Environmental Engineering Reference
In-Depth Information
Incorporates (USA), and Western Michigan
University (USA).
earth or cellulose can be used to increase
effi ciency. This method has demonstrated that the
fi ltration process is moderately effective for large
microalgae, whereby a chamber fi lter press can
achieve a concentration factor 245 times the orig-
inal concentration of
C. proboscideum
, to pro-
duce sludge with 27 % solids. The disadvantages
of membrane fi ltration include the sporadic
requirement for membrane replacement and ele-
vated production costs due to energy-intensive
pumping. It is more cost effective than the
centrifugation process for the processing of
low-culture volume (<2 m
3
/day). Due to the
cost of membrane replacement and pumping
in large-scale production (>20 m
3
/day), centrifu-
gation may be one of the most economic meth-
ods for harvesting microalgae (MacKay and
Salusbury
1988
).
11
Harvesting Microalgal
Biomass
Harvesting of microalgal biomass is one of the
important steps in producing maximum biomass.
It can actually contribute 20-30 % of total bio-
mass production cost (Chisti
2007
). The small
size of microalgal cells (typically ranging from 2
to 200
m) makes the harvesting process very
diffi cult. There are a few techniques in the
harvesting of microalgae, including centrifuga-
tion, fl occulation, fi ltration and screening, grav-
ity sedimentation, fl otation, and electrophoresis
techniques (Uduman et al.
2010
; Brennan and
Owende
2010
). The selection of harvesting tech-
niques is dependent on the physical properties of
microalgae such as size, density of the slurry,
intracellular biomass composition, and yield of
desired products (Brennan and Owende
2010
).
We summarize some of the techniques that are
currently being used for the recovery of microal-
gal biomass.
ʼ
11.2
Centrifugation
Centrifugation is a method of separating algae
from the medium using a centrifuge to cause the
algae to settle in the bottom of a fl ask or tank. It
is a useful device for both biolipid extraction
from algae and chemical separation in biodiesel.
The centrifuge works using the sedimentation
principle, where centripetal acceleration is used
to evenly distribute substances (presents in a
solution for small-scale applications) of greater
and lesser density. Centrifugation is currently
considered too expensive and to increase produc-
tion costs due to rising power consumption
(Grima et al.
2003
). This method is preferred for
harvesting algal biomass especially for produc-
ing prolonged shelf-life concentrates for aquacul-
ture usage. The main advantage of this technique
is that 95 % of the cells are effi ciently harvested
at 13,000 ×
g
by increasing the gravitation fi eld
(Greenwell et al.
2010
) thus it increases the slurry
concentration up to 150 times more and 15 %
total suspended solids are technically possible to
harvest (Mohn
1980
). It has certain disadvan-
tages, which include the high energy costs and
potentially higher maintenance requirements
(Bosma et al.
2003
).
11.1
Filtration
Conventional fi ltration may be inadequate for
biomass recovery and is most appropriate for the
harvesting of large (>70
m) microalgae such as
Coelastrum proboscideum
and
Spirulina plant-
ensis
. The small (<30
ʼ
m) microalgae like
Chlorella
,
Dunaliella
, and
Scenedesmus
cannot
be harvested using this technique (Mohn
1980
).
For recovery of smaller algal cells (<30
ʼ
m),
membrane microfi ltration and ultra-fi ltration are
a technically feasible alternative to conventional
fi ltration. Microfi ltration and ultra-fi ltration are a
form of membrane fi ltration using hydrostatic
pressure. Microfi ltration is one of the most effi -
cient and suitable methods for harvesting fragile
algal cells (Grima et al.
2003
; Mata et al.
2010
).
Conventional fi ltration works under pressure or
suction; fi ltration aids such as diatomaceous
ʼ
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