94 Marine Macro- and Microalgae: An Overview
formation of flocs. An initial vigorous mixing (100–200 rpm) of the aluminium chloride was needed
to initiate coagulation of the algae into microflocs, followed by slow mixing (50 rpm), allowing the
generation of flocs which would settle within 1 hr. Shen et al. (2013) carried out a similar investigation
on N. oculata with two cationic salts, aluminium sulfate and ferric chloride. Using single-factor and
response-surface-methodology experiments, they developed second-order polynomial models of the final
solid concentration of algae (SCA) based upon the initial algal biomass concentration (IABC), pH, and
flocculant dose (FD). The optimum flocculation conditions were predicted and validated experimentally
at IABC 1.7 g/L, pH 8.3 and FD 383.5 µM for aluminium sulfate, and IABC 2.2 g/L, pH 7.9 and FD
438.1 µM for ferric chloride, resulting in final solid concentrations of algae of 32.98 g/L and 30.10 g/L,
respectively. Biomass recovery rate was in the range 82.6 to 100% (on average 94.4%) and 60 to 100%
(on average 87.9%) for aluminium sulfate and ferric chloride, respectively. Şirin et al. (2013) performed
flocculation experiments on N. gaditana with aluminium sulfate (AS) and polyaluminium chloride
(PAC), which showed that PAC was 30% more efficient than AS regarding flocculation efficiency. With
the optimum concentration of AS (20 ppm), the flocculation efficiency was 70% after 30 min of settling.
The optimum concentration of PAC depended on the settling time. Optimum concentrations were 20 and
10 ppm for settling times of 15 and 30 minutes, and flocculation efficiency of 60 and 55%, respectively.
Wu et al. (2012a) also investigated the effect of dosage of alum on the recovery of N. oculata and found
an optimum concentration at 500 ppm and a settling time of 48 h.
Earlier work showed that both alkaline and acidic conditions could induce algal flocculation with
polyvalent metal salts. Optimal flocculation of marine microalgae can be achieved efficiently in alkaline
conditions (pH > 10.5) due to the precipitation of calcium and/or magnesium salts in the medium (Sukenik
and Shelef 1984; Sukenik et al. 1985) while the use of aluminium sulfate and iron chloride as flocculants
required acidic conditions (Garzon-Sanabria et al. 2010). Recent studies point to diverging results. On the
one hand, it was demonstrated that at high pH, algal flocculation was due to the precipitation of Ca(OH) 2
and/or Mg(OH) 2 (Wu et al. 2012b). This result was confirmed with the alga Chlorella vulgaris grown
either in a medium mimicking brackish water (Smith and Davis 2012) or in freshwater (Vandamme et al.
2012). On the other hand, Schlesinger et al. (2012) could not induce algal flocculation of nine different
marine and freshwater algae species with Mg(OH) 2 , but could with Ca(OH) 2 , NaOH, KOH, and NH 4 OH.
Parameters inducing flocculation are different for freshwater and seawater algae. Sukenik et al.
(1988) showed that flocculation with aluminium sulfate and ferric chloride of the marine algae I. galbana
required a 5- to 10-fold greater ionic strength than that for the freshwater algae C. vulgaris. Wu et al.
(2012b) showed that marine algal cultures use more Mg(OH) 2 for flocculation than the freshwater algal
cultures as a greater amount of Mg
2+
was removed from the media during flocculation of the marine algae
species.
Various other parameters, such as algal species, cell density, pH, and flocculant concentration,
can affect flocculation efficiency. Alum, which has proven to be an efficient flocculant without any pH
adjustment for the marine algae T. suecica and Chlorococcum sp. (> 90% recovery after 5 min settling),
turned out to be less effective with the marine algae Nannochloropsis salina, Dunaliella tertiolecta and
I. galbana (Elridge et al. 2012). These algae required at least twice as much coagulant to achieve similar
recoveries.
Flocculation efficiency for N. oculata, was optimal when using 48 mg/L AlCl 3 (Garzon-Sanabria et
al. 2012). However, this same study also demonstrated that flocculation efficiency also depends upon pH
and cell density in the culture as well as of the ionic strength of the growth medium. Optimal flocculation
with 48 mg/L AlCl 3 was achieved with 3 10
7
cell/mL, a pH of 5.3 and a salt concentration of 15 g/L
NaCl. Five times greater AlCl 3 dosage was required to obtain the same efficiency with lower cell density
culture (10
6
cell/mL). Salt concentration only minimally affected the efficiency of flocculation as removal
efficiency was 96% and 97% with 0 and 30 g/L NaCl, respectively, compared to the 15 g/L optimum
concentration of NaCl salt which resulted in a 98% removal efficiency.
