Environmental Engineering Reference
In-Depth Information
The dispersed ozone fl otation process is com-
monly regarded as an expensive process for water
and wastewater treatment. It is also used in harves-
ting microalgae and effl uent treatment. Recent
study on the recovery of
S. obliquus
FSP-3 with
ozonation showed effi cient recovery. The ozone-
induced fl otation leads the algal cells to become
more negatively charged and slightly increases the
hydrophobicity, favoring the fl otation in recovery
of
S. obliquus
FSP-3. However, the application of
simple air aeration provides hindrance in the
formation of fl otation (Cheng et al.
2010
).
formation of bubbles from both the electrode
causes fl ocs to fl oat. This technique is effi cient in
terms of cost but requires further optimization for
commercial feasibility (Kim et al.
2012
).
2.5
Magnetic Separation
Among the various harvesting processes, magnetic
separation has been introduced as a simple
technique for effi cient recovery of cells and bio-
molecules from liquid solution with the help of
functional magnetic particles driven by an exter-
nal magnetic fi eld (Wang et al.
2007
). Recently,
magnetic separation was used in the removal of
harmful microalgae by the functionalized mag-
netic particles. The practical application of this
technique is still limited by its complex fabrica-
tion and cost (Liu et al.
2009
). A recent study on
magnetic separation for recovery of
Botryococcus
braunii
and
C. ellipsoidea
with naked Fe
3
O
4
nanoparticles has been investigated. The recovery
effi ciency for both of the species was above 98 %
within 1 min. The magnetic nanoparticles can be
reused and recovered easily from the harvested
microalgae biomass, with effi cient biomass
recovery (Xu et al.
2011
). Magnetic harvesting of
C. vulgaris
with iron oxide magnetic particles
(IOMMs) prepared by microwave treatment under
various conditions (model environment, cultiva-
tion media, different pH) showed separation
effi ciencies of over 95 % at a 3:1 mass ratio of
IOMMs to microalgae in presence of phosphorous
limitations. The interactions of algae-IOMMs are
essential for magnetic cell separations, and phos-
phorus ions have been identifi ed as an interfering
element in the culture medium responsible for
microalgae harvesting (Prochazkova et al.
2013
).
Magnetophoresis has also been applied in the
separation of microalgae blooms in commercial
fi sh production ponds. A study conducted using
iron oxide magnetic nanoparticles (IONPs) for
microalgae separation in the aquaculture indus-
try showed recovery effi ciency of 90 % in less
than 3 min (Toh et al.
2012
). This magnetic sep-
aration technology provides effi cient micro-
algae recovery with less energy input and time.
2.4.3 Electrofl otation
The electrofl otation process involves the appli-
cation of an electric fi eld to separate the algae.
In this method, inactive metal (electrochemically
non-depositing) acts as a cathode that generates
hydrogen bubbles from water electrolysis. The
bubbles produced adhere to the microalgal fl ocs
and carries them to the surface (Chen et al.
2011
).
The electrolytic fl otation method in batch and
continuous reactors is studied for microalgae
harvesting (Alfafara et al
.
2002
). The batch study
reveals that increasing the input of electrical
power increases the removal rate of chlorophyll
and decreases the electrolysis time (Uduman
et al.
2010
). The disadvantages of electrolytic
fl otation are scaling of the cathode and the high
cost. Sandbank and Shelef (
1987
) investigated
the different microalgal recovery techniques and
concluded that electrofl otation is not the most
effective method, but it may be preferred for
harvesting marine microalgal species (Uduman
et al.
2010
). However, a new technique has been
introduced for continuous harvesting of micro-
algae via electro-coagulation-fl otation, termed
'continuous electrolytic microalgae' (CEM)
harvest. This technique generates the polarity
exchange with simultaneous harvesting and culti-
vation of microalgae. During CEM harvest, the
polarity exchange creates two distinct phases:
phase 1 (P1) and phase 2 (P2). In P1, microalgal
cells fl occulate due to the destabilization of nega-
tively charged microalgae mediated by metal
ions liberated from the dissolving electrode. In
P2, the metal ion liberation is terminated and the
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