Advancement and Challenges in Harvesting Techniques for Recovery of Microalgae Biomass - Environmental Sustainability: Role of Green Technologies - page 160

with a baffl ed hydraulic fl occulator for mixing
the process, and the overall energy consumed
per ton of fl occulated dry biomass was esti-
mated to be 0.9 kWh or the equivalent of 9.8 MJ
tone −1 biodiesel (assuming microalgae lipid con-
tent is 33 %). The cost of this harvesting process
was estimated to be 79 % lower than other con-
ventional fl occulants (Lee et al. 2010 ). The har-
vested culture medium can be effectively reused
for microalgae cultivation without affecting its
growth, and thereby the process reduces the
other additional costs such as those for water
treatment and purifi cation (Lam and Lee 2012 ).
Another study with microbial fl occulation for
harvesting marine microalgae was conducted by
Lee et al. 2010 . The growth of fl occulating
microbes was enhanced by supplementing
organic carbon (acetate, glucose, or glycerin) as
a substrate. The EPSs were induced by providing
nutrient stress, and an average recovery effi ciency
of 90 % was achieved at a low concentration of
organic substrate (0.1 g L −1 ) (Lam and Lee
2012 ).

biofuel production is believed to play a successful
role in the commercial scale (Lam and Lee 2012 ).
However, the immobilization of microalgae is
also associated with some disadvantages, such as
(1) dissolution of beads due to the high presence
of anti-gelling cations in sea water and serious
microbial attack causing loss of microalgae cells
(Mallick 2006 ), (2) rupture of beads, which is
caused by insuffi cient divalent ions (Lam and
Lee 2012 ), (3) mass transfer limitation due to the
formation of bio-fi lm as a protection layer to the
microalgae cells, hindering the transport of nutri-
ents and carbon source. Hence, the immobilized
microalgae show much slow growth compared
with free cell culture, and the problem can be
solved by co-immobilization with plant growth-
promoting bacteria (PGPB) (Gonzalez and
Bashan 2000 ).

6 Conclusions

In the literature, several methods for the harvesting
of microalgae biomass have been studied, and
many challenges associated with the method
have also come to light. The varied morphology
of microalgal species and the fi nal desired prod-
uct have led to the recovery of biomass becoming
more challenging in itself. In addition, the econom-
ical and environmental aspects, and the energy
requirements, of microalgae harvesting on a com-
mercial scale must be investigated. Centrifugation
is the most effi cient of the discussed harvesting
methods, but it has some drawbacks due to the
requirement for a high level of energy and the
associated costs. Many companies are undertaking
various research and development in the harvesting
of microalgae biomass. However, microalgae
harvesting on a large scale still needs considerable
design and technological development to reduce
the cost of the method and become more com-
mercially viable.

5.2 Immobilization Biotechnology

The growing harvesting challenges have developed
an interest in using immobilization techniques for
harvesting. The method is based on entrapment
of microalgae in a matrix and it allows continu-
ous growth of the cells within the matrix. When
the microalgae reach a stationary growth phase,
the immobilized microalgae cells will settle at the
bottom of the culture medium. In this case, the
energy requirement may not be high, as the rela-
tively large microalgae beads can be harvested
with a simple fi ltration method (e.g., sieve)
(Christenson and Sims 2011 ). Various immobili-
zation techniques have been developed and, so far,
entrapment with alginate gel is the most feasible
for immobilization of microalgae. The alginate
gel entrapment method has some advantages
like negligible toxicity, high transparency, and a
requirement for mild conditions during the immo-
bilization process (Moreno-Garrido 2008 ). The
incorporation of the immobilization technique in

Acknowledgment The work reported in this article was
fi nancially supported by a research grant [vide grant no. 22
(0600)/12/EMR-II] received from Council for Scientifi c
and Industrial Research (CSIR), Government of India,
New Delhi.