The raceway pond system is currently the most economically feasible cultivation method for mass production of algal biomass, primarily due to its relatively low capital cost and ease of operation. The pond usually consists of a closed-loop recirculation channel (oval in shape) where mixing and circulation are provided by paddlewheels to avoid algal biomass sedi­mentation. The CO2 source is sparged at the bottom of the raceway pond, as shown in Figure 12.1 (Chisti, 2007; Greenwell et al., 2010; Stephenson et al., 2010). Some raceway ponds incorporate artificial light in the system; however, this design is not practical and is economically infeasible for commercial production (Singh et al., 2011).

FIGURE 12.1 Raceway pond for algal cultivation. (Modified from Brennan and Owende, 2010.)

Raceway ponds are normally constructed with either concrete or compacted earth and lined with white plastic bags. The depth of the pond is usually 0.2-0.5 m to ensure that algae receive adequate exposure to sunlight (Brennan and Owende, 2010; Chisti, 2007). Under this cultivation system, the recorded algal biomass productivity and yield were 0.05-0.1 g/L/day and 0.3-0.5 g/L, respectively (Pulz, 2001); but are highly dependent on algal strains, cultivation conditions, and local weather.

Although raceway ponds have the advantages of low energy input and low operating cost, this system still suffers several limitations, such as massive loss of water due to high evap­oration rate and being easily contaminated by undesired microorganisms (e. g., bacteria, fun­gus, and protozoa) that could annihilate the entire algal population (Schenk et al., 2008). Hence, regular cleaning and maintenance are required in the raceway pond to ensure that the algae are growing under optimal conditions. In this regard, high lipid content and bio­mass productivity of algae for biofuel production are not the only factors to be considered, but other considerations such as fast growth rate, ease of cultivation, and ability to survive under extreme environmental conditions are equally important to ensure the existence of monoculture in a raceway pond. Chlorella, for example, can grow well in a nutrient-rich medium, Spirulina grows favorably at high pH and bicarbonate concentration, and D. salina is well adapted to a highly saline medium (Borowitzka, 1999; Brennan and Owende, 2010).