Environmental Engineering Reference
In-Depth Information
5.2
Harvesting of Microalgae
Separating algae from its medium or algal biomass concentration is called har-
vesting (Rosenberg et al.
2008
). It is a chief difficulty since algae are typically
found in water, so collection of algae is problematic and energy consuming pro-
cedure (Schenk et al.
2008
). The common methods used for harvesting are cen-
trifugation, foam fractionation (Haesman et al.
2000
), flocculation (Csordas and
Wang
2004
), membrane filtration (Knuckey et al.
2006
) and ultra-sonic separa-
tion (Rissignol et al.
2000
). Harvesting contributes to 20-30 % of total expendi-
ture (Bosma et al.
2003
). The harvesting method depends on algal species used,
cell density and culture conditions (Mata et al.
2010
). The four main steps of
harvesting are given below:
i.
Screening:
Screening is the first step in harvesting. The main principle is to introduce algae
biomass to a screen having specific aperture size, whose efficiency depends on
alga size and screen opening (Grima et al.
2003
).
a. Micro straining:
Micro strainer has a rotator drum which is covered by straining fabric, polyester
or stainless steel. The small algae aren't screened properly so there is a need of
dewatering as harvest is low (Bosma et al.
2003
). The problems encountered
by micro staining are particle fluctuations and low harvest, which are solved
by specific speed of drum rotation. The algae and bacteria also form a biofilm
on the fabric whose periodic cleaning is very essential for the process to occur
effectively (Bosma et al.
2003
; Grima et al.
2003
).
b. Vibrating screen:
Vibrating screen is mainly used for
Spirulina
harvest, which is a multicellular
and filamentous microalgae used as a food source. This process removes high
number of algae (90 %) for harvesting up to 21m
3
h
−1
. Algae slurry of 8-10 % is
formed, which covers 1/3 of area only when compared to inclined screens, which
cover 2-4 m
2
per unit (Bosma et al.
2003
).
ii.
Thickening:
Thickening is the process which increases the solid concentration of algae so that
it may be further treated. This process reduces the volume, which reduces down-
stream treatment cost. Some important methods are briefly described below:
a. Coagulation- flocculation:
Coagulation flocculation causes aggregation of algal cells into clumps for further
treatment. This process uses both organic and inorganic coagulants. Chitosan; a
water purification product; can also be used for this purpose. The chemical floc-
culation can be done as well but the main problem is that it is expensive. The
coagulant dosage used is very important and determined by bench scale jar tests
(Bosma et al. 2003; Grima et al. 2003).
