Moreno et al. 2003; Jiménez et al. 2003; Garcia-Gonzalez et al. 2003; Richmond
1992; Richmond and Cheng-Wu 2001; Laws et al. 1983; Radmann et al. 2007).
8.4.2 Closed System
The more recently developed and technologically advanced closed systems provide better choices for growing almost every microalgal strain, since the culture is
protected from invasion of contaminating organisms and hence extremophile
microalgae are unnecessary unlike open systems.
Many different PBRs have been developed in the last three decades and different
designs were recently reviewed by Wang et al. (2012). Photobioreactors are closed
systems that can be designed in a variety of configurations, e.g., tubular and flatplate reactors. They have been used in the cultivation of Chlorella sp., Porphyridium
cruentum, P. tricornutum, A. platensis, Nannochloropsis sp., Chlorella sorokiniana,
H. pluvialis, and Tetraselmis suecica, among others (Fuentes et al. 1999; Acién et al.
2001; Tredici and Zitelli 1988; Chini Zittelli et al. 2006; Ugwu et al. 2002;
García-Malea et al. 2006a, b; Cheng-Wu et al. 2001; Degen et al. 2001; Gouveia and
Oliveira 2009; Rodolfi et al. 2009; Yoo et al. 2010).
Rigid vertical column or tubular photoreactors are usually cylinders with heights
of up to 4 m and radii of 0.2 m and smaller, to allow for light penetration to increase
surface–volume ratio. The height limit is related to the gas transfer limitations and the
strength of materials used to build the columns. In high columns, varying CO 2
gradients may be established which limit CO 2 to some of the microalgae, thereby
creating pH gradients (Xu et al. 2009). Furthermore, increased column length
increases O 2 concentration in the photoreactor due to photosynthesis, which can reach
a level that can inhibit growth. Vertical column PBRs are characterized by their high
volumetric gas transfer coefficients. Gas is usually bubbled from the bottom of the
column and usually enables optimal O 2 removal as well as effectual CO 2 utilization.
Two other types PBRs used are vertical flat panels and horizontal tubular
reactors. The vertical flat panels have advantages of higher luminosity of column
reactors since they are typically thinner than column reactors, thus decreasing light
path. While horizontal column PBRs, as the name implies, are usually placed
horizontally close to the ground. This is done to attain uniform solar radiation
along the entire columns. However, generally the vertical column PBR is preferred
because productivity per unit area of ground is greater by placing the photobioreactors vertically (Cuaresma et al. 2011).
In most photoreactors microalgal growth is limited by light penetration as the
microalgae culture grows, due to self-shading. In comparison to open pond,
generally photobioreactors sustain higher volumetric productivity than open
ponds, with better capture of radiant energy, more optimal use of the cultivation
area, and variable energy consumption values for mixing and gas/liquid mass
transfer (depending on the type of photobioreactor).
8 Microalgae and Cyanobacteria Production for Feed and Food Supplements
267
1992; Richmond and Cheng-Wu 2001; Laws et al. 1983; Radmann et al. 2007).
8.4.2 Closed System
The more recently developed and technologically advanced closed systems provide better choices for growing almost every microalgal strain, since the culture is
protected from invasion of contaminating organisms and hence extremophile
microalgae are unnecessary unlike open systems.
Many different PBRs have been developed in the last three decades and different
designs were recently reviewed by Wang et al. (2012). Photobioreactors are closed
systems that can be designed in a variety of configurations, e.g., tubular and flatplate reactors. They have been used in the cultivation of Chlorella sp., Porphyridium
cruentum, P. tricornutum, A. platensis, Nannochloropsis sp., Chlorella sorokiniana,
H. pluvialis, and Tetraselmis suecica, among others (Fuentes et al. 1999; Acién et al.
2001; Tredici and Zitelli 1988; Chini Zittelli et al. 2006; Ugwu et al. 2002;
García-Malea et al. 2006a, b; Cheng-Wu et al. 2001; Degen et al. 2001; Gouveia and
Oliveira 2009; Rodolfi et al. 2009; Yoo et al. 2010).
Rigid vertical column or tubular photoreactors are usually cylinders with heights
of up to 4 m and radii of 0.2 m and smaller, to allow for light penetration to increase
surface–volume ratio. The height limit is related to the gas transfer limitations and the
strength of materials used to build the columns. In high columns, varying CO 2
gradients may be established which limit CO 2 to some of the microalgae, thereby
creating pH gradients (Xu et al. 2009). Furthermore, increased column length
increases O 2 concentration in the photoreactor due to photosynthesis, which can reach
a level that can inhibit growth. Vertical column PBRs are characterized by their high
volumetric gas transfer coefficients. Gas is usually bubbled from the bottom of the
column and usually enables optimal O 2 removal as well as effectual CO 2 utilization.
Two other types PBRs used are vertical flat panels and horizontal tubular
reactors. The vertical flat panels have advantages of higher luminosity of column
reactors since they are typically thinner than column reactors, thus decreasing light
path. While horizontal column PBRs, as the name implies, are usually placed
horizontally close to the ground. This is done to attain uniform solar radiation
along the entire columns. However, generally the vertical column PBR is preferred
because productivity per unit area of ground is greater by placing the photobioreactors vertically (Cuaresma et al. 2011).
In most photoreactors microalgal growth is limited by light penetration as the
microalgae culture grows, due to self-shading. In comparison to open pond,
generally photobioreactors sustain higher volumetric productivity than open
ponds, with better capture of radiant energy, more optimal use of the cultivation
area, and variable energy consumption values for mixing and gas/liquid mass
transfer (depending on the type of photobioreactor).
8 Microalgae and Cyanobacteria Production for Feed and Food Supplements
267