Environmental Engineering Reference
In-Depth Information
b.
Gravity sedimentation:
Gravity sedimentation process involves the solid-liquid separation, separation of
effluent as a clear liquid and feed suspension into slurry having high concentra-
tion. It separates algae in which clarity of overflow has primary importance and
algal feed suspension is dilute or concentrated when thickening of underflow is
done. This works best with heavy algae suspension (Bosma et al.
2003
).
c. Flotation:
Flotation is gravity thickening but upside down. The liquid-solid separation is
done by bubbling at the bottom of the tank rather than waiting for the particles
to settle. Once the particles reach the top of tank, thick layered slurry is formed
which can now be collected by skimming operation. This process uses light par-
ticles having low settling velocity, not allowing them to settle in the tank. The
main advantage of this process is that light algae particles are harvested in short
duration (Bosma et al.
2003
; Grima et al.
2003
).
d.
Ultrasound technique:
Ultrasound is used for harvesting algae but results show that its efficacy is less
because of small size and low particle density of algae. This technique is useful
in removing
M. aeruginosa
-a toxic algae species (as it causes the formation of
harmful algal blooms)-by its coagulation (Bosma et al.
2003
; Grima et al.
2003
).
iii.
Dewatering:
Dewatering involves the removal of water from the algae to the maximum extent
(Bosma et al.
2003
; Grima et al.
2003
). This is done by various methods; some
are described below:
a.
Filtration:
Filtration is done by forcing algae material to flow across filter medium and
water is drained out with the help of suction pump and algae mass is retained.
The main use of the process is that algae biomass of low density can be har-
vested. Filtration includes backwashing as a routine practice to avoid clogging
and fouling (Bosma et al.
2003
; Grima et al.
2003
).
b.
Centrifugation:
Centrifuges are analogous to sedimentation tanks, the one difference being the
separation by centrifugal force (higher than gravity), thus causing acceleration
of suspended particles. The higher the rotation speed of centrifuge, the quicker
the solid will spin out of the suspension. The supernatant is removed using skim-
ming tube. This method is efficient and reliable but expensive (Bosma et al.
2003; Grima et al. 2003).
iv.
Drying:
The last step of harvesting is the drying of algae, which is really challenging,
as algae are really delicate and can degrade reducing the quality of production.
Rotary dryers have sloped cylinders which dry by moving content from one side
and gravity to the other. Spray drying is an effective way of drying algae used
