Mass culture of Spirulina is often conducted in large raceway ponds made of concrete or lined with reinforced plastic approved for use for potable water. This raceway design for outdoor mass cultivation of photosynthetic microorganisms was developed
in the 1950s and is used widely in the industry.29-30 A detailed description of the design of the ponds is given by Shimamatsu.31 The ponds used in several facilities vary in size from about 2000 m2 to the largest ponds of 5000 m2 and may contain anyway between 400 and 1000 m3 of culture depending on the pond depth used. The depth can vary between 15 and 40 cm depending on season, desired algal density, and, to a certain extent, the desired biochemical composition of the final product. Mixing of the culture is mandatory in outdoor mass culture in order to facilitate light distribution and minimize self-shading in an otherwise buoyant alga-like Spirulina. Mixing is facilitated by paddle wheels. Mixing also facilitates diffusion of nutrients and maintenance of uniform temperature with depth. Generally, the higher the mixing rate and hence light availability, the higher the growth rate. Mixing also avoids scum formation. Algal mass culture is often referred to as "scum." This is a misconception derived from observations in natural lakes and ponds where lack of mixing results in the accumulation and aggregation of floating biomass. This situation does not occur under controlled outdoor production where the algal culture is being mixed continuously by paddle wheels (Figure 1.2).
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