Here we discussed different system of microalgal biomass production, their prob­lems and future considerations, which is summarized in Table 18.2.

18.2.1 Open Pond Production

Open pond production is achieved through using a special type of pond called “race­way pond” that is a shallow artificial pond and is commonly used for the cultivation of microalgae. The pond is formed into a rectangular grid, containing an oval chan­nel resembling to an automotive raceway circuit. From an aerial view, many ponds look like a rounded-corner maze (like a puzzle in which we find the way in a closed circuit). It contains a paddle wheel to make the water flow continuously around the circuit (Fig. 18.2).

Raceway ponds (RP) have been used for mass culture of microalgae (Acien Fernandez et al. 1999) since last six decades. A best suited RP for microalgal culti­vation is usually 0.3 m deep. Paddle wheel provides mixing and circulation of water in the pond. They are built with solid concrete to avoid water seepage and the floor is lined with a white sheet for enhanced light reflection (to improve the light use efficiency in photosynthesis). Culture is fed continuously in front of paddle wheel during day light and broth is harvested on completion of the loop behind the paddle wheel. The continuous (24/7) operation of paddle wheel prevents the culture sedi­mentation (Spolaore et al. 2006). Evaporation of water from the RP is a necessary evil, because it is source of water loss but also helps to maintain temperature. Because, in summer cooling effect of evaporation lowers the temperature and in winter heat-retaining capacity of water does not allow the temperature to go too low. Overall, mitigation of water loss in raceway ponds due to seepage and evaporation is an engineering challenge.

Contamination of heterotrophic microbes is another offending challenge in open cultivation systems since it may strongly influence microalgae productivity as the contaminant microbes (bacteria, fungi etc.) become parasite on microalgae. Moreover, RPs may have poor mixing of atmospheric CO2 and least penetration of sunlight (because waste water has a dark color) keeping most of the cells in dark zone. Subsequently, low biomass is produced as compared to the theoretical yield calculated for a raceway pond based on its engineering dimensions. The mixing may be improved by using an aeration system and light penetration may be enhanced by decreasing the depth of pond (maybe 0.2 m instead of 0.3 m). Despite these facts,

production of microalgal biomass for making biodiesel has been extensively studied in RPs (Sheehan et al. 1998) . Although RPs have a low biomass productivity compared with photo-bioreactors yet they balance the equation by lowering the cost of production (Terry and Raymond 1985).