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