Advancement and Challenges in Harvesting Techniques for Recovery of Microalgae Biomass - Environmental Sustainability: Role of Green Technologies - page 154

Fig. 1 Different types of microalgae harvesting process

2.1 Centrifugation 2.2 Filtration

Centrifugation as a method for harvesting
microalgae is preferred over other harvesting
techniques such as DAF and drum fi ltration
(Harun et al. 2010 ). The process is effi cient, with
biomass recovery of 80-90 % within 2-5 min
(Chen et al. 2011 ). Heasman et al. ( 2000 ) reported
that centrifugation of algal cells at 13,000 rpm
provides maximum harvesting effi ciency of
95-100 % and cell viability of 88-100 %,
whereas its effi ciency decreases to 60 % at
6,000 rpm and 40 % at 1,300 rpm. The method
is suitable for laboratory-scale operation when
the culture concentration is above 30 mg L −1 .
The method of centrifugation and cell viability
depend signifi cantly on the algal species.
Basically, the centrifugation process is reliable
for recovery of metabolites (Harun et al. 2010 ).
However, recovery of algal biomass under a
rotational velocity of 3,000 rpm is evident, with
84 % removal effi ciency of 0.2 g L −1 algal
cultures at a fl ow of 100 gal min −1 (Dassey and
Theegala 2013 ). The harvesting of microalgae
cells is accompanied by the drying process
and it responds effectively when the processed
algal biomass is increased to at least 20 % dry
weight during the dewatering stage (Dassey and
Theegala 2013 ). Although the processes are
rapid, they are also very energy intensive. The
technique involves high gravitational and shear
forces, which can damage cell structure. Large-
scale harvesting is also a great challenge in terms
of time and cost (Chen et al. 2011 ).

Filtration is one of the simplest traditional meth-
ods of separating the algal cells, but with the
advancement of modern techniques it has become
the most competitive method compared with other
harvesting options. The fi ltration process can
range from simple screening or micro-strainers to
dewatering up to complex vacuum or pressure
fi ltration systems. There are many different forms
of fi ltration, such as dead end fi ltration, microfi l-
tration, ultra fi ltration, pressure fi ltration, vacuum
fi ltration, and tangential fl ow fi ltration. This pro-
cess involves separation of a suspension through
a permeable medium, retaining the algae while
passing through the fi lter (Harun et al. 2010 ).
Filtration processes operating under pressure are
more effi cient for recovering large microalgae,
while it is not suitable for the recovery of algae
species such as Scenedesmus , Dunaliella , and
Chlorella due to their small dimensions. Various
fi ltration mediums have been studied for the
recovery of small-celled Dunaliella but have
been found to be impractical. In contrast, fi ltra-
tion through diatomaceous earth has proved
exceptional (Uduman et al. 2010 ). For appropri-
ate microalgae dewatering, different makes and
brands of pressure and vacuum fi ltration units
have been studied and the pressure belt fi lter and
vacuum fi lter thickener have been found to be
inappropriate for harvesting C. proboscideum.
However, tangential fl ow fi ltration (TFF) is one
of the high-rate microalgae harvesting methods
and is reported to recover about 70-89 % of