rapidly with high reliability. In another mode, the algal cells passes via electric field
and gets collected as result of adding negative charge to biomass particles [44].
The decrease in harvesting cost can be attained with proper design of dewatering
system and via nutrients and water recycling. The technique employed for
microalgae harvest must be able to process huge volumes of grown culture by
utilizing less energy and cost. Initially very less concentration of algal biomass
(0.5 g L
À1 ) is present in culture broth, but during the harvesting period, the broth
thickens leading to rise in concentration of biomass (up to 200 g L
À1 ). To make
harvesting more simple and effective, it is imperative to use combined technologies
instead of using single technology [45, 46]. The harvesting methods employed must
not disturb or intervene the culture medium recycling (e.g., if intervene it may cause
contamination).
7.1 Flocculation
In flocculation process when flocculating agents (bioflocculants, chemicals, metals)
are added to broth, many microalgal cells combine together to form large particles
known as flocs.
The flocs added during process should be effortlessly removed from microalgal
biomass and recoverable; otherwise these chemicals might end up as contaminants in
biomass further hindering the post-harvest operations. Another interesting approach
is reducing the pH may leads in microalgae flocculation. The flocculation happened
in microalgal cells when the pH of the medium was tuned near 4; it leads to
protonation and neutralization of carboxylic acids on cell surface of microalgae to
form flocculates [47].
7.2 Gravity-Based Technologies
7.2.1 Sedimentation
Gravity sedimentation involves the usage of cheap infrastructure and low energy for
sedimentation process as it is done in simple settling tanks.
Gravity sedimentation done in simple settling tanks is a very attractive option
because it requires very little energy and relatively low-cost infrastructure. As it is a
very slow process, this may sometimes result in degradation of biomass quality
during harvesting. Due to production of dilute slurry, the gravity sedimentation
could chiefly be used as pre-concentration mechanism before employing other
methods like filtration, flocculation, and centrifugation [48, 49]. In Israel, Algatech
has initially used gravity sedimentation for harvesting Haematococcus cysts rich in
astaxanthin (oral communication, Algatech). As the sluggish process of
Algal Biomass for Biofuels and Bioproducts
149
and gets collected as result of adding negative charge to biomass particles [44].
The decrease in harvesting cost can be attained with proper design of dewatering
system and via nutrients and water recycling. The technique employed for
microalgae harvest must be able to process huge volumes of grown culture by
utilizing less energy and cost. Initially very less concentration of algal biomass
(0.5 g L
À1 ) is present in culture broth, but during the harvesting period, the broth
thickens leading to rise in concentration of biomass (up to 200 g L
À1 ). To make
harvesting more simple and effective, it is imperative to use combined technologies
instead of using single technology [45, 46]. The harvesting methods employed must
not disturb or intervene the culture medium recycling (e.g., if intervene it may cause
contamination).
7.1 Flocculation
In flocculation process when flocculating agents (bioflocculants, chemicals, metals)
are added to broth, many microalgal cells combine together to form large particles
known as flocs.
The flocs added during process should be effortlessly removed from microalgal
biomass and recoverable; otherwise these chemicals might end up as contaminants in
biomass further hindering the post-harvest operations. Another interesting approach
is reducing the pH may leads in microalgae flocculation. The flocculation happened
in microalgal cells when the pH of the medium was tuned near 4; it leads to
protonation and neutralization of carboxylic acids on cell surface of microalgae to
form flocculates [47].
7.2 Gravity-Based Technologies
7.2.1 Sedimentation
Gravity sedimentation involves the usage of cheap infrastructure and low energy for
sedimentation process as it is done in simple settling tanks.
Gravity sedimentation done in simple settling tanks is a very attractive option
because it requires very little energy and relatively low-cost infrastructure. As it is a
very slow process, this may sometimes result in degradation of biomass quality
during harvesting. Due to production of dilute slurry, the gravity sedimentation
could chiefly be used as pre-concentration mechanism before employing other
methods like filtration, flocculation, and centrifugation [48, 49]. In Israel, Algatech
has initially used gravity sedimentation for harvesting Haematococcus cysts rich in
astaxanthin (oral communication, Algatech). As the sluggish process of
Algal Biomass for Biofuels and Bioproducts
149