Microalgal Downstream Processing: Harvesting, Drying, Extraction, Separation, and Purification 97
The biomass collected by bioflocculation inherently contains fungi, bacteria, or an additional alga
used as a flocculating agent that can interfere with the food or feed it is intended for.
Flocculation with Synthetic Polymers. Synthetic polymers have also been investigated. Knuckey
et al. (2006) developed and described a novel technique based on the adjustment of pH of the culture
to between 10 and 10.6 using NaOH, followed by the addition of Magnafloc LT-25, a non-ionic
polyacrylamide derivative polymer, to a final concentration of 0.5 mg/L. Increasing the pH of seawater to
10 or above resulted in a precipitate of calcium and/or magnesium hydroxides (Knuckey et al. 2006) which
flocculated upon addition of LT-25. Algal cells sedimented as they got trapped in this floc. The flocculate
was then harvested by siphoning off surface water after the settling period. The method was successfully
applied to harvest algal cultures of Chaetoceros calcitrans, C. muelleri, Thalassiosira pseudomonas,
Attheya septentrionalis, Nitzschia closterium, Skeletonema sp., T. suecica, and Rhodomonas salina, with
efficiencies above 80% and final concentration factor between 200- and 800-fold.
The flocculation efficiency of several cationic polyacrylamide polymers (Zetag 7550, 7570, 8110,
8140, 8180, and 8190) on algal cultures of T. suecica, Chlorococcum sp., N. salina, D. tertiolecta, and I.
galbana was found out to be less effective than with Al
3+
and Fe
3+
(Elridge et al. 2012). Polyelectrolytic
flocculants are very much subject to high salinity suspensions, such as marine culture media, as they
shrink and thus cannot act as a bridging molecule capable of initiating flocculation. Addition of an
inorganic flocculant alleviates this phenomenon and this can be achieved using lower concentrations of
the inorganic flocculant than if it was used alone. Flocculation efficiency close to 100% was achieved
(Danquah et al. 2009a) for microalgal culture of T. suecica when using the high molecular weight
synthetic cationic polyelectrolytic polymer Zetag 7650 in combination with 50 mg/L Al 2 (SO 4 ) 3 found
to be the optimal dosis of the inorganic flocculant. Flocculation using polyacrylamide polymers is not
straightforward as overdosing of the flocculant negatively impacts the flocculation efficiency (Mikulec et
al. 2015). Furthermore, this same study also showed that the optimal dose of flocculant not only depends
on the algal species where cylindrical shaped species flocculated more efficiently, but it also varies with
the life cycle of the algal biomass as the optimum dose was 0.5 mg/L in the linear phase of growth and
2 mg/L in the stationary growth phase for Scenedesmus obliquus.
Using 3-aminopropyltriethoxysilane (APTES), Farooq et al. (2013) synthesized aminoclays with a
cationic metal center (Mg
2+
or Fe
3+
, respectively Mg-APTES and Fe-APTES) presenting functional groups
of (CH 2 ) 3 NH 2 organic pendants through covalent bonding. In aqueous solutions, these aminoclays formed
sheets which presented an amorphous phyllosilicate structure capable of interacting with polysaccharidebased cell walls. When using 1 g/L of Mg-APTES on N. oculata, approximately 93 and 99% harvest
efficiencies were obtained with the marine media containing 10 g/L and 30 g/L of sea salt, respectively.
In absence of the aminoclay Mg-APTES, N. oculata did not sediment at all. This result was described
by the authors of this work as advantageous as common cationic polymer flocculants only offer limited
harvesting efficiencies in high salinity (> 5 g/L) media. Furthermore, the aminoclay showed limited
toxicity to microalgae (making possible the recycling of both clay and medium) and thus could offer the
possibility of continuous harvesting.
Flocculation using sonication
Besides the use of additives, flocculation can be induced by ultrasounds. Bosma et al. (2003) successfully
used ultrasounds to optimize the aggregation efficiency and concentration factor. They achieved 92%
separation efficiency and a concentration factor of 20. Ultrasound harvesting requires high energy
input and therefore may not be adapted to the industrial scale; however, it may be of interest if used
in combination with another flocculating method. Harvest of Microcystis aeruginosa was carried out
using the combined effect of polyaluminum chloride (PAC) with ultrasounds (Zhang et al. 2009). A short
application of sonication (1–5 s) improved the flocculation performance of algal cells and this was even
greater at a lower dosage of PAC.
