ABSTRACT
Algae can be cultivated in open or closed bioreactors. Open bioreactors are defined as any reactor that is exposed to the environment. These reactors can take many different forms, but most conform to one of the following broad categories: shallow lagoons and ponds, inclined cascade systems, circular central pivot ponds, mixed ponds, and raceway ponds (Borowitzka and Moheimani 2013). While these ponds are configured differently in terms of their construction, lining, means of propulsion/ mixing, and intensity of management, they all share the common element of being fully exposed to the external environment. Research on how to successfully cultivate microalgae using open systems was initiated in the late 1940s and early 1950s in the United States, Germany, and Japan (Cook 1950; Gummert et al. 1953; Mituya et al. 1953). While significant progress has been made over the intervening decades, the open pond systems still face serious challenges that stem from being exposed to unpredictable and uncontrollable meteorological conditions, suboptimal mixing within the culture, and exposure to many forms of contamination. These problems limit productivity, nutrient utilization efficiency, and performance stability. Despite these challenges, open ponds continue to be used and developed primarily because they are cheaper and easier to scale, build, and operate when compared to closed photobioreactors (Sheehan et al. 1998; Waltz 2009).
