Microalgal Downstream Processing: Harvesting, Drying, Extraction, Separation, and Purification 95
(Chlorella sp.) were submitted to ozone for 30 min compared to control cells not treated with ozone
(Pranowo et al. 2013).
However, the main obstacle to the use of metal salts as flocculent is that they may end up as a
contaminant of the final algal product, thus inhibiting the direct reuse of algae and implying the recovery
of the flocculent which in turn would impact the production cost.
Flocculation with Natural Polymers. The most widely used natural polymers are clay, chitosan, and
γ-polyglutamic acid. Clay minerals are interesting as flocculants because they are non-toxic and
environmentally friendly and at the same time can achieve removal efficiencies of up to 95–99%. Wu
et al. (2010) investigated the removal of the marine microalga Chattonella marina, responsible of
harmful algal blooms along the Chinese coasts, with such mineral clays, Na-montmorillonite, Na-Kaolin,
vermiculite, and palygorskite, which they modified with a Gemini surfactant, (2-bromoethyl) tetradecyl
dimethylethylammonium. Of these four clays, the Gemini modified vermiculite (6.5 mg/L) exhibited the
highest removal rate (100% in 24 hr). The stability of the Gemini modified vermiculite in sea water was
good as only 3% of the Gemini surfactant was released in the medium.
Chitosan is a material derived from chitin, the exoskeleton of crustaceans, which is a waste product
from the shrimp industry. Chitosan is one of the natural polymers most often investigated for flocculating
algae (de Godos et al. 2011; Farid et al. 2013; Divakaran and Pillai 2002; Lee et al. 2012; Beach et al.
2012) mainly due to its low cost. Chitosan is positively charged at neutral and acidic pH due to its amino
group (pKa ~ 6.5) and therefore readily binds to the negatively charged surface of algae. The flocculation
efficiency is usually compared to multivalent salts. In such a study where chitosan, ferric sulfate, and
alum were used for testing the coagulation of Neochloris oleoabundans, it was shown that chitosan was
the most effective flocculant and the optimal concentration was found to be 100 mg/L, allowing a removal
rate of 95% (Beach et al. 2012). Şirin et al. (2013) investigated the effect of chitosan on the flocculation
of N. gaditana and found 30 ppm as the optimum concentration of chitosan and flocculation efficiency
above 50% in the pH range 8.93 to 10.9. In this study, the concentrations of calcium and magnesium ions
were monitored and confirm the importance of magnesium hydroxide (Mg(OH) 2 ) precipitation in the
flocculation process with chitosan.
As previously mentioned for polyvalent salts, flocculation depends on many parameters. Divakaran
and Pillai (2002) demonstrated that effectiveness of chitosan was very sensitive to pH: a maximum
removal of 90% was achieved at pH 7.0 for the freshwater algae Spirulina, Oscillatoria and Chlorella.
The removal was lower for the marine species Synechocystis sp. This same study also showed that the
optimal chitosan concentration depended upon the concentration of alga, but a maximum of 15 mg/L
of chitosan was found sufficient to clarify all algal concentrations investigated. Ahmad et al. (2011)
also found removal efficiency of Chlorella sp. (> 99%) for chitosan, but this efficiency was affected
by chitosan concentration, mixing time and mixing rate with optimums at 10 ppm, 20 min and 150
rpm, respectively. Studies on mitigating harmful algal blooms also contribute to valuable knowledge
to algal cell removal. Chen and Pan (2012c) described the efficiency of xanthan used in combination
with clay/soil/sand and calcium hydroxide to remove the marine alga Amphidinium carterae. When used
alone, xanthan only achieved a maximum cell removal efficiency of 55%. On the other hand, the three
minerals (clay, sand, and soil) were ineffective in removing algal cells. However, when xanthan and
calcium hydroxide (20 mg/L and 100 mg/L, respectively) were used together with these three minerals
the removal efficiency increased to 83–89% within 30 min using 300 mg/L clay, sand, or soil. This
approach applied to Scenedesmus abundans cultures confirmed that using a combination of chitosan and
bentonite clay powder halved the settling time (15.3 vs. 34.2 h) (Moorthy et al. 2017).
The efficiency of removal can be affected by the pre-treatment of algae using ozone before chitosan
flocculation. It was shown to increase floc size probably due to the release of intra-cellular organic matter.
The removal of turbidity increased from 61.8% to 80.4% when algae cells (Chlorella sp.) were submitted
to ozone for 30 min compared to control cells not treated with ozone (Pranowo et al. 2013).
Naturally occurring microbial flocculants have been applied to harvest microalgae. The production
and application of such flocculants is described in the literature, though not often in connection with
microalgae removal: Protein flocculants produced by Rhodococcus erythropolis S-1 (Kurane et al. 1986)
and Bacillus sp. DP-152 (Suh et al. 1997), polyamide flocculants produced by B. subtilis DYU1 (Wu and
(Chlorella sp.) were submitted to ozone for 30 min compared to control cells not treated with ozone
(Pranowo et al. 2013).
