Chemical Flocculation Inorganic molecules such as aluminum sulfate, ferric sulfide, or lime are suitable flocculants. These molecules can neutralize the cells charge
or reduce it (Sheehan et al. 1998). However, the use of these chemicals leads to
buildup of undesirable compounds in the harvested cells which leads to issues in
using the algae as animal or human food. Instead of inorganic flocculants, the
addition of highly charged organic molecules such as polyelectrolytes is more
suitable to neutralize the algal cells. Polyelectrolytes can also physically link algal
cells together, which helps in very stable flocs formation. They also do not have the
toxicity that many of the inorganic molecules cause. This makes them a much more
attractive chemical option for flocculation.
Electroflocculation In this method an electric current is passed through the culture
to form flocs. This method is increasingly popular with very high separation
efficiencies. It can also be applicable for large-scale cultures without utilizing a
high amount of electricity.
Bioflocculation It is accomplished by stimulating limiting nitrogen or altering pH
and dissolved O 2 levels. Addition of any external material or force to the culture is
not required. Although this method does not need any additional cost for flocculants,
it generally requires a longer time and is unreliable.
1.3.2.2 Flotation
Flotation method does not require any addition of chemicals. In this method, algal
cells (flocs) are trapped at the surface by dispersed micro air bubbles in the liquid
(Phoochinda and White 2003). Some strains can naturally float at the surface of the
water as their lipid content increases.
1.3.2.3 Centrifugal Sedimentation
It is based on Stoke’s law and is widely used in many liquid–solid separations. This
technology utilizes centrifugal forces to separate substances of different densities.
Separation efficiency is based on the density and radius of desired algal cells. Algal
cells separation is based on algal cell density, residence time during centrifugation,
and settling depth. This method is very effective for algal cells separation from
liquid; however, it is not a feasible technique for large-scale algal culture due to high
energy requirement and maintenance cost (Davis et al. 2012).
1.3.2.4 Filtration
Filtration is used to separate substance according to particle and screen pore size.
Filtration can be an effective method for harvesting larger strains of algae. The algae
Spirulina sp. can be simply filtered from the culture medium. Although filtration can
be inexpensive and an effective method for harvesting algae, it has the major issue of
filter fouling and clogging, which limit its application in large-scale cultivation.
10
N. Maheshwari et al.
or reduce it (Sheehan et al. 1998). However, the use of these chemicals leads to
buildup of undesirable compounds in the harvested cells which leads to issues in
using the algae as animal or human food. Instead of inorganic flocculants, the
addition of highly charged organic molecules such as polyelectrolytes is more
suitable to neutralize the algal cells. Polyelectrolytes can also physically link algal
cells together, which helps in very stable flocs formation. They also do not have the
toxicity that many of the inorganic molecules cause. This makes them a much more
attractive chemical option for flocculation.
Electroflocculation In this method an electric current is passed through the culture
to form flocs. This method is increasingly popular with very high separation
efficiencies. It can also be applicable for large-scale cultures without utilizing a
high amount of electricity.
Bioflocculation It is accomplished by stimulating limiting nitrogen or altering pH
and dissolved O 2 levels. Addition of any external material or force to the culture is
not required. Although this method does not need any additional cost for flocculants,
it generally requires a longer time and is unreliable.
1.3.2.2 Flotation
Flotation method does not require any addition of chemicals. In this method, algal
cells (flocs) are trapped at the surface by dispersed micro air bubbles in the liquid
(Phoochinda and White 2003). Some strains can naturally float at the surface of the
water as their lipid content increases.
1.3.2.3 Centrifugal Sedimentation
It is based on Stoke’s law and is widely used in many liquid–solid separations. This
technology utilizes centrifugal forces to separate substances of different densities.
Separation efficiency is based on the density and radius of desired algal cells. Algal
cells separation is based on algal cell density, residence time during centrifugation,
and settling depth. This method is very effective for algal cells separation from
liquid; however, it is not a feasible technique for large-scale algal culture due to high
energy requirement and maintenance cost (Davis et al. 2012).
1.3.2.4 Filtration
Filtration is used to separate substance according to particle and screen pore size.
Filtration can be an effective method for harvesting larger strains of algae. The algae
Spirulina sp. can be simply filtered from the culture medium. Although filtration can
be inexpensive and an effective method for harvesting algae, it has the major issue of
filter fouling and clogging, which limit its application in large-scale cultivation.
10
N. Maheshwari et al.