Biomedical Engineering Reference
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and mixing. Bubbling of CO 2  into the bottom of reactors is generally favored, although
it achieves only moderate transfer efficiencies (13% to 20%) due to loss of CO 2  to the
atmosphere, fouling of diffusers, and poor mass transfer (Kumar et al., 2010).
Mixing must be almost continuous to prevent settling of biomass (Molina Grima,
1999) and can represent the major energy input into reactor maintenance. High rates
of mixing can also impose shear stress on microalgal cells, particularly in filamen-
tous species or those with delicate morphology (Greenwell et al., 2010). Mixing rates
are therefore a trade-off among enhanced growth rate, cell damage, and energy
requirement.
5.3 CULTIVATION SYSTEMS
A wide variety of open and closed reactor systems have been proposed for microal-
gal cultivation, possibly reflecting the diversity in the physiology and requirements
of different algal species. Ultimately, the overall goal is the continuous maintenance
of a desired algal culture under conditions for optimal productivity. High volumet-
ric and areal yields reduce cost by minimizing the reactor volume and land area
required, respectively. Important factors in achieving this include (Richmond, 2000):
• Provision of suficient light, despite daily and seasonal variations and dense
algal culture
• Optimal mixing and mass transfer, while avoiding damage to cells by shear
stress
• Minimization of deviation from optimal temperature (requires cooling in
summer and heating in winter)
• Minimization of dissolved oxygen tension
• Simple cleaning and maintenance
• Minimization of energy input requirements
• Minimization of water use (e.g., evaporation from ponds, evaporative
cooling use)
• Low capital and operating costs per unit of harvested product
5.3.1 o pen s ysteMs
Historically, the vast majority of commercial production has been carried out in
open ponds, and they are still the most widely applied reactor system in industrial
microalgal processes (Carvalho et  al., 2006). Open systems include natural water
bodies, circular ponds, raceway ponds, and cascade systems. The main constraints
on growth in open ponds are that it is impossible to control contamination, difficult
to keep the culture environment constant, and expensive to harvest the dilute biomass
(Carvalho et al., 2006). To maintain a monoculture in open ponds, highly selective
culture conditions are necessary in order to guarantee dominance by the desired
strain. For this reason, a limited number of species able to tolerate extreme condi-
tions (e.g., Spirulina, which grows at high pH, and Dunaliella , which requires high
salt concentrations) have been successfully grown. Open systems are susceptible to
changes in temperature and irradiance due to local weather and climatic conditions,
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