11.08.2015   BIOFUELS

Large-scale cultivation

There is a considerable body of information on the large-scale cultivation of micro­algae in bioreactors of various designs (Molina Grima et al., 2001; Scragg et al., 2002; Acien Fernandez et al., 2003; Chisti, 2007; de Morais and Costa, 2007a, b). The designs of photobioreactors can be divided into two types — open and closed — and the advantages and disadvantages of these types are outlined below:

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Open bioreactors:

• Natural water, raceway ponds, inclined surfaces. These can suffer from: water and CO2 loss, contamination and pollution, requirement of large area, limitation on the number of species that can be grown, no process control, dependency on weather, poor mixing and low biomass (0.1-0.2 g/l).

Closed bioreactors:

• Stirred vessels, tubular bioreactor, laminar bioreactor, plastic bag vessels. Best for high-value products, process control, continuous culture possible, all types of algae grown, flexible production, not affected by weather, high biomass (2-8 g/l).

It would appear that the two best bioreactor designs are the raceway and tubular designs. The raceway is considerably simpler but mixing is limited, temperatures vary and the biomass concentration is low. The tubular bioreactors are enclosed and with good mixing and circulation a high biomass concentration can be achieved without contamination. They are, however, more expensive to operate and require cooling during daylight. A wide range of bioreactor designs have been used to culture micro­algae and some examples of the various designs are given in Table 7.6.

Table 7.6. Alternative photobioreactor designs.

Bioreactor design

Algal species

Reference

Closed

Vertical tube

Chlorella sp. Scendesmus obliquus Spirulina sp.

de Morais and Costa (2007b)

Parabola, pipe, diamond

Chaetoceros calcitrans

Sato et al. (2006)

Tubular horizontal

Spirulina platensis

Richmond et al. (1993); Acien Fernandez et al. (2003); Molina Grima et al. (2001)

Horizontal tubular

Phaeodactylum tricornutum

Miron et al. (1999)

L-shaped

Euglena gracilis

Chae et al. (2006)

Internally illuminated stirred tank

Chlorella pyrenoidosa

Ogbonna et al. (1996)

5 l stirred tank bioreactor

Isochrysis galbana

Molina Grima et al. (1993)

Plexiglas annular

Nannochloropsis sp.

Zittelli et al. (2003)

Tubular/flat

Spirulina sp.

Tredici and Zittelli (1998)

Dual sparging column

Rhodamonas sp.

Eriksen et al. (1998)

Cone-shaped helical tubular

S. platensis

Watanabe and Hall (1996)

Helical tubular

Chlorella sp.

Scragg et al. (2003)

Vertical flat plate

Synechocystis aquatilis

Zhang et al. (2002)

Light supplied by optical fibres Stirred draft tube

Porphyridium purpureum

Fleck-Schnieder et al. (2007)

Open

Inclined

Chlorella sp.

Doucha and Livansky (2006)

Open tanks 220l

Chaetoceros sp.

Csordas and Wang (2004)

0.7 m deep polyethylene tanks

Chaetoceros sp.

Elias et al. (2003)

Raceways

Spirulina sp.

Olquin et al. (1997)

Fig. 7.8. Outline of the parameters which affect the growth of algae in bioreactors.

image129Whatever design is used, microalgal growth in bioreactors is influenced by four parameters: the supply of light, the supply of CO2, mixing (turbulence) and the build-up of oxygen (Grobbelaar, 1994) (Fig. 7.8). The maximum amount of light is about 30% saturation, and values above this can cause photobleaching, the loss of chlorophyll. The CO2 levels of 0.03% are below the optimum for growth, concentra­tion of 0.1% is more suitable, and there have been cases where 10% CO2 did not inhibit growth. In the light, microalgae can produce oxygen rapidly and a build-up of oxygen can inhibit growth. Mixing in the form of turbulence is essential to keep the cells in suspension and for gaseous exchange.

Commercial production of microalgae has been used to produce pigments, food supplements and shellfish food. The designs that have been used to produce micro­algae commercially are given in Table 7.7.