Advancement and Challenges in Harvesting Techniques for Recovery of Microalgae Biomass - Environmental Sustainability: Role of Green Technologies - page 156

2.4.1 Dissolved Air Flotation (DAF)
The DAF process is the most widely used tech-
nique in industries for effl uent treatment. The
process is based on the reduction in pressure of a
water stream that is presaturated with air. The
presaturation of the water stream can be carried
out with air at pressures higher than atmospheric
(Uduman et al. 2010 ). DAF is often used as an
effi cient clarifi cation step for treating water
containing hydrophobic matter and algae (Barrut
et al. 2013 ). The DAF method involves many
factors, such as pressure of the tank, recycle rate,
hydraulic retention time, and fl oating rate of
particles. Harvesting of microalgae with DAF
combined with fl occulation produces 6 % of
microalgal slurries. The DAF process coupled with
fl occulation is usually preferred over settling.
The method is effective at large scales, but the
usage of chemicals for fl occulation can cause
adverse effects downstream in the process
(Christenson and Sims 2011 ). A new hybrid
system has been studied for simultaneous
removal of algae and the organics produced by
algae ( Anabaena and Mycrocystis ). The system
consists of powdered activated carbon (PACs)
adsorption and DAF processes, which show low
removal effi ciency of PACs without coagulation,
while effi cacy is increased up to 70 % by con-
ventional gravity sedimentation (CGS) and up to
95 % by DAF when coagulant (i.e., polyaluminium
chloride) was used (Kwak 2009 ).

with different algal species (Molina Grima et al.
2003 ). Flocculation at an elevated pH of 11.8 and
12.0 without any addition of fl occulants showed
maximum effi ciency of 95 %, but the extremely
high pH condition can cause adverse effects.
Blanchemain and Grizeau ( 1999 ) studied recovery
of eicosapentaenoic acid (EPA) from Skeletoma
costatum , where fl occulation effi ciency of 80 %
was found, but the high pH condition of 10.2
caused cell lyses and loss of intracellular con-
tents. An increase in pH condition plays a major
role in fl occulation. As reported among the fi ve
different fl occulants (i.e. sodium hydroxide,
potassium hydroxide, calcium hydroxide, mag-
nesium hydroxide, and sodium carbonate),
fl occulation conducted with lime showed the
maximum effi ciency compared with other base
fl occulants (Vandamme et al. 2012 ).
Regardless, the method is easy and very effec-
tive for harvesting microalgae culture. However,
it is not cost effective and is unsustainable for
large-scale harvesting. Chemicals involved in
fl occulation need to be washed off before it can
be re-used, leading to extra operational costs.
Further, it can be signifi cantly hazardous to the
environment (Salim et al. 2011 ).

2.4 Flotation

Flotation methods are based on gravity separation
in which air or gas is bubbled through a solid-
liquid suspension and the gaseous molecules are
attached to the solid particles (Chen et al. 2011 ).
The dispersed micro-air bubbles are used to trap
the algal cells. In the fl otation technique, particle
size plays an important role; the smaller the par-
ticle, the more likely it is that the particle goes up
with the bubbles. The process is dependent on
size of the particle, and sizes less than 500

2.4.2 Dispersed Air Flotation
The dispersed air fl otation process is based on the
formation of bubbles by a high-speed mechanical
agitator with an air injection system (Chen et al.
2011 ). Chen et al. ( 1998 ) studied dispersed air
fl otation using three collectors, i.e. non-ionic
X-100, anionic sodium dodecylsulfate (SDS)
and N-cetyl-N-N-N-trimethylammonium bromide
(CTAB). The recovery of Scenedesmus quadri-
cauda was found to be more effective with
CTAB (Chen et al. 2011 ). Effective removal of
Ch. vulgaris by dispersed air fl otation (DiAF)
was also studied either using alum in combina-
tion with SDS or using dodecylamine alone as
the collector (Chen et al. 1998 ).

m can
be used during the process (Uduman et al. 2010 ).
Some microalgae also have a natural tendency to
fl oat on the water surface due to the high lipid
content (Brennan and Owende 2010 ). The fl ota-
tion process is divided into three types based on
bubble generation: DAF, dispersed fl otation, and
electrolytic fl otation (Chen et al. 2011 ).

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