92 Marine Macro- and Microalgae: An Overview
2
s
p
(
)
18 c
d ρ ρ g
u
μ
−
=
where u p is the sedimentation velocity of the particle (m/s), d is the particle diameter (m), r s is the particle
density (kg/m), ρ is the density of the fluid (kg/m), g is the gravitational acceleration (m/s), and m c is the
viscosity of the suspension (Ns/m).
Natural sedimentation
Naturally occurring sedimentation would obviously be the most cost effective separation method if it
was not for its slow velocity which was measured to be as low as 3 10
–7
to 7 10
–5
m/s (Choi et al. 2006)
with the consequence that most of the biomass could deteriorate during the settling time (Greenwell et
al. 2010). Griffiths et al. (2012) concluded that gravity sedimentation could be a promising technique for
the microalgae Cylindrotheca fusiformis and Tetraselmis suecica with average biomass recovery of 95%
and 89%, respectively, after 24 hr settling. Experience from traditional sedimentation units have shown
that efficient sedimentation occurs when the sedimentation velocity is greater than 10
–4
m/s (Granados
et al. 2012). Arthrospira was reported to settle spontaneously due to carbohydrate accumulation under
nutrient-stressed conditions (Markou et al. 2012). Depraetere et al. (2015) demonstrated that this natural
sedimentation was caused by an accumulation of carbohydrates that in turn increased the specific density
of the filaments. The settling velocity was 0.64 m/h allowing the biomass to be concentrated 15 times
with removal of 94% of the water. However, very seldom is natural sedimentation efficient and therefore
alternative techniques are used.
Centrifugation
From the equation above it is clear that there is a direct correlation between gravitational acceleration
and speed of sedimentation. Centrifugation up to 5,000 to 10,000 g is currently the norm in largescale centrifuges used in algae productions. For example, Adam et al. (2012) harvested the microalgae
Nannochloropsis oculata by centrifugation at around 5,000 rpm, resulting in a 30% dry weight paste
for further processing. Similarly, centrifugation of the algal suspension of Nannochloropsis sp. was
conducted at 3,000 g for 5 min in order to further extract the fatty acid eicosapentaenoic acid (Zou
et al. 2000). It is especially important to dewater the biomass to reduce drying costs in large scale
production. Most centrifugation setups tend to achieve high capture efficiency (> 90%) with low flow
rates through the centrifuge, but this requires high energy. Dassey and Theegala (2013) investigated the
use of centrifugation as a cost effective separation of microalgae. By controlling the capture efficiency
and the flow rate of the centrifugation process, the final harvesting cost could be reduced by 82%. On
the one hand when feeding the centrifuge with a flow of 0.94 L/min, the cell removal efficiency reached
94%, but this required an energy input of 20 kW h/m
3
of culture water. On the other hand, a higher feeding
flow of 23 L/min only afforded a cell removal efficiency of 17%. However, this could be achieved at a
lower energetic cost of 0.80 kW h/m
3
. Using this lower yielding running conditions, the integration of
centrifuges for harvesting microalgae could turn out to be profitable, especially if the energy supply stems
from renewable energy such as photovoltaic solar panels or wind power.
Another consideration regarding the use of centrifugation is the physical state of the biomass
harvested for further processing. The rheology of the algal cell concentrate becomes non-Newtonian
when cell suspension reaches 7–10% (v/v). And when the suspension is concentrated to as much as
15–20% of the volume as cells, the suspension is no longer fluidic and therefore non amenable to pumping
(Greenwell et al. 2010), and this is a hindrance for further downstream processing using pumps.
Flocculation
Instead of resorting to costly and energy consuming processes, simple techniques using induced
precipitation of microalgae seem to be cheap alternatives. As seen in the above mentioned equation,
2
s
p
(
)
18 c
d ρ ρ g
u
μ
−
=
where u p is the sedimentation velocity of the particle (m/s), d is the particle diameter (m), r s is the particle
density (kg/m), ρ is the density of the fluid (kg/m), g is the gravitational acceleration (m/s), and m c is the
viscosity of the suspension (Ns/m).
Natural sedimentation
Naturally occurring sedimentation would obviously be the most cost effective separation method if it
was not for its slow velocity which was measured to be as low as 3 10
–7
to 7 10
–5
m/s (Choi et al. 2006)
with the consequence that most of the biomass could deteriorate during the settling time (Greenwell et
al. 2010). Griffiths et al. (2012) concluded that gravity sedimentation could be a promising technique for
the microalgae Cylindrotheca fusiformis and Tetraselmis suecica with average biomass recovery of 95%
and 89%, respectively, after 24 hr settling. Experience from traditional sedimentation units have shown
that efficient sedimentation occurs when the sedimentation velocity is greater than 10
–4
m/s (Granados
et al. 2012). Arthrospira was reported to settle spontaneously due to carbohydrate accumulation under
nutrient-stressed conditions (Markou et al. 2012). Depraetere et al. (2015) demonstrated that this natural
sedimentation was caused by an accumulation of carbohydrates that in turn increased the specific density
of the filaments. The settling velocity was 0.64 m/h allowing the biomass to be concentrated 15 times
with removal of 94% of the water. However, very seldom is natural sedimentation efficient and therefore
alternative techniques are used.
Centrifugation
From the equation above it is clear that there is a direct correlation between gravitational acceleration
and speed of sedimentation. Centrifugation up to 5,000 to 10,000 g is currently the norm in largescale centrifuges used in algae productions. For example, Adam et al. (2012) harvested the microalgae
Nannochloropsis oculata by centrifugation at around 5,000 rpm, resulting in a 30% dry weight paste
for further processing. Similarly, centrifugation of the algal suspension of Nannochloropsis sp. was
conducted at 3,000 g for 5 min in order to further extract the fatty acid eicosapentaenoic acid (Zou
et al. 2000). It is especially important to dewater the biomass to reduce drying costs in large scale
production. Most centrifugation setups tend to achieve high capture efficiency (> 90%) with low flow
rates through the centrifuge, but this requires high energy. Dassey and Theegala (2013) investigated the
use of centrifugation as a cost effective separation of microalgae. By controlling the capture efficiency
and the flow rate of the centrifugation process, the final harvesting cost could be reduced by 82%. On
the one hand when feeding the centrifuge with a flow of 0.94 L/min, the cell removal efficiency reached
94%, but this required an energy input of 20 kW h/m
3
of culture water. On the other hand, a higher feeding
flow of 23 L/min only afforded a cell removal efficiency of 17%. However, this could be achieved at a
lower energetic cost of 0.80 kW h/m
3
. Using this lower yielding running conditions, the integration of
centrifuges for harvesting microalgae could turn out to be profitable, especially if the energy supply stems
from renewable energy such as photovoltaic solar panels or wind power.
Another consideration regarding the use of centrifugation is the physical state of the biomass
harvested for further processing. The rheology of the algal cell concentrate becomes non-Newtonian
when cell suspension reaches 7–10% (v/v). And when the suspension is concentrated to as much as
15–20% of the volume as cells, the suspension is no longer fluidic and therefore non amenable to pumping
(Greenwell et al. 2010), and this is a hindrance for further downstream processing using pumps.
Flocculation
Instead of resorting to costly and energy consuming processes, simple techniques using induced
precipitation of microalgae seem to be cheap alternatives. As seen in the above mentioned equation,