2.3.1 Open Systems
Raceway ponds systems consist of a closed-circuit recirculation channel, usually
with 0.3 m depth (Benemann and Oswald 1996; Craggs et al. 2012). Mixing and
circulation are produced by a paddlewheel, and flow is guided in the curves by
deflectors placed in the flow channel, as shown in Fig. 2. During the day, the
culture is fed continuously in front of paddlewheel, where run-off begins (Chisti
2007). The paddlewheel operates continuously to prevent sedimentation of
biomass.
In open systems, cooling is achieved only by evaporation (Chisti 2007). The
temperature varies seasonally and throughout the day, and a significant loss of
water by evaporation can be observed. As an open system, the use of CO 2 is less
efficient than in closed systems (due to loss of this compound to the atmosphere),
also representing a significant cost in the production of microalgae. In addition, the
open systems are more susceptible to contamination from other algae or microorganisms. Besides the requirement of large cultivation areas and the limitation of
cultivation period (due to contamination problems), these systems are associated to
low productivity and inefficient mixture that does not prevent the existence of
optically dark areas. However, raceway ponds have a lower production cost, as
compared to the closed photobioreactors (Chisti 2007; Harun et al. 2010; Pushparaj
et al. 1997). Additionally, the raceway ponds have the advantage of being easily
cleaned (removal of biofilm accumulated on the channels walls) (Chisti 2007).
Fig. 2 Cultivation system—raceway pond (adapted from Walter 2011)
3 Process Integration Applied to Microalgal Biofuels Production
41
Raceway ponds systems consist of a closed-circuit recirculation channel, usually
with 0.3 m depth (Benemann and Oswald 1996; Craggs et al. 2012). Mixing and
circulation are produced by a paddlewheel, and flow is guided in the curves by
deflectors placed in the flow channel, as shown in Fig. 2. During the day, the
culture is fed continuously in front of paddlewheel, where run-off begins (Chisti
2007). The paddlewheel operates continuously to prevent sedimentation of
biomass.
In open systems, cooling is achieved only by evaporation (Chisti 2007). The
temperature varies seasonally and throughout the day, and a significant loss of
water by evaporation can be observed. As an open system, the use of CO 2 is less
efficient than in closed systems (due to loss of this compound to the atmosphere),
also representing a significant cost in the production of microalgae. In addition, the
open systems are more susceptible to contamination from other algae or microorganisms. Besides the requirement of large cultivation areas and the limitation of
cultivation period (due to contamination problems), these systems are associated to
low productivity and inefficient mixture that does not prevent the existence of
optically dark areas. However, raceway ponds have a lower production cost, as
compared to the closed photobioreactors (Chisti 2007; Harun et al. 2010; Pushparaj
et al. 1997). Additionally, the raceway ponds have the advantage of being easily
cleaned (removal of biofilm accumulated on the channels walls) (Chisti 2007).
Fig. 2 Cultivation system—raceway pond (adapted from Walter 2011)
3 Process Integration Applied to Microalgal Biofuels Production
41