These 2001). The tubular photobioreactor exposes a

These are the most popular and widely used photobioreactors which
includes an array of transparent tubes in different patterns like spiral, bent
and straight. To ensure high biomass productivity the diameter of tube should
be less than 0.1 m (Chisti, 2007). The
tubular array capture sunlight and could have different orientation including horizontally,
vertically, inclined or
as a helix (Wang
et al. 2012). A tubular
photobioreactor consist of a degassing column (for gas exchange and cooling),
solar array for algae growth, the harvesting unit (to separate algae from the
suspension) and a circulation pump (Wang et al.
2012). Agitation and mixing are essential/important to stimulate the gas
exchange in the tubes (Brennan and Owende, 2010).

                                                           

Horizontal tubular photobioreactor provides a greater surface area to
volume ratio and have a better angle for incident light compared to vertical
tubular photobioreactor allowing for more efficient harvesting. However, may
requires expensive temperature control system due to high heat generation and
makes the scale up difficult (Richmond, 1987;
Wang et al. 2012). Helical tubular photobioreactor is another design
comprises of different shapes and is a hybrid of horizontal and vertical
tubular photobioreactor (Travieso et al. 2001).
The tubular photobioreactor exposes a large surface area to sunlight hence,
more suitable for outdoor mass cultures. The area size for tubular
photobioreactor may reaches up to 750 m3 (Pulz, 2001).

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1.3.2.2 Flat-plate photobioreactor

The
photobioreactor is made of small rectangular containers comprises of
transparent material. To obtain maximum exposure of solar energy the cylinders
are tilted at certain angle into direct path of light. Flat-plate
photobioreactor are of two types: pump driven and airlift flat-plate
photobioreactor. In case of pump driven photobioreactor, the mixing is achieved
by turbulence created by pumping whereas, airlift photobioreactor uses
compressed air for mixing. The photobioreactor are more suitable for culturing
since they allow proficient photosynthesis and low accumulation of dissolved O2.
However, faces problem of temperature control and adherence of algae to
reactors walls (Majid et al. 2014; Wang et al. 2012).

 

1.3.2.3 Vertical
column photobioreactors

These photobioreactors are usually cylinders having
diameter 0.2 m and height of up to 4 m. The small diameter increases the
surface-volume ratio and the height restriction is related with the gas
transfer limitations and the strength of the transparent materials used to make
the columns. The photobioreactors are aerated from the bottom which, enables
efficient CO2 utilization and O2 removal. The gas bubbles
allows the constant agitation of the medium and gentle mixing of culture with
no or very little shear stress which, leads to little cell damage compared to
impellers and pumps (Brennan and Owende, 2010;
Wang et al. 2012). These are categorized into four different types:
bubble column, internal loop airlift, split column airlift and external loop airlift column photobioreactors.
In comparison to bubble column photobioreactors, airlift photobioreactors can
withstand greater biomass production (Wang et
al. 2012).

 

1.3.3    Hybrid production systems

It is a two-stage cultivation system in which a combination of
photobioreactor and open pond method is used to grow algae. In the first stage,
algae is cultivated under controlled conditions in a photobioreactor to avoid
contamination and favor cell division. The second stage takes places in open
ponds where algal cells were adapted to ecological and nutrient stresses in
order to enhance the desired lipid production (Majid
et al. 2014).

 

1.3.4    Heterotrophic production

The method has been successfully used to cultivate algae biomass and
commercialization of high value pharmaceuticals products and food supplements (Salama et al. 2017). In the heterotrophic
production, the microalgae derive carbon and nitrogen from organic compound
present in the medium; therefore, exclude requirement for light as an energy
source. The method have advantages of controlled growth, high biomass density
and low harvesting costs. Researchers reported that high cell and biomass
densities can be achieved in fermenter under complete darkness using heterotrophic
cultivation method. Heterotrophic conditions have enhanced concentrations of Chlorella protothecoides, C. vulgaris and C. sorokiniana up to 3.4, 4.8 and 3.3, respectively, compared to
photo-autotrophic condition (Choix et al. 2012;
Liang et al. 2009; Perez?Garcia et al. 2010; Shi and Chen, 2002; Zheng et al. 2012). Microalgae cultivated using heterotrophic
method produces a range of biomass between 4 and 20 g L?1 day?1
of microalgae compared to 0.06–0.1 g L?1 day?1 and 0.36 g
L?1 day?1 in open ponds and closed photobioreactors,
respectively (Graverholt and Eriksen, 2007;
Pulz, 2001; Shen et al. 2010; Velea et al. 2018; Xiong et al. 2010).
Researchers have successfully enhanced the lipid content in microalgae through
heterotrophic cultivation e.g. under similar set of conditions, the lipid
content in heterotrophic culture of C.
protothecoides is 55.2 %, as compared to 15% (approximately 4 fold) in the
autotrophic culture of the identical microalgae (Xu
et al. 2006). According to Leyva et al.
(2014) under heterotrophic conditions C. vulgaris yielded faster growth and hoarded high lipid content
than autotrophic cultivation.

 

1.3.5    Mixotrophic production

In this method, microalgae grow in both phototrophic and heterotrophic
conditions. Microalgae undergoes photosynthesis and uses both inorganic and
organic substrate for their growth (Prokop et
al. 2015). The synergistic effect of organic substrate and light
enhances the biomass concentration and growth rate of algae. Mixotrophy makes
microalgae flexible because they process organic substrate which means growth
does not depends upon photosynthesis. Thus, during microalgae cultivation,
light is not a limiting factor as both light and organic sources can support
cell growth; example of mixotrophic algae are Spirulina platensis and C.
reinhardtii (Salama et al. 2017).
Mostly algae are primarily photolithotrophs and turns towards mixotrophy only
when the photolithotrophy is interrupted by lack of inorganic nutrients and
light. Some species of Chlorella are
both heterotroph/mixotroph and can be beneficial when exploits waste water,
which have carbon residue like digested dairy dung (Salama et al. 2017). Mixotrophic algae efficiently consumes
acetate present in the sewage waste water. For example, the presence of acetate
and minor organic fragments in anaerobically processed soybean waste water have
enhanced the growth of C. pyrenoidosa
(Zhan et al. 2016). Other than this, Chojnacka and Zieli?ska (2012) and Alkhamis and Qin (2013) investigated the effect
of phototrophic, heterotrophic and mixotrophic cultivation on growth of Spirulina sp. and Isochrysis galbana.
Their research reported higher growth rate under mixotrophic cultivation
compared to phototrophic and heterotrophic cultivations, respectively. Another
researcher reported increase in specific growth rate (3.40 d-1) and
maximum biomass dry weight (DW, 3.55 g L-1) using Chlorella sorokiniana under mixotrophic
conditions compared with heterotrophic (1.8 and 2.4 fold)  and phototrophic conditions (5.4 and 5.2
fold), respectively (Velea et al. 2018).
Thus microalgae cultivation under mixotrophic conditions can be a vital step in
production of renewable biomass to bioenergy/biofuel development.