The efficiency of removal can be affected by the pre-treatment of algae using ozone before
polyaluminium chloride 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
formation of flocs. An initial vigorous mixing (100–200 rpm) of the aluminium chloride was needed
to initiate coagulation of the algae into microflocs, followed by slow mixing (50 rpm), allowing the
generation of flocs which would settle within 1 hr. Shen et al. (2013) carried out a similar investigation
on N. oculata with two cationic salts, aluminium sulfate and ferric chloride. Using single-factor and
response-surface-methodology experiments, they developed second-order polynomial models of the final
solid concentration of algae (SCA) based upon the initial algal biomass concentration (IABC), pH, and
flocculant dose (FD). The optimum flocculation conditions were predicted and validated experimentally
at IABC 1.7 g/L, pH 8.3 and FD 383.5 µM for aluminium sulfate, and IABC 2.2 g/L, pH 7.9 and FD
438.1 µM for ferric chloride, resulting in final solid concentrations of algae of 32.98 g/L and 30.10 g/L,
respectively. Biomass recovery rate was in the range 82.6 to 100% (on average 94.4%) and 60 to 100%
(on average 87.9%) for aluminium sulfate and ferric chloride, respectively. Şirin et al. (2013) performed
flocculation experiments on N. gaditana with aluminium sulfate (AS) and polyaluminium chloride
(PAC), which showed that PAC was 30% more efficient than AS regarding flocculation efficiency. With
the optimum concentration of AS (20 ppm), the flocculation efficiency was 70% after 30 min of settling.
The optimum concentration of PAC depended on the settling time. Optimum concentrations were 20 and
10 ppm for settling times of 15 and 30 minutes, and flocculation efficiency of 60 and 55%, respectively.
Wu et al. (2012a) also investigated the effect of dosage of alum on the recovery of N. oculata and found
an optimum concentration at 500 ppm and a settling time of 48 h.
Earlier work showed that both alkaline and acidic conditions could induce algal flocculation with
polyvalent metal salts. Optimal flocculation of marine microalgae can be achieved efficiently in alkaline
conditions (pH > 10.5) due to the precipitation of calcium and/or magnesium salts in the medium (Sukenik
and Shelef 1984; Sukenik et al. 1985) while the use of aluminium sulfate and iron chloride as flocculants
required acidic conditions (Garzon-Sanabria et al. 2010). Recent studies point to diverging results. On the
one hand, it was demonstrated that at high pH, algal flocculation was due to the precipitation of Ca(OH) 2
and/or Mg(OH) 2 (Wu et al. 2012b). This result was confirmed with the alga Chlorella vulgaris grown
either in a medium mimicking brackish water (Smith and Davis 2012) or in freshwater (Vandamme et al.
2012). On the other hand, Schlesinger et al. (2012) could not induce algal flocculation of nine different
marine and freshwater algae species with Mg(OH) 2 , but could with Ca(OH) 2 , NaOH, KOH, and NH 4 OH.
Parameters inducing flocculation are different for freshwater and seawater algae. Sukenik et al.
(1988) showed that flocculation with aluminium sulfate and ferric chloride of the marine algae I. galbana
required a 5- to 10-fold greater ionic strength than that for the freshwater algae C. vulgaris. Wu et al.
(2012b) showed that marine algal cultures use more Mg(OH) 2 for flocculation than the freshwater algal
cultures as a greater amount of Mg
2+
was removed from the media during flocculation of the marine algae
species.
Various other parameters, such as algal species, cell density, pH, and flocculant concentration,
can affect flocculation efficiency. Alum, which has proven to be an efficient flocculant without any pH
adjustment for the marine algae T. suecica and Chlorococcum sp. (> 90% recovery after 5 min settling),
turned out to be less effective with the marine algae Nannochloropsis salina, Dunaliella tertiolecta and
I. galbana (Elridge et al. 2012). These algae required at least twice as much coagulant to achieve similar
recoveries.
Flocculation efficiency for N. oculata, was optimal when using 48 mg/L AlCl 3 (Garzon-Sanabria et
al. 2012). However, this same study also demonstrated that flocculation efficiency also depends upon pH
and cell density in the culture as well as of the ionic strength of the growth medium. Optimal flocculation
with 48 mg/L AlCl 3 was achieved with 3 10
7
cell/mL, a pH of 5.3 and a salt concentration of 15 g/L
NaCl. Five times greater AlCl 3 dosage was required to obtain the same efficiency with lower cell density
culture (10
6
cell/mL). Salt concentration only minimally affected the efficiency of flocculation as removal
efficiency was 96% and 97% with 0 and 30 g/L NaCl, respectively, compared to the 15 g/L optimum
concentration of NaCl salt which resulted in a 98% removal efficiency.
The efficiency of removal can be affected by the pre-treatment of algae using ozone before
polyaluminium chloride 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