2.2.1 Overview of Algae-Growing Systems
The commercial large-scale cultivation of microalgae began in earnest in the 1960s with the cultivation of Chlorella in Japan (Tsukuda et al., 1977) and the use of phytoplankton as a feedstuff for animals reared in aquaculture (Duerr et al., 1998). In the 1970s, Spirulina was harvested from Lake Texcoco in Mexico (Durand-Chastel, 1980) and produced in Thailand (Kawaguchi, 1980). By 1980, 46 large-scale facilities operated in Asia producing more than 1,000 kg of microalgae each month (Kawaguchi, 1980). The global production of microalgal biomass was estimated to be more than 5,000 dry tonnes in the year 2005 with a value of more than U.S. $1.25 billion, which excludes the value of processed products (Spolaore et al., 2006). About 3,000 dry tonnes of Spirulina are produced in China, India, Myanmar, the United States, and Japan; 2,000 dry tonnes of Chlorella are produced in Taiwan, Germany, and Japan; and 1,200 dry tonnes of Dunaliella salina are produced in Australia, Israel, the United States, and China (Spolaore et al., 2006). In 2008, the global production of microalgal biomass was estimated to be about 9,000 dry tonnes per year (Benemann, 2008).
In addition to algal biology and the intended algal products, numerous factors are considered in selecting the particular algal cultivation system to be used. These include the availability and cost of land, water, energy, nutrients, and labor, and the climate of the location (Borowitzka, 1992). The characteristics of each cultivation system, including its mixing or hydrodynamic characteristics, light utilization efficiency, ability to control temperature, ability to maintain a unialgal culture, and ease of scaling from laboratory to pilot and commercial scales also are considered (Borowitzka, 1999). The two general types of algal cultivation systems discussed in this report are open-pond systems and closed photobioreactor systems.
2.2.2 Open-Pond Systems
The majority of the large-scale microalgal production systems in commercial operation today are open-pond systems, mainly due to economic factors and ease of scale up. Most commercial-scale microalgal cultivation operations are for producing nutraceuticals, and none of them are for producing fuel. The number of microalgal species that can be grown effectively in open-pond systems is limited by the species’ ability to thrive in particularly selective environments while the ponds remain relatively free of protozoan and other algal species contamination (Borowitzka, 1999; Milledge, 2011). For example, Chlorella is grown in a nutrient-rich medium, Spirulina at high pH and bicarbonate concentration, and Dunaliella salina at high salinity (Borowitzka, 1999; Milledge, 2011).
The two most common types of open-pond systems are circular ponds and raceway ponds. Circular ponds are round ponds, with depths of 30-70 centimeters (Moheimani and Borowitzka, 2006). They are typically agitated through a centrally pivoted rotating arm. Ponds up to 45 meters in diameter have been operated in Japan and Taiwan (Becker, 1994). Oscillatoria grown in a circular pond achieved a productivity of about 15 grams dry weight per m2 per day (Sheehan et al., 1998). Mixing efficiency is poor in ponds with diameters greater than 50 meters (Shen et al., 2009). Raceway ponds (Figure 2-4 a-e) are constructed either as single units (Figure 2-4 b-e) or a group of continuous units that are joined together (Figure 2-4a). The raceway channels enable culturing algae in ponds with depths of 15-40 centimeters. The channels are constructed from concrete or compacted earth that might be lined with plastics. A paddle wheel, a propeller, or an air-lift pump operates at all times to agitate and circulate the mixture to prevent algae sedimentation (Becker, 1994; Chen et al., 2009). A key factor in open-pond design and operation is mixing, which evenly distributes