Flat-plate reactors are characterized by a large surface area and lower O2 accumu­lation than tubular reactors (Ugwu et al., 2008). They generally consist of narrow panels, with walls made of glass or stiff Perspex® (Figure 5.6a). Productivity is max­imal at minimum light path length, but again the increased yield must be traded off against increased cost of materials to hold the same volume of culture. Reactors are usually modular, with working volumes of up to 1,000 L (Carvalho et al., 2006), and can be set up vertically or at an angle to the horizontal (Lee, 2001). The panels can have an open headspace for improved gas transfer, although such an open zone can compromise sterility. They are normally cooled either by spraying the flat surface with water (which can be collected for reuse by a trough at the base of the panel), or by sandwiching two panels together (one for algal growth and one for temperature modulation) (Tredici et al., 1991). In the past, there have been problems with circula­tion in flat-plate reactors (Carvalho et al., 2006), particularly at the base and in the corners of square panels. The main advantages of flat-plate reactors are their uni­form light distribution, the fact that reactors can be tilted to maintain optimal orien­tation toward the sun, and a reduced need for pumping if the culture is mixed by air.

A promising modification to the basic flat-plate design is the alveolar panel (Figure 5.6b). Alveolar reactors have flat panels divided into a series of internal channels (or alveoli) providing structural rigidity and enabling efficient flow of cul­ture medium (Greenwell et al., 2010). The walls are made of polycarbonate, PVC, or polymethyl methacrylate (Carvalho et al. 2006). Tredici et al. (1991) used double sets of alveolar plates, placed horizontally, with culture circulated in the upper set and the lower set acting as a thermostat to control the temperature. Alveolar plates have also been placed vertically, with air bubbling from the bottom of each channel. A comparison of the productivities achieved in a range of closed PBRs is presented in Table 5.3.

5.3.3

Alternative Designs

5.3.3.1 Stirred Tank Fermenter

Conventional heterotrophic fermenters (used routinely for cultivation of nonphotosyn­thetic microorganisms) have been used for the production of microalgae, particularly for high-value products such as fine chemicals and pharmaceuticals (Mata et al., 2010). The area-to-volume ratio of a stirred tank is low; therefore, some form of inter­nal illumination (e. g., artificial lighting or sunshine directed through optical fibers) is necessary, or cultures must be grown heterotrophically (Lee, 2001; Carvalho et al., 2006). Some algae are able to grow mixotrophically or heterotrophically on organic substrates such as glucose, acetate, or peptone (Grobbelaar, 2009). In this case, part or all of the carbon and energy is supplied by the organic substrate, thereby reducing the dependence of growth rate on light and CO2 provision. Mixotrophic growth rates (where cells utilize both light and organic substrates) are often greater than purely photo-autotrophic or heterotrophic (e. g., Chlorella and Haematococcus) (Lee, 2001).

The main advantages of stirred tank reactors are the precise control over operating parameters, the ability to maintain sterility, and the wealth of experience in their oper­ation and scale-up with yeast and microbes that exists. Maintaining sterility of cultures is crucial for the production of certain high-value metabolites (e. g., pharmaceuticals). Chlorella is routinely grown in stirred tanks up to high cell density (45 g L-1), with a volumetric productivity of up to 20 g L-1d-1 (Lee, 2001). When an organic substrate is added to the medium, sterility becomes a priority as bacteria readily compete with algae for the dissolved nutrients (Lee, 2001). Stirred tanks of up to 250 L have been run (Carvalho et al. 2006). Ogbonna et al. (1999) investigated the use of stirred tanks with a combination of sunlight and internal artificial lighting, which may reduce costs. Cultivation in stirred tank systems is limited to species able to assimilate organic car­bon substrates. Not all algae are able to grow heterotrophically (Lee, 2001).