Environmental Engineering Reference
In-Depth Information
biomass (Chen et al.
2011
). Membrane technology
for recovering algal biomass is also a possible
alternative to conventional fi ltration. Membrane
technology is usually used to recover algal cells
from small aquaculture farms for feeding
shellfi sh larvae. However, the same technique is
ineffi cient for harvesting algal biomass from
large-scale cultivation processes (Molina Grima
et al.
2003
). Membrane technology also has a
problem with fouling due to the formation of an
algal layer causing pore blockages and a decrease
in permeability (Ladner et al.
2010
). However,
frequent backwash of the membrane helps to
control this fouling. The polyvinylchloride (PVC)
ultrafi ltration (UF) membrane employed with air-
assisted backwash with air scour for harvesting
Scenedesmus quadricauda
showed effi ciency
and average permeability of 46.01 g (m
2
h)
−1
and
45.50 l (m
2
h)
−1
, respectively (Zhang et al.
2010
).
Understanding the drawbacks of the higher
effi cacy and decreasing the fouling of membrane
fi ltration processes remains a challenge. In terms of
economic feasibility, membrane technologies are
not cost effective for large-scale harvesting of
algal biomass. However, harvesting small vol-
umes of algal biomass with microfi ltration (e.g.,
<2 m
3
day
−1
) is more cost effective than centrifu-
gation (Molina Grima et al.
2003
).
Microalgae also have a tendency for self-
fl occulation, known as auto-fl occulation. The
process is sensitive to pH and it increases with
the consumption of dissolved carbon dioxide.
At super saturation of calcium and phosphate
ions, the calcium phosphate precipitate will be
positively charged. Thereby algae cells which are
negatively charged serve as a solid support for
the precipitant and charge neutralization is
accomplished. A pH range of 8.5-9.0 was found
to be optimum for auto-fl occulation (Christenson
and Sims
2011
).
Polyelectrolytes are cationic polymers that are
also used as an alternative fl occulent. These are
found to be better than the non-polymerized
metal fl occulants and are effective over a wide
range of pH conditions (Molina Grima et al.
2003
). Polymer fl occulants induce the physical
linkage of the cells via bridging. Ideally, the
polymer should have high molecular weight and
charge to enhance their binding capabilities.
Tenney et al. (
1969
) also studied cationic and
anionic polyelectrolytes for harvesting
Chlorella
species. Recovery of
Chlorella
cells with cationic
polymers showed better results than anionic
polyelectrolyte. Although cationic polymers are
effective for harvesting freshwater microalgae at
a minimum concentration of 1-10 mg ml
−1
, they
can inhibit the process in a high saline environ-
ment (Molina Grima et al.
2003
). According to
reports, the fl occulation effi ciency of marine
algae decreases with an increase in the ionic
strength of the media up to 36 g L
−1
salinity. In
this regard, the optimal dosage for fl occulating
marine algae was 5- to 10-fold greater than that
for freshwater microalgae (Garzon-Sanabria
et al.
2012
). Moreover, a commercial product,
'Chitosan', is extensively used for harvesting
various microalgae. Chitosan performs fl occula-
tion at high effi ciency at a low dosage rate, but
the effi ciency is inhibited in the presence of salt.
The optimal dosage of chitosan for fl occulation
of
Thalassiosira pseudonana
and
Isochrysis
sp.
was found to be 40 mg L
−1
(Harun et al.
2010
).
However,
Chaetoceros muellari
requires a dos-
age of 150 mg L
−1
of chitosan for optimal bio-
mass recovery. The fl occulation rate varies with
the amount of Chitosan, and its effi ciency varies
2.3
Flocculation
Flocculation is considered an effective process
for large-scale harvesting of microalgae culture
(Wu et al.
2012
). The process is based on the
negative surface charge carried by the microalgal
cells that prevents self aggregation in suspension.
The surface charge can be countered by the addi-
tion of fl occulants such as multivalent cations
and cationic polymers to amass the biomass from
the culture (Harun et al.
2010
). Flocculants must
be inexpensive and non-toxic to the culture. In
addition, the fl occulent should not have any
adverse effect on further downstream processing.
Multivalent metal salts such as ferric chloride
(FeCl
3
), aluminum sulfate (Al
2
(SO
4
)
3
, alum),
and ferric sulfate (Fe
2
(SO
4
)
3
) are widely used
for fl occulation (Molina Grima et al.
2003
).
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