Open pond culture

Because the defining feature of open pond systems is that they are open to the environment, there is a potential for contamination of the medium from other species of algae or protozoa which could outcompete the species of interest and therefore reduce the productivity of the culture. Because of this limitation, open systems must utilize species that are tolerant to extreme environments. Dumliella salina is one of the most tolerant eukaryotic organisms known based on its ability to tolerate salinity ranging from 3% to 31% NaCl and temperatures ranging from lower than 0°C to above 38°C. It is due in part to these characteristics which have allowed for the creation of a large global market for -carotene from D. salina as detailed in Section 6 of this chapter. 

The other type of open pond is a circular pond, which is used mainly in Asia for the production of Chlorella. These ponds are mixed by a centrally located rotating arm (similar to those used in wastewater treatment). Thin-layer, inclined ponds consist of slightly inclined shallow trays over which a veiy thin layer of algae flows to the bottom, where the culture is collected and returned to the top. Mild mixing or wave producers, such as paddlewheel, waterjet, or air pump systems, can be used to keep the water s velocity typically close to 30 cm/s much higher velocities require excessive amounts of mixing energy. 

In this chapter, open-pond operations or other open systems are not considered as a topic of discussion. There are roughly 20 different closed bioreactor design variants Table 1.2 summarizes the basic design principles. Two major aspects are important one is whether the cells are growing as individual single cells ( planktonic ) suspended in a culture medium, or whether they need to adhere to solid surfaces or to each other (state of biomass in Table 1.2) the second is how the... 

Schenk et al. (2008) mentioned that microalgae are among the most convenient sources of biodiesel due to their all-year-round availability. Microalgae cultured in an open pond was found to have a productivity that exceeds the yield of the best oilseed crops, producing 12,000 1 ha biodiesel compared with 11901 ha for rape-seed oil. 

Usually a relatively low cell density is achieved using the raceway pond system (< 1 g dry weight/L). Czech researchers developed an inclined thin-layer cascade system, which is able to sustain much higher cell density (Masojidek et al., 2011). In this system, the turbulent flow of Chlorella cultures on inclined channels is created by gravity a pump is applied to return the cultures fi-om the bottom to the top of the channels. The system is characterized by the highly turbulent flow, thin layer of suspension (less than 1 cm), and high ratio of exposed surface area to total volume, and thus can achieve dramatically higher volumetric yield (up to 40 g/L) than open ponds (Doucha and Livansky, 2009). However, the overall areal productivity obtained from this system is around 20—25 g/m /day (Doucha and Livansky, 2009), similar to that of open ponds (20 g/m /day, Richmond et al., 1990). 

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