98 Marine Macro- and Microalgae: An Overview
Electrocoagulation flocculation
Electrocoagulation flocculation (ECF) is an alternative technique for the use of metal coagulants and
relies on the release of iron or aluminium ions from a sacrificial anode (Vandamme et al. 2013). When
using an aluminium anode during ECF, aluminium hydroxides are generated and when using an iron
anode, ferric and/or ferrous hydroxides are formed (Vandamme et al. 2011). As the aluminium or
iron anodes are oxidized, the main reaction at the cathode is the reduction of water, which generates
hydrogen gas. Comparison of the recovery of two marine microalgae species Chlorococcum sp. and
Tetraselmis sp. was carried out (Uduman et al. 2011) by electrocoagulation. High recoveries of up to 98
and 99% for Chlorococcum sp. and Tetraselmis sp., respectively, were obtained. The technique was also
evaluated for harvesting the marine microalga P. tricornutum (Vandamme et al. 2011). In this study, the
aluminium anode proved to be more efficient than the iron anode and under optimal conditions (pH 4,
sedimentation time of 30 min and stirring speed of 150 rpm) the time required to initiate flocculation as
well as the final recovery efficiencies were reproducible. Finally, the flocculation obtained was as effective
as when using alum, though with the advantage of a limited aluminium content in the harvested biomass
(below 1%). Electrocoagulation flocculation has low electricity consumption when used in seawater and
therefore could be a promising low-cost technique for harvesting microalgae. Fayad et al. (2017) also
found out that aluminium electrodes were best for harvesting C. vulgaris. 100% algae recovery was
achieved within 50 minutes by using a steering speed of 250 rpm, an inter-electrode distance of 1 cm and
a current density of 6.7 mA/cm
2
. This same study also optimized the operation parameters to minimize
energy consumption down to 1 kWh/kg microalgae: aluminium electrodes were used with a current
density of 2.9 mA/cm
2
, a stirring speed of 250 rpm and an inter-electrode distance of 1 cm at pH 4 for
60 min electrolysis. ECF harvesting of C. vulgaris in this study did not affect significantly the amount of
lipids, chlorophyls A and B, and carotenoids.
Flotation and foaming
Flotation
Wastewater treatment sludge removal often resorts to the flotation principle using dissolved air flotation
(DAF). The principle of DAF relies on the generation of micro-bubbles (< 10 mm) produced by the
decompression of a pressurized fluid. This is a very mature and effective process for removing algae
and particles in suspension allowing recovery of up to 90% and a solids concentration of 7 to 10%
dry weight after harvesting. Prior to DAF treatment the culture broth is ozonated in order to sensitize
the cells and then treated with a flocculating agent, as described in the flocculation section above. The
micro-bubbles are then generated and adhere to the flocs thus making them more buoyant. The flocs will
rise rapidly to the surface as a cell foam which can be easily removed. Hanotu et al. (2012) performed
separation of Dunaliella salina through microfiltration with the addition of coagulants and adjustment
of pH, combined with dissolved air flotation. A maximum recovery of 99.2% was obtained when using
150 mg/L ferric chloride at pH 5, with average bubble size of 86 mm.
The DAF treatment is applicable to large volumes as demonstrated in pilot to full-scale productions
(Christenson and Sims 2011).
Foaming
Foam fractionation has also been investigated. This process is the result of the combined action
of a surfactant, which has a polar and a non-polar region allowing hydrophobic interaction to rising
gas bubbles (Csordas and Wang 2004). Marine algae are known to secrete surfactants (Žutić et al.
1981), hence the logical hypothesis that foam fractionation could be a suitable harvesting technique.
Csordas and Wang (2004) investigated foam fractionation of the marine diatom Chaetoceros spp. The
Electrocoagulation flocculation
Electrocoagulation flocculation (ECF) is an alternative technique for the use of metal coagulants and
relies on the release of iron or aluminium ions from a sacrificial anode (Vandamme et al. 2013). When
using an aluminium anode during ECF, aluminium hydroxides are generated and when using an iron
anode, ferric and/or ferrous hydroxides are formed (Vandamme et al. 2011). As the aluminium or
iron anodes are oxidized, the main reaction at the cathode is the reduction of water, which generates
hydrogen gas. Comparison of the recovery of two marine microalgae species Chlorococcum sp. and
Tetraselmis sp. was carried out (Uduman et al. 2011) by electrocoagulation. High recoveries of up to 98
and 99% for Chlorococcum sp. and Tetraselmis sp., respectively, were obtained. The technique was also
evaluated for harvesting the marine microalga P. tricornutum (Vandamme et al. 2011). In this study, the
aluminium anode proved to be more efficient than the iron anode and under optimal conditions (pH 4,
sedimentation time of 30 min and stirring speed of 150 rpm) the time required to initiate flocculation as
well as the final recovery efficiencies were reproducible. Finally, the flocculation obtained was as effective
as when using alum, though with the advantage of a limited aluminium content in the harvested biomass
(below 1%). Electrocoagulation flocculation has low electricity consumption when used in seawater and
therefore could be a promising low-cost technique for harvesting microalgae. Fayad et al. (2017) also
found out that aluminium electrodes were best for harvesting C. vulgaris. 100% algae recovery was
achieved within 50 minutes by using a steering speed of 250 rpm, an inter-electrode distance of 1 cm and
a current density of 6.7 mA/cm
2
. This same study also optimized the operation parameters to minimize
energy consumption down to 1 kWh/kg microalgae: aluminium electrodes were used with a current
density of 2.9 mA/cm
2
, a stirring speed of 250 rpm and an inter-electrode distance of 1 cm at pH 4 for
60 min electrolysis. ECF harvesting of C. vulgaris in this study did not affect significantly the amount of
lipids, chlorophyls A and B, and carotenoids.
Flotation and foaming
Flotation
Wastewater treatment sludge removal often resorts to the flotation principle using dissolved air flotation
(DAF). The principle of DAF relies on the generation of micro-bubbles (< 10 mm) produced by the
decompression of a pressurized fluid. This is a very mature and effective process for removing algae
and particles in suspension allowing recovery of up to 90% and a solids concentration of 7 to 10%
dry weight after harvesting. Prior to DAF treatment the culture broth is ozonated in order to sensitize
the cells and then treated with a flocculating agent, as described in the flocculation section above. The
micro-bubbles are then generated and adhere to the flocs thus making them more buoyant. The flocs will
rise rapidly to the surface as a cell foam which can be easily removed. Hanotu et al. (2012) performed
separation of Dunaliella salina through microfiltration with the addition of coagulants and adjustment
of pH, combined with dissolved air flotation. A maximum recovery of 99.2% was obtained when using
150 mg/L ferric chloride at pH 5, with average bubble size of 86 mm.
The DAF treatment is applicable to large volumes as demonstrated in pilot to full-scale productions
(Christenson and Sims 2011).
Foaming
Foam fractionation has also been investigated. This process is the result of the combined action
of a surfactant, which has a polar and a non-polar region allowing hydrophobic interaction to rising
gas bubbles (Csordas and Wang 2004). Marine algae are known to secrete surfactants (Žutić et al.
1981), hence the logical hypothesis that foam fractionation could be a suitable harvesting technique.
Csordas and Wang (2004) investigated foam fractionation of the marine diatom Chaetoceros spp. The