Advancement and Challenges in Harvesting Techniques for Recovery of Microalgae Biomass - Environmental Sustainability: Role of Green Technologies - page 159

Carbon Capture described a closed reactor system
with a rotatable internal transparent insulator.
The reactor is designed with a harvesting chamber
where fl uid motion maintains a whirlpool to
pre-concentrate the algae before it is passed
through a roller press (Sears 2007 ). Algae to
Energy (or A2E) describe a harvester named the
'Shepherd harvester', which consists of a con-
tinuous belt and a vacuum system that moves
through the algae culture. While moving, any
algae collected on the belt are harvested by the
vacuum system before the belt passes through
the culture again (Christenson and Sims 2011 ).
Another patent application by Algaeventure
Systems Inc. also describes the use of a continu-
ous belt harvester. The harvester is based on
capillary extraction and consists of a primary and
a secondary belt (Youngs and Cook 2010 ). The
function of the primary belt is to collect algae,
and the secondary belt attached to the bottom
portion of the primary belt is compressed to drain
water after the dried biomass on the primary belt
is collected. Another two energy companies,
MBD Energy and Scipio Biofuels, also use
centrifugation methods to harvest microalgae.
MBD Energy is in collaboration with a Dutch
company, Evados, and uses their separators. The
Evados separator is a centrifuge; it allows for
easier removal of solids after concentration.
However, Scipio Biofuels also uses a centrifuga-
tion process but of a low speed for harvesting
(Christenson and Sims 2011 ). Sapphire Energy
genetically modifi ed the algae to enable con-
trolled fl occulation and simple harvesting.
Another patent application by Kent Bioenergy
describes a series of decanting procedures to
initiate the culture for fl occulation. In this method,
the fl ow of culture in a raceway pond is blocked,
and the upper layer of the water with the culture
is removed, and an equal amount of removed
culture volume is replenished. The process is
repeated a couple of times until the sediment-
ready algae suffi ciently dominate the culture
(Schwartz et al. 2010 ). In terms of biological-
based harvesting, Algenol Biofuels Inc. tried
genetic modifi cation of microalgae to avoid the
biomass harvesting step (Woods et al. 2010 ).

5 Possible Alternatives

for Economic and
Environmental Friendly
Harvesting

Recovery of microalgae biomass on a commercial
scale may be possible using different harvesting
methods, i.e. centrifugation, fi ltration, and fl oc-
culation. However, these methods also have many
disadvantages like investment of high capital,
energy, and running costs and are considered
mostly unlikely to be economical on a commercial
scale (Becker 1994 ; Benemann and Oswald 1996 ;
Lee et al. 2009 ). For commercial production of
microalgal products, the harvesting technique
must consider the harvesting cost, energy input,
and environmental effect.

5.1 Biofl occulation

Bio-based fl occulation has emerged as a new
alternative for eco-friendly harvesting of algal
biomass. Generally, biofl occulation is a dynamic
process derived from extracellular polymer
substances (EPS) synthesized by living cells.
Microorganisms such as bacteria, fungi, and
actinomycetes have been known to produce EPS
such as polysaccharides, functional proteins,
and glycoprotein, which act as biofl occulant
(Lam and Lee 2012 ). Bioflocculation with
bacteria have been also applied in wastewater
treatment. However, these processes require
additional substrates for bacterial growth, and
bacterial contaminations are promoted, which
create a hindrance to the recovery of the algal
biomass (Salim et al. 2011 ). Apart from this,
biofl occulation induced with environmental
stresses like high temperature and pH or nutri-
ent depletion may affect cell composition, and
the process is considered uneconomical on a
commercial scale (Lee et al. 2009 ). An assess-
ment was carried out to estimate the cost and
energy consumption for utilizing biofl occulation
on a commercial scale. In the assessment, the
microalgae harvesting system is incorporated