Environmental Engineering Reference
In-Depth Information
40 mg/L of chitosan is required. On the other hand,
150 mg/L of chitosan was found to complete the
fl occulation process of
Chaetoceros muellaris
(Heasman et al.
2000
). Although chemical fl occu-
lation is often reported to incur fewer operating
costs to harvest algae, this method leads to signifi -
cant pollution of the environment (Li et al.
2008a
).
The interruption of the CO
2
supply in the algal sys-
tem can cause algae to fl occulate on its own,
termed 'autofl occulation'. In most cases, this phe-
nomenon is associated with high pH due to photo-
synthetic CO
2
consumption corresponding with
precipitation of magnesium, calcium, phosphate,
and carbonate salts with algae. Where calcium
phosphate is used, excess calcium ions (positively
charged) tend to react with algae (negatively
charged) and they bind together to provide an
autofl occulation process (Sukenik and Shelef
1984
). Figure 2e shows the fl occulating agents
added in to the algal culture and starting to coagu-
late algae cells.
such as aluminum, chromium, or iron and a
univalent metal such as potassium or sodium
(e.g., AlK(SO
4
)
2
).
11.5.2 Froth Flotation
This is another method of separating algae from
the medium by adjusting the pH and bubbling air
through a column to create a froth of algae. The
algae collect in foam above the liquid level and
can be separated by suction. The pH may vary
depending on the algal species. The major disad-
vantage of the froth fl otation is that it is not eco-
nomically feasible.
12
Dehydration of Algal
Biomass
After algal biomass is harvested, it must be pro-
cessed quickly. Depending on the fi nal product,
algal biomass can be processed by dehydration or
drying (Fig. 2d). Several methods area available
to process algal biomass, namely, solar drying,
low-pressure-shelf drying, drum drying (Prakash
et al.
1997
), spray drying (Desmorieux and
Decaen
2005
), fl uidized bed drying (Leach et al.
1998
), freeze drying (Grima et al.
1994
), and
refractance window dehydration technology
(Nindo and Tang
2007
). When compared with
other dehydration processes, solar drying is the
cheapest. However, some of the major disadvan-
tages are that solar drying requires a long time
and a large space, and there is a risk of material
loss (Prakash et al.
1997
).
11.5
Flotation
Flotation is a method used in combination with
fl occulation to harvest algae. It is a simple method
by which algae can be made to fl oat on the sur-
face of the medium. Due to the increase in the
lipid content of the cells, some strains may fl oat
naturally (Bruton et al.
2009
). The main advan-
tage of this method is that it is cheap and it can be
used to harvest large-scale microalgae. A few dis-
advantages of this method are that the fl occulat-
ing agents may lead to contamination, and
evidence of its technical viability is also very lim-
ited (Grima et al.
2003
). There are two types of
fl otations:
13
Lipid Extraction
Most microalgae accumulate neutral lipids; due to
the low degree of unsaturation, these lipids are
essential for biodiesel production. There are sev-
eral methods for extracting lipids from microal-
gae, namely, the Folch method, the gravimetric
method, and the Bligh and Dyer method. Solvent
extraction (n-hexane) and gravimetric determina-
tion are the two methods often used by researchers
for the extraction of lipids from the microalgal cell
11.5.1 Dissolved Air Flotation (DAF)
Dissolved air fl otation (DAF) separates algae
from its culture by using both froth fl otation
and fl occulation (Fig. 2c). For the fl occulation
of microalgal cells, alum is used to increase the
fl oc size before applying DAF, fi ne bubbles are
supplied by an air compressor. Alum is a com-
mon name for several trivalent sulfates of metal
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