Page 234 - Advances in Eco-Fuels for a Sustainable Environment

198 Advances in Eco-Fuels for a Sustainable Environment

recover flux [66, 67] Furthermore, the development of a continuous harvest system
that integrates cultivation and extraction processes will significantly improve the
effective use of filtration-based harvest technology because the integrated system
should be able to harvest a large-scale microalgal culture and the water can be
recycled for further cultivation.

7.6.2.4 Flotation
Microalgal cells are captured by upward gas bubbles in flotation, and the microalgal
biomass is then collected in the vacuole layer on top of the suspension. Microalgal
cells with a diameter from 10 to 500μm are preferred for effective flotation. Due
to the reduced surface charges on microalgal cells, the preaggregation of various
microalgal species was shown to be effective to attain the mass required for effective
flotation [68, 69]. In general, the flotation efficiency is dependent on the size of the
created bubble: nanobubbles (0.1 μm), microbubbles (1–999μm), and fine bubbles
(1–2mm). A smaller bubble size will exhibit a longer longevity and a larger carrying
capacity due to the increased surface area-to-volume ratio [70]. Hanotu et al. [68]
emphasized that small bubbles, which rise slowly, can more easily attach to micro-
algal cells and more stably transport them to the top surface of the suspension com-
pared with large bubbles. In addition to the bubble size, the surface characteristics,
such as the hydrophobicity and the charge of the microalgae, are also crucial factors
that determine the interaction between the cells and the bubbles. In an aqueous solu-
tion, the opposing surface characteristics of the microalgal cells (negatively charged
hydrophilic) and the air bubbles (negatively charged hydrophobic) can be modified to
ensure a better contact. Compared with conventional aeration, the interaction between
freshwater microalgae, such as Chlorella vulgaris and S. obliquus FSP-3, and bubbles
was enhanced by ozone flotation even though the negative surface charge of the algal
cells became stronger by ozonation [71, 72]. Ozonation effectively produced protein-
like substances through cell lysis during flotation. Researchers emphasized that the
algal hydrophobicity played a more crucial role in the flotation of marine microalgae
than did ionic strength, which is generally regarded as a primary inhibition factor in
marine microalgae flotation.

7.6.2.5 Magnetic separation

Magnetic microalgal harvest involves the use of both functionalized magnetic parti-
cles and an external magnetic field. Because both the microalgal cells and the mag-
netic particles have negatively charged surfaces in an aqueous medium, cationic
polyelectrolytes are needed as bridges between the magnetic particles on the algal
cells [72a]. Cationic binder-modified magnetic particles and microalgal cells are
incorporated through direct linking or electrostatic interactions. After the microalgal
cells are linked with the magnetic particles, the cells can be harvested with an external
magnetic field from the aqueous solution. Electrolysis-based technologies have been
widely adopted in the water industry for the removal of various contaminants, includ-
ing microalgae. Particularly in water treatment, the advanced oxidation process (AOP)