Page 258 - Algae

Algal Culturing 241

SEMI-CONTINUOUS CULTURES
In a semi-continuous system the fresh medium is delivered to the culture all at once, by simply
opening a valve in the medium delivery line. Fresh medium flows into the culture vessel, and
spent culture flows out into a collecting vessel. Once the required medium has entered the
culture, the valve is closed, and the culture is allowed to grow for 24 h, when the procedure is
repeated. The semicontinuous technique prolongs the use of large tank cultures by partial periodic
harvesting followed immediately by topping up to the original volume and supplementing with
nutrients to achieve the original level of enrichment. The culture is grown up again, partially
harvested, etc. Semi-continuous cultures may be indoors or outdoors, but usually their duration
is unpredictable. Competitors, predators, or contaminants and metabolites eventually build up, ren-
dering the culture unsuitable for further use. As the culture is not harvested completely, the semi-
continuous method yields more algae than the batch method for a given tank size.

COMMERCIAL-SCALE CULTURES
2
Existing commercial microalgae culture systems range in volume from about 10 l to more than
9
10 l. However, aside from the specialized small-scale culture systems (,1000 l) other types of
culture systems predominate: large open ponds, circular ponds with a rotating arm to mix the
cultures, raceway ponds, or large bags.
There are several considerations as to which culture system to use. Factors to be considered
include: the biology of the alga, the cost of land, labor, energy, water, nutrients, climate (if the
culture is outdoors), and the type of final product. The various large-scale culture systems also
need to be compared on their basic properties such as their light utilization efficiency, ability to
control temperature, the hydrodynamic stress placed on the algae, the ability to maintain the
culture unialgal or axenic and how easy they are to scale up from laboratory scale to large-scale.
The final choice of system is almost always a compromise between all of these considerations to
achieve an economically acceptable outcome. Successful further development of the industry
requires significant improvements in the design and construction of the photobioreactors as well
as a better understanding of the physiology and physical properties of the microalgae to be grown.
A common feature of most of the algal species currently produced commercially (i.e., Chlor-
ella, Spirulina, and Dunaliella) is that they grow in highly selective environments, which means
that they can be grown in open air cultures and still remain relatively free of contamination by
other algae and protozoa. Thus, Chlorella grows well in nutrient-rich media, Spirulina requires a
high pH and bicarbonate concentration, and Dunaliella salina grows at very high salinity. Those
species of algae which do not have this selective advantage must be grown in closed systems.
This includes most of the marine algae grown as aquaculture feeds (e.g., Skeletonema, Chaetoceros,
Thalassiosira, Tetraselmis, and Isochrysis) and the dinoflagellate Crypthecodinium cohnii grown as
a source of long-chain polyunsaturated fatty acids, as well as almost all other species being con-
sidered for commercial mass culture. In the particular case of C. cohnii, a large scale fermentation
plant is operated by Martek Biosciences in Winchester (USA).

Outdoor Ponds

Different types of ponds have been designed and experimented with for microalgae cultivation,
which vary in size, shape, materials used for construction, and mixing device. Large outdoor
ponds can be unlined, with a natural bottom, or lined with unexpensive materials such as clay,
brick, or cement, or expensive plastics such as polyethylene, PVC sheets, glass fiber, or poly-
urethane. Unlined ponds suffer from silt suspension, percolation, and heavy contamination, and
their use is limited to a few algal species, and to particular soil and environmental conditions.
Also natural systems such as euthrophic lakes or small natural basins can be exploited for
microalgal production, provided suitable climatic conditions and sufficient nutrients. Examples