The biomass collected by bioflocculation inherently contains fungi, bacteria, or an additional alga
used as a flocculating agent that can interfere with the food or feed it is intended for.
Flocculation with Synthetic Polymers. Synthetic polymers have also been investigated. Knuckey
et al. (2006) developed and described a novel technique based on the adjustment of pH of the culture
to between 10 and 10.6 using NaOH, followed by the addition of Magnafloc LT-25, a non-ionic
polyacrylamide derivative polymer, to a final concentration of 0.5 mg/L. Increasing the pH of seawater to
10 or above resulted in a precipitate of calcium and/or magnesium hydroxides (Knuckey et al. 2006) which
flocculated upon addition of LT-25. Algal cells sedimented as they got trapped in this floc. The flocculate
was then harvested by siphoning off surface water after the settling period. The method was successfully
applied to harvest algal cultures of Chaetoceros calcitrans, C. muelleri, Thalassiosira pseudomonas,
Attheya septentrionalis, Nitzschia closterium, Skeletonema sp., T. suecica, and Rhodomonas salina, with
efficiencies above 80% and final concentration factor between 200- and 800-fold.
The flocculation efficiency of several cationic polyacrylamide polymers (Zetag 7550, 7570, 8110,
8140, 8180, and 8190) on algal cultures of T. suecica, Chlorococcum sp., N. salina, D. tertiolecta, and I.
galbana was found out to be less effective than with Al
3+
and Fe
3+
(Elridge et al. 2012). Polyelectrolytic
flocculants are very much subject to high salinity suspensions, such as marine culture media, as they
shrink and thus cannot act as a bridging molecule capable of initiating flocculation. Addition of an
inorganic flocculant alleviates this phenomenon and this can be achieved using lower concentrations of
the inorganic flocculant than if it was used alone. Flocculation efficiency close to 100% was achieved
(Danquah et al. 2009a) for microalgal culture of T. suecica when using the high molecular weight
synthetic cationic polyelectrolytic polymer Zetag 7650 in combination with 50 mg/L Al 2 (SO 4 ) 3 found
to be the optimal dosis of the inorganic flocculant. Flocculation using polyacrylamide polymers is not
straightforward as overdosing of the flocculant negatively impacts the flocculation efficiency (Mikulec et
al. 2015). Furthermore, this same study also showed that the optimal dose of flocculant not only depends
on the algal species where cylindrical shaped species flocculated more efficiently, but it also varies with
the life cycle of the algal biomass as the optimum dose was 0.5 mg/L in the linear phase of growth and
2 mg/L in the stationary growth phase for Scenedesmus obliquus.
Using 3-aminopropyltriethoxysilane (APTES), Farooq et al. (2013) synthesized aminoclays with a
cationic metal center (Mg
2+
or Fe
3+
, respectively Mg-APTES and Fe-APTES) presenting functional groups
of (CH 2 ) 3 NH 2 organic pendants through covalent bonding. In aqueous solutions, these aminoclays formed
sheets which presented an amorphous phyllosilicate structure capable of interacting with polysaccharidebased cell walls. When using 1 g/L of Mg-APTES on N. oculata, approximately 93 and 99% harvest
efficiencies were obtained with the marine media containing 10 g/L and 30 g/L of sea salt, respectively.
In absence of the aminoclay Mg-APTES, N. oculata did not sediment at all. This result was described
by the authors of this work as advantageous as common cationic polymer flocculants only offer limited
harvesting efficiencies in high salinity (> 5 g/L) media. Furthermore, the aminoclay showed limited
toxicity to microalgae (making possible the recycling of both clay and medium) and thus could offer the
possibility of continuous harvesting.
Flocculation using sonication
Besides the use of additives, flocculation can be induced by ultrasounds. Bosma et al. (2003) successfully
used ultrasounds to optimize the aggregation efficiency and concentration factor. They achieved 92%
separation efficiency and a concentration factor of 20. Ultrasound harvesting requires high energy
input and therefore may not be adapted to the industrial scale; however, it may be of interest if used
in combination with another flocculating method. Harvest of Microcystis aeruginosa was carried out
using the combined effect of polyaluminum chloride (PAC) with ultrasounds (Zhang et al. 2009). A short
application of sonication (1–5 s) improved the flocculation performance of algal cells and this was even
greater at a lower dosage of PAC.