256 Marine Macro- and Microalgae: An Overview
Settling
Settling is a harvesting technique for microalgae biomass separation from suspension because of low value
of the biomass generated from large volumes (Nurdogan and Oswald 1996). However, it is only suitable
for species with large cells/filaments such as Tetraselmis sp. CTP4 (Pereira et al. 2016) and Arthrospira
sp. (Munoz and Guieysse 2006), which minimizes the dewatering cost of biomass. Therefore, the settling
velocity of microalgal cells could be enhanced by increasing the dimension of the suspended cells, that is,
by cell aggregation, which can be achieved with chemical coagulants, resulting in large microalgal flocs
that settle rapidly to the bottom (Salim et al. 2011).
Centrifugation
Centrifugation is a common method to harvest microalgae, as no additional chemicals are needed. It is
preferred for harvesting biomass for high value-added metabolites and extended shelf-life concentrates
(Heasman et al. 2000). Biomass recovery depends on the settling characteristics of the cells, slurry
residence time in the centrifuge, and settling depth (Molina et al. 2003). Harvesting efficiency of more
than 95% (Heasman et al. 2000), and increase in slurry concentration by up to 150 times for 15% total
suspended solids are technically feasible (Mohn 1980). However, the main drawbacks of the technique
include high energy costs and potentially higher maintenance requirements due to freely moving parts
(Bosma et al. 2003).
Flocculation
Microalgal cells carry a negative charge that prevents natural aggregation of cells in suspension.
Flocculants block these surface charges on the microalgal cells, stimulating cell-cell adhesion and
generating flocs. These agents are widely used in harvesting processes as they increase the effective
particle size and facilitate aggregation, which is a step prior to other harvesting techniques such as
filtration, flotation, and/or sedimentation by centrifugation (Molina et al. 2003; Munoz and Guieysse
2006). Microalgal biomass is typically separated from large volumes of water more easily by flocculation
and sedimentation than filtration (Divakaran and Pillai 2002). Several methods have been reported
using flocculants such as chitosan, aluminium salts, polyacrylamide polymers, alum, lime, alkali
and ferric chlorides, and aluminium and ferric sulphates (Semerjian and Ayoub 2003; Knuckey et al.
2006). Aluminium salts are also widely used as flocculants, but are unsuitable for animal feed unless
the aluminium is removed. Microalgal biomass also can be separated by bio-flocculation (Salim et al.
2011) and anionic polyacrylamide (Sukenik et al. 1985; Knuckey et al. 2006) followed by pH adjustment
using alkaline solutions. This process was successfully employed to a range of species with flocculation
efficiencies approximately 80%. The use of sedimentation in combination with flocculation is reported
to be cost effective due to minimal power consumption and use of gravity for biomass settling. Recently,
calcium hydroxide has been reported as a less expensive non-toxic flocculant. Calcium is a cation that can
and should be present in animal feed, and it typically precipitates more cells than other basic flocculants
(Ami et al. 2012).
Flotation
Flotation method is based on the trapping of microalgae cells using dispersed micro-air bubbles (Wang et
al. 2008). Harvesting of microalgae biomass needs a high overflow rate, which promotes flotation during
which microalgae move upward instead of settling down. It does not require any addition of chemicals. It
is a simple method by which algae can be made to float on the surface of the suspension, being removed
as scum. Flotation is generally carried out with a combination of flocculation for microalgae harvesting.
Some strains naturally float on the surface when their lipid contents increase (Bruton et al. 2009; Hamed
2016). Froth flotation is a technique of separating microalgae from the suspension by manipulating pH
and bubbling air through a column to create algal froth that accumulates on the surface of a liquid,
which can be removed by suction. Froth flotation and drying are currently considered very expensive for
Settling
Settling is a harvesting technique for microalgae biomass separation from suspension because of low value
of the biomass generated from large volumes (Nurdogan and Oswald 1996). However, it is only suitable
for species with large cells/filaments such as Tetraselmis sp. CTP4 (Pereira et al. 2016) and Arthrospira
sp. (Munoz and Guieysse 2006), which minimizes the dewatering cost of biomass. Therefore, the settling
velocity of microalgal cells could be enhanced by increasing the dimension of the suspended cells, that is,
by cell aggregation, which can be achieved with chemical coagulants, resulting in large microalgal flocs
that settle rapidly to the bottom (Salim et al. 2011).
Centrifugation
Centrifugation is a common method to harvest microalgae, as no additional chemicals are needed. It is
preferred for harvesting biomass for high value-added metabolites and extended shelf-life concentrates
(Heasman et al. 2000). Biomass recovery depends on the settling characteristics of the cells, slurry
residence time in the centrifuge, and settling depth (Molina et al. 2003). Harvesting efficiency of more
than 95% (Heasman et al. 2000), and increase in slurry concentration by up to 150 times for 15% total
suspended solids are technically feasible (Mohn 1980). However, the main drawbacks of the technique
include high energy costs and potentially higher maintenance requirements due to freely moving parts
(Bosma et al. 2003).
Flocculation
Microalgal cells carry a negative charge that prevents natural aggregation of cells in suspension.
Flocculants block these surface charges on the microalgal cells, stimulating cell-cell adhesion and
generating flocs. These agents are widely used in harvesting processes as they increase the effective
particle size and facilitate aggregation, which is a step prior to other harvesting techniques such as
filtration, flotation, and/or sedimentation by centrifugation (Molina et al. 2003; Munoz and Guieysse
2006). Microalgal biomass is typically separated from large volumes of water more easily by flocculation
and sedimentation than filtration (Divakaran and Pillai 2002). Several methods have been reported
using flocculants such as chitosan, aluminium salts, polyacrylamide polymers, alum, lime, alkali
and ferric chlorides, and aluminium and ferric sulphates (Semerjian and Ayoub 2003; Knuckey et al.
2006). Aluminium salts are also widely used as flocculants, but are unsuitable for animal feed unless
the aluminium is removed. Microalgal biomass also can be separated by bio-flocculation (Salim et al.
2011) and anionic polyacrylamide (Sukenik et al. 1985; Knuckey et al. 2006) followed by pH adjustment
using alkaline solutions. This process was successfully employed to a range of species with flocculation
efficiencies approximately 80%. The use of sedimentation in combination with flocculation is reported
to be cost effective due to minimal power consumption and use of gravity for biomass settling. Recently,
calcium hydroxide has been reported as a less expensive non-toxic flocculant. Calcium is a cation that can
and should be present in animal feed, and it typically precipitates more cells than other basic flocculants
(Ami et al. 2012).
Flotation
Flotation method is based on the trapping of microalgae cells using dispersed micro-air bubbles (Wang et
al. 2008). Harvesting of microalgae biomass needs a high overflow rate, which promotes flotation during
which microalgae move upward instead of settling down. It does not require any addition of chemicals. It
is a simple method by which algae can be made to float on the surface of the suspension, being removed
as scum. Flotation is generally carried out with a combination of flocculation for microalgae harvesting.
Some strains naturally float on the surface when their lipid contents increase (Bruton et al. 2009; Hamed
2016). Froth flotation is a technique of separating microalgae from the suspension by manipulating pH
and bubbling air through a column to create algal froth that accumulates on the surface of a liquid,
which can be removed by suction. Froth flotation and drying are currently considered very expensive for