However, the main obstacle to the use of metal salts as flocculent is that they may end up as a
contaminant of the final algal product, thus inhibiting the direct reuse of algae and implying the recovery
of the flocculent which in turn would impact the production cost.
Flocculation with Natural Polymers. The most widely used natural polymers are clay, chitosan, and
γ-polyglutamic acid. Clay minerals are interesting as flocculants because they are non-toxic and
environmentally friendly and at the same time can achieve removal efficiencies of up to 95–99%. Wu
et al. (2010) investigated the removal of the marine microalga Chattonella marina, responsible of
harmful algal blooms along the Chinese coasts, with such mineral clays, Na-montmorillonite, Na-Kaolin,
vermiculite, and palygorskite, which they modified with a Gemini surfactant, (2-bromoethyl) tetradecyl
dimethylethylammonium. Of these four clays, the Gemini modified vermiculite (6.5 mg/L) exhibited the
highest removal rate (100% in 24 hr). The stability of the Gemini modified vermiculite in sea water was
good as only 3% of the Gemini surfactant was released in the medium.
Chitosan is a material derived from chitin, the exoskeleton of crustaceans, which is a waste product
from the shrimp industry. Chitosan is one of the natural polymers most often investigated for flocculating
algae (de Godos et al. 2011; Farid et al. 2013; Divakaran and Pillai 2002; Lee et al. 2012; Beach et al.
2012) mainly due to its low cost. Chitosan is positively charged at neutral and acidic pH due to its amino
group (pKa ~ 6.5) and therefore readily binds to the negatively charged surface of algae. The flocculation
efficiency is usually compared to multivalent salts. In such a study where chitosan, ferric sulfate, and
alum were used for testing the coagulation of Neochloris oleoabundans, it was shown that chitosan was
the most effective flocculant and the optimal concentration was found to be 100 mg/L, allowing a removal
rate of 95% (Beach et al. 2012). Şirin et al. (2013) investigated the effect of chitosan on the flocculation
of N. gaditana and found 30 ppm as the optimum concentration of chitosan and flocculation efficiency
above 50% in the pH range 8.93 to 10.9. In this study, the concentrations of calcium and magnesium ions
were monitored and confirm the importance of magnesium hydroxide (Mg(OH) 2 ) precipitation in the
flocculation process with chitosan.
As previously mentioned for polyvalent salts, flocculation depends on many parameters. Divakaran
and Pillai (2002) demonstrated that effectiveness of chitosan was very sensitive to pH: a maximum
removal of 90% was achieved at pH 7.0 for the freshwater algae Spirulina, Oscillatoria and Chlorella.
The removal was lower for the marine species Synechocystis sp. This same study also showed that the
optimal chitosan concentration depended upon the concentration of alga, but a maximum of 15 mg/L
of chitosan was found sufficient to clarify all algal concentrations investigated. Ahmad et al. (2011)
also found removal efficiency of Chlorella sp. (> 99%) for chitosan, but this efficiency was affected
by chitosan concentration, mixing time and mixing rate with optimums at 10 ppm, 20 min and 150
rpm, respectively. Studies on mitigating harmful algal blooms also contribute to valuable knowledge
to algal cell removal. Chen and Pan (2012c) described the efficiency of xanthan used in combination
with clay/soil/sand and calcium hydroxide to remove the marine alga Amphidinium carterae. When used
alone, xanthan only achieved a maximum cell removal efficiency of 55%. On the other hand, the three
minerals (clay, sand, and soil) were ineffective in removing algal cells. However, when xanthan and
calcium hydroxide (20 mg/L and 100 mg/L, respectively) were used together with these three minerals
the removal efficiency increased to 83–89% within 30 min using 300 mg/L clay, sand, or soil. This
approach applied to Scenedesmus abundans cultures confirmed that using a combination of chitosan and
bentonite clay powder halved the settling time (15.3 vs. 34.2 h) (Moorthy et al. 2017).
The efficiency of removal can be affected by the pre-treatment of algae using ozone before chitosan
flocculation. It was shown to increase floc size probably due to the release of intra-cellular organic matter.
The removal of turbidity increased from 61.8% to 80.4% when algae cells (Chlorella sp.) were submitted
to ozone for 30 min compared to control cells not treated with ozone (Pranowo et al. 2013).
Naturally occurring microbial flocculants have been applied to harvest microalgae. The production
and application of such flocculants is described in the literature, though not often in connection with
microalgae removal: Protein flocculants produced by Rhodococcus erythropolis S-1 (Kurane et al. 1986)
and Bacillus sp. DP-152 (Suh et al. 1997), polyamide flocculants produced by B. subtilis DYU1 (Wu and