204
7.3.4 Self-Immobilization
For the cell adsorption or entrapment with support materials, the additional costs for
carriers and immobilization procedures are involved, which often makes this technology economically unprofitable. By contrast, self-immobilization process (flocculation) presents a lot of advantages over immobilization such as the following:
• More cost-effective immobilization process
• Better cell growth due to less mass transfer limitation
• Less physical restriction exerted on cells
• Easier for application in the immobilization procedure
• Higher cell densities as no support materials occupy the working volume of the
bioreactors
The main disadvantage of some self-immobilization technique is that they may
cause unwanted changes in cell composition of microalgae during prolonged cultivation (Benemann and Oswald 1996), so they are more appropriate for the production of primary metabolites. The use of self-immobilization for biomass harvesting
can also be an efficient alternative to the time-consuming and expensive techniques
such as filtration and centrifugation.
Microalgal cells can stick together to form cell aggregates naturally or under
certain artificially induced conditions. In biotechnological processes, flocculation
can be carried out in various methods. Chemical flocculation using salts of Zn
2+
,
Al
3+
, and Fe
3+
is the most used technique (McGarry 1970). Flocculation can also be
induced by changing the microalgae cultivation conditions such as increasing the
medium pH, nutrient depletion, and temperature changes. Increasing the medium
pH value resulted in the high flocculation efficiency of up to 90% for freshwater
(Chlorella vulgaris, Scenedesmus sp., Chlorococcum sp.) and marine microalgae
(Nannochloropsis oculata, Phaeodactylum tricornutum) (Wu et al. 2012). It has
been revealed that the cell aggregations did not occur in waters with calcium and
magnesium deficiency (Yahi et al. 1994); thus, it was assumed that flocculation of
microalgae at high pH is caused by chemical precipitation of calcium and/or magnesium salts.
Polyelectrolytes can be used to enhance the aggregation of microalgal cells. The
surface of algal cells contains various functional groups (de la Noue and de Pauw
1988). They can be deprotonated or protonated depending on the pH and therefore
can accumulate the surface charge. Electrical methods based on electrophoresis are
also used for microalgae self-immobilization (Vandamme et al. 2012); the negatively charged surface of microalgae allows to concentrate cells by being moved in
an electrical field (Zheng et al. 2012).
Bio-flocculation techniques allow the harvesting of microalgae without addition of
chemical flocculants and the reuse of the cultivation medium. The flocculation of
microalgae induced by bacteria (alga-bacteria flocculation) has been implemented successfully in wastewater treatment (Lee et al. 2013). The usage of poly-γ-glutamic acids
from Bacillus subtilis for harvesting Nannochloropsis oculata, Phaeodactylum
S. Vasilieva et al.
7.3.4 Self-Immobilization
For the cell adsorption or entrapment with support materials, the additional costs for
carriers and immobilization procedures are involved, which often makes this technology economically unprofitable. By contrast, self-immobilization process (flocculation) presents a lot of advantages over immobilization such as the following:
• More cost-effective immobilization process
• Better cell growth due to less mass transfer limitation
• Less physical restriction exerted on cells
• Easier for application in the immobilization procedure
• Higher cell densities as no support materials occupy the working volume of the
bioreactors
The main disadvantage of some self-immobilization technique is that they may
cause unwanted changes in cell composition of microalgae during prolonged cultivation (Benemann and Oswald 1996), so they are more appropriate for the production of primary metabolites. The use of self-immobilization for biomass harvesting
can also be an efficient alternative to the time-consuming and expensive techniques
such as filtration and centrifugation.
Microalgal cells can stick together to form cell aggregates naturally or under
certain artificially induced conditions. In biotechnological processes, flocculation
can be carried out in various methods. Chemical flocculation using salts of Zn
2+
,
Al
3+
, and Fe
3+
is the most used technique (McGarry 1970). Flocculation can also be
induced by changing the microalgae cultivation conditions such as increasing the
medium pH, nutrient depletion, and temperature changes. Increasing the medium
pH value resulted in the high flocculation efficiency of up to 90% for freshwater
(Chlorella vulgaris, Scenedesmus sp., Chlorococcum sp.) and marine microalgae
(Nannochloropsis oculata, Phaeodactylum tricornutum) (Wu et al. 2012). It has
been revealed that the cell aggregations did not occur in waters with calcium and
magnesium deficiency (Yahi et al. 1994); thus, it was assumed that flocculation of
microalgae at high pH is caused by chemical precipitation of calcium and/or magnesium salts.
Polyelectrolytes can be used to enhance the aggregation of microalgal cells. The
surface of algal cells contains various functional groups (de la Noue and de Pauw
1988). They can be deprotonated or protonated depending on the pH and therefore
can accumulate the surface charge. Electrical methods based on electrophoresis are
also used for microalgae self-immobilization (Vandamme et al. 2012); the negatively charged surface of microalgae allows to concentrate cells by being moved in
an electrical field (Zheng et al. 2012).
Bio-flocculation techniques allow the harvesting of microalgae without addition of
chemical flocculants and the reuse of the cultivation medium. The flocculation of
microalgae induced by bacteria (alga-bacteria flocculation) has been implemented successfully in wastewater treatment (Lee et al. 2013). The usage of poly-γ-glutamic acids
from Bacillus subtilis for harvesting Nannochloropsis oculata, Phaeodactylum
S. Vasilieva et al.