Environmental Engineering Reference
In-Depth Information
11.3
Gravity Sedimentation
(Fe 2 (SO 4 ) 3 ), or other chemical fl occulants has
been studied extensively (McGarry 1970 ; Lee
et al. 1998 ; Papazi et al. 2010 ) and some of these
are applied in industry. Aluminium sulphate is an
effective coagulant being widely used in fl occu-
lating algal biomass ( Scenedesmus and
Chlorella ) in the wastewater treatment process
(Grima et al. 2003 ).
Although an easy and effective method, it is not
feasible and sustainable for large-scale harvesting
of microalgae because microalgae production
plants have excess cationic fl occulent that needs to
be removed from the medium before it can be re-
used, thus leading to extra operational costs
(Schenk et al. 2008 ). Flocculation can also be
induced by changing the culture conditions with
the application of extreme pH, nutrient depletion,
changes in temperature dissolved O 2 level. The pH
of the algal culture is adjusted to 10-10.6 using
NaOH, followed by the addition of non-ionic
polymer Magnfl oc LT-25. This method of harvest-
ing microalgal biomass leads to a concentration of
6-7 g L −1 (Knuckey et al. 2006 ). The process has
been successfully applied to a variety of species,
with fl occulation effi ciency of >80 %. The other
method that has been proposed for fl occulation of
microalgae is biologically induced, where bacteria
have been applied successfully in wastewater
treatment (Lee et al. 2009 ). The disadvantage of
this method is that it leads to undesirable bacte-
rial contamination of the algal production plant.
In recent years, naturally fl occulating diatom
Skeletonema has been used to form fl ocs of
Nannochloropsis (Schenk et al. 2008 ). As diatoms
have a silica-based cell wall, they require differ-
ent medium composition than most microalgal
strains used for biodiesel production, which leads
to additional cultivation costs. Chitosan is another
bio-fl occulant, and this method is very sensitive
to pH. For freshwater microalgae, the maximum
fl occulation was obtained at pH7.0, whereas, for
marine species, it is low (Divakaran and Pillai
2002 ). The remaining water can be reused for the
cultivation of algae. Heasman et al. ( 2000 ) used
chitosan as a fl occulant for Tetraselmis chui ,
Thalassiosira pseudonana , and Isochrysis sp. To
complete the fl occulation process of these strains,
Gravity sedimentation is commonly applied
and is a rapid intensive method for separating
microalgal biomass from huge volumes of
water (or wastewater). Gravity sedimentation
methods are based on Stokes's Law (Schenk
et al. 2008 ) (i.e. settling characteristics of sus-
pended solids are determined by the density
and radius of algae cells and induced sedimen-
tation velocity). Higher microalgal biomass can
be achieved by sedimentation using lamella
separators and sedimentation tanks. This
method is only suitable for harvesting of large
microalgae (ca. >70
m) such as Spirulina
(Muñoz and Guieysse 2006 ). The success of
solid removal via the gravity settling method
depends on the density of the microalgal parti-
cles. Low-density microalgal particles do not
settle well and are ineffectively separated by
settling (Edzwald 1993 ). This technique has
some advantages: it is cost effective, and it is
possible to recycle water because no potentially
toxic chemicals are used.
ʼ
11.4
Flocculation
An integrated approach is needed to minimize
the energy consumption of harvesting microal-
gae (Benemann et al. 1977 ). Several harvesting
methods have shown that fl occulation combined
with fl otation or sedimentation and followed fur-
ther by dewatering with centrifugation or fi ltra-
tion is the most promising and cost-effective
method (Schenk et al. 2008 ). This is the bulk-
harvesting technique whereby the dispersed
microalgal cells aggregate and form larger par-
ticles with a higher sedimentation rate.
Microalgal cells are negatively charged, which
prevents aggregation in aqueous suspension, so
multivalent cations or cationic polymers are use-
ful for the neutralization of microalgal cells.
Flocculation can be induced in different ways.
Induced chemical fl occulation using multivalent
metal salts like aluminum sulphate (Al 2 (SO 4 ) 3 ),
ferric
chloride
(FeCl 3 ),
ferric
sulphate
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