Review of Literature
2.6 Culturing of microalgae: mode of nutrition and reactor types
2.6.2 Various reactor systems used for algal culturing
and nutraceuticals. Closed systems comprises tubular, flat panel and column photobioreactors (Table 2.5).
Fig. 2.10 Schematic representation of the open raceway ponds used for cultivation of microalgal strains
Tubular photobioreactor contains an array of tubes arranged horizontally, vertically or inclined with an airlift system at the top of the reactor for proper mixing of air and CO2
(Fig. 2.11). The tubes are usually made up of transparent glass or acrylic or plastic materials and measures about 0.1 m or less in diameter thereby enhancing the light penetration ability.
The 25 m3 reactors at Mera Pharmaceuticals, Hawaii (Olaizola, 2003) and the 700 m3 plant in Klotze, Germany (Pulz, 2001; Janssen et al., 2002; Spolaore et al., 2006) are the largest commercial scale tubular reactor facilities constructed for growing photoautotrophic cells.
Although tubular photobioreactors are often considered the most suitable for large scale cultures of microalgae (Chisti, 2007), the length of the tubes are limited by O2 accumulation, CO2 depletion, and pH variations which are the major disadvantage with this system.
Therefore, the commercial scale plants rely on multiple reactor units with lesser surface area for effective mass transfer operations (Janssen et al., 2002).
Table 2.5 Benefits and limitations of open ponds and photobioreactor types on algal growth (modified from Brennan and Owende, 2010)
Reactor system
Benefits Limitations
Open pond Low installation cost and operational cost
Easy operation
Less energy inputs
Utilize non-agricultural land
Best for open outdoor conditions
Easy scalability
Large land requirement
Less biomass productivity compared to other reactor types
Poor light penetration
Reduced mass transfer and CO2
mixing
Risk of contamination Flat-panel
photobioreactor
Large illumination area
High biomass productivity
High mass transfer
Easy operation and sterilization
Suitable for outdoor conditions
Scale up difficulty
Difficulty in controlling temperature
Wall growth possibility
Increased hydrodynamic stress
High installation cost Tubular
photobioreactor
Large illumination area
Cheap installation and operational cost
Suitable for outdoor conditions
High biomass productivities
Reduced mass transfer
Difficulty in controlling temperature
Wall growth possibility
Large land requirement
High installation cost Column
photobioreactor
Proper CO2 mixing and less shear
Low energy consumption
Easy sterilization
Small illumination area
High installation cost
Poor light penetration
Column PBRs also have received immense attention due to their high mass transfer capabilities, mixing options and best controllable growth conditions like temperature, pH and CO2 purging. A simple reactor can be constructed by hanging a polyethylene bag vertically on a framework or in a support. Usually the inner diameter of these reactors are maintained in between 0.3 to 0.5 m and the heights at 1 to 2.5 m. These reactors do have various advantages and disadvantages as depicted in table 2.5. The flat panel (or flat plate) photobioreactors were designed which supports the growth of high cell densities with better mass transfer capabilities and avoids oxygen build up.
Fig. 2.11 Schematic representation of a simple tubular photobioreactor used for cultivation of microalgal strains (adopted and modified from Brennan and Owenede 2010)
Fig. 2.12 shows a schematic view of the flat panel closed photobioreactor that can be operated under photoautotrophic condition, heterotrophic and mixotrophic conditions.
The reactor is a thin cuboid with an inner width of 10-20 mm usually made up of thin transparent glass or polyacrylic sheet. The high dense culture is mixed or flown across this flat panel which enables proper light distribution all through the reactor (Hu et al., 1998;
Degen et al., 2001; Richmond et al., 2003). The highest densities of photoautotrophic cells, which can exceed 80 g L-1 was reported by Hu et al. (1998). Several advantages of these closed PBRs over open ponds such as (i) high photosynthetic efficiency; (ii) light availability; (iii) efficient CO2 mixing ability; (iv) ability to control the process parameters like temperature, pH etc.; and (v) less chances of contamination makes them a robust culture systems available for the growth of algae (Maity et al., 2014).
Fig. 2.12 Schematic view of a closed flat panel photobioreactor for growing algae under photoautotrophic, photoheterotrophic and mixotrophic conditions
Light is the major critical factor that influences the choice of species and the rate limiting factor in almost all culture systems (Kaewpintong et al., 2007; Walker, 2009). The light intensity decreases exponentially as per the equation 2.2 with distance from a reactor surface as the biomass concentration increases.
𝐼𝑑 𝐼0
⁄ = 𝑒(−𝛾𝑑) (2.2)
Where 𝐼𝑑 represents the light intensity at depth d, 𝐼0 is the original incident intensity, 𝛾 is the turbidity (Chen et al., 2011). Therefore, a short light path will be favorable for the optimal light transmission and to reach higher photosynthetic efficiencies. Due to these problems associated with light penetration, various photobioreactor designs with different illumination strategies have been developed to enhance the microalgae production rate and oil/lipid content (Ma and Hanna, 1999). The large natural light source available is the
sunlight in which the light intensity varies from 0 lux to 1 × 105 lux in the night and day times respectively. Moreover, with this dynamic change in light intensity the oil yield may vary between 100 and 130 m3 ha-1 while, under controlled artificial illumination the yield may rise up to 170 m3 ha-1 in laboratory conditions (Chisti, 2007). In general, the laboratory scale PBRs utilize fluorescent lamps, light emitting diodes (LEDs), optical fibers etc., as the light source. A combination of LEDs and solar excited optical fibers with solar panels was proposed as the feasible artificial illumination system with less electricity consumption Chen et al. (2011) for large scale outdoor cultivation. Still much improvement and advancement is required in this aspect to make the whole process sustainable.
In hybrid systems, both open ponds as well as closed photobioreactor operating at different modes or at different growth phases are used in combination to get better results.
In general, the first stage of growth is performed in a PBR for increasing the biomass concentration to the maximum with very less contamination, while the second stage of growth was conducted in an open pond under stress conditions for lipid accumulation. Such two stage strategies are also followed in many microalgal systems like Chlorella, Scenedesmus and Nannochloropsis sp. etc. (Rodolfi et al., 2009) for enhancing the net lipid productivity.