Harvesting of Microalgal Biomass - Biotechnological Applications of Microalgae - page 80

et al., 2011). However, the gravity settling rate for very small sized microalgae is
too low for routine high rate algae harvesting, and holding algal biomass for a long
time under dark and static conditions can result in significant biomass loss via
respiration and bacterial decomposition. Moreover, during flocculation, flocs may
float due to adsorption of tiny air bubbles and do not settle by gravitational forces.
The classical approach to gravity settling may therefore not be very efficient for
rapid biomass recovery from high-rate algal ponds. The sedimentation rate can be
increased by increasing the gravitational force via centrifugation. The latter has
very high biomass recovery (>95%) and can be applied to a wide range of microal-
gae, although it cannot be used for an algae farm producing an energy feedstock,
owing to cost constraints.
6.4 CENTRIFUGATION
Centrifugation is similar to sedimentation, wherein gravitational force is replaced by
centrifugal acceleration to enhance the concentration of solids. Particle size and den-
sity difference are the key factors in centrifugal separation. Once separated, the algae
concentrate can be obtained by simply draining the supernatant. Many research-
ers have advocated this method for reliable recovery of microalgae (Mohn, 1980;
Benemann and Oswald, 1996; Girma et al., 2003). Different types of centrifuges
have been used, and their respective reliability and efficiency have been documented
by several researchers. For example, Heasman et al. (2000) reported that 90% to
100% harvesting efficiency can be achieved via centrifugation. Sim et al. (1988)
compared centrifugation, chemical flocculation followed by dissolved air flotation
(DAF), and membrane filtration processes for harvesting algae from pilot-scale
ponds treating piggery wastewater, and they found that none of these processes were
completely satisfactory. Centrifugation was reported to be very effective but too cost
and energy intensive to be applied on a commercial scale. This kind of harvesting
is usually recommended in the production of high-value metabolites or as a second-
stage dewatering technique for concentrating algal slurries from 1% to 5% solids
to >15% solids, as it does have some limitations. Undoubtedly, it is an efficient and
reliable technique for microalgal recovery but one should also keep in mind its high
operational cost.
6. 5 FILTR ATION
Filtration is the most competitive method compared to other harvesting tech-
niques. It is most appropriately used for relatively large sized (>70 μm) algae such
as filamentous species or agglomerates. Diatomaceous earth or cellulose can be
used to increase filtration efficiency (Brennan and Owende, 2010). However, con-
ventional filtration operated under pressure or suction is not suitable for smaller
sized algae such as Chlorella , Dunaliella, and Scenedesmus. Membrane micro-
filtration and ultrafiltration are alternative filtration methods. The disadvantage of
these processes is their high cost due to the frequent replacement of membranes
and pumping costs (Pittman et al., 2011). There are many different types of filtra-
tion processes, such as dead-end filtration, microfiltration, ultrafiltration, pressure