202
Immobilization in porous materials has the following main advantages over the
gel-entrapped cells: (i) the immobilizing particles better resist compression, which
unavoidably takes place in bioreactors; (ii) the immobilization occurs under mild
conditions, and for this reason, microalgae cells retain their viability; and (iii) it is
simpler and cheaper, so it better suits for large-scale immobilization. Drawbacks of
the immobilization on porous carriers are the following: (i) microalgae cannot be
completely immobilized due to cell leakage from the carrier surface, and (ii) if the
pores are large and deep, the internal mass transfer limitation becomes significant at
high local cell densities leading to decreasing of cell metabolic activity.
Thin-layer immobilization offers a short light travel distance through the attached
microalgae. It is a significant advantage for algal biotechnologies because such systems support much higher cell densities because short light path increases light
transmission and hence the potential culture density (Solovchenko et al. 2016). The
short light path, high surface-to-volume ratio, efficient mass transfer, and high culture density in the late exponential phase are typical for thin-layer systems (Doucha
and Lívanský 2014). Essentially, the immobilized high-density microalgal culture
represents a close approximation to a high-productivity thin-layer photobioreactor
combining the advantages of this cultivation system and immobilized algae
cultivation.
Various natural and synthetic carriers are used for adsorptive immobilization of
microalgae in such systems (Gross et al. 2015). The attachment materials have to
resist moist conditions, withstand the physical force applied during mechanical harvest, and allow active propagation of microalgae cells leading to the biofilm development. Recently, printing paper (Schultze et al. 2015), cellulose acetate membrane
(Liu et al. 2013), cotton duct (Gross et al. 2015), and lignocellulose materials such
as pine sawdust, rice husk, sugar bagasse, and oak sawdust (Zhang et al. 2017) were
used for microalgae biofilm formation in thin-layer cultivation system.
7.3.3 Active Immobilization
The method of cell entrapment in the bulk volume of a carrier is the most widely
used technique for microalgae immobilization (Mallick 2006). In this method, cell
suspension is mixed with natural or synthetic monomers, and the mixture is solidified to produce a polymeric gel of various forms. Polymerization can be achieved by
physical-chemical treatments such as cross-linking of the monomers with divalent
cations (Cohen 2001) or polyvinyl alcohol and lactides (Moreno-Garrido 2008),
photodimerization, iterative freezing and thawing, and thermal gelation (Willaert
2017). Entrapped microalgae are used mainly for biotechnological applications in
biosynthesis of secondary metabolites and bioremediation of waste, as test organisms in ecotoxicology, and for removing biogenic elements, e.g., phosphorus and
nitrogen, and heavy metals from wastewater (Eroglu et al. 2015).
Synthetic polymers (acrylamide resins, polyurethanes, polyvinyl alcohol, polypropylene), proteins (gelatin, collagen), or natural polysaccharides (chitosan,
S. Vasilieva et al.
Immobilization in porous materials has the following main advantages over the
gel-entrapped cells: (i) the immobilizing particles better resist compression, which
unavoidably takes place in bioreactors; (ii) the immobilization occurs under mild
conditions, and for this reason, microalgae cells retain their viability; and (iii) it is
simpler and cheaper, so it better suits for large-scale immobilization. Drawbacks of
the immobilization on porous carriers are the following: (i) microalgae cannot be
completely immobilized due to cell leakage from the carrier surface, and (ii) if the
pores are large and deep, the internal mass transfer limitation becomes significant at
high local cell densities leading to decreasing of cell metabolic activity.
Thin-layer immobilization offers a short light travel distance through the attached
microalgae. It is a significant advantage for algal biotechnologies because such systems support much higher cell densities because short light path increases light
transmission and hence the potential culture density (Solovchenko et al. 2016). The
short light path, high surface-to-volume ratio, efficient mass transfer, and high culture density in the late exponential phase are typical for thin-layer systems (Doucha
and Lívanský 2014). Essentially, the immobilized high-density microalgal culture
represents a close approximation to a high-productivity thin-layer photobioreactor
combining the advantages of this cultivation system and immobilized algae
cultivation.
Various natural and synthetic carriers are used for adsorptive immobilization of
microalgae in such systems (Gross et al. 2015). The attachment materials have to
resist moist conditions, withstand the physical force applied during mechanical harvest, and allow active propagation of microalgae cells leading to the biofilm development. Recently, printing paper (Schultze et al. 2015), cellulose acetate membrane
(Liu et al. 2013), cotton duct (Gross et al. 2015), and lignocellulose materials such
as pine sawdust, rice husk, sugar bagasse, and oak sawdust (Zhang et al. 2017) were
used for microalgae biofilm formation in thin-layer cultivation system.
7.3.3 Active Immobilization
The method of cell entrapment in the bulk volume of a carrier is the most widely
used technique for microalgae immobilization (Mallick 2006). In this method, cell
suspension is mixed with natural or synthetic monomers, and the mixture is solidified to produce a polymeric gel of various forms. Polymerization can be achieved by
physical-chemical treatments such as cross-linking of the monomers with divalent
cations (Cohen 2001) or polyvinyl alcohol and lactides (Moreno-Garrido 2008),
photodimerization, iterative freezing and thawing, and thermal gelation (Willaert
2017). Entrapped microalgae are used mainly for biotechnological applications in
biosynthesis of secondary metabolites and bioremediation of waste, as test organisms in ecotoxicology, and for removing biogenic elements, e.g., phosphorus and
nitrogen, and heavy metals from wastewater (Eroglu et al. 2015).
Synthetic polymers (acrylamide resins, polyurethanes, polyvinyl alcohol, polypropylene), proteins (gelatin, collagen), or natural polysaccharides (chitosan,
S. Vasilieva et al.