Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 52

Improving productivity is key to achieving economic viability in large-scale, outdoor
cultures (Lee, 2001).
Microalgal cultivation has been carried out in a variety of vessels, ranging from
natural open lakes and ponds to highly complex and controlled photobioreactors
(PBRs). Typically, the term photobioreactor has been used to refer to closed systems
exclusively; however, by definition, open systems are also PBRs. A bioreactor is a
container in which living organisms are cultivated and carry out biological conver-
sions (e.g., biomass or product formation) (Grobbelaar, 2009). A PBR is a reactor in
which organisms that obtain energy from light, such as algae, plants, and certain
microbial cells (phototrophs), are used to carry out reactions (Mata et al., 2010).
Each type has advantages and disadvantages, but the overall goals are similar. In the
design of commercial algae cultivation systems, the aim is to achieve:
• Optimal volumetric and/or areal productivity
• Eficient conversion of light energy to product
• Consistency and reliability of production
• Cost effectiveness
Effective reactor design requires knowledge of both algal physiology and reac-
tor engineering, such as aspects of hydrodynamics and mass transfer (Ugwu et al.,
2008). Section 5.2 outlines the key requirements for algal growth and how these
relate to design considerations of the cultivation system. Section 5.3 describes the
range of open and closed systems that have been used for microalgal cultivation.
These are compared with respect to a range of attributes in Section 5.4.
5.2 GROWTH REQUIREMENTS AND DESIGN PARAMETERS
For optimal microalgal growth, several environmental parameters (e.g., temperature,
light intensity, pH, and nutrient concentrations) must be kept within narrow physi-
ological limits. The reactor system is critical in the provision and maintenance of a
favorable growth environment (Pulz, 2001). Hence, reactor design requires knowl-
edge of aspects of algal physiology, such as the morphology, nutrient requirements,
and stress tolerance of the species to be grown. Some of the requirements for micro-
algal growth are listed in Table 5.1, along with the consequences of under- or over-
provision, and the relevant reactor design features.
5.2.1 l iGht
Light is the principal limiting factor in the culture of photosynthetic organisms (Pulz,
2001); therefore, the intensity and utilization efficiency of the light supply are criti-
cal in reactor design (Kumar et al., 2010). The photosynthetic activity of microalgae
changes in response to light intensity in three distinct regions. At low light intensities,
cells are light limited and the photosynthetic rate increases with increasing irradiance.
Once cells become light saturated, the rate of photon absorption exceeds the rate of
electron turnover in Photosystem II (PS II), and there is no further increase in the
photosynthetic rate with increasing light intensity. Once irradiance increases above