Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 59

thus making it difficult to maintain optimal growth conditions. Low cell densities
are usually obtained, requiring large volumes to be processed during harvesting, and
thus increasing the cost of product recovery (Carvalho et al., 2006).

5.3.1.1 Natural Waters
Open systems include natural waters, lakes, and dams where the growth of the
microalgae of interest either occurs naturally or is encouraged through addition of
nutrients. Harvesting is carried out in situ; for example, Spirulina is harvested com-
mercially from Lake Texcoco in Mexico. Although naturally harvested microalgae
incur very little cost in cultivation, the productivity and product quality (biologically
and toxicologically) cannot be assured (Lee, 2001).

5.3.1.2 Circular Ponds
The first mass culture of microalgae was carried out in circular ponds (Lee, 2001).
They are generally simple, round, concrete ponds or dams, mixed by a rotating cir-
cular arm fixed in the center of the pond, or by manual stirring. The size of the pond
is limited by the strain of the water resistance against the rotating motor. The largest
reported pond is 50 m in diameter (Lee, 2001). They are commonly used in Japan,
Taiwan, Indonesia, and Ukraine for Chlorella cultivation (Lee, 2001; Pulz, 2001).

5.3.1.3 Raceway Ponds
The most commonly used design for commercial microalgal production is the
raceway pond. A raceway is an oval-shaped, single- or multiple-loop recirculation
channel (Figure 5.1), usually 15 to 20 cm deep, with mixing provided through circu-
lation by a rotating paddlewheel (Pulz, 2001; Brennan and Owende, 2010). Baffles
are often placed in the bends of the flow channel to guide the water and facilitate
mixing (Chisti, 2007). They are commonly built from concrete or packed earth,

Bae

Flow

Paddlewheel

FIGURE 5.1 Schematic diagram of a raceway pond.