The cultivation of microalgae offers advantage of carbon-neutral

The selection of technology for the cultivation/production of algal
biomass for energetic purposes is a key issue. To achieve higher yield of algal
biomass all the aspects including temperature, CO2, sunlight and
nutrients need to be controlled during cultivation. Microalgal biomass
cultivation can be proceeded using photoautotrophic, heterotrophic and
mixotropic production, which follows natural growth processes (Brennan and Owende, 2010; Salama et al. 2017).   

 

1.3.1 Photoautotrophic
cultivation

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This is the most common, technically and economically feasible method
of large scale production of algae. In this method, microalgae utilizes light
as energy source and CO2 as carbon source through a process called
photosynthesis. The lipid/fatty acid content varied from 7% to 58% during
phototropic cultivation depending upon the type of algal species used (Salama et al. 2017). Photoautotrophic
cultivation of microalgae offers advantage of carbon-neutral (utilizes CO2
as carbon source) and cost effective process. According to the
literature, microalgae cultivation deploy two different systems namely open
pond and closed bioreactors; technological viability of each of which depends
upon type of algal strain used, climatic conditions, cost of land and water (Brennan and Owende, 2010; Majid et al. 2014; Salama et al. 2017)

 

1.3.1.1 Open
pond cultivation

This is the one of the most common/simplest and oldest method of
microalgae cultivation and has been practiced since 1950s (Pulz and Scheibenbogen, 1998). The naturally
existing water bodies like ponds, lakes, lagoons, etc. are collectively called
as open ponds. The open pond usually range from 1 acre to several acres in size
and 1 cm to 100 cm in depth (Ravindran et al.
2016). The ponds are designed shallow for easy penetration of solar
radiation. There are different type of open pond system mainly categorized
based on their size, shape, and material used for their construction; which
includes unstirred open pond, circular ponds and raceway open ponds (Medipally
et al. 2015). Among these the raceway ponds are the most commonly
used artificial system (Brennan and Owende,
2010). Open ponds methods offers selective advantages for example they
can be established on minimal crop production areas, require less energy, less
technical in design and more scalable. However, major drawbacks includes high
land requirement, more prone to contamination (from air as well as ground) and
are limited by abiotic factors such as temperature, light intensity and
dissolve oxygen concentration (Harun et al.
2010). Artificial open ponds are further categorized as:

 

1.3.1.1.1 Unstirred open ponds

The unstirred open ponds are the simplest and economical but most
ineffective method for microalgae cultivation. These ponds are usually less
than a half meter in depth so that algal cell absorb light easily. According to
Shen et al. (2009) in Australia, Dunaliella salina has been commercially
cultivated in large unstirred open ponds to produce produce ?-carotene, a
nutraceutical that can sell for $54/g at retail. However, the whole system does
not have control on the factors involved in cultivation process. For example,
under the influence of gravity algal cells settles down in the form of residues
and thus reduces the availability of light and CO2 (Majid et al. 2014). Further, the low
productivity and contamination are the barriers associated with this method (Ting et al. 2017).

 

1.3.1.1.2 Circular ponds

These ponds have a centrally pivoted rotary agitator and is used for
cultivation of Chlorella in Japan,
Taiwan and Indonesia (Borowitzka and Moheimani,
2013). The diameter of a
circular pond is 45-50 m with a depth ranging from 30 and 70 cm and may be of
up to 0.5 ha in area (Borowitzka and Moheimani,
2013; Christiansen, 2011). These are constructed using concrete with a
mixing arm mounted at the center to improve mixing. However, the major
problem/disadvantage with system is that its scale is restricted to an upper
range of 1,000 m2 approximately. At this range the core pivot mixer
becomes unmanageable because of stress (Majid et al. 2014).

 

1.3.1.1.3
Raceway ponds

Among the open ponds system, the raceway ponds are extensively used for
commercial microalgae cultivation as well as for wastewater treatment. The
design is introduced by Oswald and his colleagues in early 1950s and is
cheapest to construct and use (Majid et al.
2014). These ponds are typically made of closed loop, oval shaped
recirculation channels, usually 0.2 to 0.5 m deep; which are constructed either
with concrete or compacted earth lined with plastics. The surface area of
commercial scale ponds ranges from 1 ha to 200 ha (Brennan and Owende, 2010; Hallmann, 2015). A paddle wheel is used
to derive/circulate the algae broth and nutrients in the looped channels mixes
the algal cell and prevent sedimentation. Though the microalgae’s CO2 demand
is satisfied from the surface air however, CO2 can be enhanced using
submerged aerators. Dunaliella,
Chlorella, Haematococcus and Spirulina
are some example of algal species which are cultivated in a raceway pond. There
are many systems which have been trailed for mixing and circulating algae in
the raceway ponds includes: a) Air lifts; b) Archimedes screws; c) Propellers;
d) Pumps (impellers); e) Water jets; f) Paddlewheels (Hallmann, 2015). The raceway ponds are well developed however
faces a difficulty of contamination from unwanted algae/bacteria, low
productivity and high cost (due to low cell density).

 

1.3.2
Closed photobioreactor systems

The closed photobioreactors are more versatile which are designed to
overcome the major problems/issues associated with open ponds system during
microalgae cultivation. They can be located either indoor or outdoor with
artificial light or natural light, respectively. The closed configuration of
the photobioreactors makes it a best choice to cultivate sensitive strains as
potential contaminants can be controlled easily in a closed system compared to
open ponds. Although closed system is efficient in terms of cell biomass
productivity however, biomass production is not cost economic because of cost
of the nutrient media. The photobioreactors system have various configurations
which includes: Tubular, Flat-plate, and Vertical-column photobioreactor (Brennan and Owende, 2010).

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