*3.1.3 Biomass by horizontal photobioreactors*

This is an outdoor microalgal cultivation system, which has tubes laid on the ground to form a network of loops (see **Figure 12 (b)**). A pump is used to mix the microalgal suspended culture, which raises the culture vertically periodically into a photobioreactor. Pulsed mixing at intervals produces better results than continuous mixing. *Arthrospira sp.* used as a dietary supplement was reported to have higher productivity because of a better-suited temperature range and an extended cultivation period during warm weather periods [23]. The horizontal tubular photobioreactor was assembled with three loops of 80 m each (see **Figure 12**) connected via a manifold to a bubble column used for oxygen removal, temperature control, nutrient, and antifoam addition. The horizontal photobioreactor was operated at a

**Figure 11.** *Vertical tubular photobioreactors for culturing microalgae [23].*

#### **Figure 12.**

*Horizontal tubular photobioreactor of different orientations [27]: (A) standing tubular bioreactor. The cultivation area occupied is less compared to the floor-type. (B) floor tubular reactor. This occupies a large cultivation area.*

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*Microalgae: The Multifaceted Biomass of the 21st Century*

the growth inhibitory effect of dissolved oxygen [24].

**4. Metabolic platforms for microalgal growth**

*3.1.4 Biomass by the air-lift method*

*Different types of airlift photobioreactor [26].*

**Figure 13.**

microalgae (see **Table 3**) [29].

**4.1 Autotrophic metabolism**

liquid velocity of 0.45 m s−1. To prevent high concentrations of dissolved oxygen, a superficial gas velocity (vgs) of 0.04 m s−1 was used in the bubble column [23]. The productivity of *Nannochloropsis sp*. suffered a setback in 250% air-saturated dissolved oxygen solution; and as the dissolved oxygen concentration was raised above 300%, the growth of *Nannochloropsis sp.* was stalled, and that of *Neochloris oleoabundans* was inhibited. However, increasing the airflow rate in the reactor removed

This is an outdoor microalgal cultivation technique for the production of biomass and metabolites under a highly controlled environment. By this technique, the air is moved within the system to circulate the medium in which microalgae is growing. The culture is grown in transparent tubes that lie horizontally on the ground and are connected by a network of pipes (see **Figure 13**). Air is passed through the tube such that air escapes from the end that rests inside the reactor that contains the culture and creates an effect like stirring [28]. Other configurations of the airlift reactor are an improvement over this design. The external-loop ALR is a promising configura-

tion for breakthrough scale-up *Scenedesmus sp.* biomass production [26].

Different microalgae strains acclimate in different environments, evolving their metabolic pathways to stimulate and propagate growth. However, the extent of growth depends on the composition of the culture media which can be enhanced by either inorganic or organic carbon metabolism or both. Other co-factors such as nutrient availability, pH, chemical oxygen demand (COD), and temperature also influence growth, and the accumulation of metabolites in

The photosynthetic CO2-fixation in microalgae suffices to possess a greater ability to fix CO2. Photo trophy refers to an autotrophic mode of metabolism in which

*DOI: http://dx.doi.org/10.5772/intechopen.94090*

*Microalgae: The Multifaceted Biomass of the 21st Century DOI: http://dx.doi.org/10.5772/intechopen.94090*

*Biotechnological Applications of Biomass*

*3.1.3 Biomass by horizontal photobioreactors*

*Vertical tubular photobioreactors for culturing microalgae [23].*

surface area for the interaction of light and the algal cells, increasing the time of gas mass transfer in the culture broth, and the efficient uptake of nutrients. Most times, commercial cultivation of microalgae in vertical reactor systems and reactors of other configurations is not economically viable in batch mode, due to the time taken to load, unload, and clean the reactor systems. The vertical tubular reactor can be made of alveolar panels, polyethylene sleeves, or glass tubes and supported on steel frames (see **Figure 11**). The low productivity characterizing this reactor system is overcome when the surface area to volume ratio is increased. The O2 gas mass

This is an outdoor microalgal cultivation system, which has tubes laid on the ground to form a network of loops (see **Figure 12 (b)**). A pump is used to mix the microalgal suspended culture, which raises the culture vertically periodically into a photobioreactor. Pulsed mixing at intervals produces better results than continuous mixing. *Arthrospira sp.* used as a dietary supplement was reported to have higher productivity because of a better-suited temperature range and an extended cultivation period during warm weather periods [23]. The horizontal tubular photobioreactor was assembled with three loops of 80 m each (see **Figure 12**) connected via a manifold to a bubble column used for oxygen removal, temperature control, nutrient, and antifoam addition. The horizontal photobioreactor was operated at a

*Horizontal tubular photobioreactor of different orientations [27]: (A) standing tubular bioreactor. The cultivation area occupied is less compared to the floor-type. (B) floor tubular reactor. This occupies a large* 

transfer is aided by bubbling air through the culture broth [23].

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**Figure 12.**

**Figure 11.**

*cultivation area.*

**Figure 13.** *Different types of airlift photobioreactor [26].*

liquid velocity of 0.45 m s−1. To prevent high concentrations of dissolved oxygen, a superficial gas velocity (vgs) of 0.04 m s−1 was used in the bubble column [23]. The productivity of *Nannochloropsis sp*. suffered a setback in 250% air-saturated dissolved oxygen solution; and as the dissolved oxygen concentration was raised above 300%, the growth of *Nannochloropsis sp.* was stalled, and that of *Neochloris oleoabundans* was inhibited. However, increasing the airflow rate in the reactor removed the growth inhibitory effect of dissolved oxygen [